The Investigation On Enhancement Of Heat Transfer And Instability Of Flow Boiling In Microchannels

The microchannel heat sink based on flow boiling heat transfer can produce high heat transfer coefficient and uniform wall temperature distribution,which is one of the most promising micro-scale liquid-cooling radiators for the high heat flux microelectronic devices.However,with the climbing heat flux,the flow boiling instability has become one of the most key factors that can restrict the development of microchannels,which possibly causes the mass flux and pressure drop oscillation and leads to some control and safety issues.So,there is a pressing need for understanding the mechanisms of flow boiling instability,and then improving the flow boiling stability in microchannels by reasonable structural optimizations.In this paper,several types of microchannels with different materials and structures are designed and manufactured,the flow boiling heat transfer experimental systems are set,and the continuous two-phase unstable flow boiling and pressure drop unstable flow boiling are investigated theoretically and experimentally.The mechanisms of flow boiling instabilities are studied and some methods for improving the flow boiling stability are proposed.Firstly,the straight and triangular corrugated microchannels with rectangle cross-sections are machined on silicon wafers,and a flow boiling experiment system with a group of synchronous multi-sensors is set up,the working fluid in which is driven by the high-pressure nitrogen for ensuring the low compressibility in the upstream of test section.The experiment results show that the heat transfer coefficient in triangular corrugated microchannels can be as high as 1.25 times of that in straight microchannels,and the wall temperature on the onset of boiling in corrugated microchannels is also lower than the straight microchannels under similar working conditions.They prove that the triangular corrugations can effectively enhance the flow boiling heat transfer at the cost of increasing the pressure drop in microchannels.The single-phase/two-phase interfaces in the straight microchannels are more regular than that in straight microchannels,because of the random backflow phenomenon.The nucleation cavities in triangular microchannels are much closer to inlet in comparison with the straight microchannels,which could improve the backflow and weaken the flow boiling stability,thus,the triangular corrugated microchannels have severe wall temperature and pressure drop oscillation under the working conditions of high mass flux.Secondly,the flow pattern transition and wall temperature variation under different flow boiling status are studied by using the high-speed microscope and infrared thermal camera,and a mathematical model for predicting the flow boiling stable boundaries is developed based on the single-phase forced convection heat transfer correlation and two-phase frictional pressure drop correlations.The calculation results show that the flow boiling stability can be improved by reducing the inlet temperature,increasing the heat transfer coefficient in the upstream of microchannels and installing throttle elements at inlet.Because the throttle elements could increase the pressure drop of microchannels sharply,the throttling parameters should be calculated according to the mathematical model and actual needs.Moreover,a dynamic simulation model for the continuous two-phase unstable boiling in the rectangle straight microchannels is developed based on the thermal network method and measured experiments data.The calculation results indicate that the inflection points and negative slope zone in solid/fluid temperature difference vs.heat flux curve can be eliminated by enhancing the single-phase and flow boiling heat transfer coefficient,then the energy exchange between wall and fluid is prevented and the continuous two-phase unstable flow boiling is suppressed.Thirdly,the 6063 aluminum alloy based microchannels with rectangle cross sections are designed and machined,a flow boiling experimental system with adjustable flow resistance configurations and compressibility in its upstream is set up,and the working fluid is driven by the high precision syringe pump for ensuring the precision required.The experiment and calculation results show that the pressure drop unstable flow boiling occurs when the compressibility in the upstream of microchannels is notable,which can lead to mass flux and pressure drop oscillation with high amplitudes and long period.At the same time,the flow pattern transition in microchannels is single-phase/two-phase alternating flow.The oscillation period and amplitude can be reduced by increasing the frictional pressure drop in the upstream,the vapor quality of flow boiling stable boundaries rise as well,especially under the working conditions of low mass flux.By comparison,the oscillation period and amplitude can be hardly effected after increasing the local pressure drop in the upstream,but the vapor qualities of flow boiling stable boundaries still can increase significantly.Finally,three rectangle microchannels with different depths and inlet structures are machined on the 6063 aluminum alloy,a flow boiling experiment with a closed fluid loop is set up and the effects of microchannels structures on pressure drop unstable flow boiling are studied.The experiments indicate that the hydrodynamic curve of working fluid has a negative slope zone when the working fluid is boiling in the microchannels,and the average slope of the negative slope zone increase with the increasing of microchannels' depth.The high average slope makes against the momentum exchange between the working fluid in microchannels and compressive volume,and then improves the flow boiling stability.The guiding structure at microchannels' inlet could lead to the parallel interaction and flow misdistribution,but the average slope under the working conditions of low vapor quality increase at the same time.According to the comparisons between microchannels with and without guiding structures,it can be found that the guiding structure can increase the vapor quality of stable boiling boundaries and improve the flow boiling stability under the working conditions of low vapor quality effectively.