Material Measurement Laboratory Cryogenic Engineering Conference 2015 06-29-2015 11:45 AM...

Material Measurement Laboratory

Cryogenic Engineering Conference 201506-29-2015

11:45 AM

Single-phase ambient and cryogenic temperature heat transfer coefficients in

microchannel

Seungwhan Baek and Peter E BradleyNIST

Boulder, CO 80305 USA

Material Measurement Laboratory 2

Contents

• Introduction

• Experiments

• Results

• Discussion

• Summary


Introduction• Microscale J-T cryocooler development

– Requires microchannel heat exchanger – Requires microchannel heat transfer characteristics

CCompressor

Microchannel Heat exchanger

J-T valve

Evaporator



Early prototype

Revised MCC Design


Thermal design of MCC heat exchanger

• Operating condition of MCC – Fluid flow in microchannel ( Dh < 100 μm )– Extremely low flowrate ( Re < 100 )

– Low pressure ratio (Pr<4:1)

• Cooling Performance of MCC depends on– Heat exchanger performance– Need heat exchanger/heat transfer characteristics

for MCC operating condition – No previous research at these condition)

• Two different heat exchanger in MCC– Recuperative HX: Single phase (gas or liquid)

• Phase I research

– Isothermal HX: two-phase (gas+liquid)• Phase 2 research

Recuperative HX

Isothermal HX

Dh: hydraulic diameter, Re: Reynolds number


Thermal design of heat exchanger

• Heat transfer coefficient (h) required to determine geometry of heat exchanger (Aht), heat transfer amount ()

– Geometry: Heat transfer area (Aht=L x W), thickness of the wall (th)

• Nusselt number– Dimensionless number to define heat transfer performance – Nusselt number (heat transfer characteristic) is dependent on fluid property (Pr) &

operating condition (Re)

Wall

Flow channelL

W

H

th

Flow direction

h= �̇�𝐴h𝑡(𝑇 𝑓 −𝑇𝑤)

Aht: heat transfer area (m2): heat amount (W)Tf: fluid temperature (K)Tw: wall temperature (K)

𝑁𝑢=h𝐷h

𝑘 𝑓𝑁𝑢= 𝑓 (𝑅𝑒 ,𝑃𝑟)

h: heat transfer coefficient (W/m2K)kf: thermal conductivity of fluid (W/mK)Dh: hydraulic diameter (m)Re: Reynolds number Pr: Prandtl number


10 100 1000 100001E-3

0.01

0.1

1

10

100

Yang et al (2012) Choi et al. (1991) Wu & Little (1983) Morini et al. (2012)N

usse

lt nu

mbe

r (N

u)

Reynolds number (Re)

Nusselt number: Previous research

Review of heat transfer and pressure drop characteristics of single and two-phase microchannels, Asadi, 2014, (Review paper)

• Theoretical Nusselt number: Nu=4.36 const. (circular channel, Re < 2000)

• From previous research: single-phase Nu # correlation for Re < 2000• No experiment at low temperature (T < 200 K), low flow rate (Re = 10 - 50)• Experiments do not follow theory, Differ from each other, No recent research

• Need to verify heat transfer characteristic for better design of MCC

From previous research, Nu=f(Re, Pr) for Re < 2000

Laminar TurbulentMCC Operating

regime

Researcher Diameter Correlation

Kays (1970) Dh > 1mm

Sieder & Tate (1996) Dh > 1mm

Shah and London (1978) Dh > 1mm

Shah and London (1978) Dh > 1mm

Wu & Little (1983) Dh = 150 μm

Choi et al. (1991) Dh = 81 μm

Grigull and Tratz (1965) Dh > 1mm

Nu=4.36

Previous Nu# for single phase flow (gas)


Experiments


Experiments

180 μm 110 μm 65 μm

Friction factor 300 K gas N2

Heat transfer Coefficient

70 K Liquid N2

300 K Gas N2

• Measure the single phase heat transfer coefficients• Nitrogen • microchannels (180 μm, 110 μm, 65 μm)

• Operating condition• cryogenic liquid flow (~70 K) • ambient gas flow(~300 K)


Experimental setup (schematic)

1st stage

2nd stage

GM cryocooler

Microchannel

T=64 K

Vacuum Chamber

Tinlet Twall_1 Twall_2 Twall_3Toutlet

Pressure sensor Pressure sensor

Compressor

Recuperator

Radiation shield

Flow conditioner

Constant heat flux

‘Not to scale’

Mass flow meter

Vacuum chamber

Feedthrough collar +

Test section

Compressor

GM-cryocooler 2nd stage

Microchannel assembly

GM-cryocooler 1st stage


Microchannel test section

Tout

Tin

Thermal grease

Heating element

Tw3Tw2Tw1

Epoxy

Solder

microchannel

thermocouple

LL/2L/4

Flow ‘Not to scale’

180 μm

380 μm

Unit: mm

380 μm36 AWG Do=130 μm E-type thermocouples Heating wire

Do=160 μm

Setup forDh=180 μm

Heating length=3cm


139 μm

139 μm

SEM pictures of microchannels

Dh=180 μm Dh=110 μm Dh=65 μm

Din (μm) 180 110 65Dout (μm) 380 310 160Din/Dout 0.47 0.35 0.4

Identical magnification

Thickest wallThinnest wall

100 μm

Dr. Baek’s hair cross sectionNot typical human


Scale comparison

310 μm 160 μm

106 μm

Dr. Baek hair

65 μm110 μm

Stainless steel tube

Dh= 110 μm

Stainless steeltubeDh= 65 μm

Dr. Baek Hair

OD~100 μmOD=160 μm

OD=310 μm

Thermocouple tip300 μm

Not able to measure fluid temperature and wall temperature separately.

36AWG


‘Classic’ Nu# estimation method

1. Find energy input to fluid

2. Estimate fluid temperature inside microchannel (based on linear temperature profile)

3. Measure the wall temperature

4. Determine the heat transfer coefficient

5. Calculate the Nusselt number

Microchannel

Tinlet Twall,x Toutlet

Constant heat fluxheater

𝑄 𝑓𝑙𝑜𝑤=�̇�𝑐𝑝 (𝑇 𝑜𝑢𝑡−𝑇 𝑖𝑛)=�̇� (𝑖𝑜𝑢𝑡−𝑖𝑖𝑛)

x

L

𝑄 𝑓𝑙𝑜𝑤𝑥𝐿

=�̇�𝑐𝑝 (𝑇 𝑓 , 𝑥−𝑇 𝑖𝑛 ) 𝑇 𝑓 , 𝑥=𝑄𝑓𝑙𝑜𝑤

�̇�𝑐𝑝𝐿𝑥+𝑇 𝑖𝑛

𝑇𝑤𝑎𝑙𝑙 ,𝑥

h=𝑄 𝑓𝑙𝑜𝑤

𝐴𝐻𝑇 (𝑇𝑤−𝑇 𝑓 ,𝑥)=

�̇� (𝑖𝑜𝑢𝑡−𝑖𝑖𝑛 )𝐴𝐻𝑇 (𝑇𝑤−𝑇 𝑓 , 𝑥)

𝑁𝑢=h𝐷h

𝑘 𝑓


Result & Discussion


Friction factor measurements (Gas, 300 K)

• Hydraulic characteristic of fluid in microchannels– Friction factor follows conventional theory

10 100 1000 100001E-3

0.01

0.1

1

10 180 m Exp. 110 m Exp. 65 m Exp.

Laminar Theory: 16/Re Turbulent: Blasius

f - fr

ictio

n fa

ctor

Reynolds number

Laminar Turbulent

Experimental friction factor

Laminar theory friction factor

Turbulent theory friction factor (Blasius equation)


Nu # measurement

• Nu # degrades from Re < 1000• Nu #: 180 μm > 65 μm > 110 μm• Similar trend with previous research

(Morini, Choi, Little)

10 100 1000 100001E-3

0.01

0.1

1

10

100 65 m Gas N

2 Exp.

110 m Gas N2 Exp.

180 m Gas N2 Exp.

Nu

ssel

t n

um

ber

Reynolds number

Nu=4.36

Laminar Turbulent

10 100 1000 100001E-3

0.01

0.1

1

10

100

65 m Liquid N2 Exp.

110 m Liquid N2 Exp.


Nu

ssel

t n

um

ber

Reynolds number

Nu=4.36

Laminar Turbulent

• Nu # degrades from Re > 200 • Nu # : 180 μm ~= 65 μm ~= 110 μm• No other research to compare

Gas N2 (300 K) Liquid N2 (70 K )


Scaling effectNon-D number Effect Ignored when

Kn Knudsen gas rarefaction Kn < 0.001

Ma Mach flow compressibility Ma < 0.3

Br Brinkman viscous heating Br < 0.005

λ Lambda axial conduction of wall λ < 0.01

Pe Peclet axial conduction of fluid Pe > 50

• Scaling effect can influence the thermal behavior of fluid flow in microchannels*

*Guo Z-Y and Li Z-X, 2003 International Journal of Heat and Mass Transfer 46 (1) 149-159

MicrochannelPhase Kn

(<0.001)

Ma (<0.3) Br

(<0.005)

λ(<0.01)

Pe (>50)

Re Re=1 Re=3000 Re=1 Re=3000 Re=1 Re=3000

Dh=180 μmgas 0.00007 0.0006 0.10

1.52.18 0.005 10 2340

liquid 0.00001 0.0003 0.04 0.10 0.0002 3 200

Dh=110 μmgas 0.00012 0.0018 0.27 1.63 0.006 10 2400

liquid 0.00002 0.0007 0.11 0.80 0.0002 5 200

Dh=65 μmgas 0.00021 0.0031 0.46

8.71.10 0.002 8 2430

liquid 0.00005 0.0010 0.20 0.20 0.0004 5 2700


Nu # degradation• Nu # degradation is related to axial conduction effect through the wall.• Axial conduction changes temperature profile to ‘non-linear’. (Baek et al, 2014)• Non-linear temperature profile violates the assumption in classic Nu # measurement.• Classic Nu # measurement including axial conduction effect leads to estimate ‘apparent

Nu #’.

• Apparent Nu # is neither actual nor theoretical Nu #.

• Apparent Nu # (Lin & Kandlikar, 2012) – Nu # degrades due to axial conduction effect with classic Nu# measurement method

2

1

1 4(RePr)

app app

theoryw wtheory x

f f

Nu h

Nuk ANu h

k A

Baek et al., 2014, Cryogenics 60 49-61Lin T-Y and Kandlikar S G, 2012 Journal of Heat Transfer 134 (2) 020902

(1)


Comparison of Experiment & Nuapp

• Comparison shows identical trend with experiment & equation (1).

• Comparison implies actual Nu=4.36 holds in low Re # flow.

10 100 1000 100001E-3

0.01

0.1

1

10

100 Simulations for Liquid N

2, eqn. (6)

Simulations for Gas N2, eqn. (6)

110 m

180 m

65 m

110 m

65 m Liquid N2 Exp.



65 m Gas N2 Exp.

110 m Gas N2 Exp.

180 m Gas N2 Exp.

App

aren

t Nus

selt

num

ber

Reynolds number

180 m

65 m


Summary


Summary

• Design of heat exchangers influence development of MCC– High uncertainty in operation Due to very small Dh, Low Re# , Low temperature

• The hydraulic and thermal characteristics of fluid in the microchannel are investigated by the experiments.

– Friction factors : comparable to macro-scale tubes– Nu # : decreased value @ low Re# , which are affected by axial conduction

• Axial conduction effect influence the fluid & wall temperature profile to become non-linear.

• Comparison of experimental result and theoretically derived Nuapp imply validation of Nu = 4.36 In laminar flow for single-phase fluid.

Material Measurement Laboratory

Experiment vs Previous work

22


Thank you!

Test Specimen is available!Ask Peter & Seungwhan for

observation!

Material Measurement Laboratory Cryogenic Engineering Conference 2015 06-29-2015 11:45 AM...

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