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Thermal Analysis of Gravity Effected Sintered Wick Heat Pipe
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 2179 – 2187
Thermal Analysis of Gravity Effected Sintered Wick Heat Pipe B ChNookarajua, PSV KurmaRaob, S Naga Saradac aAssociate
Professor, Department of Mechanical Engineering GokarajuRangaraju Institute of Engg& Technology, Hyderbad.. b General Manager (Retd), R&D, BHEL Ramachandra Puram, Hyderabad. c Professor Department of Mechanical Engineering, JNTUH, Kukatpally, Hyderabad
Abstract The rapid growth in technology nowadays has resulted in the densification of electronic components, which requires a highly reliable thermal management system. The heat pipes are the most promising heat transfer devices capable of transmittin g high heat fluxes rejected by the modern electronic equipment. In the present study, the experimental analysis of thermal performance of a sintered-wick heat pipe is carried out at various inclinations with gravity assisted tilt. The results have shown that the performance of heat pipe varies with the geometrical orientation of heat pipe and it is observed the heat carrying capacity of heat pipe is well pronounced at 600 gravity assisted tilt followed by 900. © 2015 2014Elsevier The Authors. Ltd. All rights reserved. © Ltd. AllElsevier rights reserved. Selection under responsibility of theofconference committee members of the 4thofInternational conferenceconference on Materialson the 4th International Selectionand andpeer-review peer-review under responsibility the conference committee members Processing and Characterization. Materials Processing and Characterization. Keywords: Electronics Cooling, Sintered wick Heat Pipe, tilt angle, Thermal performance.
1. Introduction: A heat pipe is a heat transfer device with an extremely high effective thermal conductivity. Heat pipes are evacuated vessels, typically circular in cross sections, which are back-filled with a small quantity of a working fluid. They are totally passive and are used to transfer heat from a heat source to a heat sink with minimal temperature gradients, or to isothermal surfaces.
Fig 1: Operation of Heat Pipe
* Corresponding author. Tel.: 91-98-660-55979 E-mail address:[email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.230
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Fig 2: T-S Diagram of Heat Pipe
In a Heatpipe heatenters through thecopper casing material and it will reach the liquid on the inside of the evaporator section as in Fig 1. The energy is absorbed by the liquid through conduction and it then vaporizes because the vessel is in the saturatedtwo-phase (liquid-vapor) state. Any additional added energy goes into latent heat of vaporization. During the process of vaporization, the specific volume of the water increases about 1600 times (at standard pressure). The dramatic increase in volume results in an increase of local pressure in the evaporator region. The increased pressure drives the vapor through the adiabatic region to the lower pressure condenser region. If the surface temperature of the condenser is held lower than the saturation temperature of the vapor, then condensation will occur. Capillary forces combined with a liquid pressure drop in the channels (of wick) drive the water back to the evaporator section. This cycle of conduction, vaporization, convection, condensation, and capillary flow occurs in a continuous cycle in properly functioning heat pipes. These processes are shown on a temperatureentropy (T-S) diagram in the below Fig 2. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on temperature difference between the ends of the pipe, and cannot lower temperatures beyond the ambient temperature at either end. When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapor pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporization of the working fluid reduces the temperature at the hot end of the pipe.The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapor pressure over condensing working fluid at the cooler end of the pipe, this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapor impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapor pressures are low. The speed of molecules in a gas is approximately the speed of sound, and in the absence of non-condensing gases (i.e., if there is only a gas phase present) this is the upper limit to the velocity with which they could travel in the heat pipe. In practice, the speed of the vapor through the heat pipe is limited by the rate of condensation at the cold end and far lower than the molecular speed. The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. While making heat pipes, there is no need to create a vacuum in the pipe. Simply boils the working fluid in the heat pipe until the resulting vapor has purged the non-condensing gases from the pipe and then seal the end. An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspected that water charged heat pipe would only work, when the hot end reached the boiling point (100°C) and steam was transferred to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0°C). Thus the heat pipe can
B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
operate at hot-end temperatures as low as just slightly warmer as the melting point of the working fluid. Similarly, a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid. The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. Theheat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed to evaporate one gram of water is 540 times the amount of energy needed to raise the temperature of that same one gram of water by 1 °C. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts. Heat pipes are hollow metal tubes that efficiently conduct heat from one location to another. They operate by means of a small amount of working fluid contained in a sealed tube, held under a slight vacuum. The vacuum lowers the boiling point of the working fluid, so relatively small increases in temperature vaporize the liquid which is then naturally drawn towards the colder end of the heat pipe where it condenses back to liquid. An internal wick structure then acts to return the condensed working fluid back to the hot end of the heat pipe, by a force called capillary action. When you put the edge of a paper towel in a small puddle of water, this is the force that soaks up the liquid into the paper. The crux of the situation is that some wick structures are more efficient than others, and some have limitations with respect to orientation and gravity. In the present study, capillary wick is made of sintered powder which adheres to the inner walls of the heat pipe. This acts to transport the fluid through capillary action. Choosing a sintered structure as the heat pipe wick will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. Very tight bends in the heat pipe can be achieved with this type of structure. Powder particles are diffused together and to the tube wall to form a sintered wick structure. Copper powder is the most common material used to produce sintered wick structures. Sintered wick structures using smaller powder particles can lift the working fluid a greater distance than a wick structure fabricated with larger particles. The pumping capability of a sintered wick structure is superior to the capability of a screen mesh or grooved wick structure. A sintered wick heat pipe can be used in applications with radial heat fluxes up to 250 W/cm2. In 2008, X. Zhao et. al [1] illustrated a one dimensional mathematical model for heat and mass transfer. In a miniature heat pipe with a grooved wick structure is developed and solved numerically to yield he capillary, liquid and vapor pressure distribution and the overall thermal resistance under steady-state condition. To verify the model, experimentation for measuring the maximum heat transfer rateand the thermal resistance are conducted and the maximum errors are obtained. Physical insight into transport phenomena in the miniature heat pipe with grooved wick structure has been acquired. From the results, In addition, the model is used for optimizing the wick structure thermally and hydro dynamically. It is estimated that the maximum heat transfer rate of an o.d. 15.88 mm with an optimized grooved wick structure can be enhanced up to 12.6% and the total resistance can be reduced up to 9.9%. In 2003, Jeong- se suh et al [2] describes that numerical analysis is made on the thermal performance of micro heat pipe in an axial flat grooved channel. The flow of liquid and vapor is investigated in trapezoidal grooved and the effect of variable shear stress along the interface of the liquid and vapor consideration. The result from this study are obtained in the axial variation of pressure difference between vapor and liquid, contact angle, velocity of vapor and liquid and so forth. In addition, maximum heat transfer capacity of micro heat pipe is provided by varying the operation temperature and compared with that from Schneider and devos’s model in which interfacial shear stress is neglected. In 2012, G. Kumaresan, S. Venkatachalapathy [13] provided a review on heat transfer enhancement studies of heat pipes using nano fluids. They studied the influence of various factors such as heat pipe tilt angle, charged amount of working fluid, nano particles type, size, and mass/volume fraction and its effect on the improvement of thermal efficiency, heat transfer capacity and reduction in thermal resistance. The nano fluid preparation and the analysis of its thermal characteristics also have been explained. In 2012, R. manimaran et al. [12] studied the effect of filling ratio onthermal characteristics of circular heat pipe using nano-fluid and water. The Cuo/water nanofluid is used as the working fluid of experimental heat pipes with concentration of 1.0 wt. %. The test section of the heat pipe is made of copper tube with outer diameter 22 mm, inner diameter 20.8 mm and length 600 mm. This study focuses on the effects of heat input, fill ratio and angle of inclination on the thermal efficiency and thermal resistance of heat pipe. The results indicated that the thermal efficiency increases when nano particles are added with Di-water and also the thermal resistance of the nano fluid heat pipe decreases compared with that of the heat pipe using base fluid alone.
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In 2012, Pramod R. Pachgharea and Ashish M. Mahalleb [10] presented preliminary experimental results on thermal performance of closed loop pulsating heat pipe (CLPHP). The copper capillary tube was used having internal and external diameter 2.0 mm and 3.6 mm respectively. For all experimentation, filling ratio (FR) was 50 %, number of turns was 10 and different heat inputs of 10 to 100W were supplied to PHP. For all HPs, Vertical bottom “heat mode” (+900) position is maintained. The equal lengths of evaporator, adiabatic and condenser sections were 50 mm each. Working fluids are selected as Methanol, ethanol, acetone, water and different binary mixtures. The graphs are plotted, in order to study, characteristics of the thermal resistance and average evaporator temperatures at different heat input for various working fluids. Experimental study on PHP indicated that working fluid is an important factor for the performance of PHPs. The result shows that, the thermal resistance decreases more rapidly with the increase of the heating power from 20 to 60W, whereas slowly decreases at input power above 60 W. Pure acetone gives best thermal performance in comparison with the other working fluids. No measurable difference has been recorded between the PHPs running with pure and binary mixture working fluids. In 2010, Senthilkumar R et al. [9] conducted experiments to improve the thermal performance of heat pipe using copper nano-fluid with aqueous solution of n-Butanol. The nano-fluids kept in the suspension of conventional fluids have the potential of superior heat transfer capability than the conventional fluids due to their improved thermal conductivity. In this work, the coppernano-fluid which has a 40 nm size with a concentration of 100 mg/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. The study discusses aboutthe effect of heat pipe inclination, type of working fluid and heat input on the thermal efficiency and thermal resistance. The experimental results are evaluated in terms of its performance metrics and are compared with that of DI water. 2.Experimental Design and Procedure The main principle used to determine the amount of heat transmitted from the condenser to the cooling water. Fourier’s law of heat conduction Input heat flux (q) = Q/A, Q amount of heat supplied The amount of axial heat flux transferred from evaporator section to condenser section is given by
ݍൌ
ο் ο௫
(1)
Heat transfer efficiency of heat pipe iscalculated by η= (q / qin) (2) q= m Cp ΔT Where, Q = Amount of heat transfer K=thermal conductivity of a copper rod = 374 KW/m° C A= Area of the heat pipe = (πd2) / 4 m= Water flow rate in kg/Sec. in the cooling water jacket from Rota meter. Cp= Specific heat of water, 4.18 KJ/ Kg° C. A= π D L (3) D=diameter of a heat pipe=15.88mm L=length of a heat pipe=565mm ∆Tw=difference of condensing section temperatures = T8 – T7 (°C) T8=water temperature at outlet (°C) T7=water temperature at inlet (°C) ΔX= thermocouple distance difference in mm ο்ೢ ൈ ͳͲͲ (4) Ꮈ ൌ ୧୴ୣ୬୦ୣୟ୲୧୬୮୳୲
Where,Ƞ- efficiency of a heat pipe 3. Experimental Investigations: 4.1 Experimental aspects The apparatus consists of a sintered wick heat pipe with cloth heater at evaporator end and the other end of the heat pipe is cooled with flow of water by connecting with a water jacket condenser section. The heater coil
B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
encloses the heat pipe up to a length of 200mm.The heater is enclosed with glass wool in order to ensure that there are no heat losses to the surroundings and to reduce the environmental effects. The heater is connected to a dimmer stat through which heat input given to the evaporator section of the heat pipe can be controlled. The display unit consists of dimmer stat, thermocouples, oneliter jar, and stop watch. The temperature distribution is recorded for different heat inputs and water flow rates in the evaporator and condenser sections. The Heat flux is calculated for the Sintered wick heat pipe. The cooling jacket is provided with inlet and outlet so that the direction of the water flow is opposite to that of the working fluid inside the heat pipe.
Fig 3: Experimental Setup showing Sintered Heat Pipe Oriented in different inclinations
This is done because in counter flow, more heat transfer takes place between the working fluid inside the heat pipe and the water flowing in cooling jacket when compared to parallel flow. The heat pipe is a hollow tube of length 565mm and diameter 15.88mm, filled with water as a working fluid and the maximum operating temperature is 2550C. The heat pipe is provided with markings, where the temperatures are to be measured. Experiment is doneat constant heat flux conditions and heat input of 25 W to 125W, with constant flow discharge of water at different inclinations as shown in Fig3.
Fig 4. Geometry of Heat Pipe
Fig 5. Temperature distribution along a Heat Pipe
The heat input to the evaporator was set using adimmer stat (variac) eight thermocouples were placed on the surface of the sintered wick heat pipe to measure the temperatures of evaporator, adiabatic and condenser section. Among all the thermocouples only two thrermocouples are used to measure inlet and outlet temperatures of cooling water. 4.2 Experimental results The experimental rig and a schematic view of the experimental setup are shown. With different heat flow rates and with different inclinationswere examined. Measured temperature variations along the Sintered wick heat pipe at different heat inputs and inclinations are show below.
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Fig 6. Temperature variation along the Sintered wick heat pipe
From the Figs. (6)- (8), as a result it is observed that the temperature decreases with increase in length along the heat pipe at different heat fluxes. By measuring water flow rate in kg/sec, rise in temperature of water inlet and outlet in the cooling water jacket ('TW), calculate Q/A value in W/m2 by assuming the thermal conductivity of copper material as 374W/m K. As the flow of cooling water increases the dissipation increases. The Cooling capacity of Sintered wick Heat pipe is higher as compared with thermo-syphon heat pipe having the same diameter and length.
Fig 7 (a)
Fig 7 (b)
Fig 7 (c)
Fig 7 (d)
Fig 7. Plot for length vs. temperature of heat pipe at different inclinations
Fig 8. Plot for variation of temperatures along the length of the heat pipe in different orientations
Figs. (8), From the variations the experimental results shows that, with increasing inlet heat input and Heat pipe enhances the heat transfer at exit and increases the performance of the any thermal system. However, applying higher heat energy to the evaporator decreases the performance, therefore it was found that Sintered wick Heat pipe has higher cooling capacity and higher thermal efficiency than thermo- syphon heat pipe.
B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
Figs,(9)-(11)it is observed that ,as the evaporator section is moving towards the ground the heat transporting ability of heat pipe is increasing upto600 and from the non wards it goes on decreasing. The reason for increase in efficiency of heat pipe from 0 0to 600 inclination is, as the evaporator section moves towards the ground, the condensed liquid returns to the evaporator section easily because of the assistance of gravitational forces in favor of capillary forces. The decrease in efficiency from 600 inclination of pipe is because the gravitational forces will oppose the movement of evaporated fluid from evaporator section to condenser section.
Fig 9. Plot for time vs. heater temperature of heat pipe at differentinclinations.
Fig 10. Plot for tilt angle vs. efficiency of heat pipe
The figure indicates that temperature profile along the pipe has a similar trend for different inlet heat fluxes, the results shows higher temperature at the evaporator decreased as passing through adiabatic and condenser section. In addition, the effect of increasing the inlet heat flow on the wall temperature is more significant in the evaporation section, rather than the adiabatic and condensation section. In addition, rising temperature trend happened at the upper region of condenser, more obviously at the higher heat input to the evaporator.
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Fig 11. Thermo-syphon performance at various energy inputs
The performance of a Sintered wick heat pipe at different heat inputs and inclinations are calculated and plotted. The maximum performance can be observed at input energy of 125W. In addition, the results show that the inclination has a significant effect on performance of Sintered wick heat pipe and in this study the best performance was obtained at 60o tilt angle in the results. 4. Conclusion In this paper, experiments were conducted to achieve a better understanding of sintered-wick heat pipe w i t h c o p p e r c a s i n g a n d de-ionized w a t e r as working fluid. The results obtained in the experiment and conclusions are summarized as follows. The pipe specifications are 5 6 5 m m length,15.88mm diameter heat pipe is subject different inclinations to investigate the heat pipe efficiency. a. at00inclination,the efficiencyofheatpipeis47.14% , b.at300inclination,theefficiencyofheatpipeis47.41%. c.at600inclination,theefficiencyofheatpipeis53.73% d.at900inclination,theefficiencyofheatpipeis49.28%. From the above results it is observed that, as the evaporator section is moving towards the ground the heat transporting ability of heat pipe is increasing upto600 and from then onwards it goes on decreasing. The reason for increase in efficiency of heatpipe from 0 0to 600 inclination is, as the evaporator section moves towards the ground, the condensed liquid returns to the evaporator section easily because of the assistance of gravitational forces in favor of capillary forces. The decrease in efficiency from600inclination of pipe is because the gravitational forces will oppose the movement of evaporated fluid from evaporator section to condenser section. 5. Acknowledgement The work presented in this article was funded by the All India Council for Technical Education (AICTE) under Research Promotion Scheme, File No.8023/RID/RPS-20/Pvt(II Policy)/2011-12.The authors gratefully express their thanks to the for providing financial assistance to purchase the equipment for this project and all other related expenses. 6. References 1. 2.
K. N. Shukla; A. Brusly Solomon, B C. Pillai, Mohammed Ibrahim “Thermal performance of cylindrical heat pipe using nano fluids”, Journal of thermo physics and heat transfer, vol. 24, no. 4 (2010), pp. 796-802. Ji Li, Daming Wang, G.P. Peterson, “Experimental studies on a high performance compact loop heat pipe with a square flat evaporator”. Applied Thermal Engineering, Volume 30, issue 6-7 (May, 2010), pp. 741-752.
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3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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QingjunCai and Ya-Chi Chen, “Investigations of biporous Wick Structure Dryout”. J. Heat Transfer 134(2), 021503. Xiao Huang.GeorgeFranchi, “Design and fabrication of hybrid bi-modal wick structure for heat pipe application” J Porous Mater (2008) 15:635–642. Y.M.Chen, S.C. Wu, C.I. Chu (2001); “Thermal performance of sintered miniature heat pipes”. Heat and mass transfer 37 (2001): 611-616 spinger – verlag 2001. Woo-Sung Han and Seok-Ho Rhi (2011) “Thermal characteristics of grooved heat pipe with hybrid nanofluids”. Thermal Science, vol. 15, No. 1, pp. 195-206. Somasundaram, Dhanabal, Mani (2010) “Experimental studies on porous wick flat plate heat pipe”. International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2010. M.B. Shafii and A. Faghri, Yuwen Zhang, “Analysis of heat transfer in unlooped and looped pulsating heat pipes”. International journal of Numerical Methods for heat & fluid flow, Vol. 12 iss: 5, pp.585 – 609,2006. Senthilkumar R, Vaidyanathan S, SivaramanB (2010) “Performance analysis of heat pipe using copper nanofluid with aqueous solution of n-butanol”. International Journal of Mechanical and Materials Engineering 1:4 2010. Pramod R. Pachgharea, Ashish M. Mahalle “Thermal performance of closed loop pulsating heat pipe using pure and binary working fluids”. Frontiers in Heat Pipes, 3, 033002(2012), Senthilkumar, Vaidyanathan S, Sivaraman B “Thermal analysis of heat pipe using taguchi method”. International journal of engineering science and technology: vol.2(4), 2010, 564- 569. R Manimaran, K Palaniradja, N Alagumurthi : “Effect of filling ratio on thermal characteristics of circular heat pipe using nanofluid”, Frontiers in Heat Pipes (FHP) 3 - 023004(2012). G. Kumaresan, S. Venkatachalapathy : “A review on heat transfer enhancement studies of heat pipes using nanofluids”, Frontiers in Heat Pipes (FHP) 3 - 043001(2012). R. Manoj, M. C. Kumar, R. Narasimha Rao, K. Rama, Narasimha, P. V. S. Suresh : “Performance evaluation of sodium heat pipe through parametric studies” :Frontiers in Heat Pipes (FHP) 3- 043003(2012). Dao Danh Tung, Shuichi Torii; “Heat transfer performance of a self-oscillating heat pipe using pure water and effect of inclination to this performance”. ASEAN Engineering Journal :vol.2 No.1 July 2012. Jeong Se Suh and Young Sik Park, “Analysis of thermal performance in a Micro Flat Heat Pipe with Axially Trapezoidal Groove”, Tamkang Journal of Science and Engineering, Vol.6, No.4, pp 201-206,2003
ScienceDirect Materials Today: Proceedings 2 (2015) 2179 – 2187
Thermal Analysis of Gravity Effected Sintered Wick Heat Pipe B ChNookarajua, PSV KurmaRaob, S Naga Saradac aAssociate
Professor, Department of Mechanical Engineering GokarajuRangaraju Institute of Engg& Technology, Hyderbad.. b General Manager (Retd), R&D, BHEL Ramachandra Puram, Hyderabad. c Professor Department of Mechanical Engineering, JNTUH, Kukatpally, Hyderabad
Abstract The rapid growth in technology nowadays has resulted in the densification of electronic components, which requires a highly reliable thermal management system. The heat pipes are the most promising heat transfer devices capable of transmittin g high heat fluxes rejected by the modern electronic equipment. In the present study, the experimental analysis of thermal performance of a sintered-wick heat pipe is carried out at various inclinations with gravity assisted tilt. The results have shown that the performance of heat pipe varies with the geometrical orientation of heat pipe and it is observed the heat carrying capacity of heat pipe is well pronounced at 600 gravity assisted tilt followed by 900. © 2015 2014Elsevier The Authors. Ltd. All rights reserved. © Ltd. AllElsevier rights reserved. Selection under responsibility of theofconference committee members of the 4thofInternational conferenceconference on Materialson the 4th International Selectionand andpeer-review peer-review under responsibility the conference committee members Processing and Characterization. Materials Processing and Characterization. Keywords: Electronics Cooling, Sintered wick Heat Pipe, tilt angle, Thermal performance.
1. Introduction: A heat pipe is a heat transfer device with an extremely high effective thermal conductivity. Heat pipes are evacuated vessels, typically circular in cross sections, which are back-filled with a small quantity of a working fluid. They are totally passive and are used to transfer heat from a heat source to a heat sink with minimal temperature gradients, or to isothermal surfaces.
Fig 1: Operation of Heat Pipe
* Corresponding author. Tel.: 91-98-660-55979 E-mail address:[email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.230
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B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
Fig 2: T-S Diagram of Heat Pipe
In a Heatpipe heatenters through thecopper casing material and it will reach the liquid on the inside of the evaporator section as in Fig 1. The energy is absorbed by the liquid through conduction and it then vaporizes because the vessel is in the saturatedtwo-phase (liquid-vapor) state. Any additional added energy goes into latent heat of vaporization. During the process of vaporization, the specific volume of the water increases about 1600 times (at standard pressure). The dramatic increase in volume results in an increase of local pressure in the evaporator region. The increased pressure drives the vapor through the adiabatic region to the lower pressure condenser region. If the surface temperature of the condenser is held lower than the saturation temperature of the vapor, then condensation will occur. Capillary forces combined with a liquid pressure drop in the channels (of wick) drive the water back to the evaporator section. This cycle of conduction, vaporization, convection, condensation, and capillary flow occurs in a continuous cycle in properly functioning heat pipes. These processes are shown on a temperatureentropy (T-S) diagram in the below Fig 2. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on temperature difference between the ends of the pipe, and cannot lower temperatures beyond the ambient temperature at either end. When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapor pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporization of the working fluid reduces the temperature at the hot end of the pipe.The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapor pressure over condensing working fluid at the cooler end of the pipe, this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapor impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapor pressures are low. The speed of molecules in a gas is approximately the speed of sound, and in the absence of non-condensing gases (i.e., if there is only a gas phase present) this is the upper limit to the velocity with which they could travel in the heat pipe. In practice, the speed of the vapor through the heat pipe is limited by the rate of condensation at the cold end and far lower than the molecular speed. The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. While making heat pipes, there is no need to create a vacuum in the pipe. Simply boils the working fluid in the heat pipe until the resulting vapor has purged the non-condensing gases from the pipe and then seal the end. An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspected that water charged heat pipe would only work, when the hot end reached the boiling point (100°C) and steam was transferred to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0°C). Thus the heat pipe can
B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
operate at hot-end temperatures as low as just slightly warmer as the melting point of the working fluid. Similarly, a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid. The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. Theheat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed to evaporate one gram of water is 540 times the amount of energy needed to raise the temperature of that same one gram of water by 1 °C. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts. Heat pipes are hollow metal tubes that efficiently conduct heat from one location to another. They operate by means of a small amount of working fluid contained in a sealed tube, held under a slight vacuum. The vacuum lowers the boiling point of the working fluid, so relatively small increases in temperature vaporize the liquid which is then naturally drawn towards the colder end of the heat pipe where it condenses back to liquid. An internal wick structure then acts to return the condensed working fluid back to the hot end of the heat pipe, by a force called capillary action. When you put the edge of a paper towel in a small puddle of water, this is the force that soaks up the liquid into the paper. The crux of the situation is that some wick structures are more efficient than others, and some have limitations with respect to orientation and gravity. In the present study, capillary wick is made of sintered powder which adheres to the inner walls of the heat pipe. This acts to transport the fluid through capillary action. Choosing a sintered structure as the heat pipe wick will provide high power handling, low temperature gradients and high capillary forces for anti-gravity applications. Very tight bends in the heat pipe can be achieved with this type of structure. Powder particles are diffused together and to the tube wall to form a sintered wick structure. Copper powder is the most common material used to produce sintered wick structures. Sintered wick structures using smaller powder particles can lift the working fluid a greater distance than a wick structure fabricated with larger particles. The pumping capability of a sintered wick structure is superior to the capability of a screen mesh or grooved wick structure. A sintered wick heat pipe can be used in applications with radial heat fluxes up to 250 W/cm2. In 2008, X. Zhao et. al [1] illustrated a one dimensional mathematical model for heat and mass transfer. In a miniature heat pipe with a grooved wick structure is developed and solved numerically to yield he capillary, liquid and vapor pressure distribution and the overall thermal resistance under steady-state condition. To verify the model, experimentation for measuring the maximum heat transfer rateand the thermal resistance are conducted and the maximum errors are obtained. Physical insight into transport phenomena in the miniature heat pipe with grooved wick structure has been acquired. From the results, In addition, the model is used for optimizing the wick structure thermally and hydro dynamically. It is estimated that the maximum heat transfer rate of an o.d. 15.88 mm with an optimized grooved wick structure can be enhanced up to 12.6% and the total resistance can be reduced up to 9.9%. In 2003, Jeong- se suh et al [2] describes that numerical analysis is made on the thermal performance of micro heat pipe in an axial flat grooved channel. The flow of liquid and vapor is investigated in trapezoidal grooved and the effect of variable shear stress along the interface of the liquid and vapor consideration. The result from this study are obtained in the axial variation of pressure difference between vapor and liquid, contact angle, velocity of vapor and liquid and so forth. In addition, maximum heat transfer capacity of micro heat pipe is provided by varying the operation temperature and compared with that from Schneider and devos’s model in which interfacial shear stress is neglected. In 2012, G. Kumaresan, S. Venkatachalapathy [13] provided a review on heat transfer enhancement studies of heat pipes using nano fluids. They studied the influence of various factors such as heat pipe tilt angle, charged amount of working fluid, nano particles type, size, and mass/volume fraction and its effect on the improvement of thermal efficiency, heat transfer capacity and reduction in thermal resistance. The nano fluid preparation and the analysis of its thermal characteristics also have been explained. In 2012, R. manimaran et al. [12] studied the effect of filling ratio onthermal characteristics of circular heat pipe using nano-fluid and water. The Cuo/water nanofluid is used as the working fluid of experimental heat pipes with concentration of 1.0 wt. %. The test section of the heat pipe is made of copper tube with outer diameter 22 mm, inner diameter 20.8 mm and length 600 mm. This study focuses on the effects of heat input, fill ratio and angle of inclination on the thermal efficiency and thermal resistance of heat pipe. The results indicated that the thermal efficiency increases when nano particles are added with Di-water and also the thermal resistance of the nano fluid heat pipe decreases compared with that of the heat pipe using base fluid alone.
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In 2012, Pramod R. Pachgharea and Ashish M. Mahalleb [10] presented preliminary experimental results on thermal performance of closed loop pulsating heat pipe (CLPHP). The copper capillary tube was used having internal and external diameter 2.0 mm and 3.6 mm respectively. For all experimentation, filling ratio (FR) was 50 %, number of turns was 10 and different heat inputs of 10 to 100W were supplied to PHP. For all HPs, Vertical bottom “heat mode” (+900) position is maintained. The equal lengths of evaporator, adiabatic and condenser sections were 50 mm each. Working fluids are selected as Methanol, ethanol, acetone, water and different binary mixtures. The graphs are plotted, in order to study, characteristics of the thermal resistance and average evaporator temperatures at different heat input for various working fluids. Experimental study on PHP indicated that working fluid is an important factor for the performance of PHPs. The result shows that, the thermal resistance decreases more rapidly with the increase of the heating power from 20 to 60W, whereas slowly decreases at input power above 60 W. Pure acetone gives best thermal performance in comparison with the other working fluids. No measurable difference has been recorded between the PHPs running with pure and binary mixture working fluids. In 2010, Senthilkumar R et al. [9] conducted experiments to improve the thermal performance of heat pipe using copper nano-fluid with aqueous solution of n-Butanol. The nano-fluids kept in the suspension of conventional fluids have the potential of superior heat transfer capability than the conventional fluids due to their improved thermal conductivity. In this work, the coppernano-fluid which has a 40 nm size with a concentration of 100 mg/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. The study discusses aboutthe effect of heat pipe inclination, type of working fluid and heat input on the thermal efficiency and thermal resistance. The experimental results are evaluated in terms of its performance metrics and are compared with that of DI water. 2.Experimental Design and Procedure The main principle used to determine the amount of heat transmitted from the condenser to the cooling water. Fourier’s law of heat conduction Input heat flux (q) = Q/A, Q amount of heat supplied The amount of axial heat flux transferred from evaporator section to condenser section is given by
ݍൌ
ο் ο௫
(1)
Heat transfer efficiency of heat pipe iscalculated by η= (q / qin) (2) q= m Cp ΔT Where, Q = Amount of heat transfer K=thermal conductivity of a copper rod = 374 KW/m° C A= Area of the heat pipe = (πd2) / 4 m= Water flow rate in kg/Sec. in the cooling water jacket from Rota meter. Cp= Specific heat of water, 4.18 KJ/ Kg° C. A= π D L (3) D=diameter of a heat pipe=15.88mm L=length of a heat pipe=565mm ∆Tw=difference of condensing section temperatures = T8 – T7 (°C) T8=water temperature at outlet (°C) T7=water temperature at inlet (°C) ΔX= thermocouple distance difference in mm ο்ೢ ൈ ͳͲͲ (4) Ꮈ ൌ ୧୴ୣ୬୦ୣୟ୲୧୬୮୳୲
Where,Ƞ- efficiency of a heat pipe 3. Experimental Investigations: 4.1 Experimental aspects The apparatus consists of a sintered wick heat pipe with cloth heater at evaporator end and the other end of the heat pipe is cooled with flow of water by connecting with a water jacket condenser section. The heater coil
B. Ch. Nookaraju et al. / Materials Today: Proceedings 2 (2015) 2179 – 2187
encloses the heat pipe up to a length of 200mm.The heater is enclosed with glass wool in order to ensure that there are no heat losses to the surroundings and to reduce the environmental effects. The heater is connected to a dimmer stat through which heat input given to the evaporator section of the heat pipe can be controlled. The display unit consists of dimmer stat, thermocouples, oneliter jar, and stop watch. The temperature distribution is recorded for different heat inputs and water flow rates in the evaporator and condenser sections. The Heat flux is calculated for the Sintered wick heat pipe. The cooling jacket is provided with inlet and outlet so that the direction of the water flow is opposite to that of the working fluid inside the heat pipe.
Fig 3: Experimental Setup showing Sintered Heat Pipe Oriented in different inclinations
This is done because in counter flow, more heat transfer takes place between the working fluid inside the heat pipe and the water flowing in cooling jacket when compared to parallel flow. The heat pipe is a hollow tube of length 565mm and diameter 15.88mm, filled with water as a working fluid and the maximum operating temperature is 2550C. The heat pipe is provided with markings, where the temperatures are to be measured. Experiment is doneat constant heat flux conditions and heat input of 25 W to 125W, with constant flow discharge of water at different inclinations as shown in Fig3.
Fig 4. Geometry of Heat Pipe
Fig 5. Temperature distribution along a Heat Pipe
The heat input to the evaporator was set using adimmer stat (variac) eight thermocouples were placed on the surface of the sintered wick heat pipe to measure the temperatures of evaporator, adiabatic and condenser section. Among all the thermocouples only two thrermocouples are used to measure inlet and outlet temperatures of cooling water. 4.2 Experimental results The experimental rig and a schematic view of the experimental setup are shown. With different heat flow rates and with different inclinationswere examined. Measured temperature variations along the Sintered wick heat pipe at different heat inputs and inclinations are show below.
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Fig 6. Temperature variation along the Sintered wick heat pipe
From the Figs. (6)- (8), as a result it is observed that the temperature decreases with increase in length along the heat pipe at different heat fluxes. By measuring water flow rate in kg/sec, rise in temperature of water inlet and outlet in the cooling water jacket ('TW), calculate Q/A value in W/m2 by assuming the thermal conductivity of copper material as 374W/m K. As the flow of cooling water increases the dissipation increases. The Cooling capacity of Sintered wick Heat pipe is higher as compared with thermo-syphon heat pipe having the same diameter and length.
Fig 7 (a)
Fig 7 (b)
Fig 7 (c)
Fig 7 (d)
Fig 7. Plot for length vs. temperature of heat pipe at different inclinations
Fig 8. Plot for variation of temperatures along the length of the heat pipe in different orientations
Figs. (8), From the variations the experimental results shows that, with increasing inlet heat input and Heat pipe enhances the heat transfer at exit and increases the performance of the any thermal system. However, applying higher heat energy to the evaporator decreases the performance, therefore it was found that Sintered wick Heat pipe has higher cooling capacity and higher thermal efficiency than thermo- syphon heat pipe.
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Figs,(9)-(11)it is observed that ,as the evaporator section is moving towards the ground the heat transporting ability of heat pipe is increasing upto600 and from the non wards it goes on decreasing. The reason for increase in efficiency of heat pipe from 0 0to 600 inclination is, as the evaporator section moves towards the ground, the condensed liquid returns to the evaporator section easily because of the assistance of gravitational forces in favor of capillary forces. The decrease in efficiency from 600 inclination of pipe is because the gravitational forces will oppose the movement of evaporated fluid from evaporator section to condenser section.
Fig 9. Plot for time vs. heater temperature of heat pipe at differentinclinations.
Fig 10. Plot for tilt angle vs. efficiency of heat pipe
The figure indicates that temperature profile along the pipe has a similar trend for different inlet heat fluxes, the results shows higher temperature at the evaporator decreased as passing through adiabatic and condenser section. In addition, the effect of increasing the inlet heat flow on the wall temperature is more significant in the evaporation section, rather than the adiabatic and condensation section. In addition, rising temperature trend happened at the upper region of condenser, more obviously at the higher heat input to the evaporator.
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Fig 11. Thermo-syphon performance at various energy inputs
The performance of a Sintered wick heat pipe at different heat inputs and inclinations are calculated and plotted. The maximum performance can be observed at input energy of 125W. In addition, the results show that the inclination has a significant effect on performance of Sintered wick heat pipe and in this study the best performance was obtained at 60o tilt angle in the results. 4. Conclusion In this paper, experiments were conducted to achieve a better understanding of sintered-wick heat pipe w i t h c o p p e r c a s i n g a n d de-ionized w a t e r as working fluid. The results obtained in the experiment and conclusions are summarized as follows. The pipe specifications are 5 6 5 m m length,15.88mm diameter heat pipe is subject different inclinations to investigate the heat pipe efficiency. a. at00inclination,the efficiencyofheatpipeis47.14% , b.at300inclination,theefficiencyofheatpipeis47.41%. c.at600inclination,theefficiencyofheatpipeis53.73% d.at900inclination,theefficiencyofheatpipeis49.28%. From the above results it is observed that, as the evaporator section is moving towards the ground the heat transporting ability of heat pipe is increasing upto600 and from then onwards it goes on decreasing. The reason for increase in efficiency of heatpipe from 0 0to 600 inclination is, as the evaporator section moves towards the ground, the condensed liquid returns to the evaporator section easily because of the assistance of gravitational forces in favor of capillary forces. The decrease in efficiency from600inclination of pipe is because the gravitational forces will oppose the movement of evaporated fluid from evaporator section to condenser section. 5. Acknowledgement The work presented in this article was funded by the All India Council for Technical Education (AICTE) under Research Promotion Scheme, File No.8023/RID/RPS-20/Pvt(II Policy)/2011-12.The authors gratefully express their thanks to the for providing financial assistance to purchase the equipment for this project and all other related expenses. 6. References 1. 2.
K. N. Shukla; A. Brusly Solomon, B C. Pillai, Mohammed Ibrahim “Thermal performance of cylindrical heat pipe using nano fluids”, Journal of thermo physics and heat transfer, vol. 24, no. 4 (2010), pp. 796-802. Ji Li, Daming Wang, G.P. Peterson, “Experimental studies on a high performance compact loop heat pipe with a square flat evaporator”. Applied Thermal Engineering, Volume 30, issue 6-7 (May, 2010), pp. 741-752.
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