Study On Flow And Heat Transfer Characteristics Of Two Phase Flow In Microchannel

With the increasing shortage of energy,saving energy and reducing energy consumption has become a consensus.Microchannels have been widely used in microelectronics,chemical industry and other fields due to their large specific surface area and high efficiency of heat transfer.Therefore,the study of flow state and heat transfer characteristics in microchannels has become a necessary topic.The conventional channel two phase flow mechanism is not applicable to the micro-channel two phase flow.The understanding of microchannel two phase flow and heat transfer is still insufficient.Therefore,it is very important to study the flow characteristics and heat transfer characteristics of microchannel two-phase flow.In this paper,the flow and pressure drop characteristics of parallel rectangular microchannels and the heat transfer characteristics of direct contact condensation flow are investigated by numerical simulation and experimental methods.The flow characteristics of two-phase flow in parallel microchannels are numerically simulated by air and water.Four stable flow patterns are found,which are bubble flow,slug flow,annular flow and wall flow.The influence of fluid velocity,viscosity and surface tension on the Flow pattern transformation is analyzed and the flow pattern diagram is drawn,in which the effect of viscosity on the flow pattern is relatively small.The surface tension has a great influence on bubble flow and slug flow.The pressure drop increases with the increase of the fluid velocity,but the liquid velocity has a great influence on the pressure drop,and the liquid phase plays a leading role in the pressure drop in the two-phase flow.The prediction effect of partial homogeneous flow model and split phase flow model is discussed.For the common slug flow in microchannel two-phase flow,the prediction effect of the pressure drop prediction model based on flow pattern is better,but the prediction effect gradually decreases with the increase of flow velocity.In the microchannel direct contact condensation experiment,vapor and water were used as fluid phases,and three main flow patterns,including slug flow,annular flow and fog flow,were observed by the visualization system,and The characteristics of the flow pattern in the microchannel were analyzed。The effects of channel size and fluid velocity on heat transfer and temperature distribution are analyzed.The experimental results show that microchannels of different depths have different wall temperature rise rates,but both include two stages:rapid rise and basically stable.The wall temperature of the microchannel increases with time,and gradually decreases along the direction of fluid flow.At the same point in time,the wall temperature decreases as the liquid phase velocity increases.The local Nusselt number is related to the flow velocity and channel size.The difference between the fluid temperature at the inlet and the wall temperature is small,and the heat flux density is large,so the local Nusselt number reaches the maximum value at the inlet.The increase of the heat flux density on the wall can enhance the heat transfer effect of the two-phase flow.