Parametric study of the performance of heat pipe – a review 2

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Parametric study of the performance of heat pipe – a review 2

  1. 1. INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 – International Journal of JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 1, January- February (2013), pp. 173-184 IJMET© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2012): 3.8071 (Calculated by GISI)www.jifactor.com ©IAEME PARAMETRIC STUDY OF THE PERFORMANCE OF HEAT PIPE – A REVIEW M.N.Khan, Utkarsh Gupta, Shubhansh Sinha, Shubhendu Prakash Singh, Sandeep Pathak Department of Mechanical Engineering, Krishna Institute of Engineering and Technology, Ghaziabad ABSTRACT The electronic device uses an electronic circuit which experiences an increase in operating temperature due to increase in heat flux density. Therefore, cooling becomes one of the important factors to be taken into consideration. Heat pipes are the heat transfer devices available to deal with the high density electronic cooling problem due to their high thermal conductivity, reliability and low weight. A Heat pipe uses the principles of both thermal conductivity and phase transition to manage the heat transfer between two solid interfaces. Due to the high capacity to heat transfer, heat exchanger with heat pipes has become much smaller than traditional heat exchangers in handling high heat fluxes. This paper gives you a detailed literature review about the main factors affecting the performance of heat pipe.Furthermore,the thermal resistance and heat transfer capability are affected by the influence of various parameters such as working fluid, tilt angle, fill ratio, wick structure, thermal properties, heat input and applications in different fields. KEYWORDS: Heat pipe, Working fluid, Wick structure, Tilt angle, Heat input, Applications. 1. INTRODUCTION Heat pipes are one of the most effective procedures to transport thermal energy from one point to another. It uses two principles of thermal conductivity and phase transition to efficiently manage the transfer of heat. Heat pipes contain no mechanical moving parts and typically require no maintenance. The concept of heat pipe was originally invented by Gaugler of the General Motors Corporation. In 1944, he patented a lightweight heat transfer 173
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEdevice which was essentially the present heat pipe [1], but only after Grover independentlyrediscovered it in 1960s that the remarkable properties of the heat pipe became appreciatedand serious development work followed. Grover also coined the name “heat pipe” and stated,“Within certain limitations on the manner of use, a heat pipe may be regarded as a synergisticengineering structure which is equivalent to a material having a thermal conductivity greatlyexceeding that of any known metal” [2]. The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. They are fundamentallybetter 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 theprinciple of matter phase transition). A second feature of the heat pipe is that relatively largeamounts of heat can be transported with small lightweight structures. The amount of heat thatcan be transported as latent heat of vaporization is usually several orders of magnitude largerthan can be transported as sensible heat in a conventional convective system with anequivalent temperature difference. The performance of a heat pipe is often expressed in termsof equivalent thermal conductivity. In applications where conventional cooling methods arenot suitable, heat pipes are being used very often. Once the need for heat pipe arises, the mostappropriate heat pipe needs to be selected. Often this is not an easy task. For applicationsinvolving energy conservation, the heat pipe is a prime candidate and has been used toadvantage in heat recovery systems and energy conversion devices. Conservation of energyhas never been more important before, as the cost of fuel is ever rising and the reserves arediminishing. The heat pipe provides an effective tool in a large number of applicationsassociated with conservation.2. WORKING OF HEAT PIPE Heat pipe is a very efficient instrument for transfer of heat from one end to other.Heat pipe consist of three parts namely evaporator, condenser and the wick portion shown infigure 1. Every part has its own significance in transferring the heat energy. Evaporator is theportion which receives the heat from the source. In the evaporator, the working fluid ispresent in liquid state. The heat is absorbed by liquid and it is converted into vapour phase.As vapour is formed in the evaporator portion a pressure difference is created betweenevaporator and condenser portion. This vapour pressure difference is the driving force whichtakes the vapour from evaporator to condenser. On reaching the condenser end the vapourgives out its latent heat of vaporisation and converts into liquid form. The heat absorbed atthe evaporator section is received in this way in the condenser portion. Wick portion is aporous structure made of wire mesh or grooves or sintered metal powders etc. The function ofthe wick structure is to absorb the liquid from the condenser end and transfer it to evaporatorthrough capillary action. The function of wick is very important because if wick is unable totransfer working fluid from condenser to evaporator, the heat pipe will eventually dry out andthis will stop the working of heat pipe. Hence, the parameters which are responsible for theflow of working fluid through the wick must be properly studied and applied. Heat pipe thusworks on the principle which is the combination of conduction and convection. The heattransfer process through heat pipe can be considered as heat transfer in the closed loop as theworking fluid starts from evaporator absorbs heat reaches condenser portion gives out heatand then through wick structure finally comes back to evaporator. One of the majoradvantages of the process of heat pipe is that the heat transfer in heat pipe is independent of 174
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEany external agency i.e. for transferring the heat no help is required from any external source.All the processes involved are natural and this makes the heat pipe self dependent. Moreoverit has been found that the heat pipe has high thermal conductivity, hence it finds use in theregion of high heat transfer. Because of above advantages, it has become a desirableinstrument. Fig.1. Schematic view of heat pipe3. LIMITATIONS OF HEAT PIPE During steady state operation, the maximum heat transport capability of a heat pipe isgoverned by several limitations, which must be clearly known when designing a heat pipe.The heat transfer limitations depend on the working fluid, the wick structure, the dimensionsof the heat pipe, and the heat pipe operational temperature. There are five primary heat pipetransport limitations: Fig.2. Limitations to heat transport in a heat pipe3.1 Capillary Limitation The capillary limit involves the fundamental phenomenon in heat pipe operation thatis the development of capillary pressure differences across the liquid-vapour interfaces in theevaporator and condenser. When the existing capillary pressure is insufficient in providingadequate liquid flow from the condenser to the evaporator, dry out of the evaporator wickwill occur. Therefore for the circulation of the working fluid, capillary pressure difference isthe driving potential and the maximum capillary pressure must be greater than the sum of allpressure losses inside the heat pipe. 175
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME The capillary limitation occurs when the net capillary forces generated by the vapour–liquid interfaces in the evaporator and condenser are not large enough to overcome thefrictional pressure losses due to fluid motion. This causes the heat pipe evaporator to dry outand shuts down the transfer of heat from the evaporator to the condenser. For most heat pipes,the maximum heat transfer rate due to capillary limitation can be expressed as [3].where, K is the wick permeability (m2), Aw is the wick cross-sectional area (m2), ρl is theliquid density (kg/m3), µl is the liquid viscosity (Ns/m2), reff is the wick capillary radius in theevaporator (m), g is the acceleration due to gravity (9.8 m/s2), and lt is the total length of thepipe (m) [4].3.2 Viscous Limitation In case of heat pipe operating at low temperatures, the vapour (saturation) pressure inthe evaporator region is very small and of the same magnitude as the pressure gradientrequired to drive the vapour from evaporator to condenser. In this case, the total vapourpressure is balanced by opposing viscous forces in the vapour channel. Thus, the total vapourpressure in the vapour region becomes insufficient to sustain an increased flow. This low-flow condition in the vapour region is referred to as the viscous limit. As the viscous limitoccurs at very low vapour pressures, the viscous limit is most often observed in longer heatpipes when the working fluid used is near the melting temperature (or during frozen start-upconditions) as the saturation pressure of the fluid is low. Viscous force prevents vapour flowin the heat pipe. The heat pipe thus operates below the recommended operating temperature.Increasing the heat pipe operating temperature is a potential solution or operates with analternative working fluid. The viscous limit does not represent a failure condition. In the case where the heatinput exceeds the heat input determined from the viscous limit, this results in the heat pipeoperating at a higher temperature with a corresponding increase in the saturation vapourpressure. However, this condition typically is associated with the heat pipe transitioning tobeing sonic limited.3.3 Sonic Limitation This limit is experienced in liquid metal heat pipes during start up or low-temperatureoperation due to very low vapour densities in this condition. This can lead to choked, orsonic, vapour flow. The sonic limit is typically not a factor for most heat pipes operating atroom temperature or cryogenic temperatures, except in case of very small vapour channeldiameters. With the increased vapour velocities, inertial, or dynamic, pressure effects must beincluded. Where the inertial effects of the vapour flow are significant, the heat pipe may nolonger operate in a nearly isothermal case, resulting in a significantly increased temperaturegradient along the heat pipe. The sonic limitation actually serves as an upper bound to theaxial heat transport capacity and does not necessarily result in dry out of the evaporator wickor total heat pipe failure. Any attempts to exceed the sonic limit causes an increase in both the 176
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEevaporator temperature and the axial temperature gradient along the heat pipe, thus reducingfurther the isothermal characteristics found in the vapour flow region. In other words, the heat pipe is due operating at low temperature with too much of power.The potential solution for this limitation is to create large temperature gradient so that heatpipe system carries adequate power as it warns up [5].3.4 Entrainment Limitation Basic flow conditions in a heat pipe shows that the liquid and vapour flow in oppositedirections. The interaction between the counter currents of liquid and vapour flow leads toviscous shear forces occurring at the liquid–vapour interface, which may inhibit liquid returnto the evaporator. In the most severe cases, waves may be formed and the interfacial shearforces become greater than the liquid surface tension forces, causing liquid droplets beingentrained in the vapour flow and carried to the condenser. In a majority of cases studied, thewick structure of the heat pipe was flooded (i.e. excess liquid), which allowed entrainment tooccur. The most common approach to estimating the entrainment limit in heat pipes is to usea Weber number criterion.Cotter [6] presented one of the first methods to determine the entrainment limit. This methodutilized the Weber number, defined as the ratio of the viscous shear force to the forcesresulting from the surface tension.By relating the vapour velocity and the heat transport capacity to the axial heat flux asWith the assumptions that entrainment of liquid droplets in the vapour flow & the Webernumber must be less than unity .The maximum transport capacity based on entrainment canbe written asWhere, σl is the surface tension (N/m) and rc,ave is the average capillary radius of the wick.Note that for many applications rc,ave is often approximated by reff . The entrainment limitrefers to the case of high shear forces developed as the vapour passes in the counter flowdirection over the liquid saturated wick, where the liquid may be entrained by the vapour andreturned to the condenser. This results in insufficient liquid flow of the wick structure [7].3.5 Boiling limitation At high values of heat fluxes, boiling at nucleate level may occur in the wickstructure, which causes vapour to become trapped in the wick, thus blocking return of liquid 177
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEand resulting in evaporator dry out. This phenomenon is termed as the boiling limit. It differsfrom the other limitations, as it depends on the radial or circumferential heat flux applied tothe evaporator, as opposed to the axial heat flux or total thermal power transported by theheat pipe. Determination of the heat flux or boiling limit is based on nucleate boiling theoryand is comprised of two separate phenomena: i) bubble formation & ii) Subsequent growth orcollapse of the bubbles. Bubble formation is governed by the size (and number) of nucleation sites on a solidsurface and the temperature difference between the heat pipe wall and the working fluid. Thetemperature difference, or superheat, governs the formation of bubbles. The potential solutionis to use a wick with a higher heat capacity or spread out the heat load [8].4. EFFECT OF WICK STRUCTURE Wick is one of the most important parts of heat pipe. Wick is responsible for carryingcondensed liquid from condenser to evaporator and this is necessary for working of heat pipe.Hence, study of wick structure and the factors controlling its performance is necessary. In theheat pipe, vapour flows through the cylinder and liquid flows through the wick. In absence ofgravity, electrical or any other force the pressure difference is created by capillary pressuredifference. Because of this, capillary of small radius is needed to transfer the liquid fromcondenser to evaporator. But, if the radius becomes very small, friction increases whichcauses a very large amount of pressure loss. The second problem which is encountered is thatthe small radius capillary may be choked by some vapour bubble and the flow of liquid maystop [9].The modulation of heat pipe wick thickness helps in axial capillary liquid flow, whilerestricting the increase in the wick superheat that accompany thicker, uniform wicks [10].The diameter and other structural configuration changes with the change in type of operationor input output load. For example, in thin heat pipes vapour and liquid pressure drops arelarge and therefore to keep circulating the working fluid, high capillary force is required. So,groove wick is not used for thin heat pipes since capillary force in this is low. To increase theheat carrying capacity of thin heat pipes, we use mesh wick or sintered wick. Even afterchanging the wick structure, the force developed is not enough. So, we reduce the pore radiusof wick to increase the capillary force, but in this case the liquid flow pressure drop isincreased due to decrease in permeability. So, the heat transfer capacity of thin heat pipedecreases. Hence, the channel through which the liquid flows must be enlarged to overcomethis drawback. But when liquid channel is enlarged, the vapour flow area is reduced whichagain leads to decrease in heat carrying capacity of heat pipe. So, in order to obtain balancebetween the two, an optimum thickness of liquid and vapour column must be selected [11].5. EFFECT OF FLUID CHARGE Filled ratio is the fraction (by volume) of the heat pipe which is initially filled with theliquid.(1) Low fill ratio: When the heat input is given, the fluid at the evaporator section isvaporised at a faster rate and due to the accumulation of more vapour at the condensersection, it leads to dry out phenomenon at the evaporator section, hence condensation occursat a lower rate. Thus, efficiency of heat pipe is lowered. 178
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME (2) High fill ratio: The evaporator section temperature is lower and hence evaporation occurs at a lower rate. This results in very little vapour flowing into the condenser section, reducing the thermal efficiency of the heat pipe. Filled ratio is constricted by two operational limits. At 0% filled ratio, a heat pipe with no working fluid and bare tubes only, is a heat transfer device in pure conduction mode with a very high undesirable thermal resistance. At 100% filled ratio, heat pipe operates like a single phase thermo- syphon with action maximum for a vertical heat pipe and stops for a horizontal heat pipe. The heat transfer takes place purely by axial conduction in a horizontal heat pipe [12]. A number of experiments have been performed using varying heat inputs and filled ratios. Experiments were carried out in dry mode (without working fluid) and wet mode (with working fluid in it). The dry mode experiment represents the heat transfer characteristics in an ordinary conductor, and the wet mode experiment depicts the live heat pipe characteristics. Three different working fluids namely distilled water; methanol and acetone are used in this study. The heat pipe was filled with 35%, 55%, 85% and 100% of the evaporator volume and tested for different heat input and working fluids [13]. Figures (3) to (5) show the variations of thermal resistances for different fill ratios, for the three different working fluids at different heat input. These graphs compare thermal resistances at different fill ratios of different working fluids. In general, wet run shows the reduced thermal resistances for all levels of heat input and all types of working fluids. The dry run shows the largest values of thermal resistances and it is almost constant for varying heat loads. The experiments done over the years indicate that the filling ratio and the heat input are important considering the heat transfer performance and the heat pipe performs optimally when the filling ratio ranges between 50–75%, at 50o inclination angle. The minimum performance was found for filling ratio at 25% and inclination angle at 25o [14]. In general, for working fluid fill ratios greater than 85% of evaporator volume, results are better in terms of decreased thermal resistance, increased heat transfer coefficient and reduced temperature difference across the evaporator and condenser [15]. With increase in heat input, the thermal resistance decreases and the fill ratio comes in effect because it has a great impact on the thermal resistance. For lower fill ratio, thermal resistance is higher and starts decreasing for further fill ratios.Fig.3.Variation of thermal resistance with Fig.4.Variation of thermal resistance with Fig.5.Variation of thermal resistance withdifferent heat inputs for 35% fill ratio different heat inputs for 55% fill ratio different heat inputs for 100% fill ratio 179
  8. 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME6. EFFECT OF WORKING FLUID Working fluid selection is directly connected to the properties of the fluid. Theproperty of working fluid is going to affect both the ability to transfer heat and thecompatibility with the case and wick material. A multitude of characteristics must beevaluated in order to determine the most favourable fluid for the application considering theprimary requirements which are compatible with the heat pipe material (s) such as thermalstability, wettability, reasonable vapour pressure, high latent heat and thermal conductivity,low liquid and vapour viscosities and acceptable freezing point. The Nano -fluids kept in the suspension of conventional fluids have the potential ofsuperior heat transfer capability than the conventional fluids due to their improved thermalconductivity. The copper Nano-fluid which has a 40 nm size with a concentration of 100mg/lit is kept in the suspension of the de-ionized (DI) water and an aqueous solution of n-Butanol and these fluids are used as a working medium in the heat pipe [16]. The presence of Nano-particles in the working fluid leads to a reduction in the speedof the liquid, smaller temperature difference along the heat pipe and the possibility ofreduction in size under the same operational conditions. Results manifested that the aluminaNano-fluids augmented the thermal performance of the OHP (oscillating heat pipe). Oncomparison with pure water, the maximal thermal resistance was decreased by 0.14 °C/W (or32.5%) when the power input was 58.8 W [17]. The maximum heat flux apparently increase with the increase of the massconcentration when the mass concentration is less than 1.0 wt. %. Then, they begin todecrease slowly after the mass concentration is over 1.0 wt. %. The mass concentration of 1.0wt. % corresponds also to the best input power enhancement. The maximum input power ofthe heat pipe can enhance by 42% after substituting the Nano-fluid for deionized water [18].It was found that the heat transfer rate of the CLOHP (closed loop oscillating heat pipes)using silver Nano-fluid as a working fluid was better than that the heat transfer rate whenpure water is used because the silver Nano-fluid increases the heat flux by more than10%[19].On closer scrutiny, it can be said that Nano-fluids are potential fluids to be used as workingfluid in PHP/OHP because -a. Inclusion of Nano particles can affect the start-up temperature of the PHP. However, the particle size affects the start-up temperature.b. When the Nano-particle size is reduced, the thermal conductivity of the Nano-fluid increases. However, the Nano-particles may agglomerate, settle, or coalesce to the walls with long-term operation of the Nano-fluid.c. In PHP, the use of Nano-fluids can lower operating temperatures and greater pulsations of amplitudesd. Enhanced nucleation sites and reduced bubble diameter can be obtained.e. In selection of the Nano-fluid, the surface wettability or the contact angle of the Nano- particles with the surface plays an important role.f. Among the four shapes (cylinder, blade, plate and brick) studied so far, the cylindrical shape gives the best result.Generally, water out performed the Flourinert TM liquids, in particular above approximately40 W where the liquid entrainment limit compromises the performance of the Flourinert TMcharged thermo- siphons. Even still, the Flourinert TM liquids FC-84 and FC-77 offer 180
  9. 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEadequate thermal performance below 40 W and offer the added benefit of being dielectric,which may be beneficial in some circumstances.Although the Nano-fluid has the higher heat conduction coefficient that dispels moreheat theoretically. But the higher concentration will make the higher viscosity. The higherviscosity makes the bubble difficult to produce and the force of friction causes obstruction ofthe liquid slug with tube wall becomes larger, so obstruction is relatively greater when thebubble is promoted and influences the whole efficiency of the heat transfer [20].In addition, adding too many Nano-particles to fluid would make the property of workingfluid at evaporator section tend to be in solid phase, and would make theconvection performance of Nano-fluid at evaporator section reduced. This wasdisadvantageous to the thermal efficiency of heat pipe [21]. The thermal resistance of the heatpipes with Nano-particle solution is lower than that with DI water. As a result, thehigher thermal performances of the new coolant have proved its potential as a substitute forconventional DI water in vertical circular meshed heat pipe [22].7. EFFECT OF TILT ANGLE Performance of heat pipe also depends upon how the heat pipe is placed. Heat pipecan be used in various positions. It can be horizontal, vertical or in any other angle. Inhorizontal position, gravity has no effect on the performance of heat pipe. But as the anglechanges, gravity start playing its role. With change in angle, the effect of gravity changes andthis affects the performance of heat pipe [23]. In addition, the orientation of heat pipe is alsoimportant. With change in angle, gravity can help or oppose the working of heat pipe.Depending on this, there are two types of tilt angle-favourable and adverse [24].Favourabletilt angle is that when evaporator is below and condenser is above and vice versa for adversetilt angle. When we use favourable tilt angle i.e. evaporator is below and condenser is above,the performance of heat pipe increases with increase in tilt angle. This is because; in this typeof orientation, gravity helps the movement of fluid from condenser to evaporator. So, withthe increase in rate of transfer of fluid from condenser to evaporator, the rate of heat transferincreases and hence its efficiency. Whereas, in adverse tilt angle, evaporator is above andcondenser is below and in this gravity opposes the flow of fluid from condenser to evaporatorand hence the efficiency of heat pipe falls. Hence, the heat pipe must be kept in favourabletilt angle for maximum efficiency. An increase in heat transfer rate of 39% is obtained for 2%iron oxide nano-particles, when the angle of inclination of heat pipe is 90 degrees [25].Efficiency of heat pipe usually increases with increase of angle in favourable tilt. However,when the heat pipe tilt angle exceeds a value of 60° for de-ionic water and 45° for alcohol,the heat pipe thermal efficiency tends to decrease [26].8. APPLICATION Heat pipe is very versatile device and it can be employed in various fields because ofits capacity to blend in various operations. It has been found useful in a number of fields suchas aerospace engineering, energy conversion devices, electronic cooling, biomedicalengineering etc. In addition to conventional uses, recent advances have been done to increasethe adaptive nature of heat pipe. Micro heat pies have been developed to employ it in coolingof electronic devices. Heat pipes of various shapes such as flat shaped, disk shaped, rotating, 181
  10. 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEreciprocating heat pipes have drawn attention due its multidimensional nature ofapplication[27].Heat pipe is also used as heat exchangers for the recovery of waste heat ofautomotive emissions. Wickless heat pipe is also used for numerous applications in heatrecovery system [28]. It is also used for other industrial process equipments. Carbon steelheat pipe technology is used for utilizing heat pipe as air pre heater and waste heat boiler.This is applied in field of heat recovery, environmental protection and energy conservation.Liquid metal high temperature heat pipe has been used for high-temperature hot airgenerators and heat tractors. Heat pipe also finds its use in chemical reactors includingammonia convertors [29]. Encompassing all above uses, it has become an extremely usefuldevice which facilitates the process and adds to the efficiency of the process.9. CONCLUSION The review reveals that a detailed analysis on heat pipe can be done by working ondifferent parameters like working fluid, wick structure, tilt angle, pipe material, temperatureprofiles of heat source and heat sink andenvironmental conditions. The working fluidproperties affect the ability to transfer heat and the compatibility with the case and wickmaterial. However particular working fluid can only be functional at certain temperatureranges. Also, working fluid needs a compatible vessel material to prevent corrosion orchemical reaction. Many articles reveal that Nano fluids have great potential to ascend thethermal efficiency of the heat pipe. The wick material used in heat pipe can be formed usingincorporating particles of micro-encapsulated phase change material bonded together. Use ofsuch a wick structure has the advantage of providing an additional heat absorber. This greatlyimproves heat pipe ability to absorb excess heat and prevent damage. The heat pipeorientation (read tilt angle) is important for the practical applications. It is observed throughpractical analysis that operating heat pipe in a favourable tilt position can increase heattransfer capacity.10. REFERENCES[1] Gaugler, R. S., “Heat Transfer Device”, U. S. Patent 2,350,348.[2] Trefethen, L., “On the Surface Tension Pumping of Liquids or a Possible Role of the Candlewick in Space Exploration”, G. E. Tech. Info., Ser. No. 615 D114, Feb. 1962.[3] S.W. Chi, Heat Pipe Theory and Practice, Hemisphere Publishing, Washington, DC, 1976.[4] L.W. Swanson, in: Frank Kreith (Ed.), Heat Pipe, Heat and Mass Transfer, Mechanical Engineering Handbook, CRC Press LLC, Boca Raton, 1999.[5] R.Manimaran, K.Palaniradja, N.Alagumurthi, J.Hussain ,“Factors Affecting The Thermal Performance Of Heat Pipe –A Review”, JERS/Vol. III/ Issue II/April-June, 2012/20-24.[6] T.P. Cotter, Heat pipe startup dynamics, SAE Thermionic Conversion Specialist Conference, Palo Alto, CA, 1967.[7] Jay M. Ochterbeck, Heat pipes, Heat Transfer Handbook, 1st ed., 2003. 182
  11. 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME[8] Patrik Nemec, Alexander Čaja, Milan Malcho, “Mathematical model for heat transfer limitations of heat pipe”, journal of Mathematical and Computer Modelling ,vol.57 ,pg:126–136, 2013. [9] Fabian Korn, Project Report MVK160 Heat and Mass Transport, Heat pipes and itsapplications, Dept. of Energy Sciences, Lund University, 2008.[10] J.H. Rosenfeld, J.E. Lindemuth, “Heat transfer in sintered groove heat pipes”, the International Heat Pipe Conference, Tokyo, Japan, 1999.[11] Hirofumi Aoki, Masami Ikeda and Yuichi Kimura. Frontiers in Heat Pipe , “Ultra Thin Heat Pipe and its Application”.[12] K. Mozumder, A. F. Akon, M. S. H. Chowdhury and S. C. Banik ,“performance of heat pipe for different working fluids and fill ratios”, Journal of Mechanical Engineering, Vol. ME 41, No. 2, December 2010.[13] Dr.HussainH.Ahmad ,Raqeeb H. Rajab, “An Experimental Study of Parameters Affecting a Heat Pipe. Al-Rafidain Engineering, Vol.18 ,No.3, 2010.[14] A. K. Mozumder, A. F. Akon, M. S. H. Chowdhuryand, S. C. Banik, “performance of heat pipe for different working fluids and fill ratios”, Journal of Mechanical Engineering, Transaction of the Mech. Eng. Div., The Institution of Engineers, Bangladesh, Vol. ME 41, No. 2,2010.[15] Tun-Ping Tenga, How-GaoHsua, Huai-En Mob,Chien-ChihChenc “Thermal efficiency of heat pipe with alumina nanofluid” , Journal of Alloys and Compounds,504S, S380–S384, 2010.[16] D. Wen, Y. Ding, “Effect of particle migration on heat transfer in suspensions of nanoparticles flowing through minichannels”, Microfluidic and Nano fluidic, Vol.1,pg:183-189,2005.[17] Y. Xuan and Q.Li, “Heat Transfer enhancement of Nano fluids”, International Journal of Heat and fluid Flow, vol.21, pg: 58-64, 2000.[18] ZhenHuaLiu , QunZhi Zhu, “Application of aqueous Nano fluids in a horizontal mesh heat pipe”, Energy Conversion and Management,vol.52, pg:292–300,2011.[19] H.B.Ma, C.Wilson, B. Borgmeyer, K.Park and Q.Yu, “Effect of nanofluid on the heat transport capability in an oscillating heat Pipe”, Applied Physics letters 88,143116 1- 3, 2006.[20] Yu-Hsing Lin , Shung-Wen Kang , Hui-Lun Chen ,“Effect of silver nano-fluid on pulsating heat pipe thermal performance”, Applied ThermalEngineering,vol. 28,pg :1312–1317,2008.[21] Tun-Ping Tenga,, How-GaoHsua, Huai-En Mob,Chien-Chih Chenc ,“ Thermal efficiency of heat pipe with alumina nano-fluid”, Journal of Alloys and Compounds, 504S: S380–S384,2010.[22] C.Y. Tsaia, H.T. Chiena, P.P. Dingb, B. Chanc, T.Y.Luhd, P.H. Chena, “Effect of structural character of gold nanoparticles in Nano fluid on heat pipe thermal performance”, Materials Letters,58: 1461– 1465,2004.[23] Stéphane Launay, Valérie Sartre, Jocelyn Bonjour ,“Parametric analysis of loop heat pipe operation: a literature review”, International, Volume 46, Issue 7, Pages 621– 636, July 2007.[24] R.Manimaran, K.Palaniradja, N.Alagumurthi, J.Hussain, “Factors affecting the thermal performance of heat pipe”, JERS/Vol. III/ Issue II/April-June, 2012/20-24. 183
  12. 12. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME[25] Huminic G, Huminic A, Morjan I, Dumitrache F. “Experimental study of the thermal performance of thermo syphon heat pipe using iron oxide nanoparticles”, International Journal of Heat and Mass Transfer,Vol.54, No.1-3,page no- 656- 661,2011.[26] Paisarn Naphon Pichai Assadamongkol, Teerapong Borirak ,“Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency”, International Communications in Heat and Mass Transfer, vol. 35 : page no-1316–1319,2008.[27] Garimella, S V. and Sobhan, C. B., "Recent Advances in the Modeling and Applications of Nonconventional Heat Pipes" (2001),CTRC Research Publications, Paper 13.[28] Yang, Xiugan Yuan, Guiping Lin, Beijing University of Aeronautics and Astronautics. “Waste heat recovery using heat pipe heat exchanger for heating automobile using exhaust gas”.vol. 23, no3, pp. 367-372,2003.[29] Hong Zhang , Jun Zhuang, National Technology Research and Promotion centre for Heat pipe, Nanjing University of Technology- Research, development and industrial application of heat pipe technology in China-Volume 23-Issue 9,page no-1067- 1083,2003.[30] Hitesh N Panchal, Dr. Manish Doshi, Anup Patel and Keyursinh Thakor, “Experimental Investigation On Coupling Evacuated Heat Pipe Collector On Single Basin Single Slope Solar Still Productivity” International Journal of Mechanical Engineering & Technology (IJMET), Volume 2 , Issue 1, 2011, pp. 1 - 9, Published by IAEME.[31] Ms. M.M. Shete and Dr. A.D.Desai, “Design And Development Of Test-Rig To Evaluate Performance Of Heat Pipes In Different Orientations For Mould Cooling Application” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3 , Issue 2, 2012, pp. 360 - 365, Published by IAEME.[32] Kavitha T, Rajendran A, Durairajan A and Shanmugam A, “Heat Transfer Enhancement Using Nano Fluids And Innovative Methods - An Overview” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 769 - 782, Published by IAEME. 184

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