Parametric study of the performance of heat pipe – a review 2

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Volume 4, Issue 1, January- February (2013), pp. 173-184 IJMET
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PARAMETRIC STUDY OF THE PERFORMANCE OF HEAT PIPE – A
REVIEW

M.N.Khan, Utkarsh Gupta, Shubhansh Sinha, Shubhendu Prakash Singh,
Sandeep Pathak
Department of Mechanical Engineering,
Krishna Institute of Engineering and Technology, Ghaziabad

ABSTRACT

The electronic device uses an electronic circuit which experiences an increase in
operating temperature due to increase in heat flux density. Therefore, cooling becomes one of
the important factors to be taken into consideration. Heat pipes are the heat transfer devices
available to deal with the high density electronic cooling problem due to their high thermal
conductivity, reliability and low weight. A Heat pipe uses the principles of both thermal
conductivity and phase transition to manage the heat transfer between two solid interfaces.
Due to the high capacity to heat transfer, heat exchanger with heat pipes has become much
smaller than traditional heat exchangers in handling high heat fluxes. This paper gives you a
detailed literature review about the main factors affecting the performance of heat
pipe.Furthermore,the thermal resistance and heat transfer capability are affected by the
influence of various parameters such as working fluid, tilt angle, fill ratio, wick structure,
thermal properties, heat input and applications in different fields.

KEYWORDS: Heat pipe, Working fluid, Wick structure, Tilt angle, Heat input,
Applications.

1. INTRODUCTION

Heat pipes are one of the most effective procedures to transport thermal energy from
one point to another. It uses two principles of thermal conductivity and phase transition to
efficiently manage the transfer of heat. Heat pipes contain no mechanical moving parts and
typically require no maintenance. The concept of heat pipe was originally invented by
Gaugler of the General Motors Corporation. In 1944, he patented a lightweight heat transfer

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

device which was essentially the present heat pipe [1], but only after Grover independently
rediscovered it in 1960s that the remarkable properties of the heat pipe became appreciated
and serious development work followed. Grover also coined the name “heat pipe” and stated,
“Within certain limitations on the manner of use, a heat pipe may be regarded as a synergistic
engineering structure which is equivalent to a material having a thermal conductivity greatly
exceeding that of any known metal” [2]. The advantage of heat pipes over many other heat-
dissipation mechanisms is their great efficiency in transferring heat. They are fundamentally
better at heat conduction over a distance than an equivalent cross-section of solid copper
(a heat sink alone, though simpler in design and construction, does not take advantage of the
principle of matter phase transition). A second feature of the heat pipe is that relatively large
amounts of heat can be transported with small lightweight structures. The amount of heat that
can be transported as latent heat of vaporization is usually several orders of magnitude larger
than can be transported as sensible heat in a conventional convective system with an
equivalent temperature difference. The performance of a heat pipe is often expressed in terms
of equivalent thermal conductivity. In applications where conventional cooling methods are
not suitable, heat pipes are being used very often. Once the need for heat pipe arises, the most
appropriate heat pipe needs to be selected. Often this is not an easy task. For applications
involving energy conservation, the heat pipe is a prime candidate and has been used to
advantage in heat recovery systems and energy conversion devices. Conservation of energy
has never been more important before, as the cost of fuel is ever rising and the reserves are
diminishing. The heat pipe provides an effective tool in a large number of applications
associated with conservation.

2. WORKING OF HEAT PIPE

Heat pipe is a very efficient instrument for transfer of heat from one end to other.
Heat pipe consist of three parts namely evaporator, condenser and the wick portion shown in
figure 1. Every part has its own significance in transferring the heat energy. Evaporator is the
portion which receives the heat from the source. In the evaporator, the working fluid is
present in liquid state. The heat is absorbed by liquid and it is converted into vapour phase.
As vapour is formed in the evaporator portion a pressure difference is created between
evaporator and condenser portion. This vapour pressure difference is the driving force which
takes the vapour from evaporator to condenser. On reaching the condenser end the vapour
gives out its latent heat of vaporisation and converts into liquid form. The heat absorbed at
the evaporator section is received in this way in the condenser portion. Wick portion is a
porous structure made of wire mesh or grooves or sintered metal powders etc. The function of
the wick structure is to absorb the liquid from the condenser end and transfer it to evaporator
through capillary action. The function of wick is very important because if wick is unable to
transfer working fluid from condenser to evaporator, the heat pipe will eventually dry out and
this will stop the working of heat pipe. Hence, the parameters which are responsible for the
flow of working fluid through the wick must be properly studied and applied. Heat pipe thus
works on the principle which is the combination of conduction and convection. The heat
transfer process through heat pipe can be considered as heat transfer in the closed loop as the
working fluid starts from evaporator absorbs heat reaches condenser portion gives out heat
and then through wick structure finally comes back to evaporator. One of the major
advantages of the process of heat pipe is that the heat transfer in heat pipe is independent of

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any external agency i.e. for transferring the heat no help is required from any external source.
All the processes involved are natural and this makes the heat pipe self dependent. Moreover
it has been found that the heat pipe has high thermal conductivity, hence it finds use in the
region of high heat transfer. Because of above advantages, it has become a desirable
instrument.

Fig.1. Schematic view of heat pipe

3. LIMITATIONS OF HEAT PIPE

During steady state operation, the maximum heat transport capability of a heat pipe is
governed by several limitations, which must be clearly known when designing a heat pipe.
The heat transfer limitations depend on the working fluid, the wick structure, the dimensions
of the heat pipe, and the heat pipe operational temperature. There are five primary heat pipe
transport limitations:

Fig.2. Limitations to heat transport in a heat pipe

3.1 Capillary Limitation

The capillary limit involves the fundamental phenomenon in heat pipe operation that
is the development of capillary pressure differences across the liquid-vapour interfaces in the
evaporator and condenser. When the existing capillary pressure is insufficient in providing
adequate liquid flow from the condenser to the evaporator, dry out of the evaporator wick
will occur. Therefore for the circulation of the working fluid, capillary pressure difference is
the driving potential and the maximum capillary pressure must be greater than the sum of all
pressure losses inside the heat pipe.

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The capillary limitation occurs when the net capillary forces generated by the vapour–
liquid interfaces in the evaporator and condenser are not large enough to overcome the
frictional pressure losses due to fluid motion. This causes the heat pipe evaporator to dry out
and shuts down the transfer of heat from the evaporator to the condenser. For most heat pipes,
the maximum heat transfer rate due to capillary limitation can be expressed as [3].

where, K is the wick permeability (m2), Aw is the wick cross-sectional area (m2), ρl is the
liquid density (kg/m3), µl is the liquid viscosity (Ns/m2), reff is the wick capillary radius in the
evaporator (m), g is the acceleration due to gravity (9.8 m/s2), and lt is the total length of the
pipe (m) [4].

3.2 Viscous Limitation

In case of heat pipe operating at low temperatures, the vapour (saturation) pressure in
the evaporator region is very small and of the same magnitude as the pressure gradient
required to drive the vapour from evaporator to condenser. In this case, the total vapour
pressure is balanced by opposing viscous forces in the vapour channel. Thus, the total vapour
pressure in the vapour region becomes insufficient to sustain an increased flow. This low-
flow condition in the vapour region is referred to as the viscous limit. As the viscous limit
occurs at very low vapour pressures, the viscous limit is most often observed in longer heat
pipes when the working fluid used is near the melting temperature (or during frozen start-up
conditions) as the saturation pressure of the fluid is low. Viscous force prevents vapour flow
in the heat pipe. The heat pipe thus operates below the recommended operating temperature.
Increasing the heat pipe operating temperature is a potential solution or operates with an
alternative working fluid.
The viscous limit does not represent a failure condition. In the case where the heat
input exceeds the heat input determined from the viscous limit, this results in the heat pipe
operating at a higher temperature with a corresponding increase in the saturation vapour
pressure. However, this condition typically is associated with the heat pipe transitioning to
being sonic limited.

3.3 Sonic Limitation

This limit is experienced in liquid metal heat pipes during start up or low-temperature
operation due to very low vapour densities in this condition. This can lead to choked, or
sonic, vapour flow. The sonic limit is typically not a factor for most heat pipes operating at
room temperature or cryogenic temperatures, except in case of very small vapour channel
diameters. With the increased vapour velocities, inertial, or dynamic, pressure effects must be
included. Where the inertial effects of the vapour flow are significant, the heat pipe may no
longer operate in a nearly isothermal case, resulting in a significantly increased temperature
gradient along the heat pipe. The sonic limitation actually serves as an upper bound to the
axial heat transport capacity and does not necessarily result in dry out of the evaporator wick
or total heat pipe failure. Any attempts to exceed the sonic limit causes an increase in both the

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evaporator temperature and the axial temperature gradient along the heat pipe, thus reducing
further the isothermal characteristics found in the vapour flow region.

In other words, the heat pipe is due operating at low temperature with too much of power.
The potential solution for this limitation is to create large temperature gradient so that heat
pipe system carries adequate power as it warns up [5].

3.4 Entrainment Limitation

Basic flow conditions in a heat pipe shows that the liquid and vapour flow in opposite
directions. The interaction between the counter currents of liquid and vapour flow leads to
viscous shear forces occurring at the liquid–vapour interface, which may inhibit liquid return
to the evaporator. In the most severe cases, waves may be formed and the interfacial shear
forces become greater than the liquid surface tension forces, causing liquid droplets being
entrained in the vapour flow and carried to the condenser. In a majority of cases studied, the
wick structure of the heat pipe was flooded (i.e. excess liquid), which allowed entrainment to
occur.
The most common approach to estimating the entrainment limit in heat pipes is to use
a Weber number criterion.
Cotter [6] presented one of the first methods to determine the entrainment limit. This method
utilized the Weber number, defined as the ratio of the viscous shear force to the forces
resulting from the surface tension.

By relating the vapour velocity and the heat transport capacity to the axial heat flux as

With the assumptions that entrainment of liquid droplets in the vapour flow & the Weber
number must be less than unity .The maximum transport capacity based on entrainment can
be written as

Where, σl is the surface tension (N/m) and rc,ave is the average capillary radius of the wick.
Note that for many applications rc,ave is often approximated by reff . The entrainment limit
refers to the case of high shear forces developed as the vapour passes in the counter flow
direction over the liquid saturated wick, where the liquid may be entrained by the vapour and
returned to the condenser. This results in insufficient liquid flow of the wick structure [7].

3.5 Boiling limitation

At high values of heat fluxes, boiling at nucleate level may occur in the wick
structure, which causes vapour to become trapped in the wick, thus blocking return of liquid
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and resulting in evaporator dry out. This phenomenon is termed as the boiling limit. It differs
from the other limitations, as it depends on the radial or circumferential heat flux applied to
the evaporator, as opposed to the axial heat flux or total thermal power transported by the
heat pipe. Determination of the heat flux or boiling limit is based on nucleate boiling theory
and is comprised of two separate phenomena: i) bubble formation & ii) Subsequent growth or
collapse of the bubbles.
Bubble formation is governed by the size (and number) of nucleation sites on a solid
surface and the temperature difference between the heat pipe wall and the working fluid. The
temperature difference, or superheat, governs the formation of bubbles. The potential solution
is to use a wick with a higher heat capacity or spread out the heat load [8].

4. EFFECT OF WICK STRUCTURE

Wick is one of the most important parts of heat pipe. Wick is responsible for carrying
condensed liquid from condenser to evaporator and this is necessary for working of heat pipe.
Hence, study of wick structure and the factors controlling its performance is necessary. In the
heat pipe, vapour flows through the cylinder and liquid flows through the wick. In absence of
gravity, electrical or any other force the pressure difference is created by capillary pressure
difference. Because of this, capillary of small radius is needed to transfer the liquid from
condenser to evaporator. But, if the radius becomes very small, friction increases which
causes a very large amount of pressure loss. The second problem which is encountered is that
the small radius capillary may be choked by some vapour bubble and the flow of liquid may
stop [9].The modulation of heat pipe wick thickness helps in axial capillary liquid flow, while
restricting the increase in the wick superheat that accompany thicker, uniform wicks [10].
The diameter and other structural configuration changes with the change in type of operation
or input output load. For example, in thin heat pipes vapour and liquid pressure drops are
large and therefore to keep circulating the working fluid, high capillary force is required. So,
groove wick is not used for thin heat pipes since capillary force in this is low. To increase the
heat carrying capacity of thin heat pipes, we use mesh wick or sintered wick. Even after
changing the wick structure, the force developed is not enough. So, we reduce the pore radius
of wick to increase the capillary force, but in this case the liquid flow pressure drop is
increased due to decrease in permeability. So, the heat transfer capacity of thin heat pipe
decreases. Hence, the channel through which the liquid flows must be enlarged to overcome
this drawback. But when liquid channel is enlarged, the vapour flow area is reduced which
again leads to decrease in heat carrying capacity of heat pipe. So, in order to obtain balance
between the two, an optimum thickness of liquid and vapour column must be selected [11].

5. EFFECT OF FLUID CHARGE

Filled ratio is the fraction (by volume) of the heat pipe which is initially filled with the
liquid.

(1) Low fill ratio: When the heat input is given, the fluid at the evaporator section is
vaporised at a faster rate and due to the accumulation of more vapour at the condenser
section, it leads to dry out phenomenon at the evaporator section, hence condensation occurs
at a lower rate. Thus, efficiency of heat pipe is lowered.

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(2) High fill ratio: The evaporator section temperature is lower and hence evaporation occurs
at a lower rate. This results in very little vapour flowing into the condenser section, reducing
the thermal efficiency of the heat pipe.

Filled ratio is constricted by two operational limits. At 0% filled ratio, a heat pipe
with no working fluid and bare tubes only, is a heat transfer device in pure conduction mode
with a very high undesirable thermal resistance. At 100% filled ratio, heat pipe operates like a
single phase thermo- syphon with action maximum for a vertical heat pipe and stops for a
horizontal heat pipe. The heat transfer takes place purely by axial conduction in a horizontal
heat pipe [12].
A number of experiments have been performed using varying heat inputs and filled
ratios. Experiments were carried out in dry mode (without working fluid) and wet mode (with
working fluid in it). The dry mode experiment represents the heat transfer characteristics in
an ordinary conductor, and the wet mode experiment depicts the live heat pipe characteristics.
Three different working fluids namely distilled water; methanol and acetone are used in this
study. The heat pipe was filled with 35%, 55%, 85% and 100% of the evaporator volume and
tested for different heat input and working fluids [13].
Figures (3) to (5) show the variations of thermal resistances for different fill ratios, for
the three different working fluids at different heat input. These graphs compare thermal
resistances at different fill ratios of different working fluids. In general, wet run shows the
reduced thermal resistances for all levels of heat input and all types of working fluids. The
dry run shows the largest values of thermal resistances and it is almost constant for varying
heat loads.
The experiments done over the years indicate that the filling ratio and the heat input
are important considering the heat transfer performance and the heat pipe performs optimally
when the filling ratio ranges between 50–75%, at 50o inclination angle. The minimum
performance was found for filling ratio at 25% and inclination angle at 25o [14]. In general,
for working fluid fill ratios greater than 85% of evaporator volume, results are better in terms
of decreased thermal resistance, increased heat transfer coefficient and reduced temperature
difference across the evaporator and condenser [15]. With increase in heat input, the thermal
resistance decreases and the fill ratio comes in effect because it has a great impact on the
thermal resistance. For lower fill ratio, thermal resistance is higher and starts decreasing for
further fill ratios.

Fig.3.Variation of thermal resistance with Fig.4.Variation of thermal resistance with Fig.5.Variation of thermal resistance with
different heat inputs for 35% fill ratio different heat inputs for 55% fill ratio different heat inputs for 100% fill ratio

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6. EFFECT OF WORKING FLUID

Working fluid selection is directly connected to the properties of the fluid. The
property of working fluid is going to affect both the ability to transfer heat and the
compatibility with the case and wick material. A multitude of characteristics must be
evaluated in order to determine the most favourable fluid for the application considering the
primary requirements which are compatible with the heat pipe material (s) such as thermal
stability, wettability, reasonable vapour pressure, high latent heat and thermal conductivity,
low liquid and vapour viscosities and acceptable freezing point.
The Nano -fluids kept in the suspension of conventional fluids have the potential of
superior heat transfer capability than the conventional fluids due to their improved thermal
conductivity. The copper Nano-fluid which has a 40 nm size with a concentration of 100
mg/lit is kept in the suspension of the de-ionized (DI) water and an aqueous solution of n-
Butanol and these fluids are used as a working medium in the heat pipe [16].
The presence of Nano-particles in the working fluid leads to a reduction in the speed
of the liquid, smaller temperature difference along the heat pipe and the possibility of
reduction in size under the same operational conditions. Results manifested that the alumina
Nano-fluids augmented the thermal performance of the OHP (oscillating heat pipe). On
comparison with pure water, the maximal thermal resistance was decreased by 0.14 °C/W (or
32.5%) when the power input was 58.8 W [17].
The maximum heat flux apparently increase with the increase of the mass
concentration when the mass concentration is less than 1.0 wt. %. Then, they begin to
decrease slowly after the mass concentration is over 1.0 wt. %. The mass concentration of 1.0
wt. % corresponds also to the best input power enhancement. The maximum input power of
the heat pipe can enhance by 42% after substituting the Nano-fluid for deionized water [18].
It was found that the heat transfer rate of the CLOHP (closed loop oscillating heat pipes)
using silver Nano-fluid as a working fluid was better than that the heat transfer rate when
pure water is used because the silver Nano-fluid increases the heat flux by more than
10%[19].
On closer scrutiny, it can be said that Nano-fluids are potential fluids to be used as working
fluid in PHP/OHP because -
a. Inclusion of Nano particles can affect the start-up temperature of the PHP. However, the
particle size affects the start-up temperature.
b. When the Nano-particle size is reduced, the thermal conductivity of the Nano-fluid
increases. However, the Nano-particles may agglomerate, settle, or coalesce to the walls
with long-term operation of the Nano-fluid.
c. In PHP, the use of Nano-fluids can lower operating temperatures and greater pulsations of
amplitudes
d. Enhanced nucleation sites and reduced bubble diameter can be obtained.
e. In selection of the Nano-fluid, the surface wettability or the contact angle of the Nano-
particles with the surface plays an important role.
f. Among the four shapes (cylinder, blade, plate and brick) studied so far, the cylindrical
shape gives the best result.
Generally, water out performed the Flourinert TM liquids, in particular above approximately
40 W where the liquid entrainment limit compromises the performance of the Flourinert TM
charged thermo- siphons. Even still, the Flourinert TM liquids FC-84 and FC-77 offer

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adequate thermal performance below 40 W and offer the added benefit of being dielectric,
which may be beneficial in some circumstances.
Although the Nano-fluid has the higher heat conduction coefficient that dispels more
heat theoretically. But the higher concentration will make the higher viscosity. The higher
viscosity makes the bubble difficult to produce and the force of friction causes obstruction of
the liquid slug with tube wall becomes larger, so obstruction is relatively greater when the
bubble is promoted and influences the whole efficiency of the heat transfer [20].
In addition, adding too many Nano-particles to fluid would make the property of working
fluid at evaporator section tend to be in solid phase, and would make the
convection performance of Nano-fluid at evaporator section reduced. This was
disadvantageous to the thermal efficiency of heat pipe [21]. The thermal resistance of the heat
pipes with Nano-particle solution is lower than that with DI water. As a result, the
higher thermal performances of the new coolant have proved its potential as a substitute for
conventional DI water in vertical circular meshed heat pipe [22].

7. EFFECT OF TILT ANGLE

Performance of heat pipe also depends upon how the heat pipe is placed. Heat pipe
can be used in various positions. It can be horizontal, vertical or in any other angle. In
horizontal position, gravity has no effect on the performance of heat pipe. But as the angle
changes, gravity start playing its role. With change in angle, the effect of gravity changes and
this affects the performance of heat pipe [23]. In addition, the orientation of heat pipe is also
important. With change in angle, gravity can help or oppose the working of heat pipe.
Depending on this, there are two types of tilt angle-favourable and adverse [24].Favourable
tilt angle is that when evaporator is below and condenser is above and vice versa for adverse
tilt angle. When we use favourable tilt angle i.e. evaporator is below and condenser is above,
the performance of heat pipe increases with increase in tilt angle. This is because; in this type
of orientation, gravity helps the movement of fluid from condenser to evaporator. So, with
the increase in rate of transfer of fluid from condenser to evaporator, the rate of heat transfer
increases and hence its efficiency. Whereas, in adverse tilt angle, evaporator is above and
condenser is below and in this gravity opposes the flow of fluid from condenser to evaporator
and hence the efficiency of heat pipe falls. Hence, the heat pipe must be kept in favourable
tilt angle for maximum efficiency. An increase in heat transfer rate of 39% is obtained for 2%
iron oxide nano-particles, when the angle of inclination of heat pipe is 90 degrees [25].
Efficiency of heat pipe usually increases with increase of angle in favourable tilt. However,
when the heat pipe tilt angle exceeds a value of 60° for de-ionic water and 45° for alcohol,
the heat pipe thermal efficiency tends to decrease [26].

8. APPLICATION

Heat pipe is very versatile device and it can be employed in various fields because of
its capacity to blend in various operations. It has been found useful in a number of fields such
as aerospace engineering, energy conversion devices, electronic cooling, biomedical
engineering etc. In addition to conventional uses, recent advances have been done to increase
the adaptive nature of heat pipe. Micro heat pies have been developed to employ it in cooling
of electronic devices. Heat pipes of various shapes such as flat shaped, disk shaped, rotating,

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reciprocating heat pipes have drawn attention due its multidimensional nature of
application[27].Heat pipe is also used as heat exchangers for the recovery of waste heat of
automotive emissions. Wickless heat pipe is also used for numerous applications in heat
recovery system [28]. It is also used for other industrial process equipments. Carbon steel
heat pipe technology is used for utilizing heat pipe as air pre heater and waste heat boiler.
This is applied in field of heat recovery, environmental protection and energy conservation.
Liquid metal high temperature heat pipe has been used for high-temperature hot air
generators and heat tractors. Heat pipe also finds its use in chemical reactors including
ammonia convertors [29]. Encompassing all above uses, it has become an extremely useful
device which facilitates the process and adds to the efficiency of the process.

9. CONCLUSION

The review reveals that a detailed analysis on heat pipe can be done by working on
different parameters like working fluid, wick structure, tilt angle, pipe material, temperature
profiles of heat source and heat sink andenvironmental conditions. The working fluid
properties affect the ability to transfer heat and the compatibility with the case and wick
material. However particular working fluid can only be functional at certain temperature
ranges. Also, working fluid needs a compatible vessel material to prevent corrosion or
chemical reaction. Many articles reveal that Nano fluids have great potential to ascend the
thermal efficiency of the heat pipe. The wick material used in heat pipe can be formed using
incorporating particles of micro-encapsulated phase change material bonded together. Use of
such a wick structure has the advantage of providing an additional heat absorber. This greatly
improves heat pipe ability to absorb excess heat and prevent damage. The heat pipe
orientation (read tilt angle) is important for the practical applications. It is observed through
practical analysis that operating heat pipe in a favourable tilt position can increase heat
transfer capacity.

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