Experimental And Theoretical Studies On Enhancing The Heat Transfer Characteristics Of Pulsating Heat Pipes

With the development of microelectronic technology,the heat dissipation problem has greatly affected the performance stability and service life of electronic devices.The Pulsating Heat Pipe(PHP)is one of efficient device with phase change heat transfer.It has become an effective heat dissipation device for solving the dissipation problem of high heat flux due to the advantages of simple structure,low cost and high stability.This work experimentally studied the impact of two parameters(tube diameters and working fluids)on the operating characteristics of PHP.The ultrasonic cavitation and the influence of ultrasonic action time on the performance of PHP were also tested.In addition,the numerical simulation results of thermal performance of PHP with different placement was exhibited and analyzed.The main works and conclusions are introduced as follws.(1)The testing system was designed and constructed which could experimentally study the comprehensive performance of PHP with basic variables of heating power.The heating power ranges from 5W to 25 W with an increasing step of 5W in the test.The results showed that the start-up time of PHP increased first and then decreased for the increasing of heating power and it had optimal heating power.Additionally,the heat transfer performance of PHP increased with the increasing of heating power.(2)The PHP with different diameters were tested to reveal the influence of the diameters on the heat transfer performance.Besides,the revolution of PHP was divided into the internal circulation and external circulation during its working process.The result showed that the external circulation of PHP has much higher temperature difference than that of internal circulation.So the external circulation promotes the start-up of PHP during it working process.The start-up characteristic and the heat transfer performance of PHP increased first and then decreased with the increasing of inner diameters which has the optimal diamater of 2mm.(3)The impact of working fluids(water,methanol and acetone)on the start-up characteristics and heat transfer performance of PHP were investigated with a diameter of 2mm.The results exhibited that the PHPs charged with methanol and acetone easily start-up at low heating powers,while it is more difficult to start-up for water.However,the PHP charged with water shows the better heat transfer efficiency at higher heating power.(4)The influence of ultrasonic wave on the comprehensive performance of PHP was investigated.The changes of the start-up characteristics,heat transfer performance and heat transfer efficiency per unit of PHP were tested and analyzed before and after ultrasonic wave.The results indicated that the ultrasonic could intensify the disturbance and phase transition of working fluids at low heating power and accelerate the start-up of PHP.However,the ultrasonic cavitation effect will lead excessive vapors at high heating power which not only worsen the operation stability of PHP,but aslo accelerates the generation of “dry-out phenomenon”.In addition,three different methods of applied ultrasonic were tested to reveal the different operating characteristics of PHP,which are the pre-applied ultrasound,the full-applied ultrasound and the non-applied ultrasound.The results indicated that the ultrasonic added in the start-up period was beneficial to accelerating the start-up and stable operation of PHP.(5)A novel sawtooth corrugated structure was proposed in this work.The numerical work was done to reveal the effect of locations of corrugated structure on the operation performance of PHP.The results showed that the PHP with sawtooth corrugated structure shows better heat transfer performance and start-up characteristics than that of traditional structure.