A study of flow patterns in a thermosyphon for compact heat

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. Grooten1, C. W. M. van der Geld2, L. G. M. van Deurzen3
1
2
3
Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, [email protected]
Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, [email protected]
Department of Mechanical Engineering, Technische Universiteit Eindhoven, Netherlands, [email protected]
ABSTRACT
Recently, thermosyphons have attracted interest in the
design of smaller, lighter and cheaper heat exchangers, because of their compactness, low thermal resistance, high heat
recovery effectiveness, safety and reliability. In order to understand the effects of the angle of inclination on heat transfer characteristics of a thermosyphon, a dedicated flow visualization study of flow patterns in a transparent two-phase
thermosyphon was conducted. Heat flux and angle of inclination were varied in wide ranges. The thermosyphon is made 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 inclination, 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 perimeter is wetted for β< 20º, and probably for all β< 80º. This
explains the proper functioning for each orientation of a R134a 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.
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
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
2
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 thermosyphon 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/m2,
V&cool = 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.
3
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/m2 and 3.2·104 W/m2 and increasing
angles of inclination (top to bottom),
4
V&cool = 361 – 578 l/h.
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.
Figure 5: Detailed flow pattern in the evaporator at heat flux
3.2·104 W/m2. 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 section for various angles, q’ = 2.6·104 W/m2,
V&cool = 578 l/h.
Figure 7: Proof of existence of a liquid film by observation
of drop impingement. The area between evaporator and condenser 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
CONCLUSIONS
Flow visualizations of flow patterns in a transparent twophase 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
5
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/m2. 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.
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NOMENCLATURE
D
g
L
T
t
q’
u
V
diameter, m
gravitational acceleration, m/s2
length, m
temperature, K
time, s
heat flux, W/m2K
velocity, m/s
flow rate, l/h
Greek
β angle of inclination, º
Subscipts
cool
coolant
REFERENCES
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submitted for publication
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operation limiting heat flux of long, R-134a filled
thermosyphons, submitted for publication
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