Experimental Study On The Flow Boiling Heat Transfer In An Open Microchannel

With the progress of science and technology,the number of electronic components integrated in unit area is increasing,and the heat flux of the unit area of electronic equipment is greatly improved.The performance and life of equipment are closely related to the heat dissipation capability.How to quickly and effectively take the heat generated by electronic components in time is an important and arduous task.In the phase change process,because of the absorption of latent heat of vaporization from liquid to gaseous state,the effective heat dissipation can be achieved in the phase change process.Therefore,the cooling technology based on the flow boiling heat transfer in microchannel has great application prospects in the field of heat dissipation of microelectronic devices.In conventional microchannels,with the restriction of the rib wall,the bubbles grow rapidly and fill the cross section of the channel with extending along the length of the microchannel,The blocking effect of bubbles on the channel and the asynchrony of phase transition in parallel channel make the pressure difference between parallel channels which cause the uneven distribution of fluid between parallel microchannels,and the large pulsation of flow and temperature,producing a large thermal stress which causes the thermal fatigue damage of the chip.Aiming at the problem of heat transfer deterioration caused by phase change heat transfer in ordinary microchannels,a new type of open microchannel phase change cooler is designed and manufactured in this paper,the pressure balance between channels is realized by setting the connection cavity above the parallel channel,thus improving the performance of heat transfer.The heat transfer performance of the new microchannel phase change cooler and ordinary microchannel phase change cooler is compared,and the influence of microchannel structure and placement direction on flow pattern and heat transfer characteristics in flow boiling process is explored.In order to further improve boiling heat transfer coefficient and critical heat flux,nano Ag/Ni composite structure was prepared on the surface of the new open microchannel phase change heat exchanger,and the influence of micro nano composite structure on boiling heat transfer and flow pattern was obtained through experiments.It is found that there are two heat transfer mechanisms of nucleate boiling and thin liquid film evaporation heat transfer in microchannel phase change cooler.The open microchannel has enhanced effect on nucleate boiling and thin liquid film evaporation heat transfer mode,making heat transfer coefficient significantly improved.The calculation shows that under vertical placement and horizontal placement,the average heat transfer coefficient of the open microchannel is 136%and 32%higher than that of the ordinary microchannel respectively.The influence of channel placement direction on the flow boiling heat transfer characteristics is also studied.It is found that the influence of channel placement direction on the common microchannel and the open microchannel is quite the opposite.For ordinary microchannels,the average heat transfer coefficient under horizontal placement is greater than the average heat transfer coefficient vertically placed,when the refrigerant mass flow boiling G=0.95 g/s,number Bo=0.0028,the average change ratio of thermal conductivity and has a maximum value of 1.34;The average heat transfer coefficient of the vertical connection of the open microchannel is greater than the horizontal heat transfer coefficient.When the mass flow rate is G=0.54 g/s and the boiling number is Bo=0.0019,the ratio of heat transfer coefficient reaches the maximum value of 1.53.The effect of microstructures on the boiling heat transfer in the open microchannel was studied by brush plating the Ag/Ni micro-nano composite structure on the open microchannel.It is found that the microstructures can significantly enhance the heat transfer under the different direction of the microchannel,Under vertical placement and horizontal placement,the heat transfer coefficient of the open microchannel with Ag/Ni micro-nano composite structure is 228%and 404%higher than that of the common microchannel respectively.According to the influence of placement direction,it is found that the average heat transfer coefficient of the open microchannel heat exchanger with Ag/Ni micro-nano composite structure under horizontal placement is significantly higher than that under vertical placement,and the maximum heat transfer coefficient is increased by 97%.The critical heat flux density is greatly enhanced by the brush plating micro nanocomposite structure on the surface of the open microchannel,when the mass flow rate is G=0.54g/s,the micro-nano composite structure increases the maximum surface critical heat flux density by 33%.