Study On The Characteristics Of Fluid Flow And Heat Transfer In Pulsating Heat Pipe With Wick

Because of the excellent working adaptability and equivalent thermal conductivity,the pulsating heat pipe(PHP)is regarded as a new type of heat transfer element used to solve the problem of high heating flux removal in a small space.However,the saturation pressure difference caused by the temperature difference between the hot and cold ends is the main driving force in PHP.Therefore,compared with the traditional heat pipe with wick,the temperature uniformity of PHP is not ideal,and the unicity of driving force limites the capacity of liquid supply in the evaporation section and the adaptability of gravity,which makes PHP still need to be further improved.Therefore,this article introduces the sintered wick into the design of the inner wall structure of PHP,so as to provide an additional capillary driving force for improving the wettability of the PHP’s inner wall,which especially including the capacity of liquid supply in the evaporation section,and thus enhancing the comprehensive heat transfer performance and the adaptability of gravity of PHP.Based on this idea,a new type of PHP with wick(PHPWW)is developed in this paper,and a test system for its heat transfer performance is also designed and built.This article carries out the research on the flow and heat transfer characteristics of PHPWW and traditional pulsating heat pipes with no wick(PHPWNW)under different heat loads,tube diameters and working inclination angle(gravity),which focusing on the advantages of PHPWW in the comprehensive heat transfer performance and the adaptability of gravity.Specifically,the main work and conclusions of this article are as follows:(1)In order to achieve effective comparison and verification.PHPWNWs with different tube diameters are developed,and the different inner diameters are 2mm 3mm and 4mm respectively.Acetone is used as the working fluid,and the filling ratio is 55%.Under different heat loads and working inclination angles,gas-liquid two-phase flow behavior in the PHPWNW,as well as starting characteristics,temperature uniformity and heat transfer performance are studied.The results indicate that with the increase of heat load,the PHPWNWs firstly experience static state(S)and intermittent pulsation state(S&P).Then the PHPWNWs experience pulsation(P)state and pulsation accompanied by circulation state(P&C),or experience pulsation accompanied by static and circulation state(S&P&C).At last,the PHPWNWs enter into cycle state(C),which means the PHPWNWs are in the optimum heat transfer mode.The PHPWNWs with different inner diameters cannot operate when the working inclination angle is 0°.The minimum heat loads,which correspond to different operating states when the PHPWNWs first enter into,increase as the working inclination angle decreases,so it is sensitive to the working inclination angle,and the PHPWNW with medium tube diameter 3mm has the highest sensibility.The minimum heat loads decrease as the tube diameter of PHPWNW decreases.The starting characteristics of PHPWNW with the small inner diameter 2mm is sensitive to working inclination angle,but the starting characteristics of PHPWNWs with the medium inner diameters 3mm and largre inner diameters 4mm are not sensitive to working inclination angle.When the working inclination angle is large,the smaller diameter of PHPWNW,the better the starting characteristics;when the working inclination angle is small,the larger tube diameter of PHPWNW,the better the starting characteristics.The temperature uniformity of PHPWNW shows a trend of improvement first and then deterioration with heat load increases,and it decreases with the decrease of the working inclination angle and decreases with the increase of the inner diameter of PHPWNW.The heat transfer performance of PHPWNW increases with the increase of heat load,and decreases with the decrease of working inclination angle.The heat transfer performance of PHPWNW is sensitive to working inclination angle,moreover,the sensibility to working inclination angle increases first and the decreases as the inner diameter increases.(2)PHPWWs with different inner diameters are developed,and the different inner diameters are 3.5mm 4.5mm and 5.5mm respectively.A non-contact infrared test platform for capillary driving performance is designed and built,and research on the capacity of capillary suction,which raises the working liquid level,of PHPWWs and PHPWNWs with different inner diameters is carried out.The results indicate that both PHPWWs and PHPWNWs have the capacity of capillary suction,which raises the working liquid level due to the hydrophilic nature of the copper material and surface tension.However,because of the high specific surface area of the porous structure the wick having,PHPWW’s capacity of capillary suction,which raises the working liquid level,is significantly higher than that of the PHPWNW(about 2-4 times),which can be used to enhance the wettability of the PHP’s inner wall,which especially including the capacity of liquid supply in the evaporation section.(3)PHPWWs with different inner diameters are developed,and the different inner diameters are 3.5mm 4.5mm and 5.5mm respectively.Acetone is used as the working fluid,and the filling ratio is 55%.Under different heat loads and working inclination angles,gas-liquid two-phase flow behavior in the PHPWW,as well as starting characteristics,temperature uniformity and heat transfer performance are studied,and these are compared with PHPWNW’s.The results indicate that,compared with PHPWNW,PHPWW has the same kind of working states,and PHPWW’s working sate will also change from the static state(S)to the cycle state(C)as the heat load increases.Within the same heat load range,the smaller the inner diameter the PHPWW has,the easier it is to enter cycle state(C).However,compared with PHPWNW,the inner diameter range,which makes it possible for PHPWW to enter into cycle state(C)within the same heat load range,is expanded.In particular,the comprehensive heat transfer performance and the adaptability of gravity of PHPWW are better than those of PHPWNW.The PHPWNWs with different inner diameters all can operate when the working inclination angle is 0°.And as the working inclination angle decreases,the minimum heat loads,which correspond to different operating states when the PHPWWs first enter into,decreases,which happens in PHPWWs with the inner diameters of3.5mm and 4.5mm,and it reveals that the decrease of the working inclination angle is conducive for the working state entering into cycle state(C).The minimum heat loads of PHPWW with the large tube diameter 5.5mm isn’t sensitive to the working inclination angle.Cycle state(C)is easier to form in PHPWW.Also,the starting characteristics and heat transfer performance of PHPWW have less sensitivity to working inclination angle than PHPWNW.In addition,the temperature uniformity of PHPWW also shows a trend of improvement first and then deterioration with heat load increases.Moreover,with the increase of heat load and decrease of working inclination angle,the advantage in temperature uniformity of PHPWWs gradually appears and expands compared with PHPWNWs,which means that the heat transfer performance of PHPWNW has obvious advantages over PHPWNW,and as the heat load increases and the working inclination angle decreases,the advantages continue to expand.Except for the working inclination angle 0°,the heat transfer performance of PHPWW is improved by an average of about 35.1% compared to PHPWNW.