Investigation On Thermal Performance Of Pulsating Heat Pipe With Variable Diameter

Miniaturized and integrated electronic components are confronted with the problems of over temperature and reduced reliability.As a passive heat transfer element,pulsating heat pipe(PHP)is considered to be one of the effective methods to solve this problem because of its small volume and high heat transfer efficiency.In order to better meet the heat dissipation requirements of small portable electronic equipment,three single-loop PHPs with different diameter ratios(4:4,4:3and 4:2)are constructed,and their start-up performance and heat transfer performance are numerically probed in present work.In addition,based on the above research,this paper also establishes the three-dimensional physical model of multi-turn PHP,and improves the length ratio of evaporation section/condensation section to achieve stable operation in the horizontal direction.Simultaneously,the internal flow characteristics of PHP are analyzed by Fast Fourier transform.The results show that gravity significantly affects the start-up and heat transfer performance of uniform PHP.While for variable diameter PHP,due to the existence of additional pressure difference,the sensitivity of PHP to gravity is reduced,and even the start-up phenomenon can still be observed in the horizontal position.When placed vertically,variable diameter PHP and uniform PHP show two different start-up processes,in which the former has obvious temperature overshoot and the latter maintains stable temperature oscillation.In addition,under the condition of the heating power of 20 W,the PHP with diameter ratio of 4:2 has the greatest thermal resistance,while the PHP with diameter ratio of 4:3 has the least thermal resistance.In particular,when the inclination angle is 10°,the thermal resistance of the PHP with a diameter ratio of 4:3 is reduced by 3.37% compared with the uniform PHP.Besides,the larger the diameter ratio is,the easier the PHP is to dry-out due to less liquid mass and greater flow resistance with the same filling ratio.It is also found that the multi-turn PHP with a length ratio of 1:1 displays good directional circulation characteristics in a certain inclination range(90°-30°).While the inclination angle is 0°,the PHP cannot maintain stable operation and occurs local drying.However,when the length ratio is less than 1,due to the increase of cooling area,the PHP can maintain normal operation in the horizontal position.Furthermore,the length ratio of evaporation section/condensation section significantly affects the thermal performance of PHP.Under the heating power of 80 W,the start-up time of horizontal PHP with length ratio of 1:1.2 is only one third of that with length ratio of1:1.4.Meanwhile,with the decrease of length ratio,the thermal resistance of PHP gradually decreases,especially at low and medium power,the maximum thermal resistance can be reduced by 9.1%.Besides,according to the spectrum analysis,the length ratio of evaporation section/condensation section can affect the fluid oscillation in the tube.When the length ratio is only 1:1.6,the main frequency corresponding to the temperature signal disappears,which means a stable periodic motion cannot be formed in PHP.