heat pipe ppt

8/11/2019 heat pipe ppt

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1.INTRODUCTION

In this paper, we discuss about the recent technologies about the heat pipeused in heat exchanger such as conventional, loop type etc. Now a day sorption

technology is very useful in working of sorption machinery as well as in air

conditioning cooling etc. On the other hand, heat pipe thermal control is a keyelement of heat pump, refrigeration, heat transformer etc.

Sorption machines have advantages such as short time cycle, improvedcompactness of cascading machines, increased coefficient of performance etc. In

some cases there is a possibility to combine the application of different energy

sources ( solar/ gas, solar/electricity) in the same prototype of heat pump due to heat

pipe application as heat exchangers. Actually an investigation of hydrocarbonsboiling heat transfer related to applications of heat pipes and thermosyphons as

thermal control systems in refrigeration technology, liquid hydrocarbonsgasification, electronic components, fuel cells, etc. is important.

Over a long time, compact heat pumps for miniature heat pumps andrefrigerators, Gas and cold storage systems with dimensions in order of cms are

preferred due to Small size, low weight and design flexibility. In many branches ofindustry, low temperature heads and small heat fluxes characterize operating

conditions of heat pipe heat exchangers. This is also relevant to installations forhydrocarbon gasification.

In space heating or cooling in vehicles or building, microprocessor cooling,miniature heat exchanger are very possible to use. Sorption technologies are

convenient for adsorption refrigeration and ice making systems. The fishing industryin tropical and developing countries is often an important part of the food and income

supply. A large proportion of fishing in these countries is operated by small-scalefishing vessels, mostly open boats with no refrigeration on board and fish is often

not iced onboard. It is widely and publicly recognized that post-harvesting losses are2030% in these countries due to improper handling of the fish mainly because there

is no on-board chilling. This results in a loss of food supply, loss of export revenues

and improper utilization of a limited natural resources.


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2.CONVENTIONAL HEAT PIPES

Conventional type heat pipe are those which having sintered metal powderinside those saturated with liquid . A very important feature of the HP is the ability

to transport a large amount of energy over its length with a small temperature drop

by means of liquid evaporation at the HP evaporator (heat source), vapourcondensation at the condenser (heat sink) and liquid movement in the opposite

direction inside a wick by capillary force. Essential is the possibility to change thedirection of heat flow along the HP in time and to use HPs for cooling and heating

alternately.

2.2 Miniature and microheat pipes

One of the directions of heat pipes development is miniature heat pipes

(mHP), both for passive systems for electronics cooling and for use in refrigeratingmachines. Optimization of the new copper sintered powder wick in miniature heat

pipes with outer diameter 4 mm and length of 200 mm was carried out. Themaximum heat transfer rate for these HPs is almost 50 W [14].Heat pipe family

qualified geometry is: circular tube diameter 425 mm, flat heat pipe thickness 220 mm, length 0.10.8 m, wall thickness 0.21.0 mm. Pipe materialcopper

99.95% purity, wickcopper sintered powder, wire mesh and wire bundle withthickness 0.20.8 mm.


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The second type of heat pipe is being under process by various scientific

organization including NASA, called microloop heat pipe.The main application will be electronic cooling at the chip level.

The heat pipe evaporator is constructed of silicon, such that there will be little

thermal interface resistance between the source of the heat generation, the computerchip junction, and the working fluid. The ultimate heat sink could be a box-level

spacecraft thermal bus or even the spacecraft radiator, depending on transport lengthand compensation chamber size. The device utilizes a coherent porous silicon (CPS)

wick that provides small effective pore radii. This new technology is a type of

microelectro mechanical systems (MEMS) process that allows one to drill a

pattern of micron-sized holes in a silicon wafer

3.Unconventional heat pipe in heat exchanger

3.1 Vapor-dynamic thermosyphons

Vapor-dynamic thermosyphons can provide the coupling between topping

and bottoming sorption cycles (Fig. 2). The direct coupling ensures the operating

temperatures in both cycles more favourable from the thermodynamic point of viewsince temperature drops are definitely smaller compared with conventional heatexchangers. Such thermosyphons (Fig. 4) have a low thermal resistance, their length

can reach some meters and they have the ability to transport energy to sorbent mediabeing heated by hot gases, or flames. In vapour-dynamic thermosyphons vapour and

liquid flows are separated by the wall, heat transfer is realized in the gap betweenthe inner and outer tubes.

Vapour condensation is performed on the inner surface of the outer condenser tube.The vapour-dynamic thermosyphons in order to avoid the flooding limit and increase

the maximum performance have tube separator inside used as vapour channel and a

two-phase coaxial annular channel around this separator where the vapourcondensation is produced with high efficiency . Vapour-dynamic thermosyphons can

transport up to 10 kW and more for several meters distance with its thermal

resistance R 0:030:05 K/W of heat, which is difficult to acheive in conventional

thermosyphons located horizontally.


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Fig. 2. Schematic diagram of the vapour-dynamic thermosyphon with two condensers and two valves for sorption heat pump. (1)Adsorber, (2) valve, (3) liquid pipe, (4) vapour channel, (5) electric heaters, and (6) mini-boiler.

The advantages of this thermosyphon are:

high heat transfer performance due to the vapour and liquid flow separation, there

are no interface friction losses;

low thermal resistance of thermosyphon;

vapour flow in the co-axial gap push the non-condensable gases to the gas trap,

that means the thermosyphon is eager to work with non-condensable gases inside

(Fig. 3);

Fig. 3. Thermal resistance R of vapour-dynamic thermosyphon as a function of heat input Q. (1) Water; (2) HCFC 22; (3)

water with air; and (4) HCFC 22 with an air in the gas trap.


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3.2 Loop Heat Pipes

Capillary pumped loops (CPL), and loop heat pipes (LHP), Fig. 4 are an attractivealternative for heat regulation [30]. In the LHP the capillary pumped evaporator

(Figs. 5) is used instead of a boiler. Such an evaporator is more flexible from thepoint of view of its orientation space and is more compact. In the LHP there is a

possibility to use an evaporator above the condenser. In the LHP the vapour flowsthrough the vapour channels towards the condenser and the liquid goes back the

evaporator due to the capillary pressure head of the porous wick. In the near future

an LHP should be used as thermal control devices in scientific andtelecommunication

Fig. 4. Loop heat pipe with two evaporator/condensers, liquid and vapor lines.

Fig. 5. Evaporator/condenser of a loop heat pipe.


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Specifications

The LHP sintered wick structure of the evaporator has a porosity of 45%: thelength of the unit is 280 mm, the outer diameter38 mm, evaporator body materialstainless steel (SS), maximum diameter of pores10 lm, medium diameter ofpores 35 lm, capillary pressure head 0.4 bar and the porous wick thickness8 mm.

Evaporators are compatible with water, ammonia, methanol, ethanol, acetone andmethane. The maximum heat flow rate of the evaporator is 1500 W, the thermal

resistance of the evaporator R, 0.06 K/W.

Fig. 6. Relation between the vapour temperature Tv and Q for different heat transfer direction and the HP inclination angle u 90

(evaporator above the condenser); Tcool 10; (1,2) different directions of heat transfer.


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3.3 Heat pipe panels

Another alternative to the conventional heat pipe is an aluminium(multi-channel) heat pipe panel (Fig. 7) with propane as a working fluid

to cool the low temperature sorbers of heat pumps and refrigerator.

Fig. 7. Aluminium multi-channels pulsating heat pipe panel with propane as a working fluid and silicagel monolithic sorption bed on its finned surface

The main parameters of flat heat pipe panels are: HP width70 mm, HP

height7 mm, HP length700 mm, evaporator length98 mm,

condenser length500 mm, mass0, 43 kg. HP thermal resistance R

0:05 K/W, evaporator heat transfer coefficient a 8500 W/m2

K,condenser heat transfer coefficient a 2500 W/m2 K. Heat pipe panels are

convenient as thermal control systems for the electronic components, heat

pumps and refrigerators with efficient heat recovery between different

sorption cycles. The working fluid (hydrocarbons) dynamic movement is

stable with liquid filling ratio near 0.6 of the heat pipe volume. The

propane as a good alternative to water for such heat pipes enables a

continuous motion due to the interplay between the driving and restoring

forces.


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3.4 Pulsating heat pipes

Pulsating heat pipes (PHPs) have also emerged as interesting alternatives toconventional heat pipes. PHPs have complicated thermohydrodynamic operational

characteristics. In fact, it is rare to find a combination of such events and mechanismslike bubble nucleation and collapse, bubble agglomeration and pumping action, flow

regime changes, pressure/temperature perturbations, dynamic instabilities,metastable non-equilibrium conditions, flooding or bridging etc., all together

contributing towards the thermal performance of a device. Recent literature suggests

that important milestones have been achieved in characterization of these devices.

The pulse thermal loop (PTL) is one of several oscillatory thermal transport cycles

under development that are receiving attention as a potential semi/passive, high-

power, high flux heat transport device. The PTL is unique in that it is capable ofgenerating driving pressures in excess of many mechanically pumped loops.

Fig. 8. Two configurations of pulsating heat pipe: (i) open loop an (ii) closed loop.


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3.5Spaghetti heat pipes

The small diameter (3 mm)bendable SS spaghetti heat pipes are similar to

pulsating heat pipes, but have a compact condenser and large surface evaporator. Anexample of a spaghetti heat pipe filled with ammonia, shown in Figs. 9, is

disposed inside the refrigerator chamber in such a way that food can be kept within

the refrigerating temperature range as uniformly as possible.

The spaghetti heat pipe is thermally linked with an evaporator of the sorptionrefrigerator (heat pipe condenser) and has a good thermal contact with this

evaporator.

Fig. 9. Schematic of the spaghetti heat pipe panel: (1) condenser of the heat pipe, (2) evaporator of the adsorption refrigerator,(3) porous structure, and (4) heat pipe evaporator.

Specifications

vapor plugs (bubbles) push the liquid plugs to the cold part of the unit, where vapor

bubbles collapsed with the increasing of pressure difference between vapor and


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liquid. Heat pipe thermal resistance is R 0:006 K/W. Heat pipe length is 1 m, and

heat pipe width is 0.5 m.

3.6 Sorption heat pipes

The sorption heat pipe (SHP) is a novelty and combines the enhanced heat

and mass transfer in conventional heat pipes with sorption phenomena of a sorbent

bed. Sorption heat pipe could be used as a sorption heat transfer element and becooled and heated as a heat pipe. The sorption heat pipe (Fig. 9) has a sorbent bed(adsorber/desorber and evaporator) at one end and a condenser and evaporator at the

other end. The basic principle of the sorption heat pipe operation is:

Fig. 10. Sorption heat pipe. Longitudinal cross section, [3]. (1) Vapour channel; (2) porous sorption structure; (3) finned surface ofheat pipe evaporator/condenser; (4) porous wick inside heat pipe; (5) porous valve; (6) heat pipe low temperature evaporator with

porous wick; (7) working fluid accumulated inside the evaporator; and (8) cold box with thermal insulation


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Phase 1. At the beginning of the heat pipe functioning it is necessary to desorb a

sorption structure (Fig. 9) of the heat pipe due to absorption of the heat of a heat

source. During desorption of a sorbent bed the working fluid vapor needs leave aporous structure and be condensed in the heat pipe evaporator/condenser. The vapor

is generated inside the porous structure of a sorbent bed, the vapor pressure isincreasing, and the vapor flow enters the condenser and is condensed, releasing heat

to the surroundings. Part of the cold working fluid is filtered through the porousvalve and enters the evaporator due to the pressure drop between the hot part of a

heat pipe and the evaporator. The other part of the working fluid is returned to the

sorbent bed due to capillary forces of the wick and increases the procedure of sorbentbed heating by the heater, following the microheat pipes phenomena inside the

sorbent bed. When desorption of the sorbent structure is accomplished, the source

of energy is switched off, the pressure in the sorbent bed decreased and the workingfluid is accumulated inside the evaporator.

Phase 2. After Phase 1 the porous valve is opened and the vapor pressure inside theheat pipe is equalises following the procedure of the liquid evaporation inside the

porous structure of the evaporator. During the liquid evaporation in the evaporator,the cold generation is available inside the cold box. When the liquid evaporation isaccomplished and the sorbent bed is saturated with the vapor, a porous valve is

closed and the sorbent bed begins to be cooled with the help of the heat pipecondenser.


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4. APPLICATION

4.1 Heat pipe in thermoelectric cooler

It is necessary to note that heat pipes can be applied with success in other types

of refrigerating machines, for example in thermoelectric refrigerators. Properties

of heat pipes here are used to transfer and to transform heat flow. Sometimes the

area of surface of heat input and heat output need be not identical. So, the Peltier

element size is small, and the surface of heat output should be large or have a

specific form (Fig. 10). The combination heat pipePeltier element can be

used in systems for processor cooling and in medical devices, for example in cryo-surgery. A device for local hypothermia with a heat pipe-based instrument was

developed. It has successfully passed tests and is recommended for introduction

to medical practice.

Fig. 11. Heat pipePeltier element combination.

4.2 Heat pipe heat exchangers for air preheating

Since the end of the 1970s there are some publications related to heat pipeheat exchangers. A large number were published in the USSR. The heat pipe or two-phase thermosyphon device is an important concept in heat exchangers, which can

be used in different branches of industry such as metallurgy, power, oil-refining,glass, etc.


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The heat pipe heat exchanger used for gasgas heat recovery is essentially a bundleof finned heat pipes assembled like a conventional air-cooled heat exchanger. The

heat pipe in the heat exchanger can be divided in to three parts: evaporator, adiabatic

section and condenser (Fig. 11). Passing hot flue gases over the evaporator causesthe working fluid to boil and the vapors to flow to the cold end of the tube. Cold air

flowing over the condenser in counter flow direction condenses the vapors releasinglatent heat that heats the air.

Fig. 12. Heat pipe air preheater.

Application

*Use of process waste heat for preheating process supply air

* Use process waste heat for space heating and air-conditioning* Recovery of exhaust heat from an air conditioning system in a commercial or

domestic building for preheating supply air.


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CONCLUSIONS

In present state of development, efficiency of every system is very

important. In thermal machines, there is great role of heat transfer from

one device to another device and heat exchanger are also plays a necessary

role for higher efficiency. In this way heat pipe is useful to improve

efficiency of heat transfer, ultimately it increases the efficiency of thermal

process.

Heat pipe can be easily implemented as thermal links and heatexchanger in different system to ensure the energy saving and

environmental protection.


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