Heat Transfer Report: Heat Pipe Design and Analysis

This report provides a comprehensive analysis of heat pipes, starting with an introduction to their origins and basic principles. It delves into the working mechanism of heat pipes, explaining how they utilize evaporative cooling to transfer thermal energy efficiently. The report discusses the role of evaporation and condensation of the working fluid, and the importance of temperature differences in driving the heat transfer process. It examines the components of heat pipes, including the wick structure and various types of heat pipes such as vapor chambers, variable conductance heat pipes, and diode heat pipes. Furthermore, the report reviews existing literature on cylindrical heat pipes, highlighting contributions made by researchers like Busse and others. The report concludes with a discussion of the applications and benefits of heat pipes, emphasizing their high heat transport capacity and effectiveness in various engineering applications.

Running head: HEAT TRANSFER
Heat Transfer
[Name of the Student]
[Name of the University]
[Author note]

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Table of Contents
Introduction:...............................................................................................................................2
Discussion:.................................................................................................................................3
Working principle:.....................................................................................................................3
Literature review:.......................................................................................................................7
Application of heat pipes:..........................................................................................................9
Benefits of heat pipes:..............................................................................................................10
Types of heat pipes:.................................................................................................................11
1. Vapour Chambers:...........................................................................................................11
2. variable conductance heat pipes:......................................................................................11
3. Diode heat pipes:..............................................................................................................11
Components of heat pipes:.......................................................................................................12
Conclusion:..............................................................................................................................14
References:...............................................................................................................................15

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Introduction:
Gaugler in the year of 1994 and Trefethen in the year of 1962 were the first two
persons to consider the original idea of “heat pipes”. For the basic presentation of “heat
pipes” Gaugler performed a test with a very lightweight heat transfer device. Along with this
less attention was paid to this device at that time when it was considered (Zohuri, 2016).
Trefethen was the first person to suggest this in the year of 1962. This was followed by a
patent application of this pipes in the year of 1963 by Wyatt. This was not publicized till the
year of 1964 when “George Grove” along with his co-worker at “Los Alamos National
Laboratory” made an invention regarding the concept which is totally independently. This
was done for the space program that is existing right now along with the application of this
program. Heat pipes are nothing but a two phase flow device responsible for transferring of
heat. In this device the liquid changes to vapour and again the vapour changes to liquid form
and this usually as the liquid or the vapour circulates amongst the evaporator and the
condenser which is having a high effective “thermal conductivity” (Orr et al., 2014). This
device has a high heat transport capacity. The heat pipes makes a combination of both the
principles of “thermal conductivity” along with the “phase transition” for the purpose of
transferring the heat in an effective manner. This report mainly discusses about the working
principle of heat pipes. Along with this the report also discusses about the various
components of heat pipe and the design as well.
Discussion:
Working principle:
The heat pipes are responsible for employing the evaporative cooling for the purpose
of transferring the “thermal energy” from a point to another point by means of “evaporation”

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and “condensation” of the fluid that works as a cooling fluid. The existence of the difference
in the temperature between the two ends of the pipe are mainly responsible the working of the
heat pipes. This means when a specific end of the pipe heats up the cooling fluid present
inside the pipe then the fluid gets evaporated at the end along with increasing the pressure of
the vapour that is present inside the heat pipe’s cavity (Fu et al. 2012). The vaporisation
absorbs the “latent heat of evaporation” of the working fluid and associated with this is the
reduction in the pipes end temperature. The vapour pressure is much higher than that of the
vapour which is at equilibrium. This vapour pressure is present over the hot working fluid
and is mainly present at the end of the pipes. This is also higher over the condensing working
fluid that is present al the cooler ends of the pipe. The difference of the pressure is the main
reason for driving of the mass transfer at a rapid rate towards the end where the condensation
take place. At this the excess vapour condenses which is associated by the releasing of the
latent heat along with warming up the cool part of the pipe (Tran et al., 2014). The gas that do
not condense is responsible for impeding of the flow of gas. Due to this reason the
effectiveness of the heat pipes is also reduced and this happens only at particular low
temperature where the pressure of the vapour is also low. The molecules present in the gas is
having a speed almost similar to that of sound. And for cases of gases that does not condense,
this is the upper speed limit of the molecules in the heat pipe but in general case the vapour
speed gets limited by the rate at which condensation occurs at the cold end of the pipe. This is
very much lower than the speed of the molecules. (Choi et al., 2012). This happens when the
surface for condensation is cold. The evaporation from the surface is nearly negligible if the
difference in the temperature is more than 10 degrees. This can be accessed from the curves
of the vapour pressure. It becomes very much challenging when the efficient heat transport
through the gas takes place. The main challenge is to maintain a sufficient temperature
between the condensing surface and the gas. Along with this the differences in the

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temperature also corresponds to the large amount of efficient thermal resistance. The
bottleneck present at the heat source is often less severe. This is due to high density of the gas
at that stage which is also having a high heat flux which is the maximum.
This is followed by flowing back of the condensing working fluid at the end of the
pipe which is hot. When the heat pipes are arranged vertically then the fluid might be moving
due to the gravitational force. And in case if the heat pipe is having wicks then the working
fluid will return due to the capillary action. Another interesting feature of heat pipes is the
different ranges of temperature which is very much effective. There is an assumption that the
heat pipes only work when the heated ends temperature reaches the “boiling point” that is
around 100 degrees and along with this the transferring of the steam to the cooler end also
takes place. But the “boiling point” of the water that is present inside the pipe depend upon
the absolute pressure that is present inside the pipe. In cases of pipes which are evacuated, the
vaporization of the water mainly starts from the triple point and ends at the critical point. This
happens until the heat pipes contains both liquid as well as vapour (Maydanik, Chernysheva
& Pastukhov, 2014). This helps in understanding of the fact that the various operations of the
heat pipes take place at the hot end when the temperature is slightly warmer than the melting
point of the working fluid in the hotter end of the pipe. But the maximum rate of the heat
transfer takes place only when the maximum rate of the transferring of heat is low at
temperature which is below 25 degrees. The heat pipes that are having water as their working
fluid are capable of working even if temperature is above the atmospheric boiling point
(Faghri, 2014). For long term water heat pipes the maximum temperature is around 270
degrees and the heat pipes operates at a temperature around 300 degrees for the tests taking
place at short terms.
The main working principle of the heat pipes are dependent upon “the evaporation”
and “the condensation” of the “working fluid”. The specific heat capacity is greatly increased

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by the heat of vaporization. For example, when we consider water the energy that is needed
for evaporation of one gram of water is almost 540 times greater than that of the amount of
energy that is required for rising the temperature by 1 degree of the same 1-gram amount of
water (Buschmann, 2013). Heat transfer in an effective way take place along with no moving
parts when all of the energy gets transferred to the cooler end at a rapid rate and this is
followed by condensing of the fluid at the cold end.
Fig 1: working of heat pipe
Until there is an existence of temperature difference at a larger rate between the
evaporator section and section for the condenser, then the flow circulation of the processes
related to the phase change along with the two-phase in the HP will continue. The moving of
the fluid stops when the overall temperature becomes uniform. It Starts again when there is
an existence of the temperature difference (Asirvatham, Nimmagadda & Wongwises, 2013).

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When the liquid is below the operating temperature, which means that the liquid is too
cool and this indicates that vaporization of the liquid into a gas is not possible. When the
temperature is above the operating temperature, then all the liquid will be turning to gas.
Which means that the environmental temperature will be too high and this will not let any of
the gas to condense (Qu, Wu & Cheng, 2012).
The transfer of het can take place only if it contains a saturated liquid along with its
vapour (gas phase). Vaporisation of the saturated liquid takes places which if followed by
following to the condenser. In the condenser the liquids get cooled down and after that it
again changes back to the saturated form. The “standard heat pipe” consists of the liquid that
is condensed. This is returned to the evaporator by making use of a wick structure. This will
be associated with the exertion of a capillary action on the phase of the working fluid. Wick
structures that are used in heat pipes mainly includes things like the “sintered metal powder”,
“screen”, and “grooved wicks”. Which means that this structure is having a series of grooves
which is parallel to the axis of the pipe (Sureshkumar, Mohideen & Nethaji, 2013). In case if
the condenser is located just above the evaporator against a gravitational field, then the
gravity is capable of returning the liquid. So it can be stated that this heat pipe is a
thermosyphon. Similarly, the centrifugal force is used by the rotating heat pipes to make the
liquid return from the condenser towards the evaporator.
Literature review:
The traditional form of heat pipes was cylindrical in shape and for this the majority of
the investigations that are being done is in the area of circular heat pipes (Greco et al., 2014).
The working mechanism of every heat pipes is almost same. In this report we are going to
review the work of the cylindrical heat pipes considering the flow of the fluid along with the
mechanism used for heat transfer.

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Busse made an invaluable contribution in the field of circular heat pipes. Italy had
become the focal point of different activities regarding the heat pipes in Europe under the
leadership of Busse, ISPRA. The first thing he did was analysis of the problems of drop in
pressure of the laminar vapour flow inside a long cylindrical heat pipe. This lead to the
assumption of the Poiseuille velocity profile. Along with this the similarities in the profile in
the evaporation zone were analysed. Besides this the variety of profiles having an axial
distance in the condenser zone were also analysed.
Busse et al. (1970) was able to develop a high temperature cylindrical heat pipes for
the thermionic converters. The container materials used in this pipes were Niobium,
Zirconium and Tungsten. Along with this the substances that were used in this heat pipes
were Lithium, Lead, Bismuth and Barium. The temperature range that was considered for this
heat pipes to operate was around 15000C to 20000C. In 1973 Busse was able to present the
ultimate limit for the transferring of heat in the pipes with the laminar vapour flow. The
transfer of heat gets limited either by the return of the insufficient condensate or by the
limitation of the flow of vapour. It was also reported by Busse (1980 and 1982) about the
analysing of the dry out mechanism present in the cylindrical heat pipes that is assisted by the
gravity. In 1989 Busse and Loehrke further presented a method for the purpose of predicting
the laminar subsonic flow inside the cylindrical heat pipes. Along with this they have also
described the “Velocity Profile”, along with the flow reversals that are strong in nature and
occurs inside the heat pipe.
Chun (1972) conducted an experiment for the purpose of predicting the dry out limits
present in the wicks. After the completion of the experiment a new model was proposed by
Chun for the Wick dry out. It was founded by Chun that his model will be predicting with in
the experimental dry out het pipes were very much below the values that were expected when
the liquids fully saturated the evaporator wick.

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A theoretical framework was established by Tien and Rohani (1972) for the purpose
of predicting the operational characteristics of the steady state in a two component heat pipe.
They were also capable so showing that the variations of temperature and pressure present
along the heat pipes. This was done when they applied the law of conservation of mass along
with the relations in thermodynamic relations. Taking ethanol mixtures and water as the
working fluid a series of experiment was carried out over the horizontal cylindrical heat pipes
so as to substantiate the validity of the theoretical model. The different effects of “vapour
pressure” drops the rate of vapour temperature, evaporation and condensation inside the
cylindrical heat pipes were also studied by them. For the purpose of solving the two
dimensional axisymmetric problem the “Stream Function-Vorticity approach” was used.
Along with this they also introduced the equations for energy conservation along with the
equation for thermodynamic equilibrium. This was done for the purpose of coupling the
vapour pressure and the temperature. In the year of 1973 Bankston and Smith reported about
the two dimensional analysis in the cylindrical heat pipes. The solving of the mass and
momentum equations in the laminar flow a finite difference computational method is used
which is based upon the stream function vorticity approach. This in turn eliminates the
pressure as a separate variable.
A lot of research was also done by Faghri and Thomas (1989) on the heat pipes. They
also fabricated and tested a new design of concentric annular heat pipe successfully.
Theoretical and experimental studies were carried out for the purpose of predicting the
capillary limit on the concentric annular heat pipes.
Cao and Faghri in the year of 1990 studied the changes in the phases in the heat pipes
numerically. A two dimensional compressible flow analysis model was presented by them in
respect to the high temperature cylindrical heat pipes.

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Faghri and Buchko in the year of 1991 also carried out an experimental and numerical
analysis of the various effects due to the load of heat distribution amongst the temperature of
the vapour. This was also done on temperature of the wall along with the capacity for heat
transfer of different heat pipes which are having a multiple heat source.
Jang and Faghri (1991) reported about the one dimensional transient analysis on the
cylindrical heat pipes, which treats the vapour as compressible by making use of an implicit
finite difference scheme.
Faghri and Harley (1994) also described the models for two dimensional conventional
circulars along with the gas loaded heat pipes. Incorporation of the axial conduction was done
through the walls for the purpose of analysis. Details about the mathematical model was
provided by Khrustalev and Faghri in the year of 1995 which describes the transfer of heats
through the liquid films present inside the evaporator.
Application of heat pipes:
Heat pipes are a smart investment when someone is having a device or platform that
needs any of the following:
 Heat transfer from a specified location to another location.
 Heat is to be transformed from high heat flux which is present at the evaporator
towards a lower heat flux which present at the condenser. This will be making the
process of removing all the overall heat very much easy by making use of the
methods which are conventional in nature and this includes the liquid or air cooling.
 Provide an isothermal surface.

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Benefits of heat pipes:
Some of the universal benefits of how a heat pipe is able to work with the various
applications are listed below:
 High Effective Thermal Conductivity. Heat can be transmitted over a long
distance, associated with a minimal drop in temperature.
 Passive operation. There are no moving parts, along with this there is no requirement
of energy input rather than the heat which is needed for the purpose of operating.
 Isothermal operation. The surfaces are very much isothermal, which is having a
temperature variation of about ± 5 mK.
 Long life with no maintenance. As there are no parts which is capable of moving in
the heat pipes so the possibility of wearing out also does not exists. The vacuum seal
is responsible for the prevention of the losses in liquid, along with a protective coating
which give each device a long-lasting protection from corrosion.
 Lower costs. When the temperature for operation is lowered then the devices is able
to increase the “Mean Time Between Failure” (MTBF) for the electronic assemblies.
Which also means that this would be lowering the required maintenance along with
the cost replacement (Kole & Dey, 2013). For a HVAC system, the energy that is
required for the heating and air conditioning can be reduces, associate with payback
times which is about a couple of years.
Types of heat pipes:
Heat pipes are of various kinds along with the standard and the Constant Conductance
Heat Pipes and they are listed below:

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1. Vapour Chambers:
This are thinner heat pipes which are having the similar components to that of the
tabular heat pipes that is a hallow vessel which is hermetically sealed along with a “working
fluid” and a “closed loop capillary recirculation system”. Besides this there is a series of posts
which is generally used by the vapour chamber for the purpose of preventing the collapse of
the flat top and bottom (Alizad, Vafai & Shafahi, 2012). This happens when the pressure is
lower than the atmospheric pressure.
2. variable conductance heat pipes:
The standard heat pipes are nothing but a device for constant conductance. In this type
of pipes the temperature of the heat pipes are set by the temperatures of the sink and the
source along with the thermal resistances that are coming from the source towards the heat
pipes. In this case when the temperature of the condenser or the power is greatly reduced
making the temperature of the heat pipes also drops linearly.
3. Diode heat pipes:
The transferring of heat takes place in either direction in the conventional heat pipes.
There are also other heat pipes which act as a thermal diode which means the transferring of
heat takes place only in a specified direction along with working as an insulator for the other
end (Nithyanandam & Pitchumani, 2013).
Components of heat pipes:
Heat pipes are nothing but devices which are capable of transferring heat in a passive
way. They tend to provide long life if they are designed and fabricated in a proper and right
way. Long life is considered to be critical in nature in phase of application like spacecraft
thermal control (Reay, McGlen & Kew, 2013). Heat pipes in various kinds of satellites can
easily work or operate for more than decades which does not tend to have any kind of