| With the miniaturization of instruments and the increasingly complex and diverse working environment,the single-phase heat transfer mode has become more and more difficult to meet the requirements of the heat exchange system.Therefore,phase-change heat transfer mode of evaporation and condensation in microscales has become a new promising research topic.Spray cooling and micro channel condensation have the advantages of small scale and high heat exchange efficiency,which can meet the increasingly stringent heat exchanger system requirements.In this study,the process of droplet impacting on heated surface in Leidenfrost state and the condensation process in microchannels under microgravity are studied by numerical simulation.When a droplet impacts on a heated surface above the dynamic Leidenfrost temperature,it bounces off the surface by levitating on a vapour layer.The vapour layer is of great importance for impact morphology and heat transfer.In this study,by focusing on the vapour layer,we identify that the bottom interface of the droplet,rather than remaining stable,may oscillate rapidly during the rebounding stage of the droplet in the Leidenfrost state.The interface oscillation refers to repeated formation and discharge of dimple depression at the bottom of the droplet.The results of heat transfer analysis confirm that the interface oscillation can remarkably affect the flow in the vapour layer and the heat transfer between the droplet and the heated surface.The effects of important dimensionless numbers controlling the process are analysed,including the Jakob number,the Ohnesorge number,and the Weber number.In the process of microchannel condensation under microgravity,the mass flux distribution and local heat transfer coefficient are affected by different gravity.In this study,we focus on the condensation process in square microchannels under lunar surface gravity,and analyze the effects of different gravity parameters,wall temperature boundaries and steam inflow velocities on the dryness,mass flux,refrigerant temperature and local heat transfer coefficient.The results show that the increase of the gravity will accelerate the condensation process in the microchannel,and the wall temperature and the inflow velocity also affect the change of the dryness and the local heat transfer coefficient. |
