Study On Heat Transfer Of Flow Boiling In Rectangular Channel With Extreme Wettability And Micro/Nano Structure Surface

Microchannel heat exchanger has the advantages of compact structure and high heat transfer efficiency,and it has wide application prospect in high power electronic chip cooling,aerospace and other fields.As a kind of passive heat transfer,the surface modification has a significant effect on the boiling heat transfer process.The surface modification can further enhance the boiling heat transfer performance at the microscale.Therefore,it is of great significance to explore the flow boiling heat transfer in the microchannels with different surface characteristics.Firstly,super-hydrophilic,super-hydrophobic surface aluminum-based microchannel are prepared by chemical etching and surface fluorination,the static contact angle of them are 0° and 160.2° respectively.The micro-nanometer shaped rectangular pits and convex aluminum structures of the projections are observed by SEM on the surfaces of super-hydrophobic and superhydrophobic microchannel.The results show that these microstructures are the key to the preparation of extreme wetting surfaces.Secondly,in order to investigate the flow boiling heat transfer performance in the extreme wetted surface channels,the R141 b refrigerant is used as the experimental medium,and the two-phase flow boiling heat transfer experiment is carried out in the super-hydrophobic and super-hydrophobic micro-nanostructure surface microchannels,compared with the ordinary smooth surface channel.The heat transfer mechanism is analyzed based on the bubble dynamics.The results show that the wall activation cores of super-hydrophobic surface are easy to be activated due to the need for less energy to produce bubble,and the wall superheat is 2.86 ?,which is lowest in three different surfaces microchannel.When the heat flux is less than 16kW/m2,the superhydrophobic surface channel exhibits the best heat transfer performance,and its average saturated boiling heat transfer coefficient is 1.15~1.32 times of the smooth surface channel.When the heat flux is higher than 25 k W/m2,the bubbles of super-hydrophobic surface microchannel are easy to polymerize and adhere to the heating wall,which is not conducive to the recharge of the liquid.The heat transfer coefficient is decreased and gradually lower than that of the super-hydrophilic surface and smooth surface channel.The bubble generated by the super-hydrophilic surface channel is small and easy to detachment from the wall.At the high heat flux density,the activation point of the wall is activated in a large amount.The surface property of super-hydrophilic can make the liquid replenishment timely when the bubble detachment from the wall,which enhanced the boiling heat transfer process.And the super-hydrophilic surface microchannel showed the best heat transfer performance in three kinds of surface channel.Finally,based on the experiment of boiling flow heat transfer on different surface microchannels,considering the influence of wall characteristics on boiling heat transfer,the surface energy parameters are introduced,establishing the prediction model of two-phase boiling heat transfer coefficient of support vector machine based on different surface characteristics.The absolute percentage error between the predicted value and the measured value is 2.96% and the average correlation coefficient is 96.51%,which is greatly improved when compared to not introducing surface energy parameters.