Heatpipes

International Journal of Emerging trends in Engineering and Development ISSN 2249-6149

Issue1, Vol. 2

RS Publication Page 25

Design, fabrication and performance

evaluation of axially grooved wick assisted

heat pipe Rudra Naik

1, Venugopal Varadarajan

2,G Pundarika

3

1,2,3 Department of Mechanical Engineering,

BM S College of Engineering, Bangalore, India.560019.

Abstract

The work reported here involves the fabrication and testing of an axial grooved heat pipe of

outer diameter 8mm, inner diameter 4mm and a length of 150mm.The objective of this work is

to design, fabricate and test a heat pipe with an axial grooved wick for horizontal and gravity

assisted orientations. The experimental setup consists of a heat pipe with wick, mica heater,

J-type thermocouples, temperature indicator, data acquisition system, dimmer stat, moisture

trap, pressure gauge and a vacuum pump. Orientation of heat pipe plays a vital role in

certain applications like satellites and laptops where the performances of such heat pipes

have to be studied and analyzed. In the present analysis three different working fluids i.e.

methanol, acetone and distilled water are used. The maximum heat transfer coefficient of the

grooved heat pipe for methanol, acetone and distilled water were found to be 3550 W/m2 o

C

for horizontal orientation, 1700 W/m2o

C for vertical orientation and 2400 W/m2o

C for vertical

orientation respectively. In earlier studies parameters such as type of working fluids used, fill

charge ratios and vacuum pressures have been investigated for wickless heat pipes. The effect

of orientation for both vertical and horizontal is not available in the open literature since the

wickless heat pipes do not perform well at acute angles. However it is observed that there is

considerable variation in the performance of horizontal and vertical orientation in the case of

methanol. This implies that there is a working fluid specificity that affects heat pipe

performance in the context of orientation. This analysis would help the researchers and

industry professionals in choosing appropriate working fluids for specific orientation.

KEYWORDS- Heat transfer, Grooved heat pipe, Working fluids, Capillary limit

Nomenclature

A =Cross-section area (m2 )

d =Diameter of heat pipe, diameter of wick

wire (m)


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RS Publication 6

D =Internal pipe diameter (m)

h =Heat transfer coefficient(W/m2 0C)

k =Thermal conductivity(W/m 0C)

K=Wick permeability (m2)

L =Length (m)

M =Merit number (W/m2)

P=Pressure (N/m2)

ΔpL/Δx= Sum of inertial and viscous

pressure drop

Δpv/Δx = Sum of inertial and viscous

pressure drop

ρ=Density of vapour in (kg/m3)

λ= Latent heat of the fluid in (KJ/Kg)

(ΔP)= Maximum capillary pressure

difference in (N/m2)

ΔP= Pressure gradient. (N/m2)

q =Limit

Q= Heat transport in(KW/m2)

r =Radius (m)

R= Thermal resistance (0C/W) , Gas

constant (J/kg K)

T =Temperature (0C)

V =Volume of liquid (m3)

α =Angle of inclination to the horizontal

(radians)

γ =Ratio of specific heats

η =Efficiency

μ= Dynamic viscosity in (N-s/m2)

ρ= Density (kg/m3

)

σ =Surface Tension (N/m)

Subscripts

+ =Normal hydrostatic pressure drop

a =Adiabatic

b= Boiling

c= Condenser, Capillary

cmax=Maximum capillary pressure

difference generated within capillary

wicking structure between wet and dry

points.

conv =Convection

eff =Effective

ent =Entrainment

ent= Entrainment

evaporator =Evaporator

G =Gravitational

h =hydraulic

i =Inner

l =Liquid

ll =Axial hydrostatic pressure drop

n=nucleation site

o =Outerw

p=Pipe

Ph=phase transition

s = Sonic

v= Vapor, Viscous

v=vapour space

w = wall


Issue1, Vol. 2


1. Introduction

The heat pipe is a passive heat transfer device which transfers heat from one region to another

with exceptional heat transfer capacity. It transports heat at a very high rate over a

considerable distance with a small temperature drop. It utilizes the latent heat of vaporization

of the working fluid instead of sensible heat. Hence, the effective thermal conductivity is of

several orders of magnitude higher than that of solid conductors. Heat pipes comprise of three

regions, namely the evaporator, adiabatic and the condenser region. When heat is added to the

evaporator section the same is absorbed in the form of latent heat through the vaporization

process of the working fluid and this in turn converts the liquid into vapor. This vapor reaches

the condenser and gets condensed due to a partial pressure build up inside the pipe. The

condensate returns back to the evaporator section along the walls of the heat pipe with the

assistance of gravity or due to the capillary action caused by the wick. The latent heat released

in the condenser section is taken away by means of a water jacket or an air cooling system.

Since the heat pipe utilizes the latent heat of vaporization of the working fluid instead of a

sensible heat transport, the required temperature difference is very small and still the effective

thermal conductivity may be several orders of magnitude greater than that of a best solid

conductor. Providing circumferential grooves on the internal surface of the vapor channel

enables the pumping mechanism and condenser return flow paths. Axially grooved heat pipes

are very easy to manufacture compared with circumferential grooves, because they can be

manufactured by extrusion. The authors have made an attempt to investigate the effect of

orientation on performance for the axially grooved configuration in view of providing greater

depth in understanding the heat transfer phenomenon.

Gaugler [2]

patented the heat pipe concept while working on the problem of evaporation of the

fluid to a condenser region from an evaporator region without additional work to lift the

liquid. He suggested a capillary structure as means of returning the fluid to the evaporator in

the form of a sintered wick. In 1995, Faghri [3]

developed a mathematical model to examine

the heat and mass transfer processes in a micro heat pipe, taking into account the variation of

the curvature of the free liquid surface and the interfacial shear stress due to liquid vapor

interaction.

Zhang J. et al. [4]

studied the heat transfer and fluid flow in an idealized micro heat pipe. The

temperature profile is relatively flat except the region near the evaporator. For a micro heat

pipe with larger length to width ratio, the length of the evaporator is shorter. From the vapor


Issue1, Vol. 2


pressure distribution, it was observed that the pressure varies linearly and is not strongly

affected by the length to width ratio. On evaluating, the effective thermal conductivity of a

micro heat pipe increases with increase in the evaporation area as well as length or width of

the micro heat pipe. It was also reported that a fluid with larger latent heat would produce

larger effective thermal conductivity.

Robert Richter et al [5]

developed thermodynamic aspects of heat pipe operation. In this article

the general operation and performance of heat pipes is approached from a fundamental

thermodynamic considerations. Scott et al. [6]

developed the effect of working fluid inventory

on the performance of revolving helically grooved heat pipes. Recent experiments have

shown that the capillary limit of a helically grooved heat pipe (HGHP) was increased

significantly when the transverse body force field was increased. This was due to the

geometry of a helical groove wick structure.

Khrustalev et al. [7]

developed a mathematical model for a low temperature axially grooved

heat pipe with emphasis on capillary and boiling limitations. The design of heat pipe is a very

complex process involving different physical variables such as the shape, size, weight and

volume, thermo physical properties such as working fluid, wicking structure and container

material. The thermal loads, transport length, evaporator and condenser length, operating

temperature ranges, fluid inventories and temperature environments also play a major role in

the design of the heat pipes. The present study intends to design, fabricate and test for key

performance parameters of an axially grooved wick assisted heat pipe for two different

orientations.

2 Design Considerations

The maximum heat transport capability of the heat pipe is governed by several limiting factors

which must be addressed while designing the heat pipe. There are five primary heat transport

limitations which include: viscous limit, sonic limit and capillary pumping limit, entrainment or

flooding limit and boiling limit [3]

. Each heat transport limitation is summarized as follows:


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2.1 Capillary Limit

The capillary limit is set by the pumping capacity of the wick structure. It is a strong function of

the operating orientation and the type of wick structure. The two most important properties of a

wick are the pore radius and the permeability. The pore radius determines the pumping pressure

the wick can develop. The permeability determines the frictional losses of the fluid as it flows

through the wick. There are several types of wick structures available including: grooved, screen,

cables/fibres, and sintered powder metal.

Grooved wicks have a large pore radius and a high permeability, as a result the pressure losses

are low but the pumping head is also low. Grooved wicks can transfer high heat loads in a

horizontal or gravity aided position, but cannot transfer large loads against gravity. The powder

metal wicks on the opposite end of the list have small pore radii and relatively low permeability.

Powder metal wicks are limited by pressure drops in the horizontal position but can transfer large

loads against gravity. It is recommended that the liquid fill should be at least 50% of the volume

of the evaporator [8]

.

The volume of liquid is given by:

Vl .001d le+ la+ lc 1

The fundamental drive mechanism that governs the operation of these devices emerges from the

difference in the capillary pressure across the liquid vapour interfaces in the evaporator and

condenser regions. Vaporization from the evaporator section of a heat pipe causes the meniscus

to recede into the wick and condensation in the condenser causes flooding. The point at which

the meniscus has minimum radius of curvature is referred to as the “dry” point and this usually

occurs in the evaporator at a point farthest from the condenser region. The “wet” point occurs at

that point where the vapour pressure and liquid pressure are approximately equal or where the

radius of curvature is at maximum. For a heat pipe to function properly, the net capillary pressure

difference between wet and dry points must be greater than the summation of all the pressure

losses occurring throughout the liquid and vapour flow paths. This relationship is referred to as

the capillary limitation [10]

which can be expressed as:

ΔPc m ≥ leff ∂pv

∂x dx +

∂Pl

∂xdx + ΔPh,e + ΔPh,c + ΔPll + ΔP+ (2)


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2.2 Sonic Velocity Limit

qs = 0.474λAv (ρvPv )0.5 (3)

The sonic limit heat capacity sq is always higher than the desired capacity in order to ensure that

sonic velocities will not be encountered during the heat pipe operation.

2.3 Entrainment Limit

qent = Avhfg σρg

2rhs

1/2 (4)

If entq is greater than the desired heat transport, the design operation is not hindered by

entrainment.

2.4 Boiling Limit

qb

= 2πLeff Keff Tv

λρvln

ri

rv

2σ

rn

− Δpcm (5)

Not operating within the boiling limit could result in nucleate boiling phenomenon which could

potentially cause the vapour bubbles to block the return of the working fluid back to the

evaporator.

2.5 Design Calculations

For calculations the [8,9]

volume of the fluid should always be greater than 0.6 ml. The total

evaporator volume is approximately one-third the volume of the heat pipe. The cross sectional

details are shown in figure 1.

Fig. 1 The cross sectional view of Grooved heat pipe


Issue1, Vol. 2


Vevap orator = πD2L/ 3 (6)

The heat pipe design parameters in the present experimental investigation are well within the

operational limitations of a conventional heat pipe. Provision is made for an allowance of 30%

for losses during filling and degasification of the working fluid. The total evaporator volume is

taken as 3.267ml and fill ratio of 60%.

3. Fabrication

A copper tube is selected with the OD of 8 mm, ID of 4 mm, and length of 150 mm. Boring

process is carried out. The grooving is done axially on the inner periphery of the pipe using wire

cutting process. A brass wire of 0.2mm square cross section is used for creating the grooves of

the required dimensions. This procedure is carried out with a CNC machine. Six Copper annular

fins were then copper brazed onto the pipe in the condenser region to ensure proper cooling of

the vapors. The fins were arranged at an angle of 60o

to the pipe around the condenser region of

the pipe. This ensures effective heat dissipation. The dimensions of the fins are: 50x15x1(mm3).

4. Description of the Experimental setup and the Experimental Procedure

A

B

CD

E

F

G

H

I

J

K

A Data recorderB Auto transformerC ThermocouplesD Grooved heat pipesE Cooling FanF ClinometerG Moisture trapH SS flex houseI Pressure gaugeJ Vaccum pumpK Pirani gauge

Fig. 2: The experimental test rig

The experimental setup as shown in figure 2 consists of a grooved wick assisted heat pipe, Data

acquisition system, vacuum pump, digital Pirani gauge and a power supply unit. The important

factors to be considered in the selection of heat pipe material are compatibility with working


Issue1, Vol. 2


fluid and external environment, thermal conductivity, fabrication process complexity, wet ability,

strength to weight ratio and porosity. Of the possible candidates like aluminium, copper, bronze

and stainless steel, copper is selected because of its advantages like high thermal conductivity

and better corrosion resistance. Thermocouples are used to measure the temperature variation

along the pipe length and are connected to the data acquisition system which stores and analyzes

the values. Nine thermocouples of J-type are connected along the length of the heat pipe with a

spacing of 12.5 mm between the junctions. J-type (Iron-Constantan, 1 mm diameter)

thermocouples used have a temperature range of (−40 to +750) °C and sensitivity of ~52 μV/°C,

suitable for the temperature range encountered and the sensitivity requirements. Three

thermocouples each are used for evaporator, adiabatic and condenser region respectively. A 15-

channel digital data acquisition system that can record experimental data on the computer is

used. This is essential in a project of this nature where several experiments were carried out, each

requiring over 100 temperature readings.

The DAQ records the simultaneous temperatures of the 9 thermocouples. It uses 15 Pro Scan

channel temperature scanners with a range of 0-200ºC, 230V. Additionally an RS 485 interface

facility and RS 232 to RS 435 conversion option is available. A block heater is used with a

power rating of 65W and resistance of 800Ω. In order to avoid the loss of heat, insulation is

provided surrounding the clamp. The evaporator and adiabatic section is insulated using glass

wool. A fan is installed at the condenser fins to achieve forced convection cooling. The vacuum

pump is used to maintain the required pressure within the heat pipe. An autotransformer and a

multi meter are provided to control and measure the electrical power input to the heater

respectively. The evaporator end of the pipe is sealed and the condenser end is connected to the

control valve to vary the charge volume. Working fluid is metered and charged through the fluid

inlet valve and sealed under required vacuum level. Another opening at the condenser end is

provided radially to connect the vacuum pump. An ED6 Hindvac vacuum pump with Nominal

Pumping rate of 6m3/hr and ultimate partial Pressure (on McLeod Gauge) of 5x10

-4mbar is used.

The Pump rotating speed at no load conditions is 1340-1440 rpm. Oil Capacity is 2 x 10-3

m3.

The vacuum level is measured by a digital Pirani gauge (Hindhivac Pirani Model HPS-2), which

has a pressure range of 0.001-1000mbars. The ball valve is made of Brass with a temperature

Range of (10 - 65)

0C. Ball Material type is stainless steel and Sealing Material is Poly-Tetra-

Fluro-Ethylene. When the vacuum pump is operated, there are chances that the working fluid

may get sucked into the pump. This may damage the working parts of the pump. Hence a

moisture trap is used with different levels of entry and exit points to prevent this from happening.


Issue1, Vol. 2


A tangential flow of the liquid is setup in the apparatus so that the heavy particles of the fluid are

trapped in the apparatus. It consists of a hollow cubical box of 300mm dimension. Flexible

tubing is provided between the vacuum pump, moisture trap and heat pipe. The connection

between the vacuum pump and the moisture trap is a stainless steel tube of diameter 37.5mm.

The connection between the moisture trap and the heat pipe is made using a Polyurethane tube.

Various plots are drawn to study the performance of the wick and to optimise the heat transfer

characteristics. The steady state temperature, overall heat transfer co-efficient, the thermal

resistances are calculated and the graphs are plotted. The performances of these parameters are

now compared to the working of a heat pipe without a wick.

5 Results and Discussions

The experiments were conducted at a constant working pressure of 500 mbar choosing three

different working fluids and for two different orientations and operating limits as detailed above.

The heat pipe without a working fluid represents heat transfer in a regular metallic conductor

termed as Dry run in the experiment. Its performance is considered as the base for the

evaluation of heat pipe (i.e. with working fluid in it).

Figure 3 represents variation of temperature difference between evaporator and condenser with

respect to heat input for (0 0) orientation.


Issue1, Vol. 2


Fig. 3: Effect of Heat Input on Temperature Difference between Evaporator and Condenser for 00 Orientation

There is a trend of decreasing temperature difference with increasing heat flux in case of acetone

and methanol but in case of distilled water the temperature difference increases steadily till 15

watts then stabilizes. For the dry run the maximum and minimum temperature difference are

100C and 6

0C respectively.

Figure 4 represents the variation of temperature difference between evaporator and condenser

with respect to heat input for (-900) orientation.

0

2

4

6

8

10

12

14

8 10.125 12.5 15.125 18

TE

MP

ER

AT

UR

E D

IFF

ER

NC

E (

0 C

)

HEAT INPUT (WATTS)

for 0 deg Orientation

distilled water acetone

methanol

dry run


Issue1, Vol. 2


Fig. 4: Effect of Heat Input on Temperature Difference between Evaporator and Condenser for -90 0 Orientation

The volatility of changes in temperature difference is not so pronounced in any of the working

fluids especially in the case of methanol it is irresponsive to the increasing heat flux in gravity-

assisted orientation. As modelled by faghri et al [3]

the expression for temperature difference.

Tw, o − Tv, e =1

2πRoLe

dQ

dz

Le

0

1

hhe

dz (7)

Between pipe Inner Wall and evaporator section is given by equation 16. Similar expression

exists for temperature difference between Condenser section and pipe inner wall. The difference

between Wall-Evaporator and Condenser-Wall heat transfer coefficients remains constant in case

of methanol. Hence the heat transfer is actually near isothermal. For the dry run the maximum

and minimum temperature difference are 120C and 10

0C.

In figure 5 representing enthalpy variation with heat flux for 0 0 orientation, it is observed that

there is positive rate of increase of enthalpy however the slope is maximum in cases of methanol

and minimum in case of acetone.

0

2

4

6

8

10

12

14

16

18

8 10.125 12.5 15.125 18

TE

MP

ER

AT

UR

E D

IFF

ER

NC

E (

0C

)

HEAT INPUT (W)

for -90 deg Orientation


methanol


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Fig. 5: Effect of Heat Input on Total Enthalpy for 00 Orientation

There is evidence in all the working fluids at both orientations that the temperatures at evaporator

and condenser slightly increase in the beginning as there is some sensible heat addition after

which there is a drop that is characteristic of the onset of evaporation and condensation processes

after which there is more or less isothermal operation. For the dry run the slope becomes steeper

after 12.5 watts and highest recorded enthalpy is 2400 W/m2 0

C.As seen in figure 6 representing

enthalpy variation with heat flux for -90 0 orientation the slope of increase is maximum in case of

distilled water and minimum in case methanol.

Fig. 6: Effect of Heat Input on Total Enthalpy for -90 0 Orientation

0

500

1000

1500

2000

2500

3000

3500

4000

8 10.125 12.5 15.125 18

EN

TH

AL

PY

h

HEAT INPUT (W)


distilled water

acetone

methanol

0

500

1000

1500

2000

2500

3000

8 10.125 12.5 15.125 18

EN

TH

AL

PY

h

HEAT INPUT (W)



methanol


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For dry run the slope is not as steep as in case of the horizontal orientation and achieves a

maximum enthalpy of 1050 W/m2 0

C.

In figure 7, representing the variation of thermal resistance with respect to heat flux for 0 0

orientation, the slope of decrease of thermal resistance is maximum in case of methanol and

minimum in case of distilled water.

Fig. 7: Effect of Heat Input on Thermal resistance for 00 Orientation

For dry run the thermal resistance curve is steeply declining and reaches a minimum of

0.350C/W. In gravity assisted orientation as seen in figure 8, methanol has maximum slope and

acetone forms the other end of the spectrum with minimum slope.

Fig. 8: Effect of Heat Input on Thermal resistance for -90 0 Orientation

0

0.2

0.4

0.6

0.8

1

1.2

1.4

8 10.125 12.5 15.125 18

TH

ER

MA

L R

ES

IST

AN

CE

HEAT INPUT (W)



methanol

0

0.5

1

1.5

2

2.5

8 10.125 12.5 15.125 18

TH

ER

MA

L R

ES

IST

AN

CE

HEAT INPUT (W)



methanol


Issue1, Vol. 2


This is seen owing to a wide difference in the thermo physical properties of the two working

fluids. For the dry run the maximum and minimum thermal resistances are 1.2 0C/W and 0.75

0C/W respectively.

6 Conclusions

The experiments have been conducted using three different working fluids as detailed above

in Section 5. The following conclusions were drawn.

1. The maximum heat transfer coefficient found in the horizontal orientation at 3550

W/m2 o

C at 60% fill ratio and 500 mbar pressure, 1700 W/m2 o

C at 60% fill ratio and 500

mbar vacuum pressure for vertical orientation respectively for methanol and acetone. Finally,

the maximum heat transfer coefficient of the grooved heat pipe for distilled water is found in

the vertical orientation at 2400 W/m2 o

C at 60% fill ratio and 500 mbar vacuum pressure.

These findings clearly indicate that the role of gravity is working fluid selective in influencing

performance.

2. Merit number determines the maximum possible heat transfer for a given heat pipe.

However, merit number alone cannot be the decisive parameter in rating the working fluid as it is

quite evident from the results that the merit number is high in case of distilled water compared to

acetone and methanol though methanol is seen to be the most effective working fluid. This

indicates the strong influence of geometry on the performance of heat pipe.

3. There is a significant increase in thermal resistance for the gravity assisted orientation

when compared with the horizontal position. The influence would only get compounded as the

heat input is increased to higher wattages and hence needs to be further investigated for high heat

flux conditions.

References

[1] Eastman, G.Y. The Heat Pipe. Scientific American, 1968, 218 (5), pp 38-46.

[2] Gaugler, R.S. Heat Transfer Device. US Patent, 2350348, Appl. 21st December, 1942,

Published 6 June, 1944.

[3] Faghri, Heat Pipe Science and Technology, Taylor and Francis, Washington, D.C., 1995.

[4] Zhang, J. and H. Wong "Evaporation, condensation, and flow in an idealized micro heat

pipe," Proceedings of ASME IMECE 2002 , paper No. 39196, (2002).


Issue1, Vol. 2


[5] Richter, Robert; Gottschlich, Joseph M. Thermodynamic aspects of heat pipe operation.

Journal of Thermo physics and Heat Transfer (ISSN 0887-8722), vol. 8, no. 2, p. 334-340

[6] Scott K. Thomas; R. M. Castle; Kirk L. Yerkes; The Effect of Working Fluid Inventory

on the Performance of Revolving Helically Grooved Heat Pipes, J. Heat Transfer --

February 2001 -- Volume 123, Issue 1, 120

[7] D. Khrustalev and A. Faghri ; Thermal Characteristics of Conventional and Flat Miniature

Axially Grooved Heat Pipes. J. Heat Transfer -- November 1995 -- Volume 117, Issue

4, 1048

[8] Sreenivasa, T.N, Sridhara, S. N and Pundarika.G, “Working Fluid Inventory in Miniature

Heat Pipe”, International Conference on Mechanical Engineering, BUET, Dhaka,

Bangladesh (Dec 28-30, 2005)..

[9] George P. Peterson An introduction to heat pipes: modeling, testing, and applications Wiley

series in Thermal Management of Microelectronic and Electronic systems.1994. John Wiley

& Sons.

http://adsabs.harvard.edu/cgi-bin/author_form?author=Richter,+R&fullauthor=Richter,%20Robert&charset=UTF-8&db_key=PHY

http://adsabs.harvard.edu/cgi-bin/author_form?author=Gottschlich,+J&fullauthor=Gottschlich,%20Joseph%20M.&charset=UTF-8&db_key=PHY

http://www.stormingmedia.us/authors/Thomas_Scott_K_.html

http://www.stormingmedia.us/authors/Castle_R__M_.html

http://www.stormingmedia.us/authors/Yerkes_Kirk_L_.html

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