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ABSTRACT
The paper describes the ongoing design and development
of a flexible heat pipe system which couples heat transfer
function along with its inherent flexibility, so that the
evaporator section of the heat pipe, which is connected to
the heat load does not get stressed or deformed under the
mounting conditions. Providing this flexibility will prevent
damage to the heat pipe and the system on which it is
mounted. The flexibility has been incorporated in the
adiabatic section of the heat pipe by using a flexible
metallic bellow to allow relative movement between the
two ends of the heat pipe. The primary technological
challenge faced during the execution of this project is to
integrate the capillary wick along with the flexible bellow
sub-system and ensure proper thermal operation. A
stainless steel bellow has been used as the flexible element
and has been brazed to pipes on both sides to form the
outer casing. In the present study, the flexible heat pipe has
been tested in ‘vertical heater-down position’ and ‘for
horizontal position’. Its thermal resistance is reported
under straight and bent configurations of the heat pipe;
detailed study is presently underway. As per the required
specifications, the heat pipe is expected to cater to about
100 W of thermal load dissipation, in the operating range
of 10°C to 100°C. The heat pipe has an internal diameter
of 10 mm and is 270 mm long.
INTRODUCTION
Two-phase passive devices are proven present day
solutions which can cater to a large variety of thermal
management problems at various levels. Conventional heat
pipe technology has been successfully applied in the last
forty years for the thermal management of a variety of
applications like heat exchangers, economizers, space
applications, and electronics cooling.
In general, typical heat pipes utilize the continuous
evaporation / condensation of a suitable working fluid for
two-phase heat transport utilizing latent heat in a closed
system. A typical heat pipe consists of three sections:
evaporator, adiabatic section and condenser
The present study describes the design and
development of a wicked flexible heat pipe system, which
performs the heat transfer function while not getting
stressed or deformed under the mounting conditions [4-5].
These mounting forces are likely to be generated while
attaching the heat pipe to the heat source as well as to the
condenser. Providing this flexibility is aimed at preventing
the heat producing element from damage. These mounting
forces and vibrations affect the thermal performance and
reliability of the heat pipes [6-9]. The heat pipe is designed
for a nominal heat carrying capacity of 100W at operating
temperature range of 10°C to 100°C.
MANUFACTURING OF FLEXIBLE HEAT PIPE
The proposed heat pipe has a flexible structure that is
formed on the metal pipe, such that the heat pipe can be
bent/ gets bent if external loads are experienced. In
addition, the woven mesh and the support element can be
bent together with the heat pipe without a risk of being
broken, such that the woven mesh can be maintained in
contact with the internal wall of the metal pipe to allow the
working fluid to flow smoothly in the woven mesh and to
maintain a good heat dissipating effect.
A flexible heat pipe comprising of the following is
manufactured [10-11]:
a. a metal pipe;
b. a flexible structure which is essentially a stainless
steel bellow;
c. a wick consisting of multiple layer of phosphor
bronze screen mesh, disposed inside the metal pipe;
d. a working fluid (water), filled inside the metal pipe
and attached onto the woven mesh; and
e. a support element, passed into the interior of the
woven mesh, abutted against the woven mesh, and
attached onto an internal wall of the metal pipe.
RESULTS AND DISCUSSION
Experiments are carried out to calculate the thermal
resistance of heat pipe in straight and deformed
configurations, respectively, to characterize performance of
heat pipe in the desired operating range. The heat pipe is
positioned vertically with evaporator kept downwards.
Proper insulation is applied, wherever necessary. Insulation
is removed temporarily to take IR images, as and when
required. Power input is increased in steps, both in the
forward and reverse directions. All experiments reported
here are repeated, at least three times. The thermal
resistance of a heat pipe is calculated as:
?ℎ =
? − ?
?̇
(4)
At present, we report essentially the thermal resistance of
the evaporator, due to limitations of instrumentation in the
condenser sub-section. The temperature in the adiabatic
section, the temperature in the evaporator section, thermal
resistance (between the evaporator section and the
adiabatic section) and uncertainty in thermal resistance for
the heat pipe, under different power inputs, is summarized
Tab. 1. During this trial, the heat pipe is kept in straight
configuration with coolant flowing through the condenser
at 20°C
SUMMARY AND CONCLUSIONS
The developed flexible heat pipe performs well in
deformed configuration as is evident from the IR image of
the adiabatic section in this configuration, which shows
uniform temperature across the adiabatic section. Thermal
resistance is in range of 0.1 K/W to 0.2 K/W in the
operating range of heat pipe for gravity assisted working
and 0.2 K/W to 0.25K/W for horizontal working, which is
very low as is desired for good thermal management. It is
also observed that the heat pipe can cope with sudden
power inputs and position change.