Experimental Investigations on Wickless and Wicked Multi-branch Heat Pipe in Different Orientations

Abstract

In the recent years, due to miniaturization in electronics and space application systems, significant reduction in the size of electronics devices and circuits have been observed which subsequently increases the cooling requirement of heat transfer device. In an electronic circuit of laptops, supercomputers with multiple CPUs, spacecraft etc., it is required to arrange the cooling system for multiple heat loads in the smallest possible space in view of power saving opportunities. Heat pipes have been proven to be highly effective and simple cooling devices for electronics and spacecraft applications. Conventional heat pipes work on single source and single sink methodology. One source is in contact with an evaporator of heat pipe which absorbs heat and this heat is carried away to the condenser using working substance and rejected to the heat sink. However, the concept of single heat pipe for multiple heat source is the emerging idea in the research community as far as the space constrain and compactness is concerned. In order to use single heat pipe for multiple heat source in electronics and space applications, a multibranch heat pipe with two evaporators and a condenser is developed. In the present study, initially, the experimental investigations are carried out on a wickless multi-branch heat pipe in gravity assisted mode. The start-up and dynamic characteristics are studied with different filling ratios (range: 40 - 70%), with equal heat loads (range: 0-200 W) and unequal heat loads (range: 0 -100 W) on evaporators. The results are analyzed in terms of temperature variation in axial direction, thermal resistance and heat transfer coefficient for a multi-branch thermosiphon heat pipe (MBTHP). It is found that the optimal filling ratio depends on the applied heat load under tested condition. In a thermosiphon mode, the heat pipe is capable of transporting maximum heat load of 210 W and maximum heat flux of 20.31 W/cm 2 with the maximum evaporator temperature lower than 100 °C. The minimum wickless thermal resistance of heat pipe is found to be 0.21 °C/W at 50% filling ratio and 160 W and maximum total heat transfer co-efficient is found as 6.33 kW/m2 °C. Moreover, a multibranch heat pipe with two evaporators and a condenser is developed with 20 internal grooves as a wick structure. Experimental investigation is carried out for axially grooved multibranch heat pipe (AGMBHP) by considering various parameters to obtain the best possible operating condition. AGMBHP is investigated for different filling ratios (range: 75% to 200%) in horizontal orientation for equal heat loads (0-240 W) and unequal heat loads (0-120 W) on evaporators. Heat pipe is tested for dynamic characteristic for optimum filling ratio range of 125% and 150%. Further, condenser cooling flowrate analysis is carried out to understand the performance of heat pipe. AGMBHP is capable to transmit 240 W heat load with a minimum resistance of 0.192 ℃/W in horizontal orientation. The optimum condenser cooling water flowrate is found to be 5 ml/s under tested conditions. AGMBHP is capable to provide effective thermal conductivity as high as 31,824 W/m℃ which is quite suitable for electronics cooling. Further, experimental study is performed by considering four novel types of orientations i.e. (a) horizontal orientation (HO) (b) gravity assisted orientation (GAO) (c) anti-gravity orientation (AGO) and (d) compound orientation (CO) for AGMBHP with 20 grooves. Results are analyzed in terms of start-up characteristics and total heat transfer coefficient at different heat loads. Evaporator and condenser thermal resistances are calculated and analyzed for better understanding. It is found that the horizontal orientation results in the highest overall heat transfer coefficient (2.72 kW/m2℃ at 240 W) and comparatively lower evaporator temperatures (less than 100℃ at 240 W). Maximum effective thermal conductivity is also achieved by horizontal orientation. It also resulted in lowest evaporator resistance (0.157 ℃/W) and lowest condenser resistance (0.114 ℃/W). Phenomena of temperature jump is observed, understood and elaborated for compound orientation. Finally, the comparative investigation is carried out on multi-branch heat pipe with 20, 16 and 12 numbers of axial grooves by keeping the remaining parameters constant. Results are analyzed in terms of thermal resistance; convective heat transfer coefficient and start-up temperature rise for the optimum filling ratio of each individual heat pipe. It is found that the choice of optimum number of grooves depends on the amount of heat to be transported and the duty of heat pipe for a given application. However, for the representative heat load of 240 W, heat pipe with 20 number of grooves has been proven to be optimum with the lowest temperature rise of evaporator and the lowest overall temperature variation in axial direction.