Flow Quality Measurement Based on Stratification of Flow in

Nitrogen Gas-Water and HFC-134a Refrigerant-PAG Oil Two-

Phase Flow Systems

Du-Hyun Dwayne Hwang

.A thesis subrnitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Chemical Engineering and Applied Chemistry

University of Toronto

Copyright by Du-Hyun Dwayne Hwang (2001)

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Flow Quality Measurement Based on Stratification of Flow in Nitrogen Gas-Water and HFC- 134a Refhgerant-PAG Oil Two-Phase Flow Systerns (100 1)

Du-Hyun Dwayne Hwang, M.A.Sc. Department of Chemical Engineering and Applied Chemistry

University O l Toronto

A simple method is proposed and tested to rneasure the flow quality in a closed

two-phase loop based on the use of a relatively large diameter horizontal tube to stratify

the two-phase flow. If the total rnass flow rate is known, the tlow quality can be rrliably

determined by measuring the ;as phase velocity with a hot-wire anemorneter and the void

fraction in the horizontal tube. On the other hand. i f the total mass flow rate is not known.

simplified one-dimensional momentum equations can be solved using the measured void

fraction and gas velocity to obtain both the total flow rate and flow quality. This

measurement principle was tested using nitrogen gasiwater flow loop and good agreement

was obtained between the measured and actual flow qualities. Tests were also performed

using a compressor-driven refngerant loop, where the presence of a lubricant oïl was

determined to have a detrimental effect on the determination of thermodynamic quality

based on a heat balance.

ACKNOWLEDGMENTS

I would like to express my sincere appreciation to Professor Kawaji and Dr.

Dickson for their excellent guidance and direction during al1 phases of this thesis project.

Their patience and encouragement were crucial for the successful completion of this work.

1 would also like to express my special thanks to al1 the R&D members at Halla

Clirnate Control Canada Inc., especially, Charlie Jung, Don Harvey, Wayne Whittle,

Jim Mossman, Andrew Treverton, and Leonard Park (PIant 2 Product Development

group), for their unconditional help throughout this work. Without their assistance,

encouragement and ideas, this thesis would have been a much more Formidable

undertaking.

I would like to thank everyone in Professor Kawaji's thermal-hydraulics lab ai the

University of Toronto, for helping me out, in one way or another. over the last two years.

I am thankful to the Material and Manufactunng Ontario for giving me an

industrial master's degree scholarship as well as to Halla Climate Control Canada Inc. for

providing the funding for this work.

1 owe more than I can ever manage to pay back to my family, father Kyu-Tack

Hwrng, mother Young-Ih Kim, sisten Helene and Sonia, for their sacrifice, love. and

understanding.

1 owe sincere and special thanks to Jennifer Jung, to whom 1 will be happily

manied on September 29Ih of this very year. Her love, patience, and understanding were a

blessing From God that kept me going in the times of difficulties.

Lastly. this acknowledgement would not be complrte without thanking God for

His abundant grace in my life.

TABLE OF CONTENTS

. . ............................................................................................... LIST OF TMLES vii

... LIST OF FIGURES ............................................................................................ vil1

NOMENCLATURE . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . ~ ~ . m ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ . ~ ~ ~ . ~ ~ ~ ~ ~ x

CHAPTER 1 . INTRODUCTION ....................................................................... 1

1 . 1 BXCKGROLND ..................................................................................................................... 1

7 1 . 2 OBJECTIVES ......................................................................................................................... -

........................................................... CHAPTER 2 . LITERATURE REVIECV 3 ...................................... 2.1 V.APOL:R.CO~~!PRESSIOX .UTOMOTIVE REFRIGERATIOS SYSTEM 3

2.2 Acccb iu~xro~ ................................................................................................................... 4

......................................................... 2.2.1 Evaporation of Refrigcrant in the Accumulator 9

2.2.2 Liquid Can-yover ...................................................................................................... 10

.......................................................................... 2.3 LUBRICAST I N REFRIGER=\TION SYSTEM I O

2.3.1 Polydkylene Glycol (PAG) ...................................................................................... I l

2.3.2 Solubiiity of Refngerant in Lubncant Oils ......................................................... 12

2.4 FLOW QL!.ALITY ME=\SCREMENT .................................................................................... 13

4 . 1 Void Fraction Measurement ................................................................................ 13

2.3.2 Measurement of Flow Dependent Panmeters ....................................................... 15

............................................................................. 2.4.2.1 Di fferential Pressure Method 1 6

2.4.2.2 Turbine Flow Meter Methode ........................................................................... 16

..................................................................................................... 2.4.2.3 Tracer Method 16

2.4.2.4 Flow D n g Method .............................................................................................. 17

2.43 Existing Quality Measurement Method for HFC-134a Rehgennt ......................... 17

2.3.3.1 Abdul-Razzak et al . ( 1995) ................................................................................. 18

2.4.3.2 Solberg et a1 . (2000) ............................................................................................ 19

CHAPTER 3 . THEORY .................................................................................... 20

3.1 STRATIFIED FLOW ............................................................................................................. 20

3.2 QU.AL~W MEASUREMENT [N STRATIFIED FLOW ............................................................ 3 1

3 2 . 1 Interfacial Level Gradient (ILG) Method ................................................................. 1

3.2.2 Differentiai Pressure Measurement Method ............................................................. 23

3.2.3 VelocityandVoidFraction(V&V)Method ............................................................ 24

77 3.3 FLOW RATE ME.ASCREMENT I N STRATIFIED FLOW .......................................................... - .

CHAPTER 4 . EXPERIiVIENT.4L ..................................................................... 30

4.1 ENPERIMENTXL SET-UP .................................................................................................... 30

4.1 . 1 Flow Quality bleasurement System ......................................................................... 37

.............................................................................................. 4.2 ESPERIMENTAL APPROACH 40

4.3 C.ALIBRXT[ON OF HOT-WIRE ANEMOXIETER FOR USE LVITH HFC- 1 34.4 V.4POI.R ........... 45

4.4 TEST CONDITIONS AND PROCEDURE ................................................................................. 49

5 VALLDATION OF ESPERIMENTAL SET-UP ......................................................................... 50

4.5. System Validation of the XTS .................................................................................. 5 1

4.5 2 Determination of the Xmount of PAG Oil in Circulation ........................................ 52

4.5.3 HFC-134a in the Liquid Phase of the Trvo-phase Stratifitid Flow ........................... 53 . .

4.6 DATA R E D K T 1 0 8 ............................................................................................................. 33

......................................................................................... 4.7 ESPERIMENTAL L~NCERTXINTY 56

CHAPTER 5. RESULTS AND DISCUSSION ................................................ 58 5.1 N:.,,. WATER ESPERIMEYTS .............................................................................................. 5s

5.1 . 1 Flow Quality Measurement usine V&V 'ulethod .................................................... 58

5.1.2 Determination of Total Mass Flow Rate of a Stratified Two-Phase Flow ............... 59

............................................................................................ 5.2 REFRIGERANT ESPERIMENTS 62

...................................................... 5.2.1 Flow Observation and Thermodynarnic Quality 62

5.2.2 Flow Quality and Mass Flow Measurernents using V&V biethod .......................... G4

3.7.3 Flow Quality and Mass Flow Measurements using VSrV 'vlethod based on

Moditied Calibration Coefficients ............................................................................ 66

CHAPTER 6 . CONCLUSIONS AND RECOiMMENDTIONS ................... .. 72 . 6.1 C o ~ c ~ t ~ s r o ~ s ................................................................................................................... 13

6.2 RECOMMESD.AT1ONS ......................................................................................................... 73

REFERENCES ...............................q..b...qo..q...qmq...-..q.mq..q.m......................qq........m... 75

LIST OF TABLES

LIST OF ACCCMCLATOR FCXCTIONS 1s .AN A/C SYSTEM ............................................. 8

N 2 , , , - C V . , ~ ~ ~ ~ EXPERIMEYT: FLOW COXDITIONS . WATER HEIGHT. A N D .AS ELIOMETER

OCTPCT SIGNAL ......................................................................................................... 43

RESLLTS OF Ix SITC CXLIBR.ATIOX OF HOT-LVIRE ASEMOMETER I Y H F C - I ~ ~ A

VAPOCR .................................................................................................................... 48

COSDITIO';S FOR REFRIGER..\YT ESPERIMENT ........................................................... 49

VALIDATION TEST RESCLTS ...................................................................................... 51

SA.LIPLIK RESL'LTS ON THE LIQCID FRACTION OF STR.I\TIFLED TWO-PHASE HFC-

.............................................................................................. 134.4-PAG O i t FLOIV 54

ESPERI'~[ENT.-~\L USCERT.AIXTY ................................................................................. 57

RESCLTS OF TEMPER.ATURE SCASNIXG AROCXD EVAPOR.ATOR ST.-\ND .................... 37

R ~ s c t - r s OF HFC-134.4-PAG O I L FLOW QCALITY .GD MASS FLOW MEASL~REM ENTS L'SIXG V&V METHOD BASED ON MODIFIED CALIBRATIOS ASSUMIXG .Y = I ......................................................................................................... 67

RESULTS OF HFC-134.4-PAG OIL FLOW QCALITY A N D MASS FLOW MEASUREMENTS LSING V&V METHOD BASED ON MODIFIED CALIBRATIOX ASSCMING .Y C 1 ..................... .. .................................................................................. 70

ATS PAR~METER SETTIXGS CSED FOR v . 4 ~ 1 ~ .A TION TEST RCXS ............................... 80

vii

LIST OF FIGURES

2.6 PRESSCRE-ESTHALPY DIAGRA.CI OF V..IPOCR.~OMPRESSIO .: CYCLE FOR HFC-134.1 ['I CASE OF LIQL'ID CXRRYOVER ............................................................................. 1 1

3.1 FLOW GEOMETRY CSED IN SADATOMI ET .A L.'S MODELISG ...................................... Y

3.2 POSITION OF HOT-WIRE ASEMOM ETER SENSOR ........................................................ 25

3.3 STR..ITIFIED TWO-PHASE FLOW BETWEEY TWO FLAT PLATES ................................. 2 7

4.1 SCHEMATIC OF THE EXPERIMESTAL LOOP (ATS) .................................................. 31 7 9 4.2 CONDE 3 SER COIL ...................................................................................................... J J

4.3 SCHEMATIC OF THE CONDENSER STAYD ................................................................... 33

4.4 REFRIGEMNT FLOW METER .A 'iD SIGHT GLASS ........................................................ 34

4.5 ~ ' ~ E T E R I N G VALVES CSED .AS EXPAXSION DEVICE .................................................... 34

4.6 EVAPORATOR STAND ................................................................................................. 36

4.7 !%HEMATIC OF THE HEATIXG UXIT INSIDE THE EL'APORATOR STAND ....................... 36

............................................................................................ 4.8 .ACCLML LATOR STAND 37

4.9 SCHEMATIC OF THE FLOW QL'ALITY MEASILREMEST SYSTEM .................................. 39

4.1 O SCHEMATIC OF N2,,; ILVATER LOOP ....................................................................... 41

4.1 1 COMP .A RISON OF AVERAGE VELOCITY WITH .AL XEMOMETER OLTPUT SIGNAL FOR

........................................................................ DIFFEREKT WATER LEVELS AT I OC 44

4.13 PLOT OF AVERAGE VELOCITY XGAINST ASEMOMETER OL'TPCT SIGXAL FOR hL 5 4 mm ........................................................................................................................... 44

4.13 SCHE.LI.ATIC OF THE REFRIGERWT FLOW LOOP ....................................................... 46

4 . 1 SCHEMATIC OF THE HELICAL LIQL'IDNXPOCR SEPARATOR ..................................... 47

..... 4.15 D~GITIZED IMAGE OF A LIQCID STREAM IN .A REFRIGER;\'~'T TWO-PHASE FLOW 50

4.16 SAMPLING Uxrr ............................................................ ........................................... 54

COMPARISON BETWEES ACTUAL A N D CALCGLATED FLOW QUALITIES FROM

....................................................................................... N 2 i c j W . ~ ~ ~ ~ EXPERIMEST 59

AVERAGE LIQUID HEIGHT L'SED IS ESTI.~IATIYG THE AVERAGE LIQCID VELOCITY ............................................................................................... L'SIYG EQU.ATIO': ( 18) 60

COMPARISON BETWEEN ACTCAL AND CALCYLXTED FLOW RATES FROM

N 2 1 . , / W . ~ ~ ~ ~ EXPERIME'IT ....................................................................................... 61

OBSERVATIOY OF HFC- 134.4-PAG Or L FLOW EXITISG EV.APORATOR AYD

ENTERISG STR~TIFICXT~ON TCBE :IT VARIOL'S T H E R ~ I O D Y N . A I C QC ALITY ........... 63

COMPARISON BETlVEE'I ACTC.\L .A ND CXLCULXTED FLOW RATES FROM HFC- .................................................................................... 134.v'P.AG OIL ESPERIMENT 65

PLOT OF TEMPERATCRE AGAINST ANEMOMETER OCTPL'T SIGYAL FROhl

............................................ C A L I B R ~ T I O N RUNS ASD REFRIGERANT EXPERIMENTS 66

COMPARISON BETWEEN ACTCAL AND CALCCLATED FLOW RATES FROM HFC- ............................ 134.4PAG OIL ESPERIMEYT BASED 00; MODIFIED C.~LIBRATION 68

........................... PLOT OF MEAS DEVIATIONS AGAINST ASSCMED FLOW QCALITY 71

................................................ LOGIN PROMPT OF THE ATS L.L\BVIEW PROGR~M 79

......................................................... XTS LABVIEW PROGRAM CONTROL P.ASEL Sl

NOMENCLATURE

area

conversion factor

diameter

differential or derivat

friction factor

.ive operator

acceleration due to gravity

enthalpy

height

saturation enthalpy

interfaciai area per unit axial length

a i a l distance

mass flow rate

calibration constants

pressure

heat

radius

Reynolds number

thic kness

average veloc i ty

local veloci ty

width

weight

flow quality

thermodynamic quality

a i a l distance

Lockhart-Martinelli parameter

Greek Symbols

ci

t I tC'A

1

L

\VG

\Y L

total

void fraction

di fference operator

wetted perimeter

latent heat of vaporization

dynamic viscosity

densi t y

shear stress

angle

Subscripts

gas

hot-wire anemometer

interface

liquid

wetted by gas

wetted by liquid

total

Glossary of Terms

A/C air-condi tîoning

ASHRAE American Society of Heating, Refrigerating and .4ir Conditioning Engineers

ASME

ATS

Comp.

Cond.

CT

Evap.

HWA

Hwy

O.D.

PT

ST

TC

TorT

VT

Arnencan Society of Mechanical Engineers

AccumuIator Test System

cornpressor

condenser

current transducer

evaporator

hot-wire anemometer output signal

highway

outer diameter

pressure transducer

speed transducer

thermocouple

torque transducer

voltage transducer

xii

CHAPTER 1. INTRODUCTION

1.1 Background

.Arnongst automotive A/C system engineen there have been several hypotheses

regarding the canyover of liquid reîi-igerant through an accumulator. which is basically a

liquidhapour separator and liquid reservoir installed to prevent entry of the liquid

refrigerant into a cornpressor. I t is hypothesized that liquidivapour separation. droplet

entrainment, and boiling in the reservoir are contributing to liquid canyovsr frorn the

accumulator. However, the problem has not been studied extensively to date.

Performance of accumulators is typically evaluated by measunng the overall performance

of specific NC systems, and the accumulator has not been evaiuated as a separate

component. Hence. it has been difficult to separate the accumulator function from the

effects of other parameters, such as evaporator and condenser conditions, under the hood

temperature and airflow, tube and hose length and routing, and clutch cycling. Even the

actual conditions typically experienced by accumulators have not been well establishrd.

It has become more important to understand how the accumulator functions

because of the drive to use altemate materials in the automotive industry. Accumulators

are traditionally manufactured of steel, but the industry trend is towards aluminum,

plastics, and composites in order to improve performance and reduce weight. However,

the choice of appropriate matenals for accumulaton is stymied by the lack of

understanding of the accumulator function. For instance, i t is found that aluminum

accumulators do not perform in the same marner as the steel ones, so an insulation

material is wapped around the aluminum accumulators based on a speculation that there

is deletenous boiling of refngerant in (highly conductive) aluminum compared to steel

accumulators. In general. some modifications are made to the existing accumulator

design based on the presumed effects of material properties and hypothetical accumulator

functions.

This study was proposed by Halla Climate Control Canada. Inc. (HCC) in

Belleville. Ontario to understand how the accumulato r functions, so they c m leverage

unique propenies of altemate materials when designing new. high performance

accumulators. Hence. the present work has been conducted as a joint project. and most o r

the experiments were conducted at HCC.

1 .Z Objectives

An ultimate goal of this work was to provide design engineen of accumulators For

the automotive air-conditioning ( N C ) system with a diagnostic tool to detemine the

performance characteristics when designing new. high performance accumulators. To

accomplish this task, a refngeration loop capable of simulating various operating

conditions of the automotive MC system was built and empirically validated for three

representative operating conditions, namely, idle. city driving and highway driving

conditions. Then, a method suitable For measuring the flow quality of two-phase

refngerant liquid-vapour flow into and out of the accumulator was developed and verified

for its accuracy.

Another major goal of this work was to check the effect of the presence of a

lubncant oil on the detemination of thermodynarnic quality based on a heat balance.

CHAPTER 2. LITERATURE REVIEW

2.1 Vapour-Compression Automotive Refrigeration System

A vehicle air-conditioning system conventionally includes a compressor, a

condenser, an expansion device, an accumulator and an evaporator (Kargilis, 1994) as

shown in Figure 2.1.

Expansion Device

Condenser + AccumuIator Cornpressor

Figure 2.1 Vapour-Compression Refrigeration Cycle

Refngerant is circulated through the system to transfer heat from the evaporator to

the condenser. Energy is provided to the system by the compressor which is driven by the

automobile's engine and serves to create a source of high pressure refrigerant vapour that

is allowed to pass through the condenser. The refrigerant dissipates heat in the condenser

and changes phase to a high pressure liquid. The heat dissipation is to the outside

environment and is typically achieved by blowing arnbient air over the condenser. The

refrigerant condensate then passes through an expansion device becoming a jas-liquid

hvo-phase mixture and flows into the evaporator, where an air-blower circulates warm air

from the vehicle passenger compartment over the evaporator. The consequent heat

transfer from the passenger compartrnent air to the evaporator causes the refngerant to

change to a mostly gaseous state. The refigerant is then drawn fiom the cvaporator into

the accumulator and subsequently back into the compressor as a low pressure vapour.

where i t is again compressed into high pressure vapour to repeat the cycle.

Generally, the section between the cornpressor outlet and the evaporator inlet is

refcrred to as the "high side" or "discharge side", and the section between the evaporator

outlet and the compressor inlet is referred to as the "low side" or "suction side".

2.2 Accumulator

An accumulator is essentially a vapoudliquid separator that prevents any liquid

refngerant from passing from the evaporator to the compressor as liquid refrigerant

passing to the compressor causes a drarnatic reduction in the MC system performance. A

cross-sectional view of a typical automotive AIC system accumulator and a drawing of a

typical accumulator intemal are s h o w in Figures 2.2 and 2.3, respectively. .An

accumulator is normally located between the evaporator and the compressor in the

refrigeration system (Figure 2.1). The output of the evaporator includes more than just

refngerant, carrying some lubricating oil and a small amount of water, al1 three of which

form a vapour-liquid mixture. In most systems, the two-phase mixture from the

evaporator output enters the accumulator through an inlet fitting and pours across a baffle

or so-called "defiector". The liquid collects at the bottom of the accumulator, while the

vapour nses and returns to the compressor via the "Outlet tube or J- tube". The deflector

prevents splashes of liquid from getting into the outlet tube. Entering refrigerant impinges

upon the deflector, and any liquid such as liquid refrigerant, water, and oil is separated oui

Figure 2.2 Cross-sectional View of a Typical Accurnulator

Figure 2.3 A Typical Accumulator Intemal

and deposits at the bottom of the accumulator while the gaseous refrigerant is permitted to

pass to the compressor through an open end of the J-tube that is located under the

deflector. The other end of' the J-tube is connected to the accumulator suction line, which

is in ttim connected to the compressor inlet, so that the compressor draws primarily

caseous refrigerant. L.

.A desiccant bag is placed within the accumulator for removing moisture in the

refrigerant. The desiccant material is an alumino-silicate zeolite. and water chemically

adsorbs into small pores that are present in the ortho-rhombic crystal structure. XH-7

desiccant was specially fomulated to rernove water from a mixture of HFC-134a

refrigerant and PAG Oil. ORen a tracer dye is included with the desiccant matenal. The

tracer dye dissolves into the liquid refrigerrint and is distributed rhroughout the whole ;L C

system in order to allow for early and accurate leak drtection. Another special feature of

this dye tracer is that it glows bnght green upon UV irradiation. a characteristic that nids

leak detection.

A bleed hole assembly, consisting of a srnaIl hole in the return tube and a

surrounding filter screen. is provided in the bend of the J-tube near the bottom of the

accumularor. The assembly is intended to withdraw a rnetered amount of oii in the liquid

refrigerant and return it dong with the refngerant vapour passing through the tube to the

compressor for lubrication. The size of oil bleed hole ranges between i-2 mm. depending

on the arnount of oil required by the compressor.

-4s shown in Figure 2.1, an anti-siphon hole is located in the .i-tube nrar its outlet just

below the tube-to-deflector joint to prevent liquid refkigerant from siphoning into the

suction line when the A/C system is off. The hole prevents siphoning by equalizing the

pressure inside the accumulator.

Figure 2.4 An Anti-Siphon Hole

.4nti-Siphon Hole near J-Tube Outlet

The functions of an accumulator are summarized in Table 2.1.

Table 2.1 List of Accumulator Functions in an A/C System Functions

1. Separate the hvo-phase refngerant mixture into its liquid and vapour components

2. Act as a reservoir for liquid refngerant at al1 operating conditions For it to eventually turn to a gaseous state before being retumed to the compressor

1 3. Provide for recovery of lubricating oil contained in the refrigerant I 1 4. Dampen compressor pressure pulsation I 1 5 . Remove moisture fiom the A/C system, if required I

6. Contain tracer dye for leak detection, if required 1

2.2.1 Evaporation of Refrigerant in the Accumulator

A crucial functional requirement of an accumulator is to vaporize any liquid

refngerant received from the evaporator prior to its return to the compressor. In the case

of a motor vehicle air-conditioning system. the accumulator assembly is provided with a

bracket as shown in Figure 2.5 attached ont0 a fixing point on the bodywork under the

hood, for example with a clip or strap, which embraces the accurnulator assembly. I f the

amount of refrigerant collected in the accumulator assembly is too large. Le.. if the

vapounzation of the Iiquid refrigerant is not sufficiently effective. the performance of the

air conditioning unit may deteriorate due to liquid refrigerant canyover to the compressor.

On the other hand, if the vapourization of liquid is too effective, then the system

performance may also deterionte due to excess refrigerant circulation. It is theretore

necessary to ensure adequate vapourization of the refigerant in the accumulator assembly.

(a) Bracket (b) Accumulator Assembly in Bracket

Figures 2.5 Accurnulator Bracket

1.2.2 Liquid Carryover

Potential ways in which liquid refngerant c m flow through the accumulator and

flow into the cornpressor are as follows:

The accumulator could be too srnall such that the accumulator is filled up with

liquid which pours into the outlet tube.

There is significant frothins in the accumulator and the froth cames liquid

into the outlet tube.

The Oil Bleed Orifice is too large and allows too much liquid into the tube

bottom.

The liquid gets into the anti-siphon hole.

System turbulence occurs and liquid gets into the anti-siphon hole or outlet

tube.

In case of liquid canyover, liquid refrigerant flowing towards the compressor

(Points 3' and 4' on the pressure-enthalpy diagram s h o w in Figure 2.6) flashes or

evaporates in the compressor cylinders, and this reduces the cornpressor's ability to draw

gaseous refngerant from the accumulator and eventually From the evaporator. .As a

consequence. the evapontor pressure and temperature rise resulting in reduced system

performance.

2.3 Lubricant in Refrigeration System

Refrigeration systems require lubt-icant not only to lubncate the system but also to do

some other important tasks. The lubncant oil plays an essential role in sealing

compressed rekgerant vapour behveen the suction and discharge sides as well as in

removing heat From the bearings and transferring heat from the crankcase to the

compressor exterior. Lubticant oil also reduces the compressor noise by lubricatin;

intemal moving parts (ASHRAE Handbook, 1998).

Figure 2.6 Pressure-Enthalpy Diagnm of Vapour-Compression refrigeration cycle for HFC- 134a in case of liquid carryover

2.3.1 Polyalkylene Glycol (PAG)

The lubricant used for refrigerant HFC-134a is a synthetic Iubncant generally referred to

as PXG oil. PAG oil derives either from ethylene oside or propylenc oside and is

uniquely formulated to provide lubrication to automotive compressors and system

components where ozone-friendly HFC refrigerants are used (ASHIWE Handbook.

1998). PAG oil was chosen in this work because it is soluble in HFC-134a. and hence

will not collect in the condenser, evaporator, or tubes. In ihis study, 46 ISO VG PAG oil

tvas used.

2.3.2 Solubility of Refigerant in Lubncant Oils

Due to highly soluble nature of refngerant gases in PAG oil, even with fairly

efficient oil separators ai the cornpressor outlet, the lubricant oils always find their way

into the low-pressure sides of the IVC system (Jacobs et al., 1976). The solubility of

refrigerant in lubricant oil is a function of the pressure of the gas. the temperature of the

lubricant, the nature of the sas, and the nature of the lubricant.

Halogenated refngerants such as HFC- 134a are known to have high solubility in

PAG oil ai any temperature likely to be encountered in automotive tVC systems. and dur

to this nature. thesr refngerants should be treated as a refrigerant-lubricant mixture. rather

than as a pure refrigerant (ASHRAE. 1998). For instance, addinp an appreciable amount

of lubricant to liquid refngerant c hanses the thermodynamic properties of the re figerant

so that the vapour pressures of refrigennt-lubricant mixtures at a given temperature are

always less than the vapour pressure of pure refrigerant at that temperature. Cavesiri

( 1993) reported the measurement of the solubili ty, viscosity, and densi ty of çevenl types

of PAG oil with HFC-1 34a. His measurements were attempted at up to 60 wt 96 of HFC-

13 Ja in PAG oil. However. the oil content for an automotive A/C system recommended

by many cornpressor manufacturers is between 4-7 ~ t ? ~ (Halla Intemal Document).

Unfortunately, there are no thermodynamic property data on HFC- 13la-PAG oil

mixtures available in the open literature over a pnctical concentration range used in

automotive iVC systems. This prevents reliable determination of quality or vapour mass

flow rate fraction in refrigerant two-phase flows.

2.4 Flow Quality Measurement

Flow quality is the ratio of the vapour mass flow rate to the total mass flow rate

expressed as.

any appli cations in refrigeration and air-c onditioning indi ~stnes, the knowledge of

flow quality in refrigerant two-phase liquid-vapour flocv is required, since the flow quality

is an important parameter for evaluating the performance of IVC components such as

accumulaton, evaporators, and condensers. where the entenng and/or exiting flows are

usually in a two-phase condition. Hewitt (1978) made clear that for a two-phase flow. if

the toial mass flow rate is known. then the flow quality. r. can be deduced by measuiin_r

two other parameters simultaneously, notably, either velocity or volumetnc flow raie

dong with a measure of the mixture density or void fraction. u. In this section, various

measurement methods for void fraction and other flow dependent parameters reviewed by

Hewitt ( 1978) are briefly summarized.

2.4.1 Void Fraction Measurement

Hewitt (1978) divided various two-phase void fraction measurement methods into

four categories:

Radiation attenuation or scattering methods

Impedance methods

Direct volume measurement method

Miscellaneous methods

The radiation attenuation method basically uses a finely collimated beam

generated by a radiation source, and the attenuation of the beam by the fluid is detected to

deduce the void fraction. Gamma. beta, and X rays are widely used for this technique and

numerous applications of this technique to void fraction rneasurement have been reported.

for example. by Vince and Lahey (1982), Abdul-Rmak (1995). and Badie et al. (1000).

Difficulties associated with this method arc relatively high cost. the requirement of in-situ

calibration. and laborious data processing and interpretation.

Scattering method is another radiation-related technique in which a beam of

gamma rays. X rays. or neutrons is passed through the îluid and sets scattered. The

scattered and subsequently transmitted radiation is thrn measured and interpreted to

determine the void fraction. Different applications of this method were examined by

several researchers, for instance. by Kondic and Hahn ( I W O ) and Banejee et al. ( 1978).

Impedance methods involve the rneasurement of electrical conductance and

capaci tance. Unlike the radioactive and scattenng methods, these methods do not require

data processing and yield virtually instantaneous response. In a hvo-phase mixture.

different concentrations of phases result in different dielectnc constants. and therefore.

create different electrical impedance. These differences in electrical impedance coupled

with careful calibration under conditions close to the actual measurement conditions can

be used for void fraction determination. The major difficulty with this method, however.

is that the relationship between the void fraction and the impedance depends on two-phase

flow pattern, which requires separate calibration for each flow pattern. Void fraction

measurements using this method were recently reported by Xie et al. (1990), Sadatomi et

al. ( 1993), Ali et al. (1 993), and Abdul-Razzak (1995). arnong others.

The direct volume measurement method is the most precise void Fraction

measurement technique since it involves the direct rneasurement of the volumes of both

phases in a two-phase flow. This method uses a technique called "quick-closing valves"

where two valves, one at the inlet and the other at the outlet of the test section, are

opented quickly and simultaneously in order to take a sarnple of the two-phase mixture

under investigation. The sample is then analyzed to detemine the void fraction.

Hashizume (1983) used this method to measure the void fraction in two-phase flow of

reftigerants R 12 and R22 in a horizontal pipe.

Other rnethods that are used somewhat less commonly for void fraction

measurement are acoustic techniques, measurements of average phase velocity. electro-

rnagnetic flow metering, optical methods, rnicrowave absorption. nuclear magnetic

resonance, the use of the relation betareen pressure and flow oscilIations. infrared

absorption methods, and neutron noise andysis.

2.4.1 Measurement of Flow Dependent Parameters

Hewitt (1978) descnbed the following four most widely applied methods for flow

dependent parameter measurernent. He suggested the usual practice is to use

"Instrumentation rakes". i.e. a combination of more than hvo fiow dependent

measurement rnethods.

2.4.2.1 Di fferential Pressure Method

This method makes use of differential pressure devices such as orifices and venturi

meters. The advantage of this method lies in the ease ofrnanufacturing and relatively low

cost. ASME (1959) provides detailed information on design and manufacturing of

various differential pressure type flow metering devices. However, difiçulties of this

method with two-phase flow measurement involve the flow resistance provided by the

device itself and the need for careful in situ calibration in order to obtain a good degree of

accuracy. Recent applications of this rnethod were reported by Inatsu et al. (1992).

Abdul-Razzak et al. ( 1995), and Badie et al. (3000).

2.4.2.2 Turbine Flow Meter Method

In this method, the flow rotates a rotor inside the turbine flow meter and the rate of

revolution gives an indication of the flow rate. The method yields fairly rapid response.

however. the need for in situ calibration, high cost, extreme sensitivity to flow pattern and

the difficulty associated with interpretation of signal output for a two-phase flow are the

inherent drawbacks. Recent applications of this rnethod were reponed by Kim and O'Neal

( 1994), and Abdul-Razzak (1 995).

2.4.2.3 Tracer Method

This method uses a tracer which is injected into the fluid and whose motion is

observed to detemine the velocity. For a hvo-phase flow, discrete tracers can be used to

separately label each phase. Although the potential of this method is enomous due to its

venatility, there are practical difficulties in its application due to the problem of

distributing the tracer unifomly throughout the flowing fluid, mass transfer and mixing of

the tracer at the interphase. and the changes in flow pattern and velocity between the point

of tracer injection and the point of detection. Nevertheless. Lorencez et al. ( 1997)

recently reported successful application of this method coupled with the activation of a

photochromic dye by an ultraviolet light beam.

2.4.2.4 Flow Drag Method

Typically, a perforated screen plate occupying the full cross-section of the pipe is

placed normal to the flow, and the drag force exerted on the plate is measured. The drag

screen plate is attached to a transducer that c m produce output signals proportional to the

force experienced by the screen. The output signals are then used to deduce the avcrage

mass flux. Inherent di fficulties with the use of this rnethod are the resistance conferred by

the plate on the flow and the problem in the averaging process for severely non-uni fonn

flows.

2.4.3 Existing Quality Measurement Methods for HFC- 134a Rehgerant

Attçmpts have been made to apply various hvo-phase flow quality measurement

methods to HFC-134a rehgerant flows. Below is a summary of the relevant work.

2.4.3.1 Abdul-Razzak et al. (1 995)

Abdul-Razak et al. ( 1995) performed extensive measurements to formulate an

empirical correlation between the quality and void fraction using a closed refngerant flow

loop charged with HFC-134a. They used a rnulti-ring capacitance transducer to measure

the void fraction, which was calibnted statically in situ and cornpared with a gamma

densitometer. Their data covered the quality range of 0.01 < .Y < 0.91 for the mass flux

range of 78 to 670 kg/m2-sec. and al1 flow regimss except for highly dispersed bubbly

tlow. The correlation they proposed is as follows:

where the Lockhart-Martinelli parameter. X, is de fined by

They recommended the use of the above correlation for the cases where the

thermodynarnic quality cannot be calculated using an energy balance.

Abdul-Razzak et al.'s (1995) correlation was developed using a refrigerant flow

loop charged with HFC-134a refrigerant but no lubricant, since they used a gear pump to

circulate subcooled liquid refngerant, instead of using a cornpressor to compress gaseous

refigerant. However, as mentioned earlier. al1 the automotive N C systems require

lubricants, and the refigerant-lubncant mixture has remarkably different physical

pmperties than the pure refrigerant.

2.1.3.2 Solberg et al. (7000)

Solberg et ai. (2000) used a calorimeter to measure the quality of two-phase

refrigerant flow that contained a lubncant oil. They added a known arnount of energy into

the two-phase reffigerant flow using an electric heater as it passed through the

calorimeter. AAer al1 the liquid in the flow was vaporized, the temperature and pressure

were measured to estimate the enthalpy of the refngerant exiting the calonmeter. Then.

the enthalpy at the inlet of the calorimeter. h,,, was determined from the outlet enthalpy.

hou,, by subtracting the heat input, Q.

Based on this information they estimated the quality. This method of energy balance for

the quality measurement can be achieved only by refemng to known thermodynamic data.

However, to the author's knowledge there is no enthalpy data availablc for the HFC-

134a-PAG oil mixture in the literature. A similar approach to deteminc the quality of

HFC- 1 %a-PAG oil mixture was also reported by Kim and O'Neal ( 1994).

In summary, previous works involving the quality measurement of two-phase

HFC-134a flow seemed to have ignored the effects of the existence of PAG oil on the

re fiigerant.

CHAPTER 3. THEORY

In this chapter, three new methods are proposed to measure the flow quality of

HFC-13Ja refrigerant-PAG oil hvo-phase flow on the suction side of an automotive .WC

system. These proposed methods are based on the stratification of two-phase flow in

honzontal tubes with a relatively large diameter. At last, one method is proposed to

determine both the flow quality and total mass tlow rate of a stratified two-phase flow

using a simplified one-dimensional momentum rquation.

3.1 Stratified Flow

Stratified flow is a two-phase flow pattern typically observed at relatively low

flow rates of gas and liquid in a honzontal pipeline. In this flow pattern. the gas and

liquid phases are segregated due to gavity so that the gas phase flows over the liquid

phase with a smooth or wavy interface separating the two phases. Ali et al. (1993)

investigated adiabatic cocurrent flow of air and water through a channel between two flot

plates (230 mm long and 80 mm high) with gap-widths of 0.775 mm and 1 A65 mm for

several different orientations. For horizontal flows between vertically oriented plates.

they found that for superficial gas velocities (volumetric flow rate .' channel cross-

sectional area) From 0.1 to 10 mis, stratified flow was seen at al1 superficial liquid

velocities less than 0.3 mis for a gap-width of 1.465 mm and 0.5 m/s for a gap-width of

0.778 mm. Their results indicated that if the channel height is sufficientiy large. stntified

flow can exist even at high gas flow rates unlike in smaller channels where intermittent

flow would appear.

3.2 Quality Measurement in Stratified Flow

3.2.1 Interfacial Level Gradient (ILG) Method

Based on a one-dimensional two-fluid mode1 and assuming both a smooth

interface between the phases and no phase change in a circular horizontal pipe. Sadatomi

et al. ( 1 993) developed a momentum equation for well-developed steady, cocurrent. gas-

liquid stratified flows as s h o w in Figure 3.1.

Figure 3.1 Flow Geometry used in Sadatomi et al .3 Modeling

Their mixture momentum equation took into account the effect of interfacial level

gradient (ILG), i.e., the variation in the liquid level in the flow direction, dltL!d~. It results

from different exit conditions: the hvo-phase flow flowing into a pool of liquid (dhLds >

O) or the two-phase flow exiting as a free discharge (dliL/dx c O). The two-phase mixture

momentum equation derived is as follows:

' \VG 'N'Ci - 'IVL '\VL

AG AL

where TIVG and s i ~ r = shear stresses at the fluid-wall boundary

t, = shear stress at the interface

/1vG and ItyL = channel perirnetsrs wetted by gas and liquid

1, = interfacial area per unit axial length

.AG and AL = cross-sectional areas for gas and liquid phases

Applicability of the above equation is limited to the stratified flow bounded by the

flow pattern transition boundary between the stratified and intermittent flows given by

Taitel and Dukler's ( 1 976) equation,

In a vapour-compression refrigeration system, the total mass flow rate. m,,,, can be

readily measured at the condenser outlet, since the rehgerant exitin3 the condenser is a

subcooled single-phase liquid flow. Stratification of the two-phase tlow on the suction

side can be achieved by passing the two-phase mixture through a sufficiently long pipe

with a large diameter (hereinafter referred to as the "stratification tube"), in order to

ensure a well-developed stratified flow with a smooth interface. Subsequently. hvo liquid

level measurements over some distance x can be performed to obtain the mean void

fraction (AGiA) and the interfacial level gradient, dhLidx. Then, the only unknown

parameters in the mixture momentum equation are the average velocities of the liquid

(UL) and gas (Uc). These hvo unknown parameters cm be determined by simultaneously

solving Equation (5) and the following definition of the total mass flow rate:

Once the average velocities are known. the flow quality can be calculated as follows:

The liquid density used in Equations (5) and (7) should be corrected For the presence of a

lubricant oil in the liquid phase, but unlcss the lubncant oil concentration is well known.

the liquid density will be assumed to be ihai of pure liquid refrigerant. For the vapour

phase, the lubricant oil is assumed to be absent and the vapour density in Equations ( 5 ) .

(7) and (8) c m be readily looked up in the physical property table for HFC- l3 la . provided

the temperature and pressure are known.

The advantage of this quality measurement rnethod is that it does not require any

special instrumentation, except for an accurate determination of the liquid levels or void

fractions at hvo axial points in the stratification tube.

3 2 . 2 DifferentiaI Pressure Measurement Method

A second method replaces the rneasurement of an interfacial Ievel gradient with a

pressure drop measurement in the vapour phase. In this method. hvo pressure taps

separated by a distance I are placed on top of the stratification tube and a differential

pressure transducer is used to rneasure the pressure drop in the vapour phase, APû. While

the pressure drop is being measured, one liquid level measurement is perfonned at the

mid-point behveen the two pressure taps to determine the local void Fraction. Once the

pressure drop in the vapour phase and the local void fraction (A<;/A) are known, the

friction factor definition for a turbulent flow in a srnooth pipe and the following Darcy

friction factor expression can be employed to calculate the average vapour velocity and

friction factor iteratively :

where DG is the hydraulic diameter for the vapour phase given by

Then. the flow quality can be calculated using Equation (8).

The advantage of this method is that i t requires only one

and involves no calibration of special instruments.

3.2.3 Velocity and Void Fraction (V&V) Method

liquid level measurement

This method is similar to the pressure drop method described in Section 3.7.2 in

the sense that only one liquid level measurement is required to determine the local void

Fraction and that the vapour velocity is deterrnined. Again, stratification of the two-phase

flow and the measurement of the total mass flow rate c m be achieved as descnbed in

Section 3.2.1. The third method uses a more direct measurement of the vapour velocity

by means of a hot-wire anemometer instead of a differential pressure transducer.

A challenje may arise due to the fact that a hot-wire anernometer sensor inserted

into the vapour refkigerant stream would only measure a local velocity. which must be

related to the average vapour velocity. Assuming a turbulent velocity profile in a non-

circuiar channel for the vapour stream, this challenge can be overcome by perfoming a

simple calibration to determine the conversion factor, CIi~v..,, between the anemometer

output signal, HWA. in volts. and the average vapour velocity, Uc;, as expressed by the

following equation:

UG (mis) = Cllivh HWA (volts) -+ constant (11 )

Since a turbulent velocity profile is assumed. a hirly accurate relation can be obtained

provided the anemometer sensor is placed suffkiently Car frorn the boundary layers at the

upper tube wall and the interface as shown in Figure 3.2.

Boundary Layer

Turbulent Velocity Profile

(Vapour ReMgerant)

- Hot-wire Anemometer

I

Liquid Rehgennt

Figure 3.2 Position of Hot-wire Anemometer Sensor

Another challenge may be the calibration of the hot-wire anemometer for use with

HFC- 134a, since most commercially available anemometers are factory-calibrated using

air at standard temperature and pressure. Nevenheless, most commercially available

anernometers generate lineanzed output signais. either in voitage or curent, with respect

to the fluid velocity. In addition, Smol'iakov (1983) stated that calibration of a hot-wire

anemometer practically depends solely on the fluid temperature in such a way that

calibration perfomed at different temperatures would yield different calibration curves.

In view of the above remarks, the calibration of a hot-wire anernometer for use with

different fluids, such as HFC-13Ja vapour. may be descnbed by the following equation:

Ucj (m/s) = m HWA (volts) + b (12)

where the calibration coefficients m and b are a function of temperature. Then. with data

points obtained from calibration perfomed at severai different temperatures for a number

of different flow velocities, the coefficients can be determined using:

This V&V method was chosen to measure the quality of hvo-phase HFC- 134a-

PAG oil liquid-vapour flow in this study. As well, the V&V method can be used together

with a simple method described in the next section to determine the total mass flow rate of

a stratified two-phase flow, as well as fiow quality.

3.3 Flow Rate Measurement in Stratified Flow

Consider the stratified hvo-phase flow beîween two flat plates. as depicted in

Figure 3.3. For the one-dimensional steady, fully developed liquid flow. the liquid

momentum equation reduces to

with boundary conditions given by

dv, y=h, ; r , =p-

dy

Gas CC

' 1

Liquid -+ 4 Ur.

Figure 3.3 Stratified Two-Phase Flow behveen Two Flat Plates

Integrating Equation (14) hvice and evaluating the two integration constants using

Equations ( 15) and ( 16) gives

Then, the rnean velocity is

The shear stress at the interface. r,, is given by

and the pressure gradient, dP/dx. from Darcy fiction factor expression.

The friction factor, fG, for turbulent flow is given by the Blasius relation:

The Blasius relation is valid for 31 10' 5 Reû 5 lx 10'.

Hence, if one accurately measures the vapour velocity of the stratified [low. UG, and the

liquid height, AL, for instance, as described in the V&V method, the total mass îlow rate.

m,,~, c m be determined by

This method of determinhg the total mass flow rate of a stratified two-phase flow

was tested using a nitrogen gas-water two-phase flow loop in this work.

CHAPTER 4. EXPERIMENTAL

From the survey of literature it is clear that there is a need to experimentally

confirm the validity of using the thennodynamic quality as the refrigerant-lubricant oil

two-phase flow quality. This chapter describes the apparatus and techniques used to

collect the expenmental data.

4.1 Experimental Set-Up

Figure 4.1 shows a schrrnatic of the rxpenmental loop. referred to as the

.Accumulator Test Stand (ATS). It is a closed refrigeration loop charged with HFC-1343

as the working fluid. The main components of the ATS are a compressor stand. a water-

cooled condenser stand, an insulated evaporator stand. a flow quality measurement

system. an accumulator stand. and vanous process instruments. Pnor to the

commencement of the present work, the compressor. condenser, and accumulator stands

were already built and equipped with necessary process instruments by the Research and

Devrlopment group at HCC. During the course of this work. the flow quality

measurement system and the evaporator stand were built and instmmented. -411 the AT'S

components were then put together and tweaked up in order to tum the ATS into a fully

functional automotive A/C test stand.

The compressor stand consisted of a swashplate type compressor (HCC Korea FS-

10) driven by a 10 HP variable-speed electric motor. A one-to-one pulley ratio was used

to connect the electnc motor to the compressor. The stand was equipped with a torque

sensor and a tachometer to measure the work input to the compressor. However, only the

tachometer function, i.e. measuring the compressor RPM, was utilized for this work.

Type K thermocouples and absolute pressure transducers were used to measure the

refrigerant-PAG oil mixture temperatures and pressures at both the inlet and outlet of the

compressor.

The superheated gaseous refngerant-PAG oil mixture from the compressor was

cooled as it passed through the condenser stand. The condenser itself was basically a

helical coil of 12.7-mm O.D. aluminum pipe as shown in Figure -1.2. The coil was

submerged in a water bath. which was equipped with a stirrer, a liquid level switch. a type

K thenocouple. an electncally actuated solenoid water valve, and a 1i6 HP water pump

as shown in Figure 4.3. A desired water bath temperature was manually set and whenever

the water bath temperature exceeded the set temperature, the solenoid water valve was

opened to let the fresh cold tap water into the bath. The level switch was installed near

the top of the water bath in order to prevent the water from overflowing. When the water

level was high enough to trigger the float on the switch. the liquid level switch actuated

the water pump installed at the bottom of the water bath to drain the excessive water. The

whole water bath was insulated wiih an 80-mm thick fiberglass blanket.

A Coriolis force effect mass flow meter (Endress and Hauser Promass 63F) was

installed at the condenser outlet to measure the mass flow rate of the refrigerant-PAG oil

flow. A sight glass was placed at the outlet of the flow meter (Figure 4.4) to ensure that

the flow meter chamber was completely filled with subcooled liquid refrigerant-PAG oil

mixture since any void space in the flow rneter chamber would result in inaccurate mass

flow rate readings.

Figure 4.2 Condenser Coil

Figure 4.3 Schematic of the Condenser Stand

Two metering valves were used as the expansion device to render the subcooled

refigerant-PAG oil mixture From the condenser (Figure 4.5) a nvo-phase mixture. The

two valves differed from each other in terms of the throughput, and depending on the

desired flow rate, either one or a combination of the hvo valves was used.

Figure 4.4 Refngerant Flow Meter and Sight Glass

Metering Valves -

Figure 4.5 Metering Valves used as Expansion Device

AAer the expansion through the metering valves, the two-phase HFC-134a-PAG

oil mixture was heated as it passed through the evaporator stand (Figure 4.6). The

schematic of the heating unit inside the evaporator stand is shown in Figure 4.7. The unit

consisted of an electric heater with adjustable output from O to 9 kW. a dual-tank

evaporator (2000 Ford Crown Victoria) housed in an air duct. and an air-blower used to

re-circulate the air inside the duct. The power to the electric heater was supplied by a

variable 208 VAC 3-phase controller (Load Controls). A type K thermocouple was

placed behveen the heater and the evaporator to monitor the actual temperature of the air

blown over the evaporator. For heat balance, the evaporator stand was equipped with a

power sensor (Load Controls Inc. WC) to measure the power input to the heater, and the

heating unit was insulated with a 50.8-mm thick extruded polystyrene insulation board.

Furthemore. the aluminum piping between the expansion valves and the inlet of the

stratification tube was insulated with 6-mm thick cross-linked polyethylene foam. Type K

thermocouples and absolute pressure transducers were used to measure the refrigerant-

P.4G oil mixture temperatures and pressures at both the inlet and outlet of the evaporator.

.4n accumulator stand shown in Figure 4.8 was placed between the evaporator

stand and the compressor stand. A PHN-13 1 mode1 accumulator usually found in the A/C

system of Ford F-series trucks was chosen since HCC Canada manufactures it. Type K

thermocouples and absolute pressure transducen were used to measure the refrigerant-

PAG oil mixture temperatures and pressures at the inlet as well as the outlet of the

accumulator. Also, a differentiai pressure transducer (SensuTech FDW) was used to

measure the pressure drop across the accumulator.

Figure 4.6 Evaporaior Stand

Figure 4.7 Schematic of the Heating Unit inside the Evaporator Stand

Figure 4.8 Accumulator Stand

The data from al1 the sensors and transducen were recorded every 5 seconds by a

multiplexer and a data logger, and stored on a PC-based data acquisition system.

4.1 .1 Flow Quality Measurement S ystem

A flow quality measurement system was placed between the evaporator stand and

the accumulator stand (Figure 4.1) for rneasuring the flow quality of the two-phase HFC-

134a-PAG oil mixture exiting the evaporator. Figure 1.9 shows the schematic of the flow

quality measurement systern consisting of a 2-rn long 60-mm O.D., 4.2-mm thick Pyrex

tube (stratification tube), a hot-wire anemometer (Omega FMA-904V), a type K

therrnocouple, an absolute pressure transducer, a üV light and a video camera. The

thermocouple and pressure transducer were installed to measure the temperature and

pressure of the vapour, respectiveiy, to determine the vapour density ai the point of the

vapour velocity measurement by the anemometer. The tube's inlet and outlet were

reduced to 19-mm O.D. tubing. These connections were made as smoothly as possible to

minimize the entrancdexit effects and damp out any disturbances in the flow. The

stratification tube, which was pressure-tested up to 827 kPas (120 psig) for safety was

used to stntify the two-phase flow exiting from the evaporator. The sizing of the tube

was done based on Ali et al.% work (1993) as mentioned in Section 3.1. The tip of the

anemometer probe was placed near the mid-plane of the stratification tube. and the video

camera was placed below the stratification tube vertically aligned with the anemometer

probe. Also, a 160 Watt UV lamp of 365 nm was used to irradiate the LN light at the

section of the stratification tube viewed by the video camera to facilitate detection of the

edge of the width of the liquid Stream. The images projected on a colour TV by the video

camera were then captured using a fiame-grabber sothvare. saved as jpeg files and stored

on a PC for later analysis of local void Fraction.

The anemometer output signal was logged every 5 seconds using the PC-based

data acquisition system mentioned in Section 4.1.

4.2 Experimental Approach

Pio r to applying the Velocity and Void Fraction (V&V) Method outlined in

Section 3.1.3 to the quality measurement of the hvo-phase HFC-1341-PAG oil mixture.

the method was evaluated usinj a dry nitrogen gaslwater two-phase flow loop s h o w in

Figure 1.10. Dry nitrogen gas From a high-pressure tank was passed through a dual-stage

pressure regulator, a Coriolis mass flow meter (Endress and Hauser Prornass 63A), and a

T-junction where the water was injected to obtain a two-phase mixture. Tap water was

passed through a control valve, a pressure regulator. and a Coriolis mass flow meter

(Endress and Hauser Prornass 63F) before it was injected into N,,,, flow through the T-

junction. The Nz,,,-water two-phase flow then entered the flow quality measurernent

system descnbed in the previous section and was then released to atmosphere.

Table 3.1 shows several different flow conditions tested at 19 "C, and the water

level and anemorneter output signal at each flow condition. The two flow metcrs. type K

therrnocouple. absolute pressure transducer and the anemometer output signal were

logged evey 5 seconds using the PC-based data acquisition system. .A total of 16

experimental mns were perfonned and each run represented at least 150 seconds

(normally 200 seconds) of data sampling, implying ai least 31 values (nomally 41) for

each measured quantity. With these values, time-averages were calculated. Since the

void fraction data were not logged autornatically, hvo images of water Stream viewed

from below were manually captured for each run. Before image capturing, the resolution

was determined by placing a d e r immediately below the bottom of the stratification tube

every time the video camera position was changed. The resolution was typically 108

pn/pixel. The captured images were then opened in MS Photo Editor one at a tirne and

the pixel counting was performed to determine the width, w, of the liquid stream of the

stratified two-phase flow. The following geometric formula was then used to calculate the

liquid height:

where r = inner radius of the stratification tube.

Knowing the liquid height. the cross-sectional area of the liquid phase was calculated

iising (Spiegle. 1968).

Then. the cross-sectional area of the vapour phase. A<;. was detrrmined by

In order to ensure that the anemometer sensor is placed sufficiently far from the

boundary Iayers at the upper tube wall and the interface. plots of the anemometer output

sisna1 against average velocity for three different water levels, 0.002 m. 0.003 m, and

0.004 m, were made. As depicted in Figure 4.1 1, the slopes and the y-intercepts of the

different water levels show a reasonably good agreement with each other. This confirms

that the sensor was well positioned and the liquid Ievel does not affect the memorneter

output signal provided the liquid Ievel is equal to or less than 4 mm. Hence, a plot (Figure

1.12) was made with the data points of the water level less than or equal to 4 mm to

detemine the conversion factor and constant in Equation ( 1 1). Equation ( 1 1 ) can be now

rewritten as

UG (mis) = 1.1752 HWA (volts) +- 0.197.

Equation (16) is accurate within + 6%. Subsequently, the anemometer output signals for

itL 5 4 mm in Table 1.1 were corrected for average velocity using Equation (26) and the

flow quality was calculated using Equation (8). The calculated qualities were thrn

compared with the inlet qualities to check accuracy.

Table 4.1 Nt,.,-Water Experiment: Flow Conditions. Water Height. and Anemometer

Output Signal

m m g )

3 1.3 36.8

1 49.2 23.7 30.4 18.1 11.7 45.3 19.5 15.1 18.8 23.4 17.2 39.7 19.0 36.9

m~';itcr (kg'hr)

7.60 6.76 4.30 8.97 5.28 78.8 1 8.10 1.69 1.27

13.73 13.83 1 1.95 13.57 7.56

20.6 1 14.89

mrotai

(kg/hr) 38.9 43 .G 53.5 32.7 35.7 97.5 19.8 37.0 20.7 28.8 31.6 35.3 30.8 37.2 39.6 51.8

W

(mm) 26.76 25.73 22.78 22.13 23.77 38.60 3 2 .O9 17.33 20.12 20.94 21.35 2 1 .56 27.6 1 22.05 30.91 27.84

11 L

(mm) 3.74 3.44 2.65 2.49 3.17 8.68 5.60 1.50 2.04 -.-- 7 13

2.3 1 2.56 3.0 1 2.47 5.14 3.08

HW.4 .. (volts)

2.94 3.59 4.78 2.19 2.76 t .83 1.18 4.30 t .93 1.47 1.78 2.14 1.68 4.13 1.86 3.84

1 .O 2.0 3.0 4.0

Anemometer Output Signal (Volts)

Figure -1.1 1 Companson of Average Velocity with Anemometer Output Signal for

Different Water Levels at 19 "C

Anemometer Output Signal (volts)

Figure 4-12 Plot of Average Velocity against Anemorneter Output Signal for /iL S 4 mm

4.3 Calibration of Hot-Wire Anemometer for Use with HFC-134a Vapour

A closed refiigerant flow loop s h o w in Figure 4.13 was built and used to calibrate

the hot-wire anemometer in situ For use in HFC-13la vapour. The loop was basically a

modified version of the experimental set-up showvn earlier in Figure 1.1. A gear pump

driven by a variable-speed electric motor, instead of a cornpressor, was used to circulate

the reffigerant through the loop. The liquid refrigerant from the pump was passed through

a Coriolis effect mass flow meter (Endress and Hauser Promass 63F) to measure the total

mass tlow rate of the refrigerant. and subsequently through a metenng valve. Thçn. the

refrigerant flowed into the evaporator stand (Figures 4.6 and 4.7). where known amounts

of heat were added to the liquid refrigerant for vaporization. Mostly vaporized refngerant

was then passed into a helicai liquid/vapour separator shown in Figure 4.14. The

separated liquid refrigerant was passed from the bottom of the separator through a

Coriolis mass tlow meter (Endress and Hauser Promass 63A) into a 7.57- L tank

submerged in an ice bath.

On the other hand. the vapour refrigerant from the separator was passed through a

sight glass into the flow quality measurement system identical to the one in Figure 4.9.

except the heavy duty Pyrex tube was replaced by a steel pipe of the same dimension and

geometry. The replacement was made so that the calibration could be done within a

pressure range greater than what the glass tube would have pemitted (527 kPa mau.)

without any safety concem. The vapour refrigerant then flowed into a multi-finned

radiator submerged in a cold water bath. Further cooling was made as the refiigerant fiom

the radiator flowed through the condenser stand identical to the one shown in Figure 1.3.

Figure 4.14 Schematic of the Helical LiquidVapour Separator

The subcooled liquid refrigerant then flowed into the tank where the liquid rehgerant

From the helical separator was mixed and drawn back into the gear pump to repeat the

cycle. Again, al1 the data were recorded rvery 5 seconds by a multiplexer and a data

logger, and stored on a PC-based data acquisition system.

A total of nine runs were performed and each run represented at least 900 seconds

of data sampling, implying 18 1 values for each measured quantity. With rhese values.

time-averages were calculated. For each run, the total mass flow rate (or gear pump

setting), heating power, and metenng valve opening were varied to obtain various

anemometer output signals at different temperatures. In order to ensure no refrigerant

accumulation inside the helical 1iquid;vapour separator, each run was initially performed

with no liquid refrijerant flowing from the separator to the tank. Once steady

anemometer output signal and temperature inside the stratification tube were reached, the

valve on the bypass line was opened slowly just until no liquid refngerant was detected

flowing inside the sight glass placed behveen the helical sepantor and the stratification

tube. Then, the total mass tlow rate, the mass flow rate in the bypass line. the

anemometer output signal. the temperature inside the stratification tube. and the sight

glass at the outlet of the helical separator were closely monitored for at least 15 minutes

for steadiness. Only when no change was confirmed aRer the monitoring period. the data

loging was initiated.

Of the nine data points collected. four points covering the overall temperature

range were used to determine the calibration coefficients using Equation (13) for n = 1.

After the coefficients were determined. the rest of the data points were used to check the

accuracy of the calibration. The results showed good accuracy, within +3.joh. as

summarized in Table 4.2.

Table 4.2 Results of In Situ Calibrarion of Hot-Wire Anemometer in HFC-134a Vapour

Inlet UG HWA Output Signal Calculated UG W s )

Coefficient % Error (volts) ( m/s

With the coefficients determined, Equation (12) c m now be rewntten as

Uû (mis) = 0.01768 T (K) HWA (volts) - 5.0081 HWA (volts) - 0.02321 T (K) + 6.5141. (27)

4.4 Test Conditions and Procedure

The refrijerant test conditions covered in the experirnental work are s h o w in

Table 4.3. These conditions were chosen to cover a wide range of mass flow conditions at

which the effluent from the evaporator would be sufficiently far away from the saturation

line and well within the thermodynarnic vapour region. assuming the effluent was only

pure HFC-134a. The modified Benedict-Webb-Rubin (MBWR) equation of state was

used for calculation of HFC- 134a properties (McLinden et al.. 1989). For information

regarding the operation of the ATS. refer to the ATS Operating ;Manual in Appendix.

Table 4.3 Conditions for Refrigerant Ex~eriment Mass Flow Rate 1 76 to 150 kqhr Heating Power I 3.5 to 7.5 kW

Evaoorator Out let Pressure / 270 to 335 kPa (abs) 1

Each experirnental run was perfonned at different total mass flow rate (or

compressor RPM). heating power, condenser water bath set temperature. and expansion

valve opening. The settings were varied until a desired flow condition was achieved at the

evaporator outlet (Le., the pressure and temperature at the evaporator outlet were within

the test conditions s h o w in Table 4.3). Once a desired flow condition was achieved, the

ATS was leA ruming until a steady state was reached, which took about 20 to 30 minutes.

Saturation Temperature at Evaporator Outlet 1 -3 to 4 "C i

S u~erheat 2 to 10 "C

At steady state. visual and photographic observations were made to detect the presence of

liquid flow in the stratification tube. At least hvo photographs were taken for each

experimental run using a video canera placed below the stratification tube to measure the

local void fraction (Figure 4.15). Al1 the pressure and temperature measurements were

logged automatically every 5 seconds at least for 1.5 minutes and stored on a PC for later

analysis.

Figure 4.15 Digitized Image of a Liquid Stream in a refkigerant two-phase flow

4.5 Validation of Experimental Set-Up

In the following sections, commissioning tests performed to check the

performance of the experimental set-up are presented along with the results.

4 . 5 System Validation of the ATS

The ATS was constructed as an accumulator test stand capable of simulating

various operating conditions typically encountered in automotive air-conditioning

systems. In order to venfy the qualification of the ATS as an accumulator test stand.

system validation was performed. The validation procedure involved the use of the ATS

to duplicate the wind-tunnel in-vehicle testing' data (HCC Canada Intemal Document) for

the same accurnulator mode] installed in the ATS, Le., PHN- 13 1. The wind-tunnel data

contained the information on the MC system performance at three vehicle openting

conditions. namely. idle. city dnving and highway driving conditions. Of these data. only

those values relevant to the suction linç refriperant flow were used in the system

validation, since these are the actual conditions that an accumulator would see in an .bC

system.

The results of the system validation are sumrnarized in Table 1.4.

Table 4.4 Validation Test Results

Comp. Comp. Suction P Comp. Discharge P Comp. Discharge T RPM ( k W (wa) ( O C )

Evap. Outlet T ("Cl

W i n d :\TS tunnel

Flon~ate ( k g hr) i

8

A standard accumulator test facility which allows the evduation of the accumulator performance by measuring the ovenll performance of specific MC systerns.

As s h o w in Table 4.4, the validity of the ATS as an accumulator test stand was

confirmed since the system was able to closely duplicate the wind-tunnel test data.

4.5.2 Determination of the Arnount of P.4G Oil in Circulation

In ordrr for any data obtained using the ATS to be valid. the amount of PAG oil

circulating with HFC-I34a in the loop needed to be adjusted close to the standard value.

For that reason, sampling was performed to determine the weight % (wt %) of PAG oil.

The sampling procedure and calculation of the oil concentrations were based on the HCC

Canada Standard Procedure for "Deiermining Oil In Circulation". Sampling canisters

with a volume of 75 ml were used to sarnple the subcooled liquid HFC-134a-PAG oil

mixture from the condenser. The sampling was performed at three different cornpressor

RPM's and four samplings were made at each RPM. After sarnpling, the refrigerant was

rcmoved from the sarnpling canister by s!owly bleeding the refrigerant vapour through a

fine rnetering valve. Based on the measurement of the empty weight of the sampling

canister. the weight after sarnpling, and the weight aRer bleeding off the refrigerant. the

oil concentration in the refrigerant could be calculated. Behveen different samplings, the

same arnount of PAG oil and HFC-134a removed was added back to the loop.

A commonly used PAG oil concentration for an .WC system with an FS-IO

cornpressor and a PHN-13 i accumulator is 6.5 wt % (Halla intemal Document), and the

PAG oil concentration of the ATS was ernpirically adjusted to 8 wt %.

4.5.3 HFC-134a in the Liquid Phase of the Two-phase Stratified Flow

The liquid phase of the stratified HFC-134a-PAG oil two-phase flow in the

stratification tube was sampled white the ATS was under operation to determine the

composition of the liquid phase. The sampling was necessary due to the possibility that

most of the refngerant might have vaporized or only a negligible arnount of the refrigerant

might have stayed miscible with the liquid phase as a result of the phase separation in the

stratification tube. Figure 4.16 shows the sampling unit consisting of a 75 ml sampling

canister. a 130 mm long. 6.35 mm OD thin-walled stainless steel tube. and a metering

valve for bleeding off the refrigerant aRer sampling.

Before sampling, a vacuum was pulled on the sarnpling canister down to near O

kPa (abs), and the stainless steel tube was then inserted into the stratification tube through

an O-ring sealed fitting. Following the insertion of the sarnpiing unit, the ATS was tuned

up to obtain desired steady state flow conditions. Once at steady state. the sarnpling was

performed by opening the valve on the sampling canister and sucking the liquid flowing

on the bottom of the stratification tube into the canister. Sarnpling typically lasted for 40

seconds, and the valve was then closed. Removal of the rekigerant from the sampling

canister and the weight measurements were done as described in the previous section.

One sampling was made at each of three different flow rates, and the results are

shown in Table 4.5. Although the positive values of the superheat in Table 4.5 suggest

that the refrigerant should themodynamically be entirely in the vapour state, a

considerable amount of refngerant was found to be in liquid phase.

Figure 4.16 Sampling Unit

Table 4.5 Sampling Results on the Liquid Fraction of Stratified Two-Phase HFC- 1%-

PAG Oil Flow

Flow rate (kg/hr)

12 1

Evap. Out P W a )

262.9

Evap. Out T CC)

11.1

Refrigerant wt %

14.3

Superheat ( O C )

14.1

4.6 Data Reduction

A total of 1 1 expenmental runs were performed on two-phase HFC-I34a-PAG oil

flow and each run represented at least 125 seconds (normally 150 seconds) of data

sarnpling, implying at least 26 values (nonally 3 1 ) for each measured quantity. Time-

averagi ng O F the automaticall y logged data and the image capturing/processing for

determining the cross-sectional area of the liquid phase were done as in the N2,g,-water

experiment. The resolution of the captured images was typically 70 pmipixel. The width

of the liquid Stream detemined by pixel counting was used to detemine the cross-

sectional area of the vapour flow, AG, using Equations (23)-(25). For the detemination

of the flow quality of the two-phase HFC434a-PAG oil flow exitin!: the evaporator. the

anemometer output signal values were corrected for the average refngerant vapour

velocity using Equation (27). With the correctcd velocity values. U(;. and the cross-

sectional area of the vapour phase. the flow quality was determined using Equation ( 8 )

assuming that the temperature-pressure relationship for superheated HFC- 134.1 vapour

were not affected by the existence of PAG oil.

The thennodynamic quality, -KT, of the two-phase mixture flowing out of the

evaporator was also determined assuming therrnodynamic rquilibrium and using the

following equation:

where h = enthalpy of the flowing mixture

hL,sai = saturation enthalpy of the liquid phase

A = latent heat of vaponzation

The latent heat of vaporization and the temperature-pressure relationship at saturation

were assumed to be unaffected by the existence of PAG oil.

4.7 Experimental Uncertainty

The possible sources of uncertainty in this experimental work are the instrument.

pixel counting, calibration of the hot-wire anemometer (described in Sections 4.1 and 4.3).

heat loss from the evaporator stand. and the dimension of the stratification tube. Table 4.6

surnmarizes al1 the uncertainties involved. Except for the hot-wire anernometer. al1 the

instruments were factory-calibrated and the manufacturer's calibration data or the product

specification for a given instrument was used in assessing the experimental uncertainty.

The data acquisition system used was accurate and Fast, and its uncenainty was therefore

assumed negligible enough to be zero.

The uncertainty in the pixel counting of the captured images for determining the

width of the liquid strearn in the stratification tube might have resulted from the huiness

around the edges. This could have made the counting uncertain within f 5 pixels.

The heat loss From the evaporator stand was assessed by scanning around the stand

(insulation board) with a non-contact inkared thermometer (Raytek Raynger MXZ) while

the stand was at four different heater loads. During the scanning the highest and lowest

temperatures were recorded at each heater Ioad. Table 3.7 summarizes the scanning

results and shows the evaporator stand was well insulated. Hence, the heat loss from the

evaporator stand was assumed to be zero.

Table 4.6 Experimental Uncertainty

Quantity Category Measurement

~ncertainty I 1 Source

2 . 2 "C or =0.75 96 above 0°C (whichever

is greater) -7 --.- 7 "C or 2 . 0 O 6

below 0°C

Ni. Cr-Ni, Al (Type K)

thennocoup le

.\II temperature

measurement Temperature

- -

NIST Monograp h 175

Reviseci to ITS-90 !

r 1 "9 of reading Suction stde 100 psia mas MKS calibration data sheet Pressure

500 psia m u High side

Xlso for tvater flow in

MKS calibration data sheet

=O. 1 of reading; =0.005 % of full s c a k =3.7 below 0.4 kg hr

In the bypass 1 line for HW.4 Endress and

Hauser Catalogue calibration (Section 4.3) ( Promass 63.A )

3ilc Water experiment

Instrument Mass Flow

Rate Also for S2ici tlow tn

' Benvecn zO.5 O.0 of reading;

1 Endress and / condenser and ~ 0 . 0 0 5 of full scale: I Hauser Catalogue

';,l,k Water sxueriment

=2 "h below 30 kghr

1 1 Cole-Parmcr i Caologus i (Satorius E-

expanston ( Promass 62F) valves

For sampling tec hniquc

Average vs. Local

Velocitv

I Evaporator -0.5 O& of full scale 1

Flow qurility / Refer to 1 Section 4.1 l

i heriter l ( CPC) l

I I 1

Load Controls inc. Catalogue

Hot-Wire .Anemometer Calibration

Range: 0-9 kN'

.-i measurement i I

HFC- 134a Vapour Velocitv

system 1 Rekr to 1 Section -1.3

Pixel Counting

Liquid Width

Tube dimension

=j pixel of counting i I I O.D.= 60.0 = Imm. Flow quality 1

t = 4.2 = 0.5 mm 1 meamremen: Pegasus system 1

Stratification Tube

Table 4.7 Results of Temperature Scanning around Evaporator Stand

Ambient 1 Evap. Stand Insulation Board Evap. Stand Inside 1 T ( O C )

18.6 18.5 19.2 18.8

Highest ( O C )

20.0 2 1.4 20.9 20.4

Lowest ( O C )

18.7 16.8 17.5 17.4

CHAPTER 5. RESULTS AND DISCUSSION

5.1 N2,,,-Water Erperirnents

5.1.1 Flou Qualit! 4lrasuren1ent using V&V blethod

Table 5.1 shows the rttsults of the assessrnent of the YBrV metliod for tlon yiiality

mrasuremeni. and Figure 5 . I shows the comparison brtwsen rhc iictual qualit! and ttic

calçulated t l o ~ quality using the V&V rntithod. The inlrt qualit! uas obtained tioni

separate rneasurements of the tlow rates of %,,, and tvater. and the tlow yuality u-as

dekrmincd using Equaiion ( 8 ) as outlinrd in Section 4.2. The tàct that the cornparison

between the actual qualit). and the calculated tlow quality using the V B V method yielded

ri mean deviation of 0.4 9'0 and a standard deviation of i 4.5 indicates that the \Xi'

method is accuratr and suitable to be applied to the rdrigcrnnt tlow qualit! mcasuremcnt.

Table 5.1 Rrsults of%:,,,, Woter Flow Qualit). blcasurernrnt using VBC' MethoJ

I Act ual m4;:ig,

( kg! hr ) 31.3 36.8 49.2 23.7 30.4 19.5 15.1 18.7 23 -4 17.2

36.9

Es perirnsntal

(volts) 5.99

O. 50 0.60 0.70 0.80 O .90 1 .O0

Actual Quality

Figure 5.1 Cornparison between Actual and Calculated Flow Qualities from N:,,jWater Expenment

5.1.2 Determination of Total Mass Flow Rate of a Stratified Two-Phase Flow

The method of determinin!: the total mass flow rate of a stratified two-phase flow

using a simplified one-dimensional momentum equation as outlined in Section 3.3 was

tested using the sarne experirnental results shown in Table 5.1. Since an arbitrary flow

between two flat plates was used in deriving Equation (18) and. in fact, the stratification

tube was circular, estimation was made using the liquid height, hL, in Equation ( 18) equal

to half of the rneasured liquid heights (Figure 5.2). Figure 5.3 shows the cornparison

between the actual and caIculated flow rates. The results surnmarized in Table 5.2 and

shown in Figure 5.3 indicate reasonable agreement for most of the data points. The mean

dsviation of the calculated total n m s tlow rates from the actual values LUS -8.3 * o . and

the standard deviation u-as + 19.2 %.

Figure 5.2 .Average Liquid Hright used in Estimating the .bcragr Liquid Vclocit?

using Equation 18

6 1

Table 5.2 Results of Nz,,jWater Total Mass Flow Rate Determination

Actual Flow Rate (kglhr)

n1,0,,1

k_g/hr) 38.9 43.5 53.5 32.7 35.7 28.5 31.6 35.3 20.7 30.8 47.2 51.8

Figure 5.3 Cornparison between Actual and Calculated Flow Rates from Nz(g)/Water

Experiment

Itt (mm) 1.87 1.72 1.33 1 . 3 1.5s 1.11 1.16 1.18 1.03 2.00 1.24 2.04

5.2 Refrigerant Erperiments

52.1 Flow Observation and Thermodynamic Quality

A total of 1 1 runs were perforrned to collect HFC-134a-PAG oil mixture data

using the ATS. With this data. the thermodynarnic qiiality was determined usin:: Equation

(28). In each nin, observations were made to detect the presence of liquid in the Flow

exiting the evaporator. The results are summarized in Table 5.3. Although the

thermodynamic qualities. sr. were al1 greater than unity, which means the effluents from

the evaporator should al1 be superheated vapour, the presence of liquid phase was clearly

observed in a11 the rxperimental runs. Figure 5.4 shows the pictures of the HFC-134a-

PAG oil flow exiting the evaporator and entering the stratification tube ai vanous

thermodynamic qualiiies. In every case. a wavy liquid film is seen to flow dong the inner

tube wall.

Table 5.3 Thermodynamic Quality and Flow Observation

Heater Power (Watts) 3502 3707 4679 3678 4188 438 1 48 15 5969 6490 6434 7493

Cond.

1 079.4 1098.3 1 O73 -2 1040.8 999.5 997.3 995.0 1 105.8 1214.0 1210.1 1076.0

Presence Of Liquid

Y es Y es Y es Y es Y es Yes Y es Yes Yes Y es Y es

Figure 5.4 Observation of HFC-I 3la-PAG Oil Flow Exiting Evaporator and Entenng Stratification Tube at Various Themodynamic Quality

5 - 2 2 Flow Quality and Mass Flow bleasurements using V&V Method

With the sams tsperimrntal data usrd to drtmnins the therrnodyiamic qualit! in

Section 5.2.1. the tlou quality. -Y. of HFC- l34a retiigrrant-PAG oil tu-O-phase flou

esiting the wciporator u as cietermined using the L'&V methoci. The results Lire shw n in

Table 5.4 dong ~r-ith the comparison betwcen the thcrmodynamic and calculateci t lo~r

qualitirs. Cnlikr the themiodynamic qualities. the çalculated flou qualities w r e al1 n-ell

belon unity. which is consistent \vit11 the tlow obsen~ations. The comparison. hoivever.

shows that on average the thmnodynamic qualit). was 10 times iarger than the tlou

quality. .-kiditionally. the total mass tlow rates w r e cietsrmincd and the rrsulis are

summarized in Table 5.4 and s h o w in Figure 5.5. The merin drviation of the crilculatcd

total m m tlou rates frrom the actual \.alut.s u i i s -83.4 %.

Table 5.4 Results of HFC- lj-la-PAG Oil Flow Quality and Mas5 Flow Measurrmttnts using V&V blethod

l 'c i

(kg rn') 12.6 12.8 10.8 II .? 12.7 13.3 14.2 14.2 15.2 15.8

Ci; * HW.4 il!

(volts) (min) 2.40 :.9 O. 162 2.56 4.1 0.110 2.22 3.6 0.095 2.36 5.7 0.225 2.56 3.8 0.088 3.83 2.9 -0.084 3 . 4 5 .7 O. I 1s 3.92 3 9 0.367 5.89 4.0 O. 171 4-54 4.0 0.104

Ecrip. Outlet Stratiticrition Tube 1 Calculated mt,r,l

l

l

1

-

* based on calibration coet'ficisnts

V

I

65 85 1 05 125 145

Actual Flow Rate (kglhr)

Figure 5.5 Cornparison between .4ctual and Calculated Flow Rates frorn HFC- 134dPAG Oil Experiment

The large deviations between the thermodynamic and flow qualities. and the actual

and calculated total mass flow rates could be ascribed to the vapour velocities being too

srnall, which in tum implies that the calibration coefficients in Equation (27) are not

reliable. The uncertainty in the calibration coefficients was also manifested by one of the

vapour velocities being negative. Figure 5.6 shows the temperature and vapour velocity

ranges over which hot-wire anemometer output signals were obtained in both the

calibration runs and refngerant experiments. As it can be seen fiom Figure 5.6, there was

a marked difference in the temperature and vapour velocity ranges behveen the calibration

and experimental data points. This necessitated estimating the experimental vapour

veiocities that lay outside the range of the calibration data points by extrapolation. which

predictably involves a large error.

A

I/)

5.0 - I

Ci - A

5 4.0 -: A Calibration Data V A

A A Refrigerant Data ~

270 275 280 285 290 295 300 305 Temperature (K)

Figure 5.6 Plot of Temperature against Anemometer Output Signal from Calibration Runs and Refhgerant Experiments

In the following section. modified calibration coefficients werr obtained and used

to determine the flow quality and mass flow rate using the V&V method.

5 . 3 Flow Quality and Mass Flow -Measurements using V&V Method based on

Modi fied Calibration Coefficients

Of the HFC-1341-PAG oil experiment data points in Table 5.4, four points

covering the overall temperature range were used to determine nrw calibration

coefficients using Equation (1 3) for n=l, assuming the flow quality was unity and al1 of

the S w-t % PAG oil to be flowing in the liquid strearn of the stratified MO-phase flow in

the stratification tube. With the new calibration coefficients, Equation (27) was rnodified

UG ( d s ) = 0.00262 T (K) HWA (volts) - 0.571 HWA (volts) - 0.0095 T (K) + 3.158. (29)

Using Equation (29). the vapour velocities wrre determined with the rinemorneter output

signals from the HFC-l34a-P.-\G oil rsperiments (Table 5.4). Then. the Row qualit' as

well as the total mass tlow rate w x e determincd i i s in the V&V method. The results rire

summarized in Table 5 .5 and s h o w in Figure 5.7. The thennodynamic qualit! tus . on

average. i -3 timss larger than the tlow qualit),. and the mran deviation of the ç1iIç~iliitd

total mass flot\ rates from the actiial values uas 38.8 O O . This fair agreement obt1iinr.d for

the tlow rats determination indicates that the rnocli ticil cali bration coe t'ticierits arc mcm

reliabit. than the ones previuusly rt.portt.d in Section 4.3.

Table 5 .5 Results of HFC-I Ha-PAG Oil Flow Qualit! and Mass Flow hIeris~irt.iiirnts

using V&V Method based on blodified Calibration rissuming s = 1

Strrttiticrttion Tube

* bassd on moditied calibration coeffkients

70 80 90 1 O0 110 120 130 140 150 160

Actual Flow Rate (kglhr) - - - - -- - -

Figure 5.7 Cornparison between Actual and Calculated Flow Rates from HFC- I3JdPAG Oil Experiment based on Modi fied Calibration

Nevenheiess. as shown earlier from the flow observation in Figure 5.4. the actual

fiow quality certainly was not equal ro unity. Hence. funher determination of the flow

quality and mass flow rate was done using assumed flow qualities less than unity. Table

5.6 shows the results, and Figure 5.8 shows a plot of the mean deviations of the calculated

mass flow rates from the actual mass flow rates against the assumed quality. It can be

seen in Figure 5.8 that there is a consistent trend in the mean deviation increasing with the

increasing value of flow quality assurned for determining the calibration coefficients.

Furthemore, zero mean deviation was obtained at an assumed flow quality of 0.77. With

the calibration coefficients obtained using this assumed quality, Equation (29) c m now be

rewritten as,

Uc (mis) = 0.00201 T (K) HWA (volts) - 0.438 HWA (volts) - 0.0073 T (K) - 2.422. (30)

.-!t the assumrd tlow quality of 0.77. the thermod~namic quality was. on average.

1.7 times larger than the fiou quality. As well. the standard deviations of the calculatrd

total mass tlow rates from the actual values w r e t 8.6 Oh. which is consistent \ f i th the

N2,,,,\vüur tlow results previousl! reportcd in Table 5.2. The source of error could bc the

assumptions made in dstmnining the modified calibration coefficients since the üctuül

tlow qualities uf the four espt.rimrnta1 runs used to determine the calibratiori coellicients

w r e iinlikcly qua1 tu thc iissumsd quality. nur identiccil to each other. Iloreuw-. tlic

rissumption that al1 of the 8 n-t O O O PAG oil be tlowing in the liquid Stream of the strütitied

two-phase tlow in the stratitiçation tube is not realistic sinçe the E-IFC-I ,-la w p u i i r is

known to be highly miscible in Pr\G uil.

Table 5.6 Results of HFC- 1 34a-PAG Oil Flow Quality and Mass Flow Measurements using V&V Method based on Modified Calibration assuming .Y < 1

Xssumsd Qurility

Cslculated hltoul ( kg hr

80.9 85.3 65.6 65.0 34.0 51.1 105.5 122.1 130.9 149.8 90.5 95.9 73.2 75.9 93 -9 59.9 1 t 7.9 136.8 146.8 168.0 95.3 104.2 77.0 79.9 98.8 94.3 124.1 144.2 154.7 177.2

* based on modified caiibration coefficients

Assumed Flow Quality

Figure 5.8 Plot of Mean Deviations against Assurned Flow Quaiity

CHAPTER 6 . CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

A new method of measurîng the flow quality in the suction line of an automotive

air-conditioning system has been proposed based on the use of a stratification tube to

stratify HFC-134a refrigerant-PAG oil two-phase flow in a horizontal tube without the

use of additional heating or condensing sections. By measuring the vapour velocity with a

hot-wire anemometer and the liquid level in the stratification tube, the tlow quality and

total mass flow rate can be determined. The feasibility of the proposed method was

checked by conducting experiments using a nitrogen gashater flow loop. The resuits

showed that accurate measurements of flow quality, within 26.8 %. can be made using

the present method. Fairiy good agreement was also seen between the calcuiated and

actual total mass flow rates.

.4 compressor-based HFC43Ja refrigerant-PAG oil refrigeration loop was used to

check if the presence of a lubncant oii would cause any error in the determination of

thermodynamic quality based on a heat balance. The present flow observations and the

flow quaiity rneasurements showed substantial discrepancies between the thermodynamic

quality detemined based on a heat balance and the rneasured flow quality. Hence, the

present work demonstrated that the iubricant oil cm significantly alter the thermodynarnic

relationship of pure HFC-134a ref'gerant, such that the thermodynarnic quality of the

refrîgerant-lubncant oil mixture cannot be reliably determined based on a heat balance

and equilibrium thermodynamic relationships available for pure HFC- 134a refrigerant.

Finally, reliable calibration of the hot-wire anemometer for vapour velocity

rneasurernent under a wide range of refngerant flow conditions is found to be essential for

accurate determination of the flow quality and mass flow rates using the stratification tube

rnethod.

6.2 Recommendations

In order to improve the flow quality and mass flow measurement methods

proposed in this work. the following future study and action are recommended:

The liquid height in the stratification tube was manually measured. As a result. time-

averaging was not possible. Hence. a level transmitter whic h continuousl y measures

the liquid height and sends output signals to the data acquisition system will improve

the accuracy of the height measurernent. Considcnng the low electrical conductivity

of HFC-134a, the size of the stratification tube. and the need for not disturbing the

liquid tlow, a non-intrusive vision gauge is strongly recomrnended.

Extensive calibration of the hot-wire anemometer is required to generate data points

covering the overall temperature and vapour velocity ranges typically encountered

during the operation of automotive MC systems. However, challenges can be

cxpected since it would be dificult to produce various vapour flows of high velocity

at low temperature using a pump-based refiigerant flow loop. One way of avoiding

this problem is to use a vapour velocity rneasurîng device that does not require

calibration. The best suited candidate would be a Pitot tube, provided it can be

readily inserted into the stratification tube.

a The Pyrex glass tube used as the stratification tube was chosen rnainly for the ease of

manufacturing. However, if a rectangular channel with a sufficiently large channel

height was used, better liquid Ievel and vapour velocity measurements would be

possible.

ln detennining the llow quality of HFC-134a refrigerant-PAG oil hvo-phase tlow, the

vapour density was assumed to be that of pure HFC-134a due to the unavailability of

the thermodparnic property data for the mixture over a practical concentration range.

Future experimental work to determine those property data over the concentration

range commonly encountered in automotive .WC systems will significantly improve

the accuracy of the flow quality measurement.

Abdul-Razzak, A., Shoukri, M., Chang, J. S., (1995), "Measurement of Two-Phase Refigerant Liquid-Vapor Mass Flow Rate", Final Repon. ASHRAE 722-TRP Research Projec t.

Ali. MI., Sadatomi, M. and Kawaji, M. (1993), "Adiabatic Two-Phase Flow in Channels between Two Flat Plates", The Canadian Journal of Chernical Engineering, Vol. 71. Oc tober.

ASME (19591, Fluid meter - Their Theory and Application, jth ed. New York: American Society of Mechanical Engineers.

XSHRAE Handbook ( 1 WS), Refngeration. American Society of Heating. Refrigeration. and Air-Conditioning Engineers, Inc., Atlanta.

Badie, S ., Hale, C.P., Lawrence, C.J.. Hewitt, G.F. (2000). "Pressure Gradient and Holdup in Horizontal Two-Phase Gas-Liquid Flows with Low Liquid Loading". Int. J. Multiphase Flow. vol. 26. pp. 1 5 3 - 1543.

Banejee, S., Heidnck, T. R., Saltvold, J. R., and Flemons. R. S. (1978), Measurement of Void Fraction and Mass Velocity in Transient Two-Phase Flow in "Transient Two-Phase Flow", AECL, Toronto, BP 759-834.

Cavestri, R. C., (1993). "Measurement of the Solubility, Viscosity, and Density of Synthetic Lubricants with R- 1 34aW, Final Report, ASHIWE RP-7 16 Research Project.

Halla Intemal Document, HaIla Climate Control Canada Inc., Belleville, Ontario. Canada.

Hashizume, K., (1983), "Flow Pattern, Void Fraction and Pressure Drop of Refrigerant Two-Phase Flow in a Horizontal Pipe - 1". Int. S. Multiphase Flow, vol. 9. No. 4, pp. 399-41 O.

Hewitt, G. F., (I978), "Measurement of Two Phase Flow Parameters", Academic Press.

Inatsu, H., Matsuo, H., Fujiwara, K., Yarnada, K., Nishizawa, K. (1992), "Development of Refigerant Monitoring System for Automotive Air-conditioning System", Design and Performance of Climate Control Systems (SP-916) SAE. February? pp. 29-39.

Jacobs, M. L., Scheideman, F. C., Kazem, S. M., Macken, N. A., (i976), "Oil Transport by Refrigerant Vapor", ASHRAE Transactions. Vol. 82, Part II, pp. 3 15-329.

Kargilis, A., (1994), "Design and Development of Automotive Air-Conditioning Systems", Short Course Lecture Note, University Consortium for Continuing Education

Kim, Y. C., O'Neal, D. L., (1994), "The Effect of Oil on the Two-Phase Critical Flow of Refngerant 134a through Short Tube Orifices", [nt. J. Heat Mass Transfer, Vol. 37. No. 9, pp. 1377-1385

Kondic, N. N.. and Hahn, O. J. (1970), '"Theory and Application of the Paraliel and Diverging Radiation Beam Method in Two-Phase System", 4Ih [nt. Heat Transfer Conf. Paris, 7. Paper MT 1 -5.

Lorencez. C., Nasr-Esfahany, M.. Kawaji, M. and Ojha, M. ( 1997). "Liquid Turbulence Structure at a Sheared and Wavy Gas-Liquid Interface", Int. J. Multiphase Flow, vol. 13. No. 2 , pp. 205-226.

McLinden, MO.. Gallagher, J. S., Weber. L. A.. Momson, G.. Ward. D.. Goodwin. A. R. H., Moldover, M.R., Schmidt, J. W., Chae, H. B., Bruno. T. J., Ely, J. F.. and Huber. M. L., (1989) "Measurement and Formulation of the Thermodynamic Properties of Refngerants 134a and 123". A S H U E Trans. 95(2), 763-253.

Sadatomi, M., Kawaji, LM., Lorencez, C.M. and Chang, T. ( 1993). "Prediction of Liquid Level Distribution in Horizontal Gas-Liquid Stratified Flows with Interfacial Level Gradinet". [nt. J. Multiphase Flow. vol. 19. No. 6. pp. 987-997.

Smol'iakov. A. V. ( 1983). " The measurement of turbulent fluctuations: an introduction to hot-wire anemometry and related, Berlin; New York: Springer-Verlag, pp. 65-70.

Solberg. J., Miller, N. R., and Hmjak, P. (2000). "A Sensor for Estimating the Liquid Mass Fraction of the Refngerant Exiting an Evaporator", SAE Technical Paper Series. SAE ZOO0 World Congress, Detroit, Michigan, 2000-0 1-0976.

Spiegle, M. R. (1968), "Mathematical Handbook of Formulas and Tables". McGraw-Hill Ryerson Ltd.. Equation 4.11. pp. 7.

Taitel, Y. and Dukler, A. E. (1976), A Mode1 for Predicting Flow Regime Transitions in Horizontal and Near Horizontal Gas-Liquid Flow, AiChE JI 22, 47-35.

Vince, M. A., and Lahey, R. T., Jr., (1982), "On the Development of an Objective Flow Regime Indicator", Int. J. Multiphase Flow, vol. 8, No. 2, pp. 93- 12.1.

Xie, G., Stott, A. L., Plaskowski, A., Beck, M. S., (1990), "Design of capacitance electrodes for Concentration Measurement of Two-Phase Flow", Sci. Technol.. 1, pp. 65-78.

APPENDIX

ATS Operating Manual

First time users are strongly recommended to carefully read the Robinair (Mode1 34700)

instruction manual before proceeding to the following steps. The MSDS of PAG oil and

the property data sheets of HFC- 134a should be checked in advance.

Pre-run Preparation

Leak Test

Drv nitrogen cas and SnoopTM

1. Charge the ATS with dry nitrogen gas (recommended charging pressure < 40 psig).

2. Look for leakage using SnoopTM: in case of leakage. the liquid Snoop will tum into

foam.

3. When the leak test is complete. slowly depressurize the system to atmosphenc

pressure.

Caution! Wear an appropriate cartridge-type mask and rubber gloves whilc depressurizing the system.

Vacuum decav test

1. Connect the blue liquid hose of Robinair to the Iow side charge port (on the

compressor inlet) and the red gas hose to the high side charge port (on the compressor out let).

2. Pull vacuum at least for 7 minutes (refer to the Robinair operating manual for how to pull vacuum).

3. When vacuuming is done. the pressure reading given by the low side pressure gauge on the control panel should read 30 inch Hg vac. If the pressure reading is higher than 30 inch Hg vac, go back to the leak test procedure using dry nitrogen gas and

Snoop, and double-check al1 the valve connections.

4. With both the high and low side valves on the Robinair's control panel open, wait at

least for 5 minutes. Then, closely observe the low side pressure gauge for 10-15

minutes. If there is a significant increase in pressure reading. repeat the whole leak

test procedure.

Condenser

1. Tum on the power switch on the control panel.

2. Start tilling up the condenser with iap water by setting the water valve switch to "ON" and the pump switch to "AUTO".

3 Tum the water valve switch to "AUTO" when the pump starts to pump.

It is advised to proceed to the next step while the condenser is being filled up.

1. Open the ATS LabVIEW prograrn on the cornputer connected to the ATS data acquisition system. The program c m be accessed by clicking on the "NATS.llbW icon in "C:\Prognm Files\National Instruments\LabV IEWPROJECT\ATS" or "ATS" icon on the Desktop.

2. Figure A.l shows the lopin prompt of the ATS program. Type your name in [ l ] ,

and then click on "Start " [ 2 ] and the arrow [3] buttons to execute the prograrn.

Figure

Charging-up the ATS

Charge amount: Previous preliminary runs showed that the ATS works best when charged with 1 ..)O kg of HFC- 134a.

1. Refer to the Robinair operating manual for how to charge up a system. Charging MUST be done through the high side charge port ONLY.

2. AAer the charging-up is complete, uncouple the red hose and recover the refrigerant left in the hose.

i It may corne in handy to take note of the amount recovered, as this c m be used to determine the actual amount of refngerant the system has been charged with.

Disconnect hoses with extreme caution! Ail hoses may contain liquid refrigerant under pressure.

Startine the ATS

The following instructions are given based on the validation test mns performed to

simulate three automotive A/C operating conditions, namely, highway, city driving,

and idle conditions. Various values of the adjustable parameters used for each of the

three conditions are summarized in Table A.1. The only value that is not specified in

Table A.1 is the orifice valve opening, which should be determined empirically. If

different A/C operating conditions need to be simulated, it is recommended to use the

values in Table A.l as a guideline to determine the approsimate starting points.

Then, each adjustable parameter should be adjusted to obtain the desired condition.

Before running the ATS. the amount of PAG oil circulating with HFC-l34a must be

adjusted to the standard value, which is dictated by the type of the accumulator and

compressor being used. Refer to the HCC Canada Standard Procedure for

"Determining Oil in Circulation".

ATS is designed to run at two different compressor clutch modes. clutch cycling

(steady) and non-clutch cycling (transient). The following instructions are for non-

ciutch cycling mode only.

Table A.1 ATS Panmeter Settings used for Validation Test Runs

1. Tum on the power switch on the control box of each ATS stand.

Comp Discharge P (kPa)

7 7 7 0 --- 1433 1370

NOTE: Even afier al1 the power switches are on, the compressor motor would not run unless it is manually tumed on in the ATS LabVIEW program ([SI in Figure A.2).

Comp Discharge T (OC)

56.3 57.1 71 -5

Evap Outlet T ("Cl

12.7 -2.9 -2.3

2. Set the condenser water bath temperature [ I l as required.

3. Click on [ I l to bring out the heater control prompt in Figure A.3. Switch to the

manual heater control mode by clicking on [3] in Figure A.3. Then. click on [ 2 ] again to close the prompt.

Figure A.3 Heater Control Prompt

4. Set the evaporator heating power [-Il as reqiiired.

5 Fully open the orifice valves.

6. Tum on the compressor motor [ 5 ] .

7. Slowly increase the compressor motor to 500 rpm [6]. It is normal to observe green

liquid (HFC-l34a + PAG oil + Dye tracer) passing through the sight glass behveen the flow meter and the orifice valves dunng this initial period.

NOTE: The compressor clutch will automatically get engaged whenever the rpm is greater than 350. However, it is recommended to manually put the clutch switch to "ON" [7] so that the clutch cycling mode does not get initiated.

8. Run at 500 rpm for about 15 seconds, then slowly increase the motor speed to a desired compressor rpm. Clear liquid refhgerant would normally be observed in the sight glasses within a few minutes.

9. Once liquid refrigerant starts flowing through the stratification tube, immediately reduce the orifice valve opening until the sight glass at the outlet of the flow metrr gets thoroughly filled with the liquid refngerant.

10. Adjust the orifice valve opening to obtain a desired flow rate while making sure the sight glass is completely filled with the liquid refngerant.

NOTE: Make sure the compressor outlet temperature does not exceed 121 O C at any time.

1 1. To initiate the data logging, click on [SI. Click on the same button to terminate it.

Do not forget to take note of the tirne at which the data logging was initiated !!! Each Excel file containing the experimental data gets saved using the data logging initiation time as the file name.

Shut t in~ off the ATS

1. Bnng the heater temperature setting to zero [1].

2. Put the clutch switch to "OFF" [7].

3. Slowly bnng the rnotor rpm to zero [6].

4. Fully open the orifice valves.

5 . Recover the refigerant.

6. Determine how much PAG oil was pulled out of the ATS during the recovery

process and add the same amount of unused oil to the system.

For Steps 5 and 6, refer to the Robinair operating manual For instructions on how to recover the reftigerant.

7. Tum the LabVIEW program off by clicking on [9] .

8. Dnin the condenser water bath. Make sure to tum the pump off as soon as the pump starts running dry.

Accessine the Data Files

Expenmental data logged into the ternplate Excel file can be Found in the Eollowing

location:

C:\Prograrn FilesiNationai Instnirnents\LabV1E~PROJECT\ATS\Trend Data.