Overview of heat pipe studies during the period 2010-2015

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OVERVIEW OF HEAT PIPE STUDIES DURING THE PERIOD 2010-2015
Stéphane Lipsa,
1
, Valérie Sartrea, Frédéric Lefèvrea, Sameer Khandekarb, Jocelyn Bonjoura
a CETHIL UMR5008, Université de Lyon, CNRS, INSA-Lyon, F-69621, Villeurbanne, France
b Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
ABSTRACT
Despite the numerous studies on heat pipes over the last fifty years, the development of predictive tools for the
design of heat pipes remains challenging, even for conventional technologies. As a result, heat pipes are still the
object of more than 250 scientific articles a year. The present review aims to identify and understand the current
scientific approaches followed by scientists in heat pipe science. The different types of heat pipe are reviewed in
order to identify the main phenomena involved in these systems. A brief overview of the heat pipe history is given
and the different applications are presented. A global review of the recent studies on heat pipes is then presented.
Advances are identified in terms of heat pipe characterisation, scientific issues linked to the working fluid,
understanding of phase-change heat transfer in thin liquid films and system modelling. Examples of recent works
are detailed to highlight the strategies that can be followed to answer the current issues. This global review enables
to highlight the main advances on heat pipe science of the last five years and to draw perspectives on the
forthcoming scientific results.
1. INTRODUCTION
Heat pipes are widely used in many industrial applications. They enable the transfer of high heat fluxes with low
temperature gradients by using the latent heat of vaporisation of a working fluid. The diversity of the different
kinds of heat pipes reflects the diversity of the conditions in which they are used. However, whatever the type of
heat pipe, their normal behaviour is bounded by several operating limits that depend on various phenomena. Heat
pipes are the object of thousands of scientific articles published in more than a hundred international journals.
Two long-term established series of international conferences are dedicated to them every two to three years and
they are present at all general conferences dedicated to heat transfer. However, despite the numerous studies on
heat pipes for fifty years, the development of predictive tools for their design is still challenging, even for
conventional technologies. It results in a real limitation in the spreading of heat pipes in the industry, as each new
1
Corresponding author. Email: stephane.lips@insa-lyon.fr

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heat pipe has to be carefully designed for each specific application. By means of a review of the recent works
published on heat pipes, the authors aim to understand the scientific key issues leading to this situation and the
strategies that can be implemented to progress towards a better understanding of the different types of heat pipes.
In 2007, Riffat and Ma proposed a review of developments in heat pipe technology and applications between
2000 and 2005. They noted different kinds of advances during the period. New kinds of wick structures for
conventional and flat heat pipes were proposed and new working fluids were incorporated. They highlighted the
development of new types of loop heat pipes (LHP and CPL) that were expected to be widely used in industrial
applications. They also noted studies on flexible heat pipes as well as on micro heat pipes. New applications were
studied as solar collector systems and heat pipe integrated turbines. Traditional topics as high temperature heat
pipes and space applications were also reported. For air conditioning systems, the coupling between phase-change
materials and heat pipes was investigated. Lastly, they reported the development of new models and mathematical
methods for the design of various kinds of heat pipes.
Ten years later, the research field on heat pipes has changed substantially. Figure 1 presents a word cloud
realised from the titles of the articles published on heat pipes between 2012 and 2014 (about 800 papers). In this
word cloud, the size of the words is proportional to the square root of the number of occurrence of each word. It
is a convenient tool to have a general overview of a given field (McNaught & Lam, 2010). Beside the words
directly related to the heat pipes themselves and their main components, it appears that experimental works are
dominant compared to simulations, numerical analyses and modelling. On the other side, analytical approaches
are very rare. Papers are often interested in heat pipe performance and the diversity of the applications appears,
even in the paper titles (electronic cooling, LED, air conditioning, solar collector, cryogenic applications, vehicles,
thermoelectric generators, energy storage…). One can note the importance of the working fluid (water, ethanol,
mixture) and materials (metal, copper, silicon, aluminium, glass, ceramic, copper oxide (CuO)) used in heat pipes,
as they explicitly appear in many paper titles. The link between heat pipe science and material science is
highlighted by the important occurrence of the words “nanofluid(s)” and “carbon nanotubes”, but also by the
words “material”, “sintered wick” and “process of fabrication”. Progress in manufacturing also promotes the
development of micro heat pipes as well as of light weight heat pipes.
This word cloud also gives a brief summary of the subject of the studies: effect of gravity forces and
inclination angles are studied, as well as magnetic and electric fields. The start-up procedure is the topic of several
articles and the transient response of heat pipes is studied. The word cloud even gives clues on some experimental
devices used to characterise heat pipes, such as neutron radiography.

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Figure 1 Word cloud of the titles of heat pipe articles published between 2012 and 2014 (200 top used words of about 800 papers)
The diversity of the words present in the titles is also a reflection of the diversity of the journals in which the
papers were published. The 800 papers dedicated to heat pipes between 2012 and 2014 were published in more
than 200 different international journals. Figure 2 presents the distribution of the papers in the main journals.
Despites the great number of journals publishing articles on heat pipes, about 40 % of the papers was published
in only 10 journals and almost all of them are dedicated to research on heat transfer. However, one can see that
the heat pipes also interest the communities of solar and renewable energy applications, industrial applications
and electronic cooling applications.
If the word cloud and the typology of the international journals publishing studies on heat pipes enable to
highlight the main keywords describing the current research on heat pipes, it does not give information on the
content of the articles.
The present review article aims to draw a summary of the research dedicated to heat pipes during the period
2010-2015. Its goal is not to be exhaustive, nor to discuss the different scientific results, but is rather to give an
overview of the various approaches used by the current heat pipe research community. In a first part, the various
types of heat pipes are reviewed in order to identify the main phenomena involved in these systems. Then, as
knowing the past is useful to understand the present, a brief history and an overview of the applications are
presented. The recent advances are finally detailed and classified in terms of system characterisation, scientific
issues involving the working fluid, phase-change heat transfer in thin films and heat pipe modelling.
Using
performance
Loop
Operation
Visualization
heat
pipe
THERMAL
Experimental
study
Thermosyphon
Transfer
Investigation
System
Pulsating
Cooling
Effect
Nanofluid
solar
oscillating
Wick
Flat
Fluid
Temperature
closed
ANALYSIS
Application
Working
VAPOR
Exchanger
Design
Numerical
collector
Evaporator
water
CHAMBER
MICRO
SIMULATION
flow
ENERGY
MINIATURE
MODEL
HIGH
novel
HEATING
Plate
POWER
Development
Structure
BASED
Pipes
STORAGE
Thermosiphon
Influence
Electronic
cryogenic
Modeling
enhancement
Research
EXPERIMENT
Sintered
Optimization
Theoretical
LED
Generation
recovery
tube
capillary
Surface
Integrated
Carbon
thermoelectric
Air
Type
ratio
Copper
Liquid
Open
Technology
GAS
evacuated
Phase
Boiling
Used
filling
Module
heater
LATENTunit
Review
Magnetic
Control
Pump
Different
Fin
New
mesh
Hybrid
ROTATING
Transient
Dynamic
CHARACTERIZATION
resistance
management
vertical
Vehicle
USE
operating
EMBEDDED
evaluation
Method
Condensation
FABRICATION
grooved
assisted
PASSIVEprocess
Part
IMPROVEMENT
aluminum
Two
BUBBLE
Efficiency
electric
METAL
Measurement
pressure
Operational
rate
Field
RESPONSE
Modelling
Conductivity
screen
GROOVE
material
Mixture
Distribution
POROUS
TRANSPORT
Dissipation
waste
SMALL
Flux
Enhanced
Three
applied
approach
analytical
WALL
Inclination
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Low
ceramic
Comparative
CUO
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single
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Characteristics
Nanofluids
EFFECTS
WICKS
APPLICATIONS
Fluids
systems
non
Conditions
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Multi
Shaped
THERMOSYPHONS
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Regions
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studies
Parameters
Self
condensable
Double
concentrated
Turn
Investigations Efficient
LIGHT
Disk
U
Glass
Start
Thermo
Ethanol
steady

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Figure 2 Distribution of the recent papers (2012-2014) on heat pipes in the various international journals
2. BRIEF OVERVIEW OF THE HEAT PIPE TECHNOLOGIES, HISTORY AND APPLICATIONS
2.1. The various heat pipe technologies
A heat pipe is a system which is able to transfer high heat fluxes from a heat source to a heat sink with a low
thermal resistance using liquid-vapour phase change (Reay et al., 2013). It consists of a cavity filled by a fluid at
saturation. The liquid evaporates at the contact of the heat source and condenses close to the heat sink. The way
the vapour and the liquid flow to the condenser and to the evaporator respectively depends on the type of heat
pipe. The main types of heat pipes are summarised in figure 3. The distinction can be made between conventional
heat pipes, loop heat pipes and oscillating heat pipes.
The family of conventional heat pipes comprises thermosyphons, cylindrical heat pipes, flat plate heat pipes
and rotating heat pipes. The liquid and vapour flows are countercurrent within the heat pipe body. The liquid
flows from the condenser to the evaporator owing to either gravity, capillary, centrifugal forces or a combination
of these forces. In capillary heat pipes, the capillary structure (grooves, meshes or porous medium) has to be
continuous from the condenser to the evaporator.
The generic term ‘loop heat pipes’ refers to loop heat pipes themselves (LHPs), but also capillary pumped
loops (CPLs) and two-phase loop thermosyphons (also called closed-loop thermosyphons). In these systems, the
liquid and vapour flow in separate lines. For LHPs and CPLs, the sum of frictional and gravitational pressure
drops are compensated by the capillary forces in the capillary structure placed at the evaporator only. A CPL
differs from a LHP by the place of the reservoir, which has a great importance on the overall system behaviour.
In closed-loop thermosyphons, the gravitational forces compensate for the frictional pressure drop.
Distribution of the recent papers about heat pipes in the various scientifics journals
0%
2%
4%
6%
8%
10%
12% Heat transfer
Energy and thermodynamics
Solar and renewable energy
Industry and mechanical engineering
Electronic and electricity
Micro- and nano- scale engineeing
Cryogenic application
Cold and refrigeration
Other
Percentage of articles

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Figure 3 Different heat pipe technologies
The oscillating heat pipes, also called pulsating heat pipes (PHPs) are made of a single meandering tube placed
between the heat source and the heat sink. Its diameter, close to the fluid capillary length, leads to a distribution
of the fluid within the tube into liquid plugs and vapour slugs. The violent vaporisation of multiple liquid slugs
in the evaporator, associated to the condensation of multiple vapour plugs at the condenser, generates self-
sustained oscillations of the fluid. It leads to an efficient heat transfer from the heat source to the heat sink, both
by latent and sensible heat. These systems are cheap and easy to manufacture, but their behaviour is difficult to
predict and they are currently sparsely used in the industry.
Despite the strong differences between the various heat pipe technologies, there are several phenomena shared
by these systems. Obviously, liquid-vapour phase-change heat transfer is present in all heat pipes. The phase
change occurs at the scale of the capillary structure or at the scale of the thin liquid films present in the system.
condenser
q
h) Pulsating heat pipe (PHP)
vapor
line
condenser
q
liquid
line
g) Capillary pumped loop
d) Rotating heat pipe
q
condenser
vapor
liquid
evaporator
evaporator
evaporator
capillary structure
reservoir
vapor
channels
Gravity assisted heat
pipes
Capillary heat pipes
Other heat pipes
e) Two-phase loop thermosyphon
condenser
q
g
a) Thermosyphon
evaporator
condenser
q
g
liquid
vapor
evaporator
b) cylindrical heat pipe
condenser
q
liquid
capillary
structure
vapor
evaporator
c) Flat plate heat pipe / vapor chamber
q
condenser
vap. liq.
evaporator
capillary
structure
Conventional heat pipes Loop and pulsating heat pipes
vapor
channels
condenser
q
f) Loop heat pipe
reservoirevaporator
vapor
line
liquid
line
capillary structure

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The capillary forces are indeed almost never negligible. Moreover, as the fluid is always heated through a wall,
the interactions between the working fluid and the wall, mainly wetting effects, are of great importance. Lastly,
there is always a coupling between hydrodynamic and thermal phenomena, as the working fluid follows a
thermodynamic cycle in the systems.
In the following of the document, research works on heat pipes are classified according to the type of studies
and not to the type of systems. The similarity of the phenomena involved in all heat pipes induces that the progress
in understanding of one kind of heat pipe generally helps to progress on the other kinds.
2.2. History and applications
In order to have an overview of the research in heat pipe science, a global analysis of the studies published on
heat pipes is realised. Figure 4 presents the evolution of the number of articles related to heat pipes indexed on
the Web of ScienceTM database between 1975 and 2014.
Table 1 summarises the exact query used to plot figure 4. Several technologies are distinguished by filtering
the content on the article title only, so the present analysis does not pretend to be exhaustive, but rather aims to
give the general trends of research in heat pipes during the last forty years. The category “others” in figure 4 refers
to all papers for which the type of heat pipe has not been identified through the title. They often refer to
conventional cylindrical heat pipes, but also to studies dedicated to phenomena involved in heat pipes, in general.
The heat pipe science began during the sixties and conventional heat pipes were soon widely used in space
applications, for instance to transfer the heat dissipated by the components to the radiators. To try to reduce the
weight of the systems, CPLs and LHPs were invented by the NASA during the sixties and by the Russian Federal
Space Agency during the seventies, respectively. However, these technologies were not reliable enough during
this period.
During the eighties, terrestrial applications of heat pipes were developed, mainly with thermosyphons because
of the difficulty overcoming the gravity forces. Thermosyphons have been widely used in industrial applications,
as well as in the heat exchangers. The development of electric locomotives also motivated the use of heat pipes
in mobile applications.
During the nineties, new types of heat pipes were invented and more and more studied. Micro heat pipes
appeared thanks to the progress of micro technologies. They aimed to reduce the thermal contact resistance
between the electronic component and the heat sink by directly integrating the heat pipe into the silicon substrate
of the electronic component. At the same time, the progresses in porous material technologies enabled the
implementation of CPLs and LHPs in spacecrafts.

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Since 2000, the number of papers dedicated to heat pipes increases continuously and reaches now about 250
papers per year. According to Larsen and von Ins (2010), the general annual growth rate of scientific publications
is close to 5%, which implies a doubling time of about 15 years. With a doubling time of about 8 years, the growth
rate of publications dedicated to heat pipes is much higher than the growth rate of all scientific publications. This
enhanced research effort is mainly motivated by the increase of the heat flux density dissipated by electronic
components, which creates a need for efficient and reliable cooling systems. Heat pipes, especially CPLs and
LHPs, are thus developed for terrestrial applications and the systems need to be optimised and perfectly
understood in order to deal with gravitational forces and acceleration forces for on-board vehicle applications. At
the same time, rising energy prices favor the use of heat pipes in numerous applications, either as a passive system
to remove heat, to improve the efficiency of heat recovery systems, or to homogenise the temperature of various
systems. In parallel, the continuous progress in new materials and manufacturing processes enables the spreading
of heat pipes in many other industrial applications.
In figure 4, one can note the important development of research on CPLs / LHPs over the last 15 years and
on PHPs over the last 10 years. Together, they represent currently one third of the papers devoted to heat pipes.
The development of reliable LHP would open the use of heat pipes in many applications, as they enable to transfer
heat over a longer distance than other types of heat pipes, while having a low sensitivity to gravitational and
acceleration forces. The development of PHPs is mainly motivated by the low cost of these kinds of systems. One
can also note the remaining importance of research on thermosyphons, despite the age of the early research on
this field. These systems are more and more optimised and studies are aimed to push back their operating limits,
especially in terms of heat power, heat flux density and operating temperature.
Figure 4 Number of papers dedicated to heat pipes according to the database of Web of ScienceTM
0
100
200
300
400
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
Number of pulications
Year
Vapour chambers
PHP
CPL
LHP
Flat plate heat pipe
Micro heat pipe
Rotating heat pipe
Thermosyphon
Others (conventionnal heat pipes and general studies)

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Table 1 Queries corresponding to Figure 4
Type of heat pipe
Query (in title)
Total
results
All kinds of heat
pipes
"heat pipe(s)" or thermosyphon(s) or thermosiphon(s) or “vapo(u)r
chamber(s)” or “capillary pumped loop(s)”
3634
Thermosyphon*
thermosyphon(s) or thermosiphon(s)
820
Rotating heat pipe
rotating and "heat pipe(s)"
49
Micro heat pipe
“micro heat pipe(s)”
112
Flat heat pipe
“Flat (plate) heat pipe(s)”
119
LHP
“loop heat pipe(s)”
325
CPL
“capillary pumped loop(s)”
75
PHP
(pulsating or oscillating) and “heat pipe(s)”
334
Vapour chambers
“vapo(u)r chamber(s)”
117
*the term “thermosyphon” can also refer to “single-phase thermosyphons”. A rapid analysis of the abstracts shows that single-phase
thermosyphons represent between 15 and 20 % of the papers mentioning the term “thermosyphons” in their title.
Scientific issues involved in heat pipes are usually classified in four categories: evaporation heat transfer,
condensation heat transfer, flow patterns in heat pipes and capillary flows. In the present article, another
classification of the studies available in the literature over the period 2010-2015 was selected:
- The research motivated by the heat pipe characterisation,
- The scientific issues linked to the working fluid behaviour,
- The studies aiming to predict the phase change heat transfer in thin films,
- The development of new heat pipe models.
This classification enables to highlight not only the different scientific issues but also the different scientific
approaches that are used by the research teams working on heat pipes. Indeed, the present paper is rather dedicated
to the different approaches and current ways to progress on the heat pipe understanding than directly to the
scientific results themselves.

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3. RESEARCH MOTIVATED BY THE HEAT PIPE CHARACTERISATION
3.1. Determination of the heat pipe performance
From an industrial point of view, an important outcome of heat pipe studies is the determination of the overall
performance of the various heat pipes. The performance can be typically expressed in terms of system thermal
resistance and of system capacity to function in given operating conditions (imposed heat flux, ambient
temperature, acceleration, orientation…). Many studies are thus devoted to the determination of the heat pipe
performance and the numerous prototypes that are tested enable to build important databases for each type of heat
pipes. The thermal resistance of a heat pipe usually depends on the heat transfer at the evaporator and at the
condenser and thus on many parameters as the working fluid, the fluid fill charge and the effective thermal
conductivity of the wick (when exists), but also on other phenomena as the operating regime, or the partial or
total dry-out of the evaporator. For instance, the thermal performance of LHPs is often characterised by their
operating curve (vapour or reservoir temperature as a function of the heat power) and not by their thermal
resistance. Determining a heat pipe performance also leads to determine its operating limits, often detected by a
sudden increase of the thermal resistance or of the operating temperature. If the different limits are well known
from decades, it is not so trivial to link the observed heat pipe behavior to a particular limit.
As an example, Maydanik et al. (2014) recently proposed a review on the performance of loop heat pipes with
flat evaporators. The performance of the various geometries is compared, as well as the impact of the working
fluid and the materials. Recommendations were then proposed in order to achieve a good performance when
designing the evaporator of a LHP. For instance, they advised to use a copper/water combination for the wick
material and the working fluid for the temperature range 70-100°C. For lower temperatures, ammonia can be used
but with a compatible material only. They also concluded that LHPs are mainly interesting when the distance
between the heat source and the heat sink exceeds 200 mm and when a loop thermosyphon cannot be used. This
kind of review is very important for the community as it enables to summarise the data scattering in a large number
of articles.
The review articles that deal with heat pipe performance can also focus on a specific application instead of a
specific type of heat pipe. For instance, Srimuang and Amatachaya (2012) proposed a review on heat pipe heat
exchangers presented in the literature. Their effectiveness ranged from 0.16 to 0.825 and the authors concluded
that the four main influent parameters were the inlet temperature in the evaporator section, the hot and cold air
velocities, the fins geometry and the working fluid inside the heat pipe. This example highlights the fact that the
main parameters strongly depend on the application in which the heat pipe is used and not only on the type of
heat pipe itself.

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Moreover, the performance of a heat pipe is not only limited to its thermal performance. For instance, Zhang
et al. (2014a) studied the socio-economic performance of a solar water heating system that includes a heat pump
and a new type of two-phase loop thermosyphon. They highlighted the fact that three factors have to be prioritised:
the energy efficiency, the economic revenue and the environmental benefit. They concluded that the performance
of their novel loop thermosyphon, compared to traditional ones, depends strongly on the selected criterion and on
the location of the system (London, Shanghai or Hong-Kong in their study). This example shows that reducing
the heat pipe performance to its only thermal resistance minimises the challenges that have to face the heat pipe
scientific community.
Besides academic papers, many patents are filed, in which specific geometries and configurations are
proposed. Patents can deal with specific parts of heat pipe, as condensers (Fried et al., 2013) or wicks (Asfia et
al., 2014). Additional parts are also proposed, as a reservoir filled with adsorbent material in order to deal with
freezing problems (Bonjour et al., 2013). The large number of patents filed each year shows the strong links
between the academic research on heat pipes and their industrial applications.
In a general way, each time a new heat pipe design is proposed, the first step in studying a prototype is to
determine its performance. As an example, Lachassagne et al. (2012, 2013) proposed a new kind of LHP, called
CPLIP (Capillary Pumped Loop for Integrated Power), with a reservoir located above the evaporator. In their first
paper they determined its performance, and subsequently proposed a model in steady-state conditions. Heat pipes
also have to be tested in various operating conditions. For instance, Mameli et al. (2014) characterised a closed
loop pulsating heat pipe in microgravity conditions. They showed that their heat pipe is more affected by the
variation of the gravity than by the level of gravity itself. They also concluded that the performance of their heat
pipe is similar in microgravity and placed horizontally in normal gravity conditions. This kind of conclusion is
important for the community to limit the number of tests required in microgravity, as they are expensive and
difficult to perform.
Eventually, the last step, after a prototype characterisation, is to study its behaviour in the real system. Studies
can be found, for instance, for solar applications (Zhang et al., 2014b), HVAC applications (Zhenying Wang et
al., 2015) or electronic cooling applications (Kim & Kim, 2014). Most of the time, this kind of study focuses on
the transient behaviour of the systems and the authors often compare their measurements with the results of
transient numerical models. Indeed, the scientific goals of these studies are often to check the relevance of the
models that are developed to predict the heat pipe performance for each specific application.

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3.2. Capillary structure characterisation
For capillary heat pipes, the development of models requires the knowledge of the properties of their capillary
structure filled with the working fluids as they have a direct effect on the heat pipe performance. Consequently,
the capillary structures are the object of great attention in heat pipe science. New ways of manufacturing new
capillary structures are developed continuously. For instance, Singh et al. (2014) presented their fabrication
technique for a sintered aluminium evaporator of a Loop Heat Pipe. Santos et al. (2012) proposed an evaporator
made of ceramic. Their experience helps in developing evaporators that will be more advanced in the future.
Besides developing new capillary structures with new techniques and new materials, a strong challenge is to
determine the properties of existing capillary structures and to develop predictive tools that can be used in heat
pipe models. The measurement of the permeability of a porous medium can be performed easily (Ameli et al.,
2013; Hansen et al., 2015). However, the effective permeability of the capillary structure inside a heat pipe may
be different from the bulk permeability of the medium because of the influence of the liquid-vapour interface.
The measure of the global porosity can be easily performed knowing the mass and the volume of the wick.
This method was used by Deng et al. (2013) as an example. However, it does not give any information about the
distribution of the porosity in the wick, which is important to know in some configurations like biporous wicks.
The measurement of the effective pore radius is also complicated because the contact angle between the fluid and
the wick affects its value. For instance, Singh et al. (2014) used the bubble point testing method, which enables
to determine the largest pore radius whereas Becker et al. (Becker et al., 2011) characterised their wick by the
measure of the smallest radius of curvature of the interface before the depriming of the wick. To characterise the
pore size distribution, other methods need to be used, as the mercury injection or the imbibition (Dullien, 2012)
but these methods remain challenging in practice.
From a thermal point of view, the determination of the equivalent thermal conductivity of the capillary
structure is even more difficult. It can be performed with a flash method, for instance (Ababneh et al., 2014), but
the measured value takes into account only conduction through the capillary structure and does not consider
evaporation or condensation phenomena. More sophisticated set-ups have to be developed to take into account
these phenomena (Iverson et al., 2007) but there is a lack of experimental data and studies in this field.
3.3. Example of contribution of inverse methods
When direct measurements cannot be performed, another approach consists in using inverse methods. This
approach is illustrated in this section with a novel method proposed by Revil-Baudard and Lips (2015). The
authors aimed to determine the capillary structure properties from measurements of the overall performance of a
flat plate heat pipe under different inclination angles. The determination of the thermal properties was based on

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an analytical method that was directly inverted: the equivalent thermal conductivities of the capillary structure at
the condenser and at the evaporator were thus the outputs of the inverse method, whereas the temperature
measurements along the heat pipe were the inputs (figure 5). This technique enabled a direct comparison between
the properties of the capillary structure of various heat pipes even if the global thermal resistances of the systems
were different. It was the first step for the construction of an experimental equivalent thermal conductivity
database for the development of predictive tools.
Figure 5 Example of comparison between an experimental temperature profile and the corresponding
profile calculated by the inverse method for a flat plate heat pipe (Revil-Baudard & Lips, 2015)
The determination of the hydrodynamic properties of the capillary structure, i.e. its effective permeability and
its effective pore radius is more difficult, as no direct measurement can be performed in a non-transparent heat
pipe. The method is based on the measurement of the capillary limit for different inclination angles. The capillary
limit is reached when the sum of the gravitational and frictional pressure drops is equal to the maximum capillary
pressure that the wick structure can sustain. It leads to a dry-out at the evaporator and thus to an increase of the
thermal resistance of the heat pipe. When the heat pipe is tilted in unfavourable positions, the capillary limit
decreases because of the effect of the gravitational pressure drop. The frictional pressure drop and the effective
pore radius of the capillary structure can be estimated by assuming that the capillary pressure at the capillary limit
is constant whatever the inclination angle (figure 6). This method has been successfully tested on a grooved flat
plate heat pipe and validated by means of microscopy measurements. However, more studies are required to use
this method with other capillary structures, as the assumption of a constant capillary pressure at the capillary limit
of the heat pipe is not trivial.
0 50 100 150 200
36
37
38
39
40
41
Position along the heat pipe [mm]
Temperature [°C]
Experiments
Inverse method
Saturation temperature

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Figure 6 Example of evolution of the gravitational pressure drop as a function of the heat transfer rate
transferred by phase change (Levap = 140 mm) (Revil-Baudard & Lips, 2015)
This example deals with flat heat pipes, but inverse methods can be used for a wide range of studies and are
often the only way to determine accurately various parameters. For instance, Mehta and Khandekar (2014) used
an inverse method to determine the heat transfer coefficient between the wall and a Taylor bubble train flow in a
mini-channel of square cross-section (5 mm x 5 mm). They coupled experimental data from IR visualisation with
a numerical model that takes into account the 3D conduction in the wall. The protocol for measuring the local
heat transfer coefficient was far from being simple and they concluded that the major challenges were to get a
sufficient spatial resolution to determine local temperature gradients and to minimise the conjugate heat transfer
effect in the system. This effect depends strongly on the wall thickness, but also on the frequency and length of
the bubbles in the flow.
As a conclusion, even if a large number of papers are devoted to the determination of the heat pipe
performance and/or the capillary structure properties, there is often a need in understanding the phenomena that
take place in the heat pipe itself, as the models often assume a given heat pipe behaviour, which is not always
verified experimentally.
4. SCIENTIFIC ISSUES LINKED TO THE WORKING FLUID BEHAVIOUR
4.1. Towards a better understanding of the operating regimes and the fluid behaviour
The performance of a heat pipe often depends on numerous parameters and specific studies focus on the
understanding of the heat pipe behaviour itself. For conventional capillary heat pipes, the operating conditions
are bounded by several limits that all lead to a dry-out at the evaporator. For thermosyphons, pulsating heat pipes
and loop heat pipes, several operating regimes can be observed and recent progresses have been made in their
characterisation. As an example, Miscevic et al. (2012) and Kaled et al. (2012) studied the flow regimes and the
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Heat flux transferred by phase change [W]
ΔPf
ΔPg

14
transient behaviour of a CPL, respectively. They concluded that the pseudo-periodicity of the system was affected
by the fluid motion and had a direct influence on the pressure drops in the loop.
Operating regime is also of a great importance for PHPs. Karthikeyan et al. (2014) studied the self-sustained
oscillations in a PHP by means of an infrared camera (figure 7). They characterised the different operating regimes
and their impact on the heat pipe performance.
Figure 7 Experimental characterisation of the operating regimes of a PHP by means of a thermal camera (V.
K. Karthikeyan et al., 2014)
Thermal measurements help to characterise the heat pipe behaviour, but it is often important to deal with the
behaviour of the working fluid inside the capillary structure itself. Heat pipes are often studied as black boxes and
thus no direct observation can be performed inside the system. For the last ten years, many research teams studied
transparent heat pipes, which allow a better understanding of the physical phenomena that take place inside the
system.
As an example, Lips et al. (2010; 2011) used a confocal microscope to measure the pressure drops of the
liquid inside 1D and 2D capillary structures. These experimental results enabled the validation of the
hydrodynamic models of the liquid flow in grooves. However, this technology cannot be used to visualise
liquid/vapour interfaces in other capillary structures, like meshes or sintered wicks: the prediction of the pressure
drops in this type of capillary structure remains an important challenge. The confocal microscopy was also

15
successfully used for the measurement of the condensing film thickness in a silicon heat pipe (Lefvre et al.,
2010). Indeed, the geometry of the condensing film is of great importance in heat pipes, as it directly affects the
thermal resistance of the condenser. The contribution of the confocal microscopy to the knowledge of flat heat
pipe behaviour is discussed here as an example, but transparent systems were also widely used for the study of
thermosyphons (Smith et al., 2014), pulsating heat pipes (Ji et al., 2013) and loop heat pipes (Xu et al., 2014).
In order to study the liquid-vapour interface shape in details, some studies focus on a sub-system of the heat
pipe only. For instance, El Achkar et al. (2012) studied the condensation of n-pentane in a micro-channel that
represents the condenser of a LHP. They measured the size and the frequency of the vapour bubbles during the
condensation and quantified the distribution between sensible and latent heats of phase change. On the other hand,
Mottet et al. (2015) developed a test bench dedicated to the study of partial dry-out in the evaporator of a LHP.
A pseudo evaporator was designed and experimental results were compared to model predictions.
These examples of experimental works highlight the importance of the direct visualisation in these kinds of
systems: the distribution of the liquid and vapour phases in the capillary structure of a heat pipe is far from being
trivial and the numerical models are often limited by an imperfect understanding of the fluid behaviour. This lack
of knowledge directly impacts the accuracy of the heat pipe design tools as the prediction of their thermal
performance mainly depends on the liquid film thicknesses in the condenser and evaporator.
4.2. Research on new fluids
Besides research on the fluid behaviour inside the heat pipe, some studies are dedicated to new kinds of fluids
themselves. The choice of the fluid is indeed of a great importance and it is not so trivial to choose an appropriate
fluid for the appropriate heat pipe technology for a specific application. The fluid properties must show a good
trade-off between high latent heat of vaporisation, surface tension and thermal conductivity and a low viscosity
for the whole range of operating temperatures. A given operating temperature corresponds to a given operating
pressure that the heat pipe must be able to withstand. The compatibility between the working fluid and the other
materials must also be perfect. This problem currently limits the use of flexible and/or lightweight heat pipes
because of the poor chemical stability of materials like plastics or polymer composites. An example of
compatibility table proposed by Mishkinis et al. (2010) is presented on figure 8. This table shows that studies on
fluid/material compatibilities can be contradictory and that more studies are required to get a complete knowledge
of the possible interactions between the working fluids and the wick and structural materials.

16
Figure 8 Compatibility table between the working fluids and the wick and structural materials (Mishkinis et
al., 2010)
Other criteria, like toxicity for the humans and the environment must also be taken into account. As an example,
water can be an appropriate working fluid for an operating temperature range from 50°C to 150°C, but problems
of low pressure and high pressure can occur out of this range. Moreover, freezing can also be a problem for heat
pipes with certain kinds of capillary structures. Studies on new fluids are thus necessary.
For instance, MacGregor et al. (2013) proposed a comparison of different working fluid performances for
thermosyphons in order to replace R134a, widely used, but subjected to a ban in the near future. Other authors
also tested their prototypes with new refrigerants, as R1234ze, which was found to be an efficient working fluid
for loop heat pipes (Yeo et al., 2014). For cryogenic applications, superfluid helium was also tested (Gully, 2015).
In a general way, the definition of an efficient working fluid is still a subject of discussion. Since the initial
approach of Chi (1976) to define a figure of merit for cylindrical grooved heat pipes, other figures of merit were
proposed. For instance, Launay et al. (2010) proposed figures of merit for the working fluid in loop heat pipes,
whereas Arab and Abbas (2014) proposed a model to predict the thermal performance of a trapezoidal grooved
heat pipe when changing the working fluid.
Besides new fluids themselves, contemporary heat pipe research is mainly focusing on two new fluid families:
the self-rewetting fluids and the nano-fluids.

17
The self-rewetting fluids exhibit an increase of their surface tension when their temperature increases. They
often consist of a dilute aqueous solution of alcohol: in a certain range of temperatures, a concentration gradient
occurs at the evaporator and the Marangoni effect adds to the temperature effect and helps in draining the fluid
from the condenser to the evaporator. Firstly studied for space applications (Savino et al., 2010), self-rewetting
fluids have now been successfully tested in conventional heat pipes (Senthilkumar et al., 2012), thermosyphons
(M. Karthikeyan et al., 2013) and even oscillating heat pipes (Hu et al., 2014). The self-rewetting fluids have
proven their potential efficiency, but more studies are still required to precisely predict their behaviour.
A nanofluid consists in a liquid base in which nanometric size metallic/ non-metallic/ ceramic particles are
incorporated, quite often with surfactants to ensure their stability. Depending on their material, the nanoparticles
enable to get a working fluid with enhanced or tailor-made thermo-physical properties. In the last years, numerous
studies on nanofluids in heat pipes have been published. Liu and Li (2012) performed a review of the dedicated
studies and concluded that depending on the type of nanoparticles, their size and their concentration, nanofluids
could significantly increase the heat pipe performances, both in terms of thermal resistance and of maximum heat
removal capacity. The major effect of nanofluids seems to be the surface structuration of the evaporator, which
affects the wettability of the wall, as well as the boiling phenomenon (Stutz et al., 2011). These conclusions are
shared by other review articles recently published (Sureshkumar et al., 2013 ; Alawi et al., 2014). At present
however, there are conflicting results and opinions on the efficacy of such fluids in heat pipe and thermosyphon
systems. For instance, Khandekar et al. (2008) observed a decrease of the thermal performance of a PHP when
nanofluids are used as working fluids. The nanoparticle clogging affected the boiling phenomenon and the authors
tried to quantify the effects of the various mechanisms. More focussed research is this area needs to be undertaken
for clarity (Buschmann, 2013). The effect of the wall structuration on the heat transfer is further discussed in the
section 5 of the present manuscript.
Figure 9 shows the number of publications dedicated to nanofluids in heat pipes according to the database of
Web of ScienceTM. The number of papers increased rapidly between 2006 and 2010, but has decreased since then.
Finally, the studies on nanofluids can be considered as studies on liquid-vapour phase change heat transfer on
nanostructured surface. However, the change in structural morphology of the wall depends on numerous
parameters, as it is time-dependent and involves many aging phenomena of the nanofluid (aggregation,
deposition, etc.). As a conclusion, if the positive effect of nanofluids on heat pipe performance is well admitted,
they mainly affect the heat transfer coefficient during the phase change heat transfer and the pressure drops in the
capillary structure. No phenomenon specific to heat pipes was observed, which could explain the decrease of the
number of articles dedicated to nanofluids in heat pipes since 2010.

18
Figure 9 Number of publications dedicated to nanofluids in heat pipes according to the database of Web of
ScienceTM
5. STUDIES AIMING TO PREDICT THE PHASE CHANGE HEAT TRANSFER IN THIN FILMS
Yet, wall structuration (should it be nanostructuration or microstructuration) can still be viewed as a promising
track for the improvement of the efficiency of heat pipes. Heat pipes are certainly one applicative motivation of
many research works on the boiling or condensation heat transfer enhancement using structured surfaces.
Regarding condensation, the objective is quite often to promote dropwise condensation by modifying the fluid
wettability: making a surface (super-)hydrophobic facilitates the roll-off of the droplets, which improves the
overall condensation heat transfer. (Super-)hydrophobic surfaces are generally obtained by acting on the surface
chemical or physical characteristics to decrease the surface energy, or by acting on the surface roughness (Bisetto
et al., 2014). Different processes of chemical coating of the solid substrate with a low surface-energy substance
are currently being developed and evaluated, as explained by Sikarwar et al. (2011).
Regarding boiling (i.e. for an application to thermosiphons), heat transfer enhancement techniques have been
studied for decades, from the emergence of extended surfaces in the 1970ies-1990ies to the development, like for
condensation, of microstructured surfaces and more recently nanostructured surfaces. Extended surfaces are
manufactured based on standard machining and metal processing. Microstructured surfaces can be obtained by
means of porous coating, or by deformation of extended surfaces (e.g. low fins compression, fin bending and
cutting, etc.), as described Poniewski and Thome (2008). Nanostructuration relies on chemical processes
(oxidation, etching, etc.) or nanoelectromechanical systems (NEMS). Enhanced surfaces are usually used for two
distinct purposes: either to increase the critical heat flux, or to increase the heat transfer coefficient.
Beyond a simple increase in heat transfer area, extended surfaces can be employed to optimise the distribution
of active nucleation sites or to favour the liquid flow around the heated wall. In addition, when reducing the size
of the structures that form the extension down to micrometer or nanometer size, additional phenomena can come
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into play, as reviewed by Kim et al. (2015) among many other reviews on the subject of interest. For instance,
while nucleation takes place within the pores of a porous coating, liquid can be fed to the nucleation site by means
of capillary pumping. Micro- or nanoscale geometric effects (size, but also liquid flow distribution) can also
favour the bubble initiation, growth or detachment, while the micro- or nanostructure will modify the fluid wetting
characteristics, which will ultimately affect both the heat transfer coefficient and CHF. As an example, the effect
of the nanostructuration with a Fe203 nanofluid on the boiling curve of a 100 µm platinum wire is presented on
figure 10 (Stutz et al., 2011): depending on the nano-particle coating duration (and thus on the shape of the
nanostructuration), the heat transfer coefficient can be increased or decreased but the CHF is always increased by
this nano-particle coating.
Figure 10 Example of the effect of the nanostructuration of the wall on the boiling curve obtained with γ-
Fe203 nanoparticle on a 100 µm platinum wire. τc is the nano-particle coating duration with pure water (Stutz et
al., 2011).
Lastly, there are also many attempts to take the benefit of the new possibilities offered by micro- and
nanotechnologies to improve the wick structures that come into play in all the capillary heat pipes (Ranjan et al.,
2011) where capillary evaporation occurs, especially LHPs. As a matter of fact, while the performance of these
heat pipes is usually limited by both the ability of the wick to ensure capillary pumping and by its thermal
resistance, micro- or (probably better) nanoscale objects (e.g. nanowires, nanotubes, etc.) should help improve
both wicks characteristics as leading to smaller menisci (i.e. increased capillary pumping), as they usually have a
greater thermal conductivity, and as they allow for an enhancement of evaporation (Plawsky et al., 2014).

20
However, the difficulty in predicting the thermal performance of a heat pipe actually lies in the predominance
of heat transfer through very thin liquid films. Numerous studies aim to determine the heat transfer coefficients
during condensation or evaporation, but the models fail to reproduce the measurements, even for simple
geometries as grooves (Lips et al, 2010). Studies dealing with micro-heat pipes (and thus with very simple
geometries) are often limited to theoretical considerations and there is a lack of experimental validation (Liu &
Chen, 2013). For PHPs, Khandekar et al. (2010) pointed that too many fundamental phenomena still need to be
understood to achieve a complete model because of the pulsating and/or oscillating character of the Taylor bubble
flow. Experimental set-ups were developed to study the evaporation and condensation phenomena in capillary
tubes. For instance, Chauris et al. (2015) studied the evaporation of the thin film deposited by a moving meniscus.
They highlighted the phenomena involved in the process and quantified the impact of each phenomenon on the
global heat transfer by comparing their experimental results with a numerical model. They concluded that during
the transit of a meniscus, most of the energy is transferred through the thin liquid film deposited by the meniscus,
but the impact of the meniscus itself is not negligible.
Other recent studies focus on heat transfer in thin films, but with no direct application to heat pipes. As an
example, Srinivasan et al. (2015) performed an experimental study that aims to understand the mechanisms
occurring during the evaporation of an isolated liquid slug. They focused particularly on the drainage of the thin
liquid film and found a good agreement with the Taylor’s law predictions. Kunkelmann et al. (2012) studied
experimentally and theoretically the effect of the three-phase contact line velocity on the heat transfer. They
concluded that the heat transfer in the contact line zone mainly depends on the micro-layer evaporation and on
the transient conduction in the wall. At the scale of the liquid thin film, the properties of the wall are indeed found
to be not negligible and the coupling of the phenomena is complex.

21
To highlight this complexity, a study performed by Rao et al. (2015) and dedicated to the understanding of a
single-branch pulsating heat pipe is taken here as an example. This study illustrated the coupling of thermal and
hydrodynamic phenomena, which could lead to self-sustained oscillations. The experimental set-up of the authors
consisted of a single vertical and transparent capillary tube, closed on one side, and connected to a reservoir on
the other side. Two transparent water heat exchangers, acting as an evaporator and a condenser, were located
along the heat pipe. This experimental set-up did not intend to represent a real pulsating heat pipe but it could be
viewed as a model to study some fundamental physical phenomena while avoiding the hydraulic interactions
between the different branches of a real pulsating heat pipe. Depending on the temperatures of the reservoir, the
evaporator and the condenser, self-sustained oscillations of the meniscus were indeed observed. The authors
simultaneously measured the meniscus position, the triple contact line position, the vapour pressure and the
vapour temperature. They described the four stage cycle associated to the meniscus oscillation and highlighted
the main phenomena involved in the process (figure 11).
The observed cycle was a result of a balance between the pressure of the reservoir, the frictional pressure
drop, the variation of pressure due to the compression and expansion phenomena and the variation of pressure
due to the change of vapour mass, resulting from condensation and evaporation of the fluid. The four stage shape
was due to the difference of time scale between the hydrodynamic phenomena and the phase change phenomena.
The authors showed that the vapour was always superheated, which is in accordance with the conclusions of Gully
et al. (2013). It enabled them to determine the instantaneous mass of vapour and to estimate by a thermal model
the rate of evaporation and condensation in various zones of the liquid-vapour interface during a cycle.
Condensation occurred mainly on the liquid film and evaporation occurred on the liquid film but it is particularly
significant close to the triple contact line. For both evaporation and condensation, the phase change rate was
maximum when the meniscus was at its second bottom-most position.
Even if the conclusions of this study cannot be directly used to explain the overall principle of a real pulsating
heat pipe because of its too simplified behaviour, this study is a good example of the complexity of the balance
between thermal and hydrodynamic phenomena that can occur in this kind of systems. This is particularly
important in a pulsating heat pipe, but also in all kinds of heat pipes where evaporation through thin liquid films
occurs. In the case of a pulsating heat pipe, no complete model is able to predict their behaviour, but the progress
in the understanding of the phenomena contributes to more realistic models.

22
Figure 11 Description of the four zone cycle and the associated phenomena (Rao et al., 2015)
6. THE DEVELOPMENT OF NEW HEAT PIPE MODELS
During the past few years, heat pipe models have indeed been improved. Both analytical and numerical models
were proposed at the scale either of a single phenomenon or of the system. The goal of the present article is not
to precisely describe the complete set of equations on which the models published in the literature are based, but
rather to give a brief overview of the panel of models that are still under development today.
Concerning conventional heat pipes (thermosyphons and capillary heat pipes), two types of studies were
published: the progress in CFD modelling enabled the development of 3D thermal and hydrodynamic models
(Jung-Chang Wang, 2012), but in parallel, analytical models are proposed (Lips & Lefèvre, 2014). The first ones
enable a better integration of the heat pipes in a more complex system, whereas the second ones give simple and
accurate engineering tools for the design of the heat pipes themselves. Some other specific studies aim to
determine the wick properties by means of detailed thermal and hydrodynamic models at the pore scale (Ranjan

23
et al., 2012). These three different approaches are very complementary and each of them leads to a better
understanding of the phenomena involved in each type of conventional heat pipe.
Several studies are devoted to the modelling of loop heat pipes. Siedel et al. (2015b) proposed a
comprehensive review of the steady-state modelling works. They highlighted the high number of models available
and noted that most of them are numerical. The same authors proposed, in another article, a complete analytical
model, requiring a low computational time compared to numerical ones (Siedel et al., 2015a). These models are
able to reproduce the experiments, but their limit lies in the lack of knowledge of the wick properties (permeability
and effective thermal conductivity), of the accommodation coefficient of the working fluid and of the thermal
contact resistance between the wall and the wick structure. The presence, or absence, of a vapour zone at the
contact between the porous medium and the heat source is also a source of discussion. Mottet et al. (2015)
developed a 3D model of a wicked LHP evaporator. They used a mesoscale approach with a pore network model.
Their model enabled to highlight phenomena that a 2D model could not reproduce and the authors made the
distinction between different regimes governed by different phenomena. The best regime is found to be when a
two-phase liquid-vapor zone forms just at the contact of the evaporator casing. Their simulation enables to guide
the design of new wicks for LHPs.
At the scale of the system, transient models of LHPs have been proposed. For instance, Kaled et al. (2012)
proposed a model classically based on the energy, mass and momentum balances for the evaporator-reservoir, the
condenser and the transport lines. They concluded that the fluid motion participates in the pseudo-periodic
behaviour of the system. In parallel, Nishikawara et al. (2013) proposed a transient model that correctly predicts
the experimental data, despite the presence of an overshoot temperature when the heat load changes, which is not
observed experimentally.
An important part of the modelling works published in the last few years are devoted to pulsating heat pipes.
On one side, the increasing number of experimental databases enables the development of empirical correlations
(Qu & Wang, 2013). On the other side, some 3D CFD models are proposed (Lin et al., 2013) and
phenomenological models are implemented. They show a good ability to reproduce the chaotic behaviour of PHPs
(Nikolayev, 2011). In all cases, these models still have to be improved in order to take into account all physical
phenomena, especially at the scale of the thin liquid film and the triple contact line. Detailed models already exist
to understand these phenomena, but their experimental validation remains challenging (Nikolayev, 2010).
7. SUMMARY AND CONCLUSIONS
This brief review of recent studies focused on heat pipes enables to highlight the main approaches used by the
research teams to increase the understanding of the various types of systems. Both experimental and theoretical

24
works are proposed and the scale of interest of the studies varies from the system size itself to the scale of the
very thin liquid film present in the evaporation and condensation zones.
During the past five years, some significant advances have been achieved:
- The understanding of evaporation and condensation phenomena on a capillary scale has been improved,
mainly thanks to new systems of visualisation and instrumentation.
- New fluids and new materials have been successfully tested and enabled to increase the performance of heat
pipes.
- Major progresses in the understanding of LHPs have now enabled to develop this technology at an industrial
scale. The models are now able to predict the experiments satisfactorily even if improvements could still be
achieved in the prediction of transient behaviour and thermal resistances involved in the systems. From a
technological point of view, new geometries and types of capillary structure have also been proposed (bi-porous
wick, multi-layer wick, ceramic wick …). A new challenge is now the miniaturisation of the systems.
- The more impressive advances are probably related to the PHPs. Five years ago, even the main phenomena
were not well identified. Models are now almost able to reflect their chaotic behaviour and reliable predictive
tools can be expected in the coming few years.
However, several scientific questions still need to be answered:
- The predictive tools strongly depend on the capillary structure properties and on the heat transfer coefficient
during condensation and evaporation. Limited progress has been realised in developing satisfactory models or
correlations, even for simple geometries.
- Several phenomena still have to be better understood to be correctly taken into account in the models. Boiling
in the capillary structure of a flat heat pipe and coalescence and break-up of liquid slugs and vapour plugs in
pulsating heat pipes are only two examples.
- The evaporation and condensation processes implying thin liquid films are not yet fully understood, and
especially the influence of the wall properties on these phenomena. If the case of fully wetting fluids is supported
by some theoretical backgrounds, partial wetting, which is present in real engineering systems, still needs to be
understood.
To answer these questions, more studies will be required. First of all, experimental studies with visualisation
are essential because of the need to understand the coupling between the different phenomena involved in these
systems. The lack of experimental data is even more acute for the phenomena that act at the micro-scale. For
instance, without the knowledge of the accommodation coefficient, the thermal models of phase change heat
transfer at the triple contact line cannot be validated. The real mechanisms leading to the onset of nucleate boiling
are also not yet fully understood and this strongly limits the predictability of the numerical models as they almost

25
always need to be fitted with experimental data. Besides this, there is a lack of reliable convective condensation
models at low flow rates, able to predict the heat transfer coefficients in small and bended tubes such as the LHP
condensers. Another challenge will be to couple these microscale models to the system scale models. This
difficulty comes partially from the fact that the physical, topological and chemical properties of the materials are
often poorly known. To answer this issue, more interactions should be created between the heat pipe research
community and the material science research community.
One can also expect that the progresses in other research fields will bring new tools enabling to improve the
current systems. For instance, the progress in high frequency micro-electronics opens the way for active control
of heat pipes and the continuous development of new materials awake hopes of real flexible and lightweight heat
pipes if the current problems of fluid/material compatibility on plastic heat pipes are solved. Anyway, one can
conclude that the study of heat pipes will remain a challenging and exciting topic at least for the next couple of
decades.
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