Study On Flow Boiling Heat Transfer Of Microchannel Cooling Unit For High Power IGBT

In this paper,high power cooling requirements of IGBT modules in high-speed train traction inverters were taken as research backgrounds.Based on the investigation of traditional IGBT cooling technologies and combined with high efficiency and compactness of micro-scale flow boiling heat transfer,the IGBT module heat dissipation scheme based on microchannels cooling unit was proposed.A natural circulation cooling system and a forced circulation cooling system were established.The study contents included:start-up and heat transfer characteristics of microchannels unit-natural circulation cooling system;flow boiling heat transfer laws of R134a in microchannels;transitions of flow patterns and heat transfer mechanisms in microchannels;microchannels structural optimization based on experimental data and theoretical research.The start-up process of natural circulation cooling system was divided into three stages,and all the points in the loop constituted a stable thermodynamic cycle after successfully started-up.Heat transfer performance of the system was analyzed.Charging ratio and heat flux were the main factors affecting heat transfer of microchannels unit under this system.Bubble detachment diameter and frequency were both single-valued functions of heat flux in the nuclear boiling.The higher the heat flux was,the larger the detachment frequency was,thus a greater pressure difference between evaporater and condenser would be,which led to a larger mass flux and accelerated the start-up and flow patterns transition.Based on the existing studies,a criterion for the transition from bubbly flow to slug flow in microchannels under adaptive conditions was proposed.Heat flux,mass flux and saturation temperature were the main factors affecting the heat transfer of microchannel unit under forced circulation.The heat transfer coefficient changed with increasing heat flux in an"M-shaped"curve,and increased with increasing mass flux and saturation temperature.The higher the saturation temperature was,the lower the wall superheat and local vapor quality would be,thus the flow patterns were more inclined to the nuclear boiling region.On the contrary,the flow patterns were more inclined to the convection evaporation region.Heat transfer performance of the slug flow was the best in the nuclear boiling.While the annular flow liquid film reached the thinnest in convective evaporation,heat transfer coefficient showed a maximum value.In the transition zone of slug-annular flow,the two mechanisms coexisted.Conversion process between the two mechanisms inhibited flow boiling heat transfer.Considering the influences of various factors on vapor quality and flow patterns,the correlation for predicting vapor quality in transition of slug-annular flow was proposed,and it was proved to be well predicted.Shortening the length of channels in a certain range was proved to be able to control vapor quality at the middle and outlet of the channels,thereby controlling the mainstream flow pattern to be slug flow or annular flow with the thinest liquid film,which helped to improve heat transfer coefficient of the microchannels unit.The channel length to diameter ratio was established as a function of the midpoint quality,which could be used to guide the optimal channel length and its segmented structure design under the condition of known IGBT module size.