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1. INTRODUCTION

A heat pipe is a device that efficiently transports thermal energy from its one

point to the other. It utilizes the latent heat of the vaporized working fluid instead of the

sensible heat. As a result, the effective thermal conductivity may be several orders of

magnitudes higher than that of the good solid conductors. A heat pipe consists of a sealed

container, a wick structure, a small amount of working fluid that is just sufficient to saturate

the wick and it is in equilibrium with its own vapour. The operating pressure inside the heat

pipe is the vapour pressure of its working fluid. The length of the heat pipe can be divided

into three parts viz. evaporator section, adiabatic section and condenser section. In a standard

heat pipe, the inside of the container is lined with a wicking material. Space for the vapor

travel is provided inside the container. A heat pipe or heat pin is a heat-transfer device that

combines the principles of both thermal conductivity and phase transition to efficiently

manage the transfer of heat between two solid interfaces. At the hot interface within a heat

pipe, which is typically at a very low pressure, a liquid in contact with a thermally conductive

solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels

along the heat pipe to the cold interface, condenses back into a liquid, releasing the latent

heat. The liquid then returns to the hot interface through either capillary action or gravity

action where it evaporates once more and repeats the cycle. In addition, the internal pressure

of the heat pipe can be set or adjusted to facilitate the phase change depending on the

demands of the working conditions of the thermally managed system.(1)

FIG 1: Heat Pipe(1)

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2. HISTORY OF HEAT PIPE

The general principle of heat pipes using gravity (commonly classified as two

phase thermosyphons) dates back to the steam age. The modern concept for a capillary driven

heat pipe was first suggested by R.S. Gaugler of General Motors in 1942 who patented the

idea.(2)

The benefits of employing capillary action were independently developed and first

demonstrated by George Grover at Los Alamos National Laboratory in 1963 and

subsequently published in the Journal of Applied Physics in 1964. Grover noted in his

notebook.

"Heat transfer via capillary movement of fluids. The "pumping" action of surface

tension forces may be sufficient to move liquids from a cold temperature zone to a high

temperature zone (with subsequent return in vapor form using as the driving force, the

difference in vapor pressure at the two temperatures) to be of interest in transferring heat

from the hot to the cold zone. Such a closed system, requiring no external pumps, may be of

particular interest in space reactors in moving heat from the reactor core to a radiating

system. In the absence of gravity, the forces must only be such as to overcome the capillary

and the drag of the returning vapor through its channels."

Between 1964 and 1966, RCA was the first corporation to undertake research and

development of heat pipes for commercial applications (though their work was mostly funded

by the US government). During the late 1960s NASA played a large role in heat pipe

development by funding a significant amount of research on their applications and reliability

in space flight following from Grover's suggestion. NASA‟s attraction to heat pipe cooling

systems was understandable given their low weight, high heat flux, and zero power draw.

Their primary interest however was based on the fact that the system wouldn‟t be adversely

affected by operating in a zero gravity environment. The first application of heat pipes in the

space program was in thermal equilibration of satellite transponders. As satellites orbit, one

side is exposed to the direct radiation of the sun while the opposite side is completely dark

and exposed to the deep cold of outer space. This causes severe discrepancies in the

temperature (and thus reliability and accuracy) of the transponders. The heat pipe cooling

system designed for this purpose managed the high heat fluxes and demonstrated flawless

operation with and without the influence of gravity. The developed cooling system was the

first description and usage of variable conductance heat pipes to actively regulate heat flow or

evaporator temperature.

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Starting in the 1980s Sony began incorporating heat pipes into the cooling schemes

for some of its commercial electronic products in place of both forced convection and passive

finned heat sinks. Initially they were used in tuners & amplifiers, soon spreading to other

high heat flux electronics applications. During the late 1990s increasingly hot microcomputer

CPUs spurred a threefold increase in the number of U.S. heat pipe patent applications. As

heat pipes transferred from a specialized industrial heat transfer component to a consumer

commodity most development and production moved from the U.S. to Asia. Modern CPU

heat pipes are typically made from copper and use water as the working fluid.(2)

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3. BASIC COMPONENTS OF A HEAT PIPE

The basic components of a heat pipe are(5)

1. The container

2. The working fluid

3. The wick or capillary structure

3.1 Container:

The function of the container is to isolate the working fluid from the outside

environment. It has to be there for leak proof, maintain the pressure differential across the

walls, and enable transfer of thermal energy to take place from and into the working fluid.

The prime requirements are:(5)

1. Compatibility (Both with working fluid and External environment)

2. Porosity

3. Wettability

4. Ease of fabrication including welding, machinability and ductility

5. Thermal conductivity

6. Strength to weight ratio

3.2 Working fluid:

The first consideration in the identification of the working fluid is the operating vapor

temperature range. Within the approximate temperature band, several possible working fluids

may exist and a variety of characteristics must be examined in order to determine the most

acceptable of these fluids for the application considered.

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The prime requirements are: (5)

1. Compatibility with wick and wall materials

2. Good thermal stability

3. Wettability of wick and wall materials

4. High latent heat

5. High thermal conductivity

6. Low liquid and vapour viscosities

Examples of Working fluids

TABLE 1: Working Fluids and their properties.(6)

MEDIUM MELTING POINT(0C)

BOILING POINT AT ATM PRESSURE(0C)

USEFUL RANGE(0C)

Helium -271 -261 -271 to 269

Ammonium -78 -33 -60 to 100 Water 0 100 30 to 200 Silver `960 2212 1800 to 2300

Choosing the Working Fluid

Chi (1976) developed a parameter of gauging the effectiveness of a working fluid called

the liquid transport factor:(11)

Where

Latent heat of vaporization and

Surface tension.

l subscript means for liquid

3.3 Wick:

The wick structure in a heat pipe facilitates liquid return from the evaporator from

the condenser. The main purposes of wick are to generate the capillary pressure, and to

distribute the liquid around the evaporator section of heat pipe. The commonly used wick

structure is a wrapped screen wick

l

llN

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FIG 2: The Wicking Structure(( Created by carving out grooves on the interior core of

the Heat Pipe)(11)

There are various types of wicks used in Heat Pipe, some of them are:

1. Screen Mesh Wick

2. Sintered Powder Wick

FIG 3: Screen Mesh Wick(Utilizes multiple wire layer to create a porous wick)(11)

The wicking structure

.

Screen Mesh Wick

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FIG 4: Sintered Powder Wick(Utilizes densely packed metal spheres)(11)

Purpose of the Wick:-

• Transports working fluid from the Condenser to the Evaporator.

• Provides liquid flow even against gravity.

How the Wick Works:-

• Liquid flows in a wick due to capillary action.

• Intermolecular forces between the wick and the fluid are stronger than the forces

within the fluid.

A resultant increase in surface tension occurs.

Sintered Powder Wick

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4. TYPES OF HEAT PIPES

4.1 Thermosyphon-

Gravity assisted wickless heat pipe. Gravity is used to force the condensate back into

the evaporator. Therefore, condenser must be above the evaporator in a gravity field.

4.2 Leading edge-

Placed in the leading edge of hypersonic vehicles to cool high heat fluxes near the

wing leading edge.

4.3 Rotating and revolving-

Condensate returned to the evaporator through centrifugal force. No capillary wicks

required. Used to cool turbine components and armatures for electric motors.

4.4 Cryogenic-

Low temperature heat pipe. Used to cool optical instruments in Space.(2)

4.5 Flat Plate-

Much like traditional cylindrical heat pipes but are rectangular. Used to cool and

flatten temperatures of semiconductor or transistor packages assembled in arrays on

the top of the heat pipe.

FIG 5: Flat Plate Heat Pipe(3)

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4.6 Micro heat pipes-

Small heat pipes that are noncircular and use angled corners as liquid arteries.

Characterized by the equation(2)

(rC /rh ) 1

Where rc is the capillary radius, and rh is the hydraulic radius of the flow channel. Employed

in cooling semiconductors (improve thermal control), laser diodes, photovoltaic cells,

medical devices.

4.7 Variable conductance-

Allows variable heat fluxes into the evaporator while evaporator temperature

remains constant by pushing a non- condensable gas into the condenser when heat fluxes are

low and moving the gas out of the condenser when heat fluxes are high, thereby, increasing

condenser surface area. They come in various forms like excess-liquid or gas-loaded form.

Used in electronics cooling.(4)

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5. OPERATING PRINCIPLE

Figure shows the working principle of a heat pipe. Thermal input at the evaporator

region vaporizes the working fluid and this vapour travels to the condenser section through

the inner core of heat pipe. At the condenser region, the vapor of the working fluid condenses

and the latent heat is rejected via condensation. The condensate returns to the evaporator by

means of capillary action in the wick.

FIG 6: Operating Principle of Heat Pipe

As previously mentioned there is liquid vapour equilibrium inside the heat pipe.

When thermal energy is supplied to the evaporator, this equilibrium breaks down as the

working fluid evaporates. The generated vapour is at a higher pressure than the section

through the vapor space provided. Vapor condenses giving away its latent heat of

vapourization to the heat sink. The capillary pressure created in the menisci of the wick,

pumps the condensed fluid back to the evaporator section. The cycle repeats and the thermal

energy is continuously transported from the evaporator to condenser in the form of latent heat

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of vaporization. When the thermal energy is applied to the evaporator, the liquid recedes into

the pores of the wick and thus the menisci at the liquid-vapour interface are highly curved.

This phenomenon is shown in figure. At the condenser end, the menisci at the liquid-vapor

interface are nearly flat during the condensation due to the difference in the curvature of

menisci driving force that circulates the fluid against the liquid and vapor pressure losses and

body forces such as gravity.

Heat pipes contain no mechanical moving parts and typically require no maintenance,

though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of

the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's

effectiveness at transferring heat. This is significant when the working fluid's vapour pressure

is low.(5)

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6. THERMODYNAMIC CYCLE

• 1-2 Heat applied to evaporator through external sources vapourises working fluid to a

saturated (2‟) or superheated (2) vapour.

• 2-3 Vapour pressure drives vapour through adiabatic section to condenser.

• 3-4 Vapour condenses, releasing heat to a heat sink.

• 4-1 Capillary pressure created by menisci in wick pumps condensed fluid into

evaporator section.

• Process starts over.(11)

Ideal Thermodynamic Cycle:-

FIG 7: Ideal Thermodynamic Cycle(11)

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7. EXPERIMENTAL PROCEDURE

A copper tube of suitable length is cleaned thoroughly with suitable cleaning

agents. Screen mesh acts as a wick is wound around a coil in layers and inserted into the

copper tube intact. It is then closed by end caps at both ends. Thermocouples are equally

spaced at various positions of the heat pipe. The mica sheet is wound over the evaporator

region of the heat pipe since mica is a good electrical insulator and a thermal conductor. A

heating coil is wound over the mica sheet in a uniformly spaced manner. The two end of the

heating coil are connected to the electric power input. A few centimeter thick cover of glass

wool is provided over the entire region of the heat pipe over the glass wool covering, the heat

pipe is covered with thick PUF insulation which is normally provided in automobiles.

The heat pipe is evacuated to a pressure of -1.36atm for about 2hours using a

vacuum pump. The heat pipe is tested for holding the vacuum for about twelve hours. After

vacuum test,R-12working fluid is filled in the heat pipe for specified pressure which can be

indicated by the pressure gauge. The coolant water supply is provided to the heat pipe and

can be controlled by a valve. The thermocouples on the heat pipe are connected to the

temperature scanner. A voltmeter is connected in parallel to the dimmerstat. Dimmerstat is

supplied with AC current. The temperature scanner is connected to an electric power inlet

through a voltage stabilizer. The ambient pressure and ambient temperature are noted. The

heat pipe evaporator region heating coil is connected to the electric power inlet. Coolant

water is supplied to the condenser coolant chamber. The dimmerstat initially is at no-load

condition. The load on the dimmerstat is varied very slowly till the required power is

obtained.(5)

Power can be calculated using the equation P=VI cosФ

Where cosФ is the power factor, (0.8 for A.C supply).

HEAT PIPE TEST

The copper tube heat pipe of 25.4 mm inner diameter and thickness employs a five

layered 100x100 brass screen mesh wick. The length of the evaporator, adiabatic and

condenser sections are 100, 50 and 150 mm respectively. The temperature of the heat pipe are

measured using a copper-constantan T-type thermocouples arranged at ten positions equally

spaced along a line on the periphery of the heat pipe. Additionally, two thermocouples are

provided to measure the temperature of coolant inlet and outlet temperatures. The evaporator

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region is heated by an electric heating coil wound over a mica sheet. The condenser region is

cooled using coolant flowing through condenser coolant chamber. Electric power input is

varied by using dimmer stat. The thermocouples are connected to the 16-channel temperature

scanner.(5)

EXPERIMENT

The experimental heat pipe is initially at room temperature. The coolant water enters

the condenser cooling chamber at this temperature and coolant is allowed to flow at a

particular flow rate. The initial pressure reading is to be noted from the pressure gauge

connected at the evaporator end. The ambient thermocouple temperatures are noted using

thermocouples. Initially, the dimmer stat is to be kept at no-load condition. The load on the

dimmer stat is slowly varied till it reaches the required value. The electric power is supplied

to the electric heating coil which is wound over the evaporator section. The temperature at

each position on the heat pipe can be measured by using the thermocouples connected at

equal intervals. The initial temperature readings are taken in steps of 2 minutes and in later

stages the time interval increases to 5 minutes. After 30-35 minutes the system will reach the

steady state conditions.(5)

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8. STUDY OF VARIOUS PARAMETERS

8.1 EFFECT OF POWER

The first experiment is to find out how the temperature profile varies with respect

to the variation provided to the electric power supply given the electric heating coil in the

evaporator region. The temperature profile with electric power supply of 25 W is plotted for a

time period of nearly one hour till it reaches steady state condition. Then the electric power is

varied to 50 W and 80 W to plot corresponding temperature profiles. The variation of

temperature profile is then compared.(5)

8.2 EFFECT OF PRESSURE

The pressure inside the heat pipe plays an important role in the temperature profile

plotting. The pressure is varied by controlling the quantity of the working fluid supplied to

heat pipe. The plotting of temperature profile is done on different pressure values. The

combined effect of pressure inside the heat pipe and the power supplied to electric coil at the

evaporator can also be obtained by varying both parameters.(5)

8.3 EFFECT OF COOLANT SUPPLY

The coolant supplied for circulation over the condenser coolant chamber is water. The

coolant flow rate is the ratio of coolant volume circulating in unit time. The variation in

temperature profile is analyzed at various coolant flow rates.

8.4 EFFECT OF WORKING FLUID

The effects of various working fluids like R-12,R-22 and R-134a are analyzed in the

experiment. These are commonly used refrigerants.(5)

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9. HEAT PIPE APPLICATIONS

Heat pipes are extensively used in many modern computer systems, where increased

power requirements and subsequent increases in heat emission have resulted in greater

demands on cooling systems.

9.1 Electronics cooling-

Small high performance components cause high heat fluxes and high heat dissipation

demands. Used to cool transistors and high density semiconductors Heat pipes are

typically used to move heat away from components such as CPUs and GPUs to heat sinks

where thermal energy may be dissipated into the environment.

.

A laptop heat pipe system cross section view

FIG 8: laptop heat pipe system(9)

9.2 Aerospace-

Cool satellite solar array, as well as shuttle leading edge during reentry. For

nuclear power cells for space craft. Grover and his colleagues were working on cooling

systems for nuclear power cells for space craft, where extreme thermal conditions are

found. Heat pipes have since been used extensively in spacecraft as a means for

managing internal temperature conditions.(7)

9.3 Heat exchangers-

Power industries use heat pipe heat exchangers as air heaters on boilers.

9.4 Solar Thermal-

Heat pipes are also being widely used in solar thermal water heating

applications in combination with evacuated tube solar collector arrays. In these

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applications, distilled water is commonly used as the heat transfer fluid inside a sealed

length of copper tubing that is located within an evacuated glass tube and oriented

towards the sun.

9.5 Permafrost cooling-

Building on permafrost is difficult because heat from the structure can thaw

the permafrost. To avoid the risk of destabilization, heat pipes are used in some cases.

For example, on the Trans-Alaska Pipeline System residual ground heat remaining in

the oil, as well as that produced by friction and turbulence in the moving oil could

conduct down the pipe's support legs and melt the permafrost on which the supports

are anchored. This would cause the pipeline to sink and possibly sustain damage. To

prevent this each vertical support member has been mounted with 4 vertical heat

pipes.(2)

FIG 9: Alaska pipeline support legs cooled by heat pipes to keep permafrost frozen.(9)

Heat pipes are also used to dissipate heat alongside parts of the Qinghai–

Tibet Railway. The embankment and track absorb the Sun's heat. Vertical heat pipes

either side of the formation prevent that heat spreading any further into the surrounding

ground.

9.6 Cooking-

The first commercial heat pipe product was the "Thermal Magic

Cooking Pin", developed by Energy Conversion Systems, Inc., and first sold in

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1966.The cooking pins used water as the working fluid. The container was stainless

steel, with an inner copper layer for compatibility. During operation, the heat pipe is

poked through the roast. One end of the pipe extends into the oven where it draws heat

to the middle of the roast. The high effective conductivity of the heat pipe cut the co

Nuclear

9.7 Reactor Cooling-

Since the early 1990s, numerous nuclear reactor power systems have been

proposed using heat pipes for transporting heat between the reactor core and power

conversion system.

The first nuclear reactor to produce electricity using heat pipes,

Demonstration Using Flattop Fission was first operated September 13, 2012.(8)

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10. MAIN HEAT TRANSFER LIMITATIONS

Heat pipes must be tuned to particular cooling conditions. The choice of pipe

material, size and coolant all have an effect on the optimal temperatures in which heat pipes

work. When heated above a certain temperature, all of the working fluid in the heat pipe will

vaporize and the condensation process will cease to occur; in such conditions, the heat pipe's

thermal conductivity is effectively reduced to the heat conduction properties of its solid metal

casing alone. As most heat pipes are constructed of copper (a metal with high heat

conductivity), an overheated heat pipe will generally continue to conduct heat at around 1/80

of the original conductivity. In addition, below a certain temperature, the working fluid will

not undergo phase change, and the thermal conductivity will be reduced to that of the solid

metal casing. One of the key criteria for the selection of a working fluid is the desired

operational temperature range of the application. The lower temperature limit typically occurs

a few degrees above the freezing point of the working fluid. Most manufacturers cannot make

a traditional heat pipe smaller than 3mm in diameter due to material limitations (though

1.6mm thin sheets can be fabricated). Experiments have been conducted with micro heat

pipes, which use piping with sharp edges, such as triangular or rhombus-like tubing. In these

cases, the sharp edges transfer the fluid through capillary action, and no wick is necessary.(9)

10.1 Capillary limit-

It occurs when the capillary pressure is too low to provide enough liquid

to the evaporator from the condenser. Leads to dry out in the evaporator. Dry out

prevents the thermodynamic cycle from continuing and the heat pipe no longer

functions properly.

10.2 Boiling Limit-

It occurs when the radial heat flux into the heat pipe causes the liquid in

the wick to boil and evaporate causing dry out.

10.3 Entrainment Limit-

At high vapour velocities, droplets of liquid in the wick are torn from the

wick and sent into the vapour. Results in dry out.

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10.6 Sonic limit-

It occurs when the vapour velocity reaches sonic speed at the evaporator

and any increase in pressure difference will not speed up the flow; like choked flow in

converging-diverging nozzle. Usually occurs during startup of heat pipe.

10.5 Viscous Limit-

At low temperatures the vapour pressure difference between the

condenser and the evaporator may not be enough to overcome viscous forces. The

vapour from the evaporator doesn‟t move to the condenser and the thermodynamic

cycle doesn‟t occur.

FIG 10: Usage limitations of heat pipes(9)

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11. DESIGNING OF A HEAT PIPE

11.1 STAGES IN THE DESIGN

(i) Select wick and Wall materials

(ii) Select working fluids

Criteria - limitations

Pressure, Priming, Handling, Purity etc.

(iii) Examine wick types:

Homogeneous rejected Arterial selected

(iv) Determine artery sizes

(v) Examine radial resistance to heat flow

(vi) Examine overall pressure balance of proposed design

(vii) Select final configuration

11.2 CONTAINER DESIGN

• Things that should be considered for container design:

– Operating temperature range of the heat pipe.

– Internal operating pressure and container structural integrity.

– Evaporator and condenser size and shape.

– Possibility of external corrosion.

– Prevent leaks.

– Compatibility with wick and working fluid.

• Stresses:

– Since the heat pipe is like a pressure vessel it must satisfy ASME pressure

vessel codes.

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Typical materials

The container materials are selected according to the working fluids used

– Aluminum: used when working fluid are ammonia,acetone,freon11,freon21,

heptanes etc.

– Stainless steel : used when working fluid are water, ammonia, methanol,

acetone, dowtherm etc.

– Copper : used when working fluid are water, methanol, acetone etc.

– Tungstone: used when working fluid are lead and silver are used

– Refractory materials or linings : High temperature heat pipes may use to

prevent corrosion.

11.3 SAMPLE DESIGN

Problem : A heat pipe is required which will be capable of transferring a minimum of

15 W at vapor temperatures between 0 and 80 0C over a distance of 1 m in zero

gravity (a satellite application) . Restraints on the design are such that the evaporator

and condenser sections are each 8 cm long , located at each end of the heat pipe , and

the maximum permissible temperature drop between the outside wall of the evaporator

and the outside wall of the condenser is 60 C. Because of Height and volume

limitation, the cross - sectional area of the vapour space should not exceed 0 . 197 cm2 .

SELECTION OF MATERIAL

• The selection of wick and wall material is based on various criteria.

• The heat pipe must also with stand bonding temperatures.

• In this problem mass being an important parameter.

• So Aluminum alloy (HT30) is chosen for the wall and Stainless Steel for the Wick.

WORKING FLUID

Working fluid compatible with the wall and wick materials, based on available data,

includes(6)

:

• Freon 11

• Freon 113

• Acetone

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TABLE 2: Generalized Results of Experimental Compatibility tests(3)

WORKING FLUID COMPATIBLE MATERIAL INCOMPATIBLE

MATERIAL

WATER Stainless Steel, Copper, Silica,

Nickel, Titanium

Aluminum, Inconel

AMMONIA Aluminum, Stainless Steel, Cold

Rolled Steel, Iron, Nickel

Copper

METHANOL Stainless Steel, Iron, Copper,

Brass, Silica

Aluminum

ACETONE Aluminum, Stainless Steel,

Copper, Brass, Silica

--------------------

FREON -11 Aluminum --------------------

FREON – 21 Aluminum, Iron --------------------

FREON -113 Aluminum --------------------

HEPTANE Aluminum --------------------

DOWTHERM Stainless Steel, Copper, Silica --------------------

LITHIUM Tungsten, Tantalum,

Molybdenum, Niobium

Stainless Steel,

Nickel, Inconel,

Titanium

SODIUM Stainless Steel, Inconel, Nickel,

Niobium

Titanium

CESIUM Titanium, Niobium ---------------------

MERCURY Stainless Steel Molybdenum,

Nickel, Tantalum,

Inconel

LEAD Tungsten, Tantalum Stainless Steel,

Nickel

SILVER Tungsten, Tantalum Rhenium

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11.4 CONCLUSION ON SELECTION OF WORKING FLUID

The materials chosen depend on the temperature conditions in which the heat pipe

must operate, with coolants ranging from liquid helium for extremely low temperature

applications (2–4 K) to mercury (523–923 K) & sodium (873–1473 K) and even indium

(2000–3000 K) for extremely high temperatures. The vast majority of heat pipes for low

temperature applications use some combination of ammonia (213–373 K), alcohol (methanol

(283–403 K) or ethanol (273–403 K)) or water (303–473 K) as working fluid. Water, for

instance, at low pressure will boil at just above 273 K (0 degrees Celsius) and so can start to

effectively transfer latent heat at this low temperature.

After the various examinations like Sonic Limit, Entrainment Limit, Wicking Limit,

Priming of the Wick, Acetone is selected.(6)

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12. ADVANTAGES

The advantage of heat pipes over many other heat-dissipation mechanisms is their

great efficiency in transferring heat. They are fundamentally better at heat conduction over a

distance than an equivalent cross-section of solid copper (a heat sink alone, though simpler in

design and construction, does not take advantage of the principle of matter phase transition).

Some heat pipes have demonstrated a heat flux of more than 230 MW/m².

Active control of heat flux can be affected by adding a variable volume liquid

reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir

of inert immiscible gas attached to the condensing section. Varying the gas reservoir pressure

changes the volume of gas charged to the condenser which in turn limits the area available for

vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be

accommodated with a single design. A modified heat pipe with a reservoir having no

capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal

diode. This heat pipe will transfer heat in one direction, acting as an insulator in the other.(1)

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13. CONCLUSION

Heat pipes are equipment that effectively transfers heat from source to sink. Heat

pipes are preferred over other heat transfer equipment since the effective thermal conductivity

of heat pipes are several orders higher than that of good solid conductors because of the use

of vaporized working fluid. Its basic components are a container, working fluid and a wick.

Various types of heat pipes are there based on its function. Various parameters influencing

the working of a heat pipe are studied. The design parameters of heat pipe are studied. The

applications and limitations of heat pipes were noted.(1)

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REFERENCES

1. Shaikh Nushad, HEAT PIPES, 2012,Excel journal of engineering technology and

management”,Vol 3, No 1

2. NASA-CR-25C8, HEAT PIPES, Midwest Research Institute, Washington D C,

January 1975

3. J P Hollman, HEAT TRANSFER Seventh Edition(1992) „The Heat Pipes’ (p.640-

645)

4. Gaugler, Richard (1944). “Heat Transfer Devices”. Dayton, Ohio: U.S. Patent

Office(2350348).

5. Chi, S.W, “Heat Pipe Theory And Practice-A Source Book”, Hemisphere publishing

Company

6. P. K. Nag, 1996, “Heat And Mass Transfer”. Eighth Edition. T. M. H(p.654-665)

7. Jim Danneskiold, Los Alamos-developed heat pipes ease space flight. Los Alamos

News Release, April 26, 2000.

8. Mani Annamali and Somasundaram Dhanabal, Experimental studies on porous wick

plate heat pipe 2010) IIT, Madras

9. Fabian Korn, 2010, “Heat pipes And Its Applications”. Lund, Sweden,

10. L. Boukhris, A. Boudjemai, A. Roubache and M.Bensadda, 2011, “Satellite thermal

control: cooling by diphasic loop”, World academy of science, engineering and

technology,59

11. Rice, Graham, HEAT PIPE (www.thermopedia.com) downloaded on 8-10-2013