Volume 11 Issue1, 2023

HEAT PIPE PERFORMANCE WITH VARYING WORKING

FLUIDS AND FILL PROPORTIONS

A. K. Mozumdehjk1,*, A. F. Akonh1, M. S. H. Chowdhury1 and S. C. Banik2

Abstract: Miniature heat pipes of 5 mm in diameter and 150 mm in length with a 10 W thermal capacity
are designed, fabricated, and tested. Different thermal loads were used in experiments to evaluate heat pipe
performance with and without working fluid. Common solvents including water, methanol, and acetone
were utilized in the experiment. Thermocouples were used to record the temperature profile throughout the
length of the heat pipe. Thermal resistance and the total heat transfer coefficient were used to measure the
heat pipe's effectiveness. It was found that the performance characteristics changed depending on the
quantity of liquid stocked. Finally, the optimal liquid fill ratio is determined in terms of reduced temperature
differential, increased heat transfer coefficient, and decreased thermal resistance. Researchers in this area
will benefit greatly from having access to this study's published data. The Miniature heat pipe has its highest
overall heat transfer coefficient while using acetone as the working fluid.

Keywords: Heat Exchanger, Supply of Working Fluid, and Filling Efficiency

INTRODUCTION
The heat pipe is a closed, hollow tube filled with a given in the specialist literature to ensure the
liquid having a boiling point near to the target mathematical models are accurate.An analytical
temperature. The tube is submerged in two different formula for the minimal meniscus radius was
temperatures, one at each end. The pipe's job is to developed using the momentum conservation and
carry heat from the warmer area to the cooler Laplace-Young equations.limit was achieved in
one.Both theoretical and practical studies were micro- and nano-sized heat pipes 4. The impacts of
conducted to determine heat pipe's dominant contact angle, vapor pressure drop, tilt angle,
factors and defining features.1, 2. Garcia et al. groove size, and channel angle were included into
looked into the capillary structure of micro heat these calculations, as were shear forces at the
pipes to develop a mathematical model and its liquid/solid and liquid/vapor interfaces.
numerical solution for laminar two-phase flow of In light of the latest experimental results 5, the
liquid and vapor of working fluid. 3. In the original analytical model created by Cotter to
mathematical model, the vapor flows in a single estimate the maximum heat transmission capacity
dimension while the liquid flows in a quasi-single in micro heat pipes has been reevaluated. Although
dimension in a steady state. For a micro heat pipe it produces trends that are inagreement with the
with a capillary structure with a cross section in the actual data, the original model greatly overpredicts
shape of a four-tipped asteroid, the authors provide the maximum heat transfer capacity since it
data on the longitudinal distributions of the mass assumed a fixed evaporator zone, as is the case with
flow rate, the pressure, and the traverse section area most models. A semi-empirical correlation has
of the phases, as well as the curvature radius of the been created in an attempt to give a more precise
liquid-vapor interface. Maximum heat transfer forecasting tool.
capacity in respect to capillary pressure is
investigated. Results gained are verified with data
1
Department of Mechanical Engineering, Bangladesh University of Engineering & Technology,Dhaka-1000, Bangladesh.
2
Department of Mechanical Engineering, Chittagong University of Engineering & Technology, Chittagong, Bangladesh
*Corresponding email: aloke@me.buet.ac.bd
The intricate nature of micro heat pipe heat that uses the boiling heat-transfer process in a
transfer necessitated the development of a compact area. Based on this idea, two wickless
mathematical model 6 to reliably predict the network heat-pipes (or thermal spreaders) are
heat transport capacities and temperature developed, manufactured, and tested. The
gradients that contribute to the overall axial copper or aluminum thermal spreaders are
temperature decrease. The model's fluid and wickless, cross-grooved heat transmission
heat transfer are regulated using a third-order devices. They disperse a localized heat source
ordinary differential equation. analytical across a considerably broader area. Therefore,
solution for the two-dimensional heat air cooling may be used to disperse the high heat
conduction equation that governs the macro flux produced by the concentrated heat source.
evaporating film region in the triangular Water and methanol are used to put the network
corners; effects of the vapor flow on the liquid of heat pipes through their paces in a variety of
flow in the micro heat pipe; flow and operational situations and orientations with
condensation of the thin film due to surface respect to the gravity vector. Total heat input is
tension in the condenser; and capillary flow 393 W, with maximal heat fluxes of roughly 40
along the axial direction of the micro heat pipe. W/cm2 for methanol and 110W/cm2 for water.
In comparison to micro heat pipes, the effects of METHOD OF EXPERIMENT
the working limit, liquid locking, and length A copper tube with an inner diameter of 5 mm
give MHP its own distinct physical phenomena. and an outside diameter of 8 mm is used to create
In other words, micro heat pipes smaller than 1 the heat pipe (Fig. 1). An evaporator heater of 230
mm are susceptible to the liquid blockage volts and 50 watts was constructed out of a Ni-Cr
phenomenon, which causes condenser liquid to wire of 8 millimeters in diameter and 50
gather at the condenser's outlet and results in millimeters in length. Asbestos is used for
incomplete heat transfer. In MHP, if the
insulation in the heat pipe's evaporator and
condenser is cooled too much, the vapor
temperature will drop, resulting in a lower adiabatic sections to reduce heat loss. Both a
maximum heat transfer rate. Significant impacts variac and a multimeter were supplied for use in
due to the heat pipe's total length and the regulating and monitoring the power supply. We
capillary limit show, too, among the heat pipe's measured temperatures using K-Type
operational restrictions. 7. The requirements, thermocouple wires (the positions of the
design restrictions, and financial resources of thermocouples are shown in Fig. 1; thermocouple
the project's creators will determine which heat 1 is located 20 mm from the base, while
pipe cooling method will be the most suitable. thermocouples 2, 3, 4, 5, and 6 are located at 40,
A simple analytical model was created to 70, 90, 120, and 140 mm). To get an accurate
estimate the performance of counter-flow heat reading, a simple 8-channel digital thermometer
exchanger units using thermosyphon 8, and the is employed. The condenser end has five copper
properties of such units were tested fins brazed onto it; each fin is 50 mm in length,
experimentally. Heat pipes or a two-phase 15 mm in breadth, and 0.5 mm thick. Both dry run
closed thermosyphon are used as the heat- (no working fluid in the tube) and wet run
transfer element. No matter the geometry of the (working fluid present) conditions were used in
element bundle, the maximum heat-transfer rate the experiments. When there is no fluid within the
is proportional to the ratio of the heated-to-
heat pipe, it acts much like a metal conductor.
cooled lengths of the heat-transfer elements. In
their work, Zuo and Gunnerson 9 analyzed the Heat pipes (those containing a working fluid) are
heat transport in an angled two-phase closed evaluated according on how well they operate.
thermosyphon. Because the thin layer of liquid Turning the heat "on" and keeping tabs on the
on the top side is simpler to boil off, the steadily rising temperature until a steady state is
inclination-induced circumferential flow is bad reached is a common protocol. To optimize the
for drying out but beneficial for flooding fluid stockpile, the performance of the micro heat
because the thick film on the underneath pipe was investigated by repeating the
corresponds to a significant gravity force. Cao experiments with varying heat inputs and fill
and Gao 10 looked into a network heat pipe idea ratios.
 Fig. 1: Schematic Arrangement of experimental setup

METHOD OF EXPERIMENT by toggling the selection switch. This procedure

Evaporator, adiabatic, and condenser sections are was repeated using varying amounts of heat input,
the three components that make up the test section. fill ratios, and workingfluids. Multiple graphs were
Three liquids (distilled water, methanol, and generated to analyze the micro heat pipe's operation
acetone) were tested for their heat transfer and determine the best way to distribute the fluid
properties. The properties were also evaluated in a stock. By adjusting the variac's output voltage, we
dry run test. This led to the creation of two small were able to provide varying degrees of heat
heat pipes. Before the real thing input.The term "fill ratio" refers to the extent to
provided that both ends of the heat pipe were which the working fluids fill the evaporator
properly sealed. Both the bottom and the top of the section's capacity. Each of the three working fluids
heat pipe that contained liquid were hermetically was tested at fill ratios of 35%, 55%, 85%, and
sealed with corks. The evaporator was wrapped 100% of the evaporator capacity.After achieving
with Ni-Cr thermic wire. The line supply was steady state, temperatures were recorded at each of
routed via a variac to power the heater. Forced the six locations along the heat pipe's outside for
convection was accomplished by attaching fins to each of the three working fluids and each of the fill
the condenser and pointing a fan in their ratios.
direction.Glue was used to secure six pairs of
thermocouple wires to the chassis. Each set of EQUATIONS USED IN MATHEMATICS
thermocouples was fused together initially at the The heat pipe's efficiency is enhanced in a
top, and it was made sure that, apart from that one roundabout way thanks to thermal resistance -
location, the thermocouples would not come into
contact with one another. They were later joined to Results and Suggestions
the main body. Connecting cables were used to Both dry (no working fluid) and wet
connect the thermocouple probes' other ends to the (with working fluid) experimental conditions
digital thermocouple reader. Six thermocouples were used. Experiments conducted in dry
were installed on the heat pipe's outside, two in mode are analogous to the heat transfer
each of the evaporator and condenser portions. characteristics of a standard conductor,
Each segment has thermocouples installed every 20 whereas those conducted in wet mode are
millimeters.Wet run (with the working fluid within analogous to those of a real heat pipe. In this
the tube) and dry run (without the working fluid investigation, the temperature-adaptive
inside) experiments were carried out. When there is properties of three commonly used working
no fluid within the heat pipe, it acts much like a fluids—distilled water, methanol, and
metal conductor. The efficiency of the heat pipe acetone—are compared. Different heat inputs
(with the working fluid within) is used as a and working fluids were used to conduct
benchmark for its overall rating. Heat was applied experiments in which the heat pipe was filled
to the heat pipe, and the temperature increase was to 35, 55, 85, and 100 percent of the evaporator
measured intermittently until the system reached volume.Temperature data collected at various
steady state. Once equilibrium was reached, the axial distances along the heat pipe body is used
temperatures at each of the six sites were recorded to construct an axial temperature profile.
 170

140

Temperature (oC)
110

80

50

20
10 30 50 70 90 110 130
Distance (mm)
Fig. 2: Axial temperature profile for DRY RUN
Figures 2–5 depict the axial temperature distribution throughout the heat pipe during dry run and wet run
(55% fill ratios).
Figure 2 depicts the temperature changes in the evaporator, adiabatic section, and condenser as a function
of distance during a dry run. The results demonstrate that when heat is added, the temperature gradient
between the condenser and evaporator becomes steeper. Since a steeper temperature gradient is

100 2W
4W
6W
8W
80 10W
Temperature (oC)

60
Te  Tc
R= Q
C/W (1)
And the overall heat transfer co-efficient is given by- 20
 Fig. 3: Axial temperature profile for Water With55%fill ratio
12

10

Thermal Resistance (oC/W)

8

6
Dry RunWater MethanolAcetone

0
0 2 4 612 8 10
Power (Watt)
Fig. 4: Axial temperature profile for Methanol With 55% fill ratio
70
2W
4W
6W
8W
10W

Fig. 6: variations of thermal resistance with different heat inputs for 35% fill ratio
12

60 10
Thermal Resistance (oC/W)
Temperature (oC)

8
5
0
6

40 4
Dry RunWater MethanolAcetone

2
30
10 30 50 70 90 110 130
Distance (mm) 0
Fig. 5: Axial temperature profile for Acetone With 55% fill ratio
necessary in the case of simple conduction heat The slopes of the axial temperature distributions
transfer to enhance heat transmission. are less for methanol than they are for water or
When compared to dry run, where the axial dry run, and they are much smaller for acetone as
temperature distribution has steeper slopes for the working fluid than they are for water or
the same heat inputs, the wet run has shallower methanol.
slopes, suggesting enhanced heat transmission at Changes in Thermal Resistance (R) as a
even lower temperatures.At 10W heat input, the Function of Temperature: varying fill ratios for
heat pipe stops working as seen by the sudden the three working fluids result in varying thermal
shift in the slope of the axial temperature resistances, as shown in Figures 6-8, for the same
distribution for water (Fig. 3). At this point, more amount of heat input. These charts are helpful for
heat is lost via the evaporator than is gained by comparing thermal resistances of various
the condenser. All the other working fluids working fluids at varying fill ratios.
follow the same general patterns. Temperature-resistance differences with material
Fig. 7: Variations of thermal resistance with different heat inputs for 55% fill ratio
12

10

Thermal Resistance (oC/W)

Dry RunWater MethanolAcetone

0
0 2 4 6 8 10 12
Power (Watt)
Fig. 8: Variations of thermal resistance with different heat inputs for 100% fill ratio
Above three figures (Figs. 6-8), we see the heat
inputs for dry run and wet run (for 35%, 55%, and Variations in the global heat transfer coefficients
100%). The lower thermal resistances shown in for a fill factor of one hundred percent
wet run are consistent across heat input intensities
and material kinds. liquids used for mechanical According to the results of the dry run, the forced
operations. The greatest thermal resistance values convective heat transfer at the fin end has
are seen during the dry run, and they remain an overall heat transfer co-efficient of
almost constant throughout a wide range of heat around 2000 W/m2-°C. By increasing
inputs. Acetone has the lowest thermal resistance the heat transfer rate via evaporation and
for any fill ratio and any heat input. The graph condensation inside the heat pipe,
pattern in Fig. 6 shows that, just as with other fill charging the heat pipe with working
ratios, lower thermal resistances may be fluids results in a significant increase in
produced for larger heat loads when using 35% heat transfer co-efficient.
fill. ratios. Figure 7 shows that at a fill ratio of
55%, the thermal resistance values fall for Heat transfer coefficients for water at 35% fill
methanol and acetone but rise for water after 6W. ratio (Fig. 9) are almost constant,
The efficiency of water increases to 6W at whereas those for working fluid
55%.Water and methanol exhibit almost identical methanol rise slightly with increasing
thermal resistance values at 100% fill ratio (Fig. heat input. The heat transfer coefficient
8), but acetone displays minimal values for for acetone dramatically rises.
greater heat inputs.
 Fig. 9: Overall heat transfer co-efficients withdifferent heat inputs for 35% fill ratio
20000
Dry Run
Heat Transfer Coefficient (W/m2K)

Water
16000 Methanol
Acetone

12000

8000

4000

0
0 2 4 6 8 10 12
Power (Watt)

Fig. 10: Overall heat transfer co-efficients withdifferent heat inputs for 85% fill ratio
quickly with enhanced heat capacity.
Fig. 10 shows that at a fill ratio of 85%, the heat transfer co-efficient value decreases very slowly for water,
grows very slowly for methanol, and increases fluids.
extremely fast for acetone as the working fluid.
Heat transfer coefficients for water and methanol
are quite comparable at 100% fill ratio (Fig. 11),
but those for acetone are higher for increasing In Figs. 12 and 13, we have loads of 6 W and 10
heat inputs. W, respectively. At any given fill ratio, acetone
exhibits the smallest temperature fluctuations.
Acetone-filled heat pipes have a very high heat Acetone was shown to be the most effective
transfer coefficient. However, the burn out at working fluid throughout the investigated
maximum heat input limits this monotonic rise in temperature ranges.
heat transfer co-efficient value with load. It was When working with methanol and water, the fill
previously mentioned that this condition leads to ratio has a negligible impact on the temperature
"starving" in the evaporator section because the gap between the two phases. At greater fill ratios,
rate of condensate return is lower than the rate of however, acetone exhibits a smaller temperature
evaporation. differential. When using acetone as the working
Calculating the Ideal Fluid Filling Ratio fluid, the optimal result is achieved with a 100%
Evaporator-to-condenser temperature differential fill ratio of the evaporator volume, resulting in a
vs working fluid fill ratio as a percentage of minimal temperature gradient between the two
evaporator volume for all three heat transfer components.
60

50
Temp. Difference (T e-Tc ) oC

40
 CONCLUSIONS pages), February 1998, Ma, H. B., and
Peterson, G. P., "The Minimum Meniscus
The design, fabrication, and testing of a Radius and Capillary Heat Transport Limit in
miniaturized heat pipe with a power output of 10
Micro Heat Pipes," doi:10.1115/1.2830046.
W have all met with great success. Various power
outputs (2W, 4W, 6W, 8W, and 10W) are shown, [6] [5] The Heat Transport Capacity of Micro
each with its own unique operational Heat Pipes, by J. M. Ha and G. P. Peterson, J.
characteristics.In wet run, the system achieves Heat Transfer, Volume 120, Issue 4, 1064 (8
steady state faster than in dry run. The research pages), November 1998,
yields the following results: When heat loads are doi:10.1115/1.2825891.
increased, the steady-state temperature rises.
While the dry run's axial temperature distribution [7] [6] May 1999, Peterson, G. P., and Ma, H. B.,
slopes upward in response to added heat, the wet "Temperature Response of Heat Transport in a
run's axial temperature distribution slopes Micro Heat Pipe", J. Heat Transfer, 121(2),
downward on average. Wet run reduces the total 438 (8 pages), doi:10.1115/1.2825997.
thermal resistance of the working heat pipe
compared to dry run. The thermal resistance was [8] The third edition of Heat Pipes by Paul Dunn
10.5 °C/W in the dry run and 7.25 °C/W in the and David A. Reay was published in 1982 by
wet run, both with a 2W heat input capacity. In Pergamon Press in New York.
the range of inputs measured for acetone and [9] In February 1978, Lee Y. and Bedrossian A.
methanol, the total heat transfer coefficient of published "The characteristics of heat
heat pipe rises with increase in heat input, but the exchangers using heat pipes or
value for a heat pipe filled with water remains thermosyphons" in the International Journal of
practically constant. Heat pipe performance in
Heat and Mass Transfer (Volume 21, Issue 2),
terms of temperature difference is demonstrated
pages 221-229.
to be least affected by the fill ratio of working
fluid as a percentage of evaporator volume when [10] The Journal of Heat Transfer, Volume 117,
water and methanol are employed as working Pages 1073–1075, 1995, "Heat Transfer
fluids. The acetone fill ratio has a negative effect Analysis of an Inclined Two Phase Closed
on the temperature differential between the Thermosyphon" [9] Zuo, Z. J., and Gunnerson,
evaporator and condenser. There is a minimum F. S."Wickless network heat pipes for high
temperature differential between the evaporator heat flux spreading applications," Cao, Y., and
and the condenser when the evaporator is filled to Gao, M., June 2002 Applications of wickless
100% of its capacity with acetone as the working network heat pipes for dispersing high heat
fluid. The heat transfer coefficient, thermal
flux, International Journal of Heat and Mass
resistance, and temperature differential between
Transfer, 45(12), 2539-2547(9).
the evaporator and condenser all improve with fill
ratios of working fluid more than 85% of the
capacity of the evaporator.
REFERENCES
[1] July 2009 "study of two phases heat transport
capacity in a micro heat pipe", WSEAS
Transactions on Information Science and
Applications, Volume 6, Issue 7, pp. 1104-
1114, ISSN:1790-0832 Hsu, C. H., Kung, K.
Y., Hu, S., and Chang, C. C.
[2] In March 1994, "A heat pipe transient analysis
model," International Journal of Heat and
Mass Transfer, [2] Tourniera, J. M., and El-
Genka, M. S.
[3] Pages 753-762, Issue 5 of Volume 37
[4] [3] Universidad, Ciencia y Tecnologia,
Volume 4, Issue 14, Pages 59–66, June 2000,
"Thermal Fluid Dynamic Steady State
Behavior of Micro Heat Pipes" Garcia, F., J.
Segura, and S. Zarea
[5] [4] J. Heat Transfer, Vol. 120, No. 1, p. 227 (7