Study On Flow And Enhanced Heat Transfer In Parallel Rectangular Sudden Expansion Microchannels

With the progress and development of microelectronics and integrated circuits,electronic chips are developing towards the direction of multi-function,high-power,high-degree and micro-miniaturization.With the increasing power,the higher heat flux produced by electronic chips can not be removed,the surface temperature rises sharply,and the service life of products decreases,which seriously restricts the development of high and new technologies.Traditional air cooling technology can not meet the needs of high heat flux electronic chips.In liquid cooling technology,microchannel has the advantages of small structure,less refrigerant consumption and high heat dissipation,which makes microchannel an ideal radiator for electronic devices,and attracts more and more scholars'attention.The research on flow and heat transfer in microchannels has gradually developed from simple microchannel structures such as rectangular,triangular and circular to complex microchannel structures.In this paper,a parallel rectangular sudden expansion microchannel was designed and fabricated.Using water,ethanol and nitrogen as working fluid,the flow and heat transfer characteristics of water and ethanol flowing through the microchannel alone,the flow and heat transfer characteristics of ethanol/nitrogen gas-liquid two-phase under heating and the flow pattern of gas-liquid two-phase under non-heating conditions were studied by theoretical analysis and experimental research.The single-phase refrigerant study shows that for flow characteristics,the pressure drop at inlet and outlet of refrigerant increases with the increase of Reynolds number,and at the same flow rate,the pressure drop at inlet and outlet of ethanol is larger than that of water;at the same heat flux,the pressure drop at inlet and outlet of refrigerant increases with the increase of flow rate,but with the increase of flow rate,the pressure drop difference between different heat flux decreases.For heat transfer characteristics,the bottom temperature of water and ethanol microchannels decreases with the increase of Reynolds number,but after Re=225,the wall temperature of microchannels decreases gradually;under the same conditions,the average heat transfer coefficient of water can reach 7 000~10 000 W/(m~2·K),while that of ethanol can only reach 3 600~5 500 W/(m~2·K),and the heat transfer capacity of water is obviously greater than that of ethanol.The results of two-phase refrigerant study show that for flow characteristics,the pressure drop at the inlet and outlet of ethanol/nitrogen gas-liquid two-phase refrigerant increases with the increase of liquid phase conversion speed and gas phase conversion speed;with the decrease of the gas phase conversion velocity,the friction multiplier of the liquid phase decreases slowly with the increase of X,and the curves of different gas phase conversion speed are almost connected into a curve,and the C value of the curve is equal to 2.In terms of heat transfer characteristics,the heat transfer capacity of ethanol/nitrogen is better than that of ethanol in single phase.With the increase of nitrogen flow rate,the stronger the ability to promote ethanol flow,the stronger the disturbance,and the heat transfer is enhanced.When nitrogen flow rate increases from 0.089m/s to 0.268 m/s,the wall temperature of microchannel can be reduced by 2?3?.For gas-liquid two-phase flow pattern,bubbly flow,plug flow,slug flow and annular flow occur in turn with the increase of gas velocity in the process of ethanol/nitrogen gas-liquid two-phase working fluid flow.When gas enters the liquid storage tank,due to the different distance between each channel and the inlet,the flow distribution is uneven,and the gas content in the middle channel is greater than that in the two sides of the channel.