Numerical Analysis of Performance of Closed-

Loop Pulsating Heat Pipe

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

Master of Technology in

Mechanical Engineering with Specialization in “Thermal Engineering”

by

Ashutosh Kr Singh

Department of Mechanical Engineering National Institute of Technology

Rourkela June 2013

Numerical Analysis of Performance of Closed-

Loop Pulsating Heat Pipe

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

Master of Technology

in Mechanical Engineering with Specialization in

“Thermal Engineering”

by

Ashutosh Kr Singh

Under the guidance of Prof.A.K.Satapathy

Department of Mechanical Engineering National Institute of Technology

Rourkela June 2013

National Institute of Technology Rourkela

CERTIFICATE

This is to certify that the thesis entitled, “Numerical Analysis of performance

of closed loop pulsating heat pipe” submitted by Mr. Ashutosh Kr singh in partial

fulfillment of the requirements for the award of Master of Technology Degree in

Mechanical Engineering with specialization in Thermal Engineering at the

National Institute of Technology, Rourkela (Deemed University) is an authentic

work carried out by her under my supervision and guidance.

To the best of my knowledge, the matter embodied in the thesis has not been

submitted to any other University/ Institute for the award of any degree or

diploma.

Date: Prof. ASHOK KUMAR SATAPATHY

Department of Mechanical Engineering

National Institute of Technology

Rourkela – 769008

i

ACKNOWLEDGEMENT

I would like to express my deep sense of gratitude and respect to my supervisor Prof. Ashok

Kumar Satapathy for his excellent guidance, suggestions and constructive criticism. Working

under his supervision greatly contributed in improving quality of my research work and in

developing my engineering and management skills.

I am extremely fortunate to be involved in such an exciting and challenging research project. It

gave me an opportunity to work in a new environment of Fluent. This project has increased my

thinking and understanding capability.

I would like to express my thanks to all my friends, all staffs and faculty members of

mechanical engineering department for making my stay in N.I.T. Rourkela a pleasant and

memorable experience.

I would like to thank all whose direct and indirect support helped me in completing my thesis

in time.

Lastly I would like to convey my heartiest gratitude to my parents for their unconditional love

and support.

Ashutosh Kr Singh

Roll No. 211ME3193

Department of Mechanical Engg.

National Institute of technology

ii

CONTENTS

Acknowledgement i

Contents ii

Abstract iv

List of Tables v

List of Figures vi

Abbreviations and Acronyms viii

CHAPTER-1

INTRODUCTION 1

1.1 Introduction 2

1.2 Heat pipe 3

1.2.1 Working principle of heat pipe 3

1.3 Some special type of heat pipe 4

1.3.1 Pulsating heat pipe 4

1.3.2 Working principle of CLPHPs 5

1.3.3 Capillary pumped loop heat pipe 6

1.4 Parameter affect the performance of closed loop pulsating heat pipe 7

1.5 Advantages 8

1.6 Applications 8

1.7 Objectives of work 8

1.8 Organization of the thesis 9

CHAPTER-2

LITERATURE REVIEW 10

2.1 Literature survey 11

CHAPTER-3

PROBLEM FORMULATION 17

iii

3.1 Introduction 18

3.2 Governing equations 19

3.3 Volume of fluid 19

3.3.1 Volume fraction 20

3.4 Boundary conditions 20

CHAPTER-4

CFD MODELING 22

4.1 Introduction 23

4.2 CFD programs 23

4.2.1 The pre-processor 24

4.2.2 The main solver 25

4.2.3 The post-processor 26

4.3 Overview of FLUENT package 27

4.4 CFD procedure 37

4.4.1 Geometry creation 28

4.4.2 Mesh generation 28

4.4.3 Flow specification 29

CHAPTER-5

RESULTS AND DISCUSSIONS 31

5.1 Results and discussions 32

CHAPTER-6

CONCLUSIONS AND FUTURE SCOPE 50

6.1 Conclusions 51

6.2 Future scope 51

References

iv

ABSTRACT

This presents a computational study on the heat transfer characteristics of closed loop pulsating

heat pipe (CLPHPs). However modeling of a CLPHP system in GAMBIT has many challenging

issues due to the complexity and multi-physics nature of the system. So, the closed loop

pulsating heat pipe modeled here has no wick material inside it as it present in heat pipe. The

closed loop pulsating heat pipe has no complex structure so it is to be modeled. Flow

visualization was conducted for the closed loop pulsating heat pipe using ANSYS Fluent 13.0.

With appropriate boundary conditions we can visualize the behavior of the model and make

predictions regarding its performance. Water-water vapor and ethyl alcohol ant ethyl alcohol

vapor are taken as the working fluid and heat flux is supplied at the inlet. Phenomena such as

nucleation boiling, formation of slug and propagation of inertia wave were observed in the

closed loop PHPs. Also the analysis has been done to know the behavior of CLPHPs under

varying supply of heat flux at the inlet (evaporator).for this, the output heat flux is obtain at

outlet (condenser) and find out how the heat flux is varying for different heat flux and the

different working fluid.

Keywords— two phase flow, convective heat transfer, boiling and condensation.

v

List of Tables

Serial No. Description Page No.

Table 3.1 Properties of working fluids 21

Table 4.1 Relaxation factors 30

Table 5.1 The heat flux output at condenser for different values of heat flux

input at evaporator for water-water vapor working fluid

42

Table 5.2 The heat flux output at condenser for different values of heat flux

input at evaporator for ethyl alcohol-ethyl alcohol vapor

46

vi

List of Figures

Serial No. Description Page No.

Fig. 1.1 Schematic diagram of heat pipe 3

Fig.1.2 Schematic diagram of pulsating heat pipe 4

Fig.1.3 Schematic diagram of CLPHPs 5

Fig.1.4 Schematic diagram of capillary pumped loop heat pipe 7

Fig.3.1 Schematic diagram of geometry considered for the analysis 18

Fig.4.1 Overview of modeling process 24

Fig.4.2 A small section of geometry meshed in Gambit 29

Fig.5.1(a) Position of vapor slug at t=5 s 32

Fig.5.1(b) Position of vapor slug at t=15 s 33

Fig.5.1(c) Position of vapor slug at t=25 s 33

Fig.5.1(d) Position of vapor slug at t=35 s 34

Fig.5.1(e) Position of vapor slug at t=50 s 34

Fig.5.1(f) Position of vapor slug at t=55 s 35

Fig.5.1(g) Position of vapor slug at t=60 s 35

Fig.5.1(h) Position of vapor slug at t=65 s 36

Fig.5.1(i) Position of vapor slug at t=70 s 36

Fig.5.2 (a) Temperature variation inside the channel at t=50 s 37

vii

Fig.5.2(b) Temperature variation inside the channel at t=55 s 37

Fig.5.2(c) Temperature variation inside the channel at t=60 s 38

Fig.5.2(d) Temperature variation inside the channel at t=65 s 38

Fig.5.2(e) Temperature variation inside the channel at t=70 s 39

Fig.5.2(f) Temperature variation inside the channel at t=75 s 39

Fig.5.2(g) Temperature variation inside the channel at t=80 s 40

Fig.5.2(h) Temperature variation inside the channel at t=85 s 40

Fig.5.3 Pressure variation inside the channel 41

Fig.5.4 Heat flux vs. time for water-water vapor working fluid 45

Fig.5.5 Heat flux vs. time for ethyl alcohol-ethyl alcohol vapor 49

viii

ABBREVIATIONS & ACRONYMS

Bo Bond Number

Cp specific heat, J/kgK

D diameter, mm

E ̈ Eotvos Number

g acceleration due to gravity, m/s2

k thermal conductivity, W/mK

Q heat flux, W

Greek symbols

α volume fraction

µ dynamic viscosity, kg/ms

ρ density. kg/m3

σ surface tension, N/m

Subscripts

crit critical

liq liquid

vap vapor

1 | P a g e

CHAPTER-1

INTRODUCTION

2 | P a g e

1.1 INTRODUCTION

Close Loop Pulsating heat pipes (CLPHPs) typically suited for microelectronics cooling consists

of a plain meandering tube of capillary dimensions with many U-turns and joined end to end.

The pipe is first evacuated and then filled partially with a working fluid. If the diameters of Close

Loop Pulsating heat pipe is not too large, the fluid distributes itself into an arrangement of liquid

slugs separated by vapor bubbles. One end of this tube bundle receives heat transferring it to the

other end by a pulsating action of the liquid–vapor/slug-bubble system. The liquid and vapor

slug/bubble transport is caused by the thermally induced pressure pulsations inside the device

and no external mechanical power is required. The type of fluid and the operating pressure inside

the pulsating heat pipe depend on the operating temperature of the heat pipe. The region between

evaporator and condenser is adiabatic. The heat is transfer from evaporator to condenser by the

means of pulsating action of vapor slug and liquid slug. This pulsation appears as a non-

equilibrium chaotic process, whose continuous operation requires non-equilibrium conditions

inside the tube in some of the parallel channels. For Close Loop Pulsating heat pipes (CLPHPs),

no external power source is needed to either initiate or sustain the fluid motion or the transfer of

heat. The purpose of this project is to understand how CLPHPs operate and to be able to

understand how various parameters (geometry, fill ratio, materials, working fluid, etc.) affect its

performance. Understanding its operation is further complicated by the non-equilibrium nature of

the evaporation and condensation process, bubble growth and collapse and the coupled response

of the multiphase fluid dynamics among the different channels.

3 | P a g e

1.2 Heat Pipe

The heat pipe has two region i.e. evaporator and condenser. There is adiabatic region which

separates condenser and evaporator. The heat pipe has wall, the wick structure and the space for

the working fluid.

Fig. 1.1 Schematic diagram of Heat pipe

1.2.1 Working principle of Heat Pipe

The heat is absorbed in the evaporator region and is carried out through the pipe by the

evaporation of the fluid by absorbing the heat. The high temperature vapor movies toward the

condenser by the action of buoyancy force. At the condenser it rejects the heat by convection and

coverts into liquid droplets. These droplets move to the evaporator due to gravity though the

wick material.

4 | P a g e

1.3 Some special types of Heat Pipe

Pulsating(oscillating) heat pipe

capillary pumped loops heat pipe (CPLHPs)

Micro-heat pipe

Rotating heat pipe

1.3.1 Pulsating heat pipe

Pulsating heat pipe has many numbers of U-turns of tube with capillary diameter. These tubes

are evacuated and partially filled with the working fluid. When the diameter of the tube is so

small, preferably <2mm then the working fluid distributed itself in the form of vapor slug and

liquid slug. When it compare with the convectional heat pipe, it has no wick material inside the

tube.

Fig.1.2 Schematic diagram of pulsating heat pipe

5 | P a g e

There are two type of pulsating heat pipe.

Closed loop pulsating heat pipe

Open loop pulsating heat pipe

Closed loop pulsating heat pipe perform better than open loop devices because of the fluid

circulation that is superposed upon the oscillations within the loop. By using the check valve

within the loop, the performance of the closed loop heat pipe is further improved. The

installation of the check valve is very difficult and costly because of the small nature of the

device. So, the closed loop pulsating heat pipe without a check valve is mostly used.

1.3.2 WORKING PRINCIPLE OF CLPHPs

Fig. 1.3 schematic diagram of CLPHPs

6 | P a g e

One end of the CLPHPs tube bundle receives heat, transferring it to the other by a pulsating

action of the working fluid, generating, in general, a capillary slug flow. While in operation,

there exists a temperature gradient between the heated and cooled end. Small temperature

differences also exist amongst the individual ’U’ bends of the evaporator and condenser due to

local non-uniform heat transfer rates which are always present in real systems. Since each tube

section between the evaporator and the condenser has a different volumetric distribution of the

working fluid, the pressure drop associated with each sub-section is different. This causes

pressure imbalances leading to thermally driven two-phase flow instabilities eventually

responsible for the thermo fluidic transport. Bubble generation processes in the heater tubes

sections and condensation processes at the other end create a sustained ‘non-equilibrium’ state as

the internal pressure tries to equalize within the closed system. Thus, a self-sustained thermally

driven oscillating flow is obtained. There occurs no 'classical steady state' in CLPHPs operation

as far as the internal hydrodynamics is concerned. Instead, pressure waves and fluid pulsations

are generated in each of the individual tube sections, which interact with each other generating

secondary/ ternary reflections with perturbations. It will be appreciated that CLPHPs are

complex heat transfer systems with a very strong thermo-hydrodynamic coupling governing the

thermal performance. The cooling philosophy draws inspiration from conventional heat pipes on

one hand and single phase forced flow liquid cooling on the other. Thus, the net heat transfer is a

combination of the sensible heat of the liquid plugs and the latent heat of the vapor bubbles. The

construction of CLPHPs is such that on a macro level, heat transfer can be compared to an

extended surface ‘fin’ system. Simultaneously, the internal fluid flow may be compared to flow

boiling in narrow channels.

7 | P a g e

1.3.3 Capillary pumped loop heat pipe

For systems where the heat fluxes are very high or where the heat from the heat source needs to

be moved far away. In the loop heat pipe, the vapor travels around in a loop where it condenses

and returns to the evaporator. The evaporation in capillary pumped loop heat pipe takes place on

the surface of the wick adjacent to the evaporator wall. Vapor removal channel must be

incorporated in the wick or evaporator wall to ensure that the vapor can flow from the wick to

the vapor line with an acceptable pressure drop.

Fig. 1.4 schematic diagram of capillary pumped loop heat pipe

1.4 Parameter affects the performance of closed loop pulsating heat pipe

Working fluid

Internal diameter

8 | P a g e

Total tube length

Length of evaporator , adiabatic section and condenser

Number of turns or loops

Filling ratio

Inclination angle

1.5 Advantages:

The size of the heat pipe varies from 10mm to 15m long.

Thermal conductivity of the heat pipe is several times greater than that of the best solid

conductor.

The relative weight of the heat pipe is very less compare to the solid conductor.

There is no wick material is used in the closed loop pulsating heat pipe as compare to the

heat pipe.

1.6 Applications:

Heat pipe heat exchanger is used to cool the electronic equipment in a closed cabinet.

Heat pipes designed for use in the thermal control of the nuclear reactor.

Heat pipe is used in the space craft heat rejection.

Heat pipe is used to remove heat from leading edge of hypersonic aircraft.

1.7 Objectives of Work

The objective of the present work is to study the closed loop pulsating heat pipe with a single

turn using ANSYS FLUENT 13.0. In this work, the visualization of the working fluid has been

done. Inside the tube, how the liquid and vapor slug is moving inside the capillary tube. Apart

9 | P a g e

from this, for the different heat flux supplied at the evaporator what is the amount of heat flux is

rejected at the condenser. The above step is repeated for the other working fluid such as water

and ethyl alcohol. Also the filling ratio is taken in consideration.

1.8 Organization of the Thesis

This thesis comprises of six chapters excluding references.

Chapter 1 gives the brief introduction of heat pipe, closed loop pulsating heat pipe; parameter

affects the performance of closed loop pulsating heat pipe, advantages applications and with the

objective of work.

In chapter 2 I have given a brief literature review about the topic and research which are related

to my present work.

Chapter 3 deals with the introduction of my problem with its governing equations and boundary

conditions.

In chapter 4 CFD modeling of the problem has been done.

Chapter 5 deals with the results and discussions of my research work for all the considerer

boundary conditions.

Chapter 6 gives the conclusion and scope of future work.

10 | P a g e

CHAPTER-2

LITERATURE REVIEW

11 | P a g e

2.1 Literature Survey:

Thermal management of electronics is a recent issue which is increasingly gaining importance in

line with the advancement in packaging technology. Some of the key areas requiring synchronal

research for successful thermal management are material science, packaging concepts,

fabrication technology and novel cooling strategies. Focusing on the later area, my work

attempts to analyze the performance of Pulsating Heat Pipes (PHPs), especially suited for

thermal management of electronics. A satisfactory progress has been achieved in the last decade

in the understanding of these devices but quite a few phenomenon of the device operation still

remain unexplored or unclear. However with the development achieved so far, the prospects for

this unprecedented technology seem quite promising.

Contemporary trends in thermal management of electronic devices are very demanding and the

limits are being focused in every aspect of design. Market requirements include: (a) Thermal

resistance from chip to heat sink < 1 K/W, (b) High heat transport capability up to 250 W, (c)

Heat flux spreading up to 60 W/cm2, (d) Mechanical and thermal compatibility, (e) Long term

reliability, (f) Miniaturization, and (g) Low cost. These demands pose a simultaneous challenge

of managing increased power levels and fluxes [1, 2]. With such specific boundary conditions in

mind, neoteric cooling/ heat transfer strategies are continuously being asked i.e. development of

pool boiling and jet impingement cooling which are phase change techniques, and more recently

mini/micro channel flow boiling concepts [3]. In line with these developments is the introduction

of pulsating heat pipes in the early nineties [4-7], as a very promising heat transfer technology,

especially suited for thermal management of electronics.

In the Pulsating Heat Pipe, filling results in a natural, uncontrolled, asymmetric liquid-vapor,

plug-bubble distribution (uneven void fraction) in the tube sections, due to the dominance of

12 | P a g e

surface tension forces [8]. One end of the CLPHP tube bundle receives heat, after that transfer it

to the other by a pulsating action of the working fluid, which in general generate a capillary slug

flow. Since each tube section between the evaporator and the condenser has a different

volumetric distribution of the working fluid, the pressure drop linked with each sub-section is

different. There occurs no 'classical steady state' in CLPHP operation as far as the internal

hydrodynamics is mattered. In fact, in each of the individual tube sections pressure waves and

fluid pulsations are generated, which interact with each other generating secondary/ ternary

reflections with perturbations [8, 9]. The construction of CLPHPs is such that on a macro level,

heat transfer can be confronted to an extended surface ‘fin’ system.

CLPHPs may never be as good as equivalent heat pipe or thermosyphon systems which are

based on pure latent heat transfer. However as compared to an equivalent metallic finned array,

at least there will be an advantage in weight. Finally, there is always a reliability advantage

because of the absence of an external mechanical pump [10].

The available experimental results and trends indicate that any attempt to analyze CLPHPs

must address two strongly interdependent crucial aspects simultaneously, viz. system ‘thermo’

and ‘hydrodynamics’. A multi turn PHP is a thermo-hydrodynamic provenance of a reverse

thermosyphon (without control valve), a bubble pump and a two-phase loop [11]. The applicable

equations in these cases have been outlined by Khandekar et al. [12].

Looking into the available literature, it can be seen that six major thermo-mechanical

parameters have emerged as the primary design parameters affecting the PHP system dynamics

[13].These parameters are internal diameter of the PHP tube, input heat flux to the device,

volumetric filling ratio of the working fluid, total number of turns, device orientation with

respect to gravity, and working fluid thermo-physical properties [13]. Other conditions which

13 | P a g e

affect the operation are use of flow direction control check valves, tube cross sectional shape,

tube material and fluid combination, and rigidity of the tube material, etc. Various flow patterns

other than capillary slug flow, e.g. bubbly flow, developing or semi-annular flow and fully

developed annular flow (in case of CLPHPs) have also been reported which have a significant

effect on the thermal performance of the device [14-19]. A comprehensive theory of the complex

thermo-hydrodynamic phenomena governing the operation of PHPs is not yet available [20].

The internal tube diameter is one of the important parameter which essentially defines a

CLPHP. The physical behavior is attached to the ‘pulsating’ mode only under a certain range of

diameters. The critical Bond number (or Eötvös) criterion gives the tentative design rule for the

diameter [7, 10]:

(Eö)crit 2crit

(1)

Dcrit

(2)

This criterion fixes that individual liquid slugs and vapor bubbles are formed in the device and

they do not agglomerate so that phase separation does not occur, if the device is kept

isothermally in a non-operating period. For a given specified heat power, a decrease in the

diameter will increase the dissipative losses and lead to poor performance, while an increase in

the diameter much above the critical diameter will change the phenomenological operation of the

device. It will no more behave as a pulsating heat pipe but will transform into an interconnected

array of two phase thermosyphons. The applied heat flux affects the internal bubble dynamics,

sizes and agglomeration/breaking patterns, level of perturbations and flow instabilities, and flow

pattern transition from capillary slug flow to semi-annular and annular [20, 21]. CLPHPs are

14 | P a g e

basically suitable for high heat flux operation. Since the input heat provides the pumping power,

below a certain level, no oscillations occurs.

Experimental results so far reveal that there is an optimum filling ratio for proper CLPHP

operation (in the pulsating mode of operation). This optimum, however, is not accurately defined

but generally is a plateau around 40% fills charge. A too high filling ratio above the optimum

leads to a decrease in the overall degree of freedom as there are not enough bubbles for liquid

pumping. At 100% filling ratio, the device acts as single phase buoyancy driven thermosyphon

[18].

If the total heat through input is defined, an increase in the number of turns leads to a decrease

in heat flux handled per turn. Thus, an optimum number of turns exists for a given heat through

input. However, in the present work analysis has to be done for single turn only.

Apart from simplicity of design, one of the strongest factors in favor of pulsating heat pipes is

that their thermal performance is independent of the operating orientation. However there are

some disputing trends in performance with device orientation, or horizontal as well as anti-

gravity (heater-up) operation was not achieved at all [15, 17, 22]. Several results from other

sources for a multi-turn CLPHP suggest that horizontal operation is possible although not as

good as the vertical operation [14, 23, 24]. Some studies indicate near complete performance

independence with orientation [6, 7, 25]. These apparently contradictory and uncomplimentary

results from literature seem to suggest that requirements for an orientation independent operation

are:

(a) Sufficiently large number of PHP turns, which is responsible for a higher degree of internal

perturbations and inhomogeneity of the system,

(b) A high input heat flux leading to higher ‘pumping power’ and enhanced instabilities,

15 | P a g e

(c) These two aspects are not mutually exclusive and must simultaneously be satisfied.

The results of Akachi [6, 7], Charoensawan et al. [26] and Khandekar et al. [27] tend to support

the first hypothesis. They conclude that a certain critical number of turns are required to make

horizontal operation possible and also to bridge the performance gap between vertical and

horizontal operation. The second hypothesis is tentatively supported by the fact that even for

vertical operation, there is a critical minimum input heat flux requirement to initiate self-excited

oscillations [9-11, 16, 17, 20, 28]. In the absence of gravity, this minimum heat flux is likely to

be higher.

The net heat transfer is a combination of the sensible heat of the liquid plugs and the latent heat

of the vapor bubbles. If the internal flow pattern remains predominantly in the slug flow regime

(as in case of OLPHPs and in case of CLPHPs at low heat fluxes), then it has been demonstrated

that latent heat will not play a dominant role in the overall heat transfer [9, 28]. The critical heat

flux decreased as the section length increased, and increased with increase in the latent heat of

vaporization [34].

Experimental results on OLPHPs have been reported in the original patent by Akachi [4-6] for a

power range of 5 to 90 W in top and bottom heating mode with an average thermal resistance

ranging from 0.64 to 1.16 K/W (R-142b). Maezawa et al. [23] studied an OLPHP consisting of

20 turns of copper tube (ID 1.0 mm) of total length 24 m. R-142b was used as the working fluid.

Fill charge and inclination were varied and the temperature fluctuations at the adiabatic wall

section were also recorded. Kawara et al. [29] have undertaken a visualization study of an

OLPHP employing proton radiography visualization. TS-Heatronics Co. Ltd., Japan have

developed a range of PHPs including design variations termed as ‘Heat Lane’ and ‘Kenzan’ fins

[30]. Material combinations, e.g. SS-liquid N2, Al-R142 and copper with water, methanol, R113

16 | P a g e

and R142b have been tested. Maezawa et al. [24] have tested another set of OLPHPs with R142b

and water as the working fluid with a filling ratio of 50%. More recently Rittidech et al. [31, 32]

investigated the effect of inclination angles and working fluid properties for an OLPHP made of

copper tubes (ID 2.03 mm, Le = La = Lc = 50 mm, 100 mm and 150 mm, Lt = 10 m). R123,

ethanol and water were used as working fluids with a filling ratio of 50%. However, no work has

been reported till date on investigating the performance of a Pulsating Heat Pipe numerically.

Thus the work to be done here is different from the work done by others as here study will be

made using ANSYS FLUENT 13.0. Mathematical modeling and theoretical analysis of PHPs has

been attempted in the recent past with many simplified approaches. These may be categorized as

follows [8]:

(a) Comparing PHP action to equivalent single spring-mass-damper system [36],

(b) Kinematic analysis by comparison with a multiple spring-mass-damper system [37],

(c) Applying fundamental equations of conservation of mass, momentum and energy to a

specified PHP control volume [38-40],

(d) Mathematical analysis highlighting the existence of ‘chaos’ [41, 42]

(e) Modeling by artificial neural networks, [43].

(f) Modeling by semi-empirical correlations based on non-dimensional numbers [26, 27].

These available models do not actually represent the complete thermo-hydrodynamics of the

PHPs. Modeling of CLPHPs continues to pose a challenge.

So, it may be concluded that the work on CLPHPs to be done here on ANSYS FLUENT 13.0

may contribute to face this challenge.

17 | P a g e

CHAPTER-3

PROBLEM FORMULATION

18 | P a g e

3.1 Introduction

Analysis of the performance of CLPHPs is done using computational fluid dynamics method.

For this geometry is modeled in 2D in Gambit 2.2.30. A schematic diagram of the geometry is

shown in figure 3.1. The length and the breadth of the channel (here channel instead of pipe is

said as model is in 2D) is 110 mm and 60 mm respectively and the pipe is assumed to be made of

copper. Water and water vapor is taken as the working fluid which flows in the channel of width

2 mm.

Fig 3.1Schematic diagram of geometry considered for the analysis

After creating geometric models and meshing has done in Gambit 2.2.30. Then the model was

analyzed by varying the wall heat flux at evaporator for a particular filling ratio. The two case of

the working fluid was considered. The working fluid taken in consideration was water-water

vapor and ethyl alcohol-ethyl alcohol vapor.

19 | P a g e

3.2 Governing Equations

Applying boundary conditions, the governing equations for convective heat transfer are as

follows:

The critical Bond number (or Eötvös) criterion gives the tentative design rule for the diameter

(Eö)crit 2crit

(1)

Dcrit

(2)

3.3 Volume of Fluid:

In computational fluid dynamics, the Volume of fluid method is one of the most well-known

methods for volume tracking and locating the free surface. The motion of all phases is modeled

by solving a single set of transport equations with appropriate jump boundary conditions at the

Interface. The VOF model can model two or more immiscible fluids by solving a single set of

momentum equations and tracking the volume fraction of each of the fluids throughout the

domain. It is generally used to figure out a time dependent solution but for problems which are

concerned with steady state solution; it is possible to perform a steady state calculation. A steady

state VOF calculation is practical only when the solution is independent of the initial conditions

and there are distinct inflow boundaries for the individual phases. Typical applications include

the motion of large bubbles in a liquid, the motion of liquid after a dam break, the prediction of

jet breakup, and the steady or transient tracking of any liquid-gas interface. In general, the steady

20 | P a g e

or transient VOF formulation relies on the fact that two or more fluids (or phases) are not

interpenetrating.

3.3.1 Volume fraction

In VOF model the variables and properties in any given cell are either purely representative of

one of the phases, or representative of a mixture of the phase, depending upon the volume

fraction values. In other words, if the qth

fluid’s volume fraction in the cell is denoted as αq then

the following three conditions are possible:

αq =0 : the cell is empty(of the qth

fluid).

αq =1 : the cell is full (of the qth

fluid).

0<αq <1 : the cell contains the interface.

3.4 Boundary Conditions

The analysis of the model has been done under two sections.

i) In the first case the water and water vapor is taken as the working fluid with filling ratio of 70

% and the heat flux of 30W, 50W and 100W has applied at evaporator. The initial working fluid

temperature is 300K and the ambient temperature is 298K.

ii) In the second case the ethyl alcohol and ethyl alcohol vapor is taken as the working fluid with

filling ratio of 70 % and the heat flux of 30W, 50W and 100W has applied at evaporator. The

initial working fluid temperature is 300K and the ambient temperature is 298K.

In the work reported here, water and water vapor and also ethyl alcohol liquid and ethyl alcohol

vapor as the working fluid for the analysis. Fluid properties are assumed to be constant with

temperature. The properties of water and water vapor and also ethyl alcohol liquid and ethyl

alcohol vapor considered for the analysis is given in table 3.1

21 | P a g e

The temperature of the fluid is taken as 300K and the convective heat transfer is considered at

the condenser. The ambient temperature is 298K.

Table 3.1 Properties of working fluids

Description Symbol Water

liquid

Water

vapor

Ethyl

alcohol

liquid

Ethyl

alcohol

vapor

Units

Density ρ 1000 0.5542 790 2.06 kg/m3

Dynamic

Viscosity

µ 0.001003 0.0000134 0.0012 0.0000108 kg/ms

Specific Heat Cp 4182 2014 2470 2407 J/kgK

Thermal

Conductivity

k 0.6 0.0261 0.182 0.0145 W/mK

22 | P a g e

CHAPTER-4

CFD MODELING

23 | P a g e

4.1 Introduction

The invention of high speed digital computers, combined with the development of accurate

numerical methods for solving physical problems, has revolutionized the way we study and

practice fluid dynamics and heat transfer. This approach is called Computational Fluid Dynamics

or CFD in short, and it has made it possible to analyze complex flow geometries with the same

ease as that faced while solving idealized problems using conventional methods. CFD may thus

be regarded as a zone of study combining fluid dynamics and numerical analysis. Historically,

the earlier development of CFD in the 1960s and 1970s was driven by the need of the aerospace

industries. Modern CFD, however, has applications across all disciplines – civil, mechanical,

electrical, electronics, chemical, aerospace, ocean, and biomedical engineering being a few of

them. CFD substitutes testing and experimentation, and reduces the total time of testing and

designing. Fig. 4.1 gives the overview of the CFD modeling process.

4.2 CFD Programs

The development of affordable high performance computing hardware and the availability of

user-friendly interfaces have led to the development of commercial CFD packages. Before these

CFD packages came into the ordinary use, one had to write his own code to carry out a CFD

analysis. The programs were usually different for different problems, although some part of the

code of one program could be used in another. The programs were inadequately tested and

reliability of the results was often questioned. Today, well tested commercial CFD packages not

only have made CFD analysis a routine design tool in industry, but are also helping the research

engineer in focusing on the physical system more effectively.

24 | P a g e

Fig.4.1 Overview of Modeling Process

All established CFD software contain three elements (i) a pre-processor, (ii) the main

solver, and (iii) a post-processor

4.2.1 The Pre-Processor

Pre-processing is the first step of CFD analysis in which the user

(a) Defines the modeling objectives,

25 | P a g e

(b) Identifies the computational domain, and

(c) Designs and creates the grid system

The process of CFD modeling starts with an understanding of the actual problem and

identification of the computational domain. This is followed by generations of the mesh

structure, which is the most important portion of the pre-processing activity. It is believed that

more than 50% of the time spent by a CFD analyst goes towards mesh generation. Both

computation time and accuracy of solution depend on the mesh structure. Optimal grids are

generally non-uniform – finer in areas where large variation of variables is expected and coarser

in regions where relatively little changes is expected. In order to reduce the difficulties of

engineers and maximize productivity, all the major CFD programs include provision for

importing shape and geometry information from CAD packages like AutoCAD and I-DEAS, and

mesh information from other packages like GAMBIT.

4.2.2 The Main Solver

The solver is the heart of CFD software. It sets up the equations which are selected according to

the options chosen by the analyst and grid points generated by the pre-processor, and solves them

to compute the flow field.

The process incorporates the following tasks:

• selecting appropriate physical model,

• defining material properties,

• prescribing boundary conditions,

26 | P a g e

• providing initial solutions,

• setting up solver controls,

• setting up convergence criteria,

• solving equation set, and

• saving results

Once the model is completely set, the solution is initialized consequently calculation starts and

intermediate results can be monitored at every time step from iteration to iteration. The progress

of the solution process get displayed on the screen in terms of the residuals, a measure of the

extent to which the governing equations are not satisfied.

4.2.3 The Post-processor

The post-processor is the last part of CFD software. It helps the user to analyze the results and

get useful data. The results may be displayed as vector plots of vector quantities like velocity,

contour plots of scalar variables, for example pressure and temperature, streamlines and

animation in case of unsteady simulation. Global parameters like skin friction coefficient lift

coefficient, Nusselt number and Colburn factor etc. may be computed through appropriate formulas.

These data from a CFD post-processor can also be exported to visualization software for better

display and to software for better graph plotting.

Various general-purpose CFD packages have been published in the past decade. Important

among them are: PHOENICS, FLUENT, STAR-CD, CFX, CFD-ACE, ANSWER, CFD++,

FLOW-3D and COMPACT. Generally all these packages are based on the finite volume method.

CFD packages have also been developed for special applications. FLOTHERM and ICEPAK for

electronics cooling, CFX-TASCFLOW and FINE/TURBO for turbo machinery and ORCA for

27 | P a g e

mixing process analysis are some examples. Most CFD software packages contain their own

mesh generators and post processors. Some popular visualization software used with CFD

packages are TECPLOT and FIELDVIEW.

4.3 Overview of FLUENT Package

FLUENT is a state-of-the-art computer program for modeling heat transfer and fluid flow in

complex geometries. FLUENT provides complete mesh flexibility, solving one’s flow problems

with unstructured grids that can be generated about complex geometries with relative ease.

Supported grid types include 2D triangular/quadrilateral. 3D FLUENT also allows user to refine

or coarsen grid based on the flow solution.

FLUENT is written in the C computer language and makes full use of the flexibility and power

offered by the language. As a result, true dynamic memory allocation, efficient data structures,

and flexible solver control (user defined functions) are all made possible. In addition, FLUENT

uses a client/server architecture, which allows it to run separate simultaneous processes on client

desktop workstations and powerful computer servers, for efficient execution, interactive control,

and complete flexibility of machine or operating system type.

All functions necessary to compute a solution and display the results are accessible in

FLUENT through an interactive, menu-driven interface. The user interface is written in a

language called Scheme, a dialect of LISP. The advanced user can customize and enhance the

interface by writing menu macros and functions.

4.4 CFD Procedure

For numerical analysis in CFD, it requires five stages such as:

Geometry creation

28 | P a g e

Grid generation

Flow specification

Calculation and numerical solution

Results

Based on control volume method, 3-D analysis of fluid flow and heat transfer for the helical

coiled tube is done on ANSYS FLUENT 13.0 software. All the above mentioned processes are

done using the three CFD tools which are pre-processor, solver and post-processor.

4.4.1 Geometry Creation

Geometry of closed loop pulsating heat pipe (CLPHs) is modeled using GAMBIT 2.2.30 as

shown in fig. 3.1. The length and the breadth of the channel (here channel instead of pipe is said

as model is in 2D) is 110 mm and 60 mm respectively and the pipe is assumed to be made of

copper. Water is taken as the working fluid which flows in the channel of width 2 mm.

4.4.2 Mesh Generation

The fig.4.2 shows the grid of a section of geometry. It depicts that the domain was meshed with

rectangular cells. Grid independence was studied by doing different simulation with taking

different no cells. The quadrilateral grid has generated on all the faces with aspect ratio 1. The

total number of grid generated is 23252.

29 | P a g e

Fig.4.2 A small section of geometry meshed in Gambit

4.4.3 Flow Specification

The assumptions used in this model were

The flow was transient and turbulence. The fluid density was constant throughout the

computational domain.

Water-water vapor and ethyl alcohol-ethyl alcohol vapor are the working fluid. The fluid

properties (ρ, μ and specific heat) being constant throughout the computational domain.

Except evaporator and condenser part, the other length of the CLPHPs has kept at

adiabatic condition.

For this present analysis the method applied is explained below. All the governing equations

used in present analysis were solved by using ANSYS FLUENT 13.0 finite volume

30 | P a g e

commercial code. Implicit first order upwind scheme was used for solving governing

equation. The convergence criterion was fixed such that the residual value was lower than

1e-6. The VOF model is used to analysis. The constant heat flux is supplied at the evaporator

end. This heat flux is varied in different condition. The convective heat transfer is considered

at the condenser end. The remaining length of the pipe is at adiabatic condition. The

turbulence model applied for present analysis was Reynolds stress model. Relaxation factor

have been kept to default values. Refer table 4.1 for values.

Table 4.1 Relaxation factors

Pressure Density Energy Momentum Turbulent

dissipation rate

Body Force

0.3 1 1 0.7 0.8 1

31 | P a g e

CHAPTER-5

RESULTS AND DISCUSSION

32 | P a g e

5.1 Results and Discussion

Figure 5.1 shows the contour of phases of the working fluid. This contour shows the fraction of

water and water vapor inside the channel. Using these contours the oscillating behavior of

working fluid inside a closed loop pulsating heat pipe has shown. As can be seen from the

figures that vapor slug is moving to and fro at different time interval. It is also concluded that

when the liquid slug and the vapor slug is moving in forward direction, it carried out the heat

from the evaporator and transport heat flux to the condenser end by oscillating behavior. During

this process, there is change of phase from liquid phase to vapor phase and vice versa.

(a) Position of vapor slug at t= 5 s

33 | P a g e

(b) Position of vapor slug at t= 15 s

(c) Position of vapor slug at t= 25 s

34 | P a g e

(d) Position of vapor slug at t=35 s

(e) Position of vapor slug at t= 50 sec

35 | P a g e

(f) Position of vapor slug at t= 55 s

(g) Position of vapor slug at t= 60 s

36 | P a g e

(h) Position of vapor slug at t= 65 s

(i) Position of vapor slug at t= 70 s

Figure 5.1 the oscillating behavior of working fluid

37 | P a g e

(a) Temperature variation inside the channel at t=50 s

(b) Temperature variation inside the channel at t=55 s

38 | P a g e

(c) Temperature variation inside the channel at t=60 s

(d) Temperature variation inside the channel at t=65 s

39 | P a g e

(e) Temperature variation inside the channel at t=70 s

(f) Temperature variation inside the channel at t=75 s

40 | P a g e

(g) Temperature variation inside the channel at t=80 s

(h) Temperature variation inside the channel at t=85 s

Fig. 5.2 the variation of temperature in channel due to oscillating heating

Figure 5.2 shows the variation of temperature at different time. The two curves show the

temperature in upper channel and the lower channel. From the figure it is observed when the

41 | P a g e

liquid moving in forward direction then it absorbed heat from the evaporator. This is the

reason for the temperature rise in the upper part of the channel near the evaporator end while

the heat flux is rejected at the condenser that why the temperature in the lower part of the

channel decrease. When the working fluid moves in backward direction, again the heat flux

is absorbed at the evaporator by the working fluid. So the temperature increases in the lower

part of the channel.at the condenser, the heat flux is further rejected so the temperature in the

upper part of the channel going to be further decrease.

Fig.5.3 pressure variation inside the channel

Figure 5.3 shows the variation of pressure inside the channel. In the some part of the channel,

the pressure is too high while in the other part the pressure is too low. This pressure difference

causes the working fluid to move to and fro.

42 | P a g e

Table 5.1 shows the heat flux output at the condenser for different value of heat flux input at the

evaporator. The working fluid is taken as two phase fluid i.e. water liquid and water vapor.

From the table we observed that for the heat flux input the heat flux output variation start earlier

than the lower value of heat flux input.

Table 5.1 the heat flux output at condenser for different values of heat flux input at evaporator

for water-water vapor working fluid

Time in sec Heat output(W) for

30W input

Heat output(W) for

50W input

Heat output(W) for

100W input

0 0 0 0

5 20.64159 19.08212 19.84356

10 17.98927 17.16788 16.95148

15 17.26441 15.49101 15.17172

20 14.96384 15.53616 13.36375

25 13.95291 18.2872 12.16009

30 12.31902 17.24688 10.7149

35 10.89966 14.9366 9.827559

40 9.511117 13.29477 8.606341

45 8.434875 11.92502 7.790276

43 | P a g e

50 10.46197 10.30404 7.8195

55 9.251165 9.76943 16.05724

60 8.536623 9.123188 11.40452

65 8.443436 15.48048 13.83295

70 8.877634 11.19273 10.04448

75 6.310759 11.29016 9.305625

80 6.006869 12.6132 11.36196

85 5.281271 9.774879 8.08682

90 5.362241 8.251593 7.841149

95 5.678013 7.048686 6.656506

100 4.799042 9.553208 5.779629

105 4.066377 8.059357 5.144253

110 3.831323 5.572752 6.739488

115 3.743459 5.718899 5.833715

120 3.921056 5.078343 3.893465

125 4.998725 5.107701 5.368751

44 | P a g e

130 2.965296 3.840414 5.336428

135 2.583455 3.610587 4.879534

140 3.927177 3.879366 3.528817

145 2.836003 3.493398 3.865195

150 2.190431 3.140001 3.911274

155 1.879695 3.319992 3.293952

160 1.722643 2.635528 2.797819

165 1.684039 2.914893 2.938489

170 1.635763 1.892423 2.683431

175 1.976357 1.72561 2.203906

180 1.831121 1.717581 1.841486

185 2.181299 1.506687 1.497033

190 1.615289 1.280394 1.357307

195 1.604695 1.262742 1.245869

200 1.59982 1.346909 1.128268

45 | P a g e

Fig.5.4 Heat flux vs. time for water-water vapor working fluid

Figure 5.4 shows the variation of heat flux at condenser with time for different heat flux at

evaporator. It is clearly visible from graph that the heat transfer behavior is oscillating which

satisfies the thermosyphon phenomenon that occurs in CLPHPs.

Table 5.2 shows the heat flux output at the condenser for different value of heat flux input at the

evaporator. The working fluid is taken as two phase fluid i.e. Ethyl alcohol liquid and ethyl

alcohol vapor. When we compare the data for heat flux out for 30W of heat flux input, we found

that the variation in the output heat flux for ethyl alcohol start earlier than water. It has been also

observed that the fluctuation rate in the case of alcohol is higher than the water. So oscillation is

faster in case of alcohol than the water.

Table 5.2 the heat flux output at condenser for different value of heat flux input at evaporator for

ethyl alcohol-ethyl alcohol vapor working fluid.

0

5

10

15

20

25

0 50 100 150 200

he

at t

ran

sfe

r ra

te(W

)

time (s)

heat transfer rate vs time for Water

Qin=30W

Qin=50W

Qin=100W

46 | P a g e

Time in sec Heat output(W) for 30W input

0 0

5 15.689

10 12.10302

15 13.53325

20 10.34767

25 6.97627

30 5.595694

35 4.111408

40 3.288454

45 2.32275

50 2.433071

51 2.826646

52 4.168352

53 5.757897

54 6.169416

55 2.679956

56 2.502211

57 4.115772

47 | P a g e

58 2.837486

59 1.740256

60 1.872791

61 2.641127

62 3.720674

63 3.610297

64 1.9428

65 1.641023

66 1.598649

67 1.525151

68 1.502425

70 1.352532

71 2.523652

72 2.128292

73 1.276367

74 1.306113

75 1.98288

76 2.465005

77 3.066896

48 | P a g e

78 1.336126

79 1.308021

80 1.270359

81 1.323168

82 1.361207

83 1.527749

84 1.747146

85 1.048199

86 1.320493

87 1.130706

88 1.027092

89 1.021945

90 1.229296

49 | P a g e

Fig.5.5 Heat flux vs. time for ethyl alcohol-ethyl alcohol vapor

Figure 5.5 shows the variation of heat flux at condenser with time for different heat flux at

evaporator. It is clearly visible from graph the frequent oscillation has been attended much

earlier than the water. And also it has observed that the oscillation occur in short interval of time

compare to water.

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 52 54 56 58 60 62 64 66 68 71 73 75 77 79 81 83 85 87 89

He

at t

ran

sfe

r ra

te (

Q)

in W

Time in sec

heat transfer rate Vs time for Alchohal

Qin=30w

50 | P a g e

CHAPTER-6

CONCLUSIONS AND FUTURE SCOPE

51 | P a g e

6.1 Conclusion

Through the CFD methodology, this work investigates the flow and heat transfer phenomena in a

closed loop pulsating heat pipe. Effects of heat flux at evaporator and different working fluid

have been also studied. Several important conclusions could be drawn from the present

simulations and would be presented as follows

There is pressure variation inside the tube because of increase in volume of the working

fluid by absorbing heat at one end which causes the transport of vapor slug and liquid.

After a certain time of interval, the oscillating behavior of working fluid becomes more

frequent which causes oscillation in heat flux at the output that means at some moment

cooling effect will be more and at some time it will be less.

Alcohol attends frequent oscillation earlier as compared to water which signifies that

fluid with lower specific heat will give cooling effect much earlier than the fluid with

higher specific heat.

For higher the value of input heat flux, the oscillation starts in lesser time as compare to

the lower value of heat input.

6.2 Future Scope

The works which are required to be done in future are:

Analysis of CLPHPs is done by taking the different working material like water, R-123

and ethanol and comparing their result.

3-d modeling of CLPHPs and its analysis.

By varying the filling ratio, the above results should be find out and comparing their

results.

52 | P a g e

References

[1] Azar K., The History of Power Dissipation, Electronics Cooling, Vol. 6, No. 1, (2000):

Available at http://electronics-cooling.com/ html/articles.html.

[2] Bar-Cohen, Trends in Packaging of Computer Systems, in Cooling of Electronic Systems,

edited by Kakac S., Yüncü H. and Hijikata K., Kulwar Acad. Pub., (1994): pp. 17-45

[3] Lin S., Sefiane K. and Christy J., Prospects of Confined Flow Boiling in Thermal

Management of Microsystems, Applied Thermal Engg. Vol. 22, (2002): pp. 825-837

[4] Akachi H., U. S. patent # 4921041, 1990.

[5] Akachi H., U. S. patent # 5219020, 1993.

[6] Akachi H., U. S. patent # 5490558, 1996.

[7] Akachi H., Polášek F. and Štulc P., Pulsating Heat Pipes, Proc. 5th Int. Heat Pipe Symp.,

(1996):pp. 208-217, Melbourne, Australia

[8] Khandekar S., Schneider M. and Groll M., Mathematical Modeling of Pulsating Heat Pipes:

State of the Art and Future Challenges, Proc. 5th ASME/ISHMT Int. Heat and Mass Transfer

Conf., ISBN-0-07-047443-5, (2002): pp. 856- 862, Kolkata, India

[9] Groll M. and Khandekar S., Pulsating Heat Pipes: A Challenge and Still Unsolved Problem in

Heat Pipe Science, Archives of Thermodynamics, ISSN 1231-0956, Vol. 23/4., (2002): pp. 17-

28

[10] Khandekar S. and Groll M., On the Definition of Pulsating Heat Pipes: An Overview, Proc.

5th Minsk Int. Conf. (Heat Pipes, Heat Pumps and Refrigerators), Minsk, Belarus, 2003.

[11] Khandekar S. and Groll M., An Insight into Thermo-Hydraulic Coupling in Pulsating Heat

Pipes, Int. J. of Thermal Sciences, ISSN 1290- 0729, Vol. 43/1, (2004): pp. 13-20

53 | P a g e

[12] Khandekar S., Manyam S., Groll M. and Pandey M., Two-phase Flow Modeling in Closed

Loop Pulsating Heat Pipes, Proc. 13th Int. Heat Pipe Conf., Shanghai, China, 2004.

[13] Khandekar S., Thermo-hydrodynamics of Closed Loop Pulsating Heat Pipes, Doctoral

Dissertation, Universität Stuttgart, Germany, 2004.

[14]Gi. K., Sato F. and Maezawa S., Flow Visualization Experiment on Oscillating Heat Pipe,

Proc. 11th Int. Heat Pipe Conf., (1999): pp. 149- 153, Tokyo, Japan

[15] Lee W., Jung H., Kim J. and Kim J., Flow Visualization of Oscillating Capillary Tube Heat

Pipe, Proc. 11th IHPC, (1999): pp. 355-360,Tokyo, Japan

[16] Tong B. Y., Wong T. N. and Ooi K., Closed- Loop Pulsating Heat Pipe, Applied Thermal

Engineering, ISSN 1359-4311, Vol. 21/18, (2001): pp. 1845-1862

[17] Qu W. and Ma T., Experimental Investigation on Flow and Heat Transfer of a Pulsating

Heat Pipe, Proc. 12th Int. Heat Pipe Conf., (2002): pp. 226- 231, Moscow, Russia

[18] Khandekar S., Groll M., Charoensawan P. and Terdtoon P., Pulsating Heat Pipes:

Thermofluidic Characteristics and Comparative Study with Single Phase Thermosyphon, Proc.

12th Int. Heat Trans. Conf., ISBN-2-84299-307-1, Vol. 4, (2002):pp. 459-464, Grenoble, France

[19] Charoensawan P., Heat Transfer Characteristics of Closed Loop Oscillating Heat Pipes,

Ph.D. Thesis, Chiang Mai University, Chiang Mai, Thailand, 2003.

[20] Groll M. and Khandekar S., Pulsating Heat Pipes: Progress and Prospects, Proc. 3rd Int.

Conf. on Energy and Environment, ISBN 7- 5323-7335-5/TK-22, Vol. 1, (2003): pp. 723-730,

Shanghai, China.

[21] Khandekar S., Dollinger N. and Groll M., Understanding Operational Regimes of Pulsating

Heat Pipes: An Experimental Study, Applied Thermal Engineering, ISSN 1359- 4311, Vol. 23/6,

(2003): pp. 707-719

54 | P a g e

[22] Khandekar S., Schneider M., Schäfer P., Kulenovic R. and Groll M., Thermofluiddynamic

Study of Flat Plate Closed Loop Pulsating Heat Pipes, Microscale Thermophysical Engg., ISSN

1089-3954, Vol. 6/4, (2002): pp. 303-318

[23] Maezawa S., Gi A., Minamisawa K. and Akachi H., Thermal Performance of Capillary

Tube Thermosyphon, Proc. 9th Int. Heat Pipe Conf., Vol. 2, (1995):pp. 791-795, Albuquerque,

New Mexico

[24] Maezawa S., Nakajima R., Gi K. and Akachi H., Experimental Chaos in Oscillating

Capillary Tube Heat Pipes, Preprints 10th Int. Heat Pipe Conf., Vol. 3/4, Session F, Stuttgart,

Germany, 1997.

[25] Akachi H. and Polášek F., Thermal Control of IGBT Modules in Traction Drives by

Pulsating Heat Pipes, Preprints 10th Int. Heat Pipe Conf., Vol. 3, (1997): pp. 8-12, Stuttgart,

Germany

[26] Charoensawan P., Khandekar S., Groll M. and Terdtoon P., Closed Loop Pulsating Heat

Pipes-Part A: Parametric Experimental Investigations, App. Thermal Engg., ISSN 1359-4311,

Vol. 23/16, (2003): pp. 2009-2020

[27] Khandekar S., Charoensawan P., Groll M. and Terdtoon P., Closed Loop Pulsating Heat

Pipes-Part B: Visualization and Semi- Empirical Modeling, Applied Thermal Engineering, ISSN

1359-4311, Vol. 23/16, (2003): pp. 2021-2033

[28] Nishio S., Nagata S., Baba S. and Shirakashi R., Thermal Performance of SEMOS Heat

Pipes, Proc. 12th Int. Heat Trans. Conf., ISBN 2-84299-307-1, Vol. 4, (2002): pp. 477-482,

Grenoble, France

55 | P a g e

[29] Kawara Z., Takahashi O., Serizawa A. and Kohno M., Visualization of Flow in Heat Pipe

by Proton Radiography, Proc. 24th Visualization and Information Symp., (1996): pp. 27-23,

Osaka, Japan,.

[30] Akachi H. and Miyazaki Y., Stereo-Type Heat Lane Heat Sink, Preprints 10th Int. Heat Pipe

Conf., Vol. 3/4-F, Stuttgart, Germany, 1997.

[31] Rittidech, S., Terdtoon, P., Tantakom,P., Murakami, M., and Jompakdee, W., Effect of

Inclination Angles, Evaporation and Working Fluid Properties on Heat Transfer Characteristic of

A Closed-End Oscillating Heat Pipe, Procs. of the 6th Int. Heat pipe Symp., Chiang Mai,

Thailand, 2000.

[32] Rittidech, S., Terdtoon, P., Murakami, M., Kamonpet, P., and Jompakdee, W., Effect of

Evaporator Section Lengths, Number of Turns and Working Fluid on Internal Flow Patterns of A

Closed-End Oscillating Heat Pipe at Normal Operating Condition, Procs. of the 7th Int. Heat

Pipe Symp., Jeju, Korea, 2003.

[33] Terdtoon, P., A Closed Look to a Closed-End Oscillating Heat Pipe, Procs. Of the 7th Int.

Heat Pipe Symp., Jeju, Korea, 2003.

[34] Katpradit, T., Wongratanaphisan, T., Terdtoon, P., Akbarzadeh A., and Kamonpet P.,

Correlation to Predict Heat Transfer Characteristics of A Closed End Oscillating Heat Pipe at

Critical State, Applied Thermal Engineering, Elsevier Science, ISSN 1359- 4311, (article in

press), 2004.

[35] Panyoyai, N., Effects of Meandering Turns and Aspect Ratios on Internal Flow Patterns of

Closed End Oscillating Heat Pipes at Critical State, Master Thesis, Chiang Mai University, 2004.

[36] Zuo Z. J. and North M. T., Miniature High Heat Flux Heat Pipes for Cooling Electronics,

Proc. of SEE 2000, Hong Kong,( 2000): pp. 573-579

56 | P a g e

[37] Wong T., Tong B. Y., Lim S. M. and Ooi K. T., Theoretical Modeling of Pulsating Heat

Pipe, Proc. 11th Int. Heat Pipe Conf., (1999): pp. 159- 163, Tokyo, Japan

[38] Swanepoel G., Taylor A. and Dobson R., Theoretical Modeling of Pulsating Heat Pipes,

Proc. Int. Heat Pipe Symp. (CD), (2000):pp. 227-234, Chiang Mai, Thailand

[39] Shafii M. B., Faghri A. and Zhang Y., Thermal Modeling of Unlooped and Looped

Pulsating Heat Pipes, ASME J. Heat Tran., Vol. 123 (2001):pp. 1159-1172

[40] Zhang Y. and Faghri A., Heat Transfer in a Pulsating Heat Pipe with Open End, Int. J. Heat

Mass Transfer, Vol. (2002):755-764

[41] Reay David and Kew Peter, Heat Pipes Theory, Design and Application, London,

Butterworth-Heinemann, 2006

[42] Peterson, G.P., An Introduction to Heat Pipes Modeling, Testing and Application, New

York, Jhon Wiley & sons, Inc., 1994