HEAT 2008, Fifth International Conference on

Transport Phenomena In Multiphase Systems

June 30 - July 3, 2008, Bialystok, Poland

A study of flow patterns in a thermosyphon

for compact heat exchanger applications

M. H. M. Grooten

1, C. W. M. van der Geld

2, L. G. M. van Deurzen

3

1Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, m.h.m.grooten@tue.nl

2Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, c.w.m.v.d.geld@tue.nl

3Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, l.g.m.v.deurzen@student.tue.nl

ABSTRACT

Recently, thermosyphons have attracted interest in the

design of smaller, lighter and cheaper heat exchangers, be-

cause of their compactness, low thermal resistance, high heat

recovery effectiveness, safety and reliability. In order to un-

derstand the effects of the angle of inclination on heat trans-

fer characteristics of a thermosyphon, a dedicated flow visu-

alization study of flow patterns in a transparent two-phase

thermosyphon was conducted. Heat flux and angle of incli-

nation were varied in wide ranges. The thermosyphon is ma-

de of glass with an inner diameter of 16 mm and a total

length of 290 mm. Acetone is the working fluid at a filling

ratio of 80%. The results show that at all angles of in-

clination, β: (1) vapor plugs exist at heat fluxes less than 14

kW/m2; (2) an annular condensate film flow with a wavy

structure exists at heat fluxes between 14 kW/m2 and 32

kW/m2; (3) waves from condenser to evaporator propagate

faster with increasing heat flux. These waves explain the

corresponding heat transfer enhancement. The whole peri-

meter is wetted for β< 20º, and probably for all β< 80º. This

explains the proper functioning for each orientation of a R-

134a filled copper thermosyphon found in a previous study.

INTRODUCTION

Thermosyphons, or heat pipes, are suitable devices to

transfer heat between two gas streams. Recently,

thermosyphons have attracted more interest in the shift

towards smaller, lighter and cheaper heat exchangers,

because of their compactness, low thermal resistance, high

heat recovery effectiveness, safety and reliability. Vasiliev

(1998, 2005) and Zhang and Zhuang (2003) give overviews

of applications and advantages of thermosyphons and heat

pipes as heat transferring devices.

Scope of our recent study was the typical application of

a heat pipe equipped air-to-air heat exchanger, where two

plate heat exchangers were coupled with multiple

thermosyphons (Hagens et al., 2007). Further research on

this typical application revealed rather interesting effects of

the angle of inclination on heat transfer characteristics

(Grooten and Van der Geld, submitted for publication). The

most important heat transfer characteristics measured

include: condensation and evaporation heat transfer

coefficients, heat flux, and saturation temperature. No

consensus has been found in the literature on the effect of

the angle of inclination, β. This lack of consensus is due to

many differences in operating conditions, fluid composition

and geometry (Chato, 1962; Hahne and Gross, 1981; Larkin,

1982; Negishi and Sawada, 1983; Wen and Guo, 1984;

Hahne et al., 1987; Wang and Ma, 1991; Lock and Kirchner,

1992; Kudritskii, 1994; Zuo and Gunnerson, 1995; Shiraishi

et al., 1995, 1997; Terdtoon et al., 1998, 1999; Payakaruk et

al., 2000). So far, little attention has been paid to the fluid

flow structure in a thermosyphon at inclination and its

effects on heat transfer ability. Some visualization studies

were performed on inclined two-phase thermosyphons

(Negishi and Sawada, 1983; Shiraishi et al., 1995, 1997;

Terdtoon et al., 1998, 1999; Hahne et al., 1987), but not

with acetone as working fluid and not with views of the flow

in both the evaporator and the adiabatic section of the

thermosyphon, where flow structures are observed best.

Knowledge of the flow structures in thermosyphons is

essential to understand the effects of the angle of inclination

on heat transfer characteristics.

Therefore, a dedicated flow visualization study of flow

patterns in a transparent two-phase thermosyphon is

conducted, see Fig. 1. The thermosyphon is filled with

acetone. Operating conditions and working fluid are selected

to mimic those conditions in previous research with R-134a

filled copper thermosyphons (Hagens et al., 2007; Grooten

and Van der Geld, submitted for publication). The objective

is to observe the trend in changes of the flow patterns at

various inclination angles from β = 0º up to 80º from

vertical, and at heat fluxes up to 32 kW/m2. Detailed

recordings of the flow patterns at the evaporator side and the

condensate film flow will be presented and analyzed.

2

Figure 1: Schematic view of the setup.

EXPERIMENTAL

The total length of the thermosyphon is 290 mm, of

which the evaporator section is 100 mm, the condenser

section is 110 mm and the adiabatic section is 80 mm. The

thermosyphon (Fig. 1) is made of a glass tube of 16 mm

diameter, with a smooth inner surface. The working fluid is

acetone. Acetone on glass has about the same static contact

angle as R-134a on copper, see the Introduction. The static

contact angle of acetone on glass is 3.6º (Landolt and

Bornstein, 1956), the critical temperature is 508 K, the triple

point is at 177 K, and the boiling point at atmospheric

pressure is 329 K (NIST, 2005). Only acetone is present in

the thermosyphon. The fluid is saturated between 177 and

508 K. The thermosyphon operates at relatively low

pressures, up to atmospheric, and at room temperature, so

the glass thermosyphon is operated safely and heat losses are

negligible.

The filling ratio is 80%, defined as the volume of liquid

plus the volume that would be obtained if all vapor is

condensed to liquid, divided by the volume of the

evaporator. From recent research (Grooten and Van der

Geld, submitted for publication), the filling ratio was found

not to be crucial for heat transfer characteristics of

thermosyphons as long as dry-out of the evaporator is

avoided. Dry-out of the evaporator does not occur with a

filling ratio of 80% acetone. The filling ratio is not

optimized since the heat pipe was found to be functioning

properly under all desired test conditions. Focus of the

present study is on explaining physical phenomena, not on

designing heat exchangers.

The thermosyphon is filled with acetone according to

the following procedure:

� the upper valve is opened and the tube is evacuated

� the lower valve is connected with a container with

acetone and is opened after evacuation of the tube

� acetone is sucked into the tube until the lower valve is

closed.

Experiments are performed at angles of inclination with

the vertical of β = 0, 5, 10, 15, 20, 30, 60 and 80º. Angles

are measured with a Stabila protractor, which is 0.3º

accurate.

Input heat fluxes vary from 0 to 32 kW/m2 and are

controlled by an electric heated wire wound in the

evaporator wall. The wire is connected to a Belotti variator.

No heating or cooling occurs in the adiabatic section.

For cooling, a water jacket with tap water surrounds the

condenser. The water flow is measured with a Porter &

Fischer D049 rotameter with a maximum capacity of 722 l/h,

accurate to ±5% of the measured value after calibration.

Temperatures are measured at several positions at the

thermosyphon outer wall and at the coolant inlet with K-type

thermocouples. The thermocouples are accurate to 0.2 ºC

after calibration. The operating temperature is the averaged

wall temperature of two thermocouples at the adiabatic

section. During measurements this temperature is typically

50 ºC. Cooling water inlet temperature is kept typically at 15

ºC. The thermocouples are read with a digital thermocouple

thermometer Fluke 2190A and a thermocouple selector

Fluke Y2001.

Recordings are carried out according to the following

procedure:

� the thermosyphon is placed at the desired angle

� the condenser cooling is controlled

� the heat input with the electric heater is controlled

� steady temperatures are reached

� the flow patterns are recorded

3

Figure 2: Schematic top view of the setup.

Two cameras record the flow structures: a Sony

camcorder DCR-PC9E with 25 frames per second and a

PCO 1200HS high speed camera operated at 1000 frames

per second. A halogen light behind tissue paper ensures

diffuse light for clear visualizations, see Fig. 2. The thermo-

syphon is placed behind Plexiglas for safety.

Figure 3. Plug flow in the evaporator section, development

in time in steps of 0.1 s (left to right) and angles of

inclination of β = 0º, 30º, 60º and 80º downwards

respectively. q’ = 1.4·104 W/m

2, coolV& = 361 l/h .

RESULTS

The following results will be presented.

Visualizations of flow patterns in the evaporator section:

� Flow development in time at constant heat flux and

increasing angle of inclination with the vertical.

� Various angles of inclination and increasing heat flux.

Detailed flow pattern visualizations at various angles of

inclination at constant heat flux.

Visualizations of flow patterns in the adiabatic section:

� At constant heat flux and increasing angle of inclination

with the vertical.

4

� Wetting characteristics at inclination up to 20º

At heat fluxes below 14 kW/m2, plug flow is observed in

the vertical evaporator, see Fig 3 (β = 0º). In plug flow, the

following repetitive behavior occurs in the evaporator: at

time 0 s, no vapor bubbles are observed and the liquid is at

rest. At time 0.1 s, a large bubble originates in the middle of

the evaporator, rises and subsequently disintegrates at the

liquid vapor interface at time 0.2 s. Turbulent motion of

bubble remnants continues at the liquid vapor surface until

time is 0.4 s.

At other angles of inclination, a similar type of flow

pattern is observed, see Fig. 3. Apparently, heat

accumulation takes place at or near the center of the

evaporator section, resulting in sudden rapid growth of a

boiling bubble.

At heat fluxes exceeding 14 kW/m2, the flow pattern in

the evaporator changes from plug flow to pool boiling.

Although only movies can show this, the stills of Fig. 4

indicate the pool boiling regime for 20 kW/m2 and 32

kW/m2 at inclination angles of 0º to 80º. Take into

consideration that the higher the angle of inclination, the

more the interface is tilted. The higher the heat flux, the

more agitation occurs in the liquid due to boiling, at each

angle of inclination.

It is practically impossible to capture the motion of

vapor bubbles in stills. That is why in Fig. 5 these motions

are indicated with white arrows. The figure shows the effect

of the angle of inclination on the flow pattern in the

evaporator at a heat flux of 32 kW/m2. In non-vertical

position of the thermosyphon, vapor bubbles first rise nearly

vertically and continue to rise lopsided, parallel with the

upper side of the container of the thermosyphon. Vapor

bubbles at the upper wall rise much faster than bubbles at the

lower wall and at β = 20º vapor bubbles are found to

circulate at the lower wall, possibly induced by liquid

circulation.

Figure 4: Flow patterns in the evaporator section at heat

fluxes of 2.0·104 W/m

2 and 3.2·10

4 W/m

2 and increasing

angles of inclination (top to bottom), coolV& = 361 – 578 l/h.

5

Figure 5: Detailed flow pattern in the evaporator at heat flux

3.2·104 W/m

2. From left to right, one vertical and two

lopsided cases: β = 0º, 20º and 80º. Arrows indicate general

bubble flow directions.

Figure 6: Flow pattern of the liquid film at the adiabatic sec-

tion for various angles, q’ = 2.6·104 W/m

2, coolV& = 578 l/h.

Figure 7: Proof of existence of a liquid film by observation

of drop impingement. The area between evaporator and con-

denser is shown. Development in time from left to right.

The flow pattern in the adiabatic section between the

evaporator and the condenser section shows a liquid film

flowing downwards along the wall, see Fig. 6. The liquid

film clearly has a wavy interface and flows at a velocity in

the order of 1 m/s. By inclining the thermosyphon from the

vertical to β = 30º, the wavy liquid film concentrates at the

lower part of the wall, as shown in Fig. 6. Figure 7 shows

drop impingements that prove that up to an angle of β = 20º

the wall is still fully wet, although the wavy flow is only

observed at the lower part of the wall. In Fig. 7, two

situations are shown where a droplet escapes from the

evaporator section and hits the adiabatic wall of the

thermosyphon. Both droplets spread out and induce waves,

proving that a liquid surface and a liquid film are present.

ANALYSIS

At heat fluxes below 14 kW/m2 and at all angles of

inclination measured, we found a plug flow in the

evaporator, as shown in Fig. 3. This is in agreement with

observations by Negishi and Sawada (1983). They found a

‘dashing motion’ of a big bubble rushing into the condenser;

this was for ethanol at filling ratios above 40% and water at

filling ratios above 60%.

At heat fluxes exceeding 14 kW/m2, we found pool

boiling in the evaporator and liquid returns to the evaporator

as an annular flow at β = 0º. This is in agreement with flow

patterns observed by Shiraishi et al. (1995) for R-113. At

inclined positions, Shiraishi et al. observed a stratified flow

as basic flow pattern, which agrees with our observations,

Fig’s 3 through 6. In more detail, however, some differences

are found: Shiraishi observed liquid disturbance waves

propagating upwards, which were not observed in the

present experiments. These liquid disturbance waves and

impingement of liquid droplets, splashing from the pool

boiling in the evaporator section, were concluded to be more

important in wetting the evaporator upper wall than the

filmwise returning condensate flow. However, in the present

research it was shown that without disturbance waves and

with only incidental liquid droplets splashing upwards from

the boiling liquid on to the upper wall, the upper wall was

still fully wet, see Fig. 7. Splashing was observed for angles

of inclination up to β = 20º. The condensate film flows

downwards along the wall. Moreover, no dry patches were

observed in the present visualizations. At angles of

inclinations exceeding β = 20º, no droplet impingements

were observed and the presence of a liquid film at the upper

evaporator wall can only be deduced from the low value of

the contact angle (3.6º).

The pool boiling in the evaporator at heat fluxes

exceeding 14 kW/m2 is in agreement with findings of

Terdtoon et al. (1998), who visualized a R123 filled

thermosyphon with a filling ratio of 80%. Thermal

conditions were comparable to our measurements, but flow

patterns were observed at the evaporator section only. For a

length to diameter ratio similar to our geometry, Terdtoon et

al. found a bubbly flow with coagulation of bubbles at the

upper wall of the evaporator for angles of inclination of β =

0, 60 and 85º. However, Terdtoon et al. did not observe plug

flow as in our case at heat fluxes below 14 kW/m2.

CONCLUSIONS

Flow visualizations of flow patterns in a transparent two-

phase thermosyphon were conducted at inclination angles

with the vertical from 0º up to 80º and heat fluxes up to 32

kW/m2. The working fluid was acetone with a filling ratio of

6

80%. Detailed flow patterns of the boiling mixture in the

evaporator section and the condensate film in the adiabatic

section of the thermosyphon were compared with flow

visualization studies found in literature. The conclusions

from our present work are summarized below.

Vapor plugs exist at all angles of inclination at heat

fluxes below 14 kW/m2.

An annular condensate film flow with a wavy structure

exists at heat fluxes between 14 kW/m2 and 32 kW/m

2. In

literature, this is regarded as the ‘normal’ operation mode of

this type of thermosyphon. The condensate waves travel

downwards at a typical velocity of 1 m/s.

Waves from condenser to evaporator propagate faster

with increasing heat flux; these waves explain the

corresponding enhancement of condensation heat transfer

coefficients with increasing heat flux that Grooten and Van

der Geld (submitted for publication) measured for R-134a

filled copper thermosyphons.

The whole perimeter is probably wetted at each angle of

inclination, see the proof for β = 20º in Fig. 7. This explains

the proper functioning that Grooten and Van der Geld

(submitted for publication) measured for each orientation of

an R-134a filled copper thermosyphon.

NOMENCLATURE

D diameter, m

g gravitational acceleration, m/s2

L length, m

T temperature, K

t time, s

q’ heat flux, W/m2K

u velocity, m/s

V flow rate, l/h

Greek

β angle of inclination, º

Subscipts

cool coolant

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