Study On The Uniformity Of Phase Distribution In Two-phase Flow Heat Exchangers With Small Channels

New energy vehicles are the development trend for a long time in the future.Battery thermal management of the power battery,is an essential method to ensure the performance,safety and life span of new energy vehicles.Heat pump technology is a highly potential battery thermal management technology,which has obvious advantages in environmental adaptability and energy saving.However,the current development of battery heat pump thermal management technology is not perfect,and there are many problems that need to be solved urgently.Among them,the maldistribution of two-phase refrigerant in the header and each branch pipe is one of the most prominent problems affecting the performance of the heat pump system.Based on a kind of cold plate type micro channel two-phase flow heat exchanger structure which can be used in electric vehicle heat pump technology,a two-phase flow visualization phase separation experimental platform with R1233zd(E)as working fluid was built.On the one hand,the two-phase flow characteristics in each channel with different inlet conditions(upper inlet,middle inlet and lower inlet)and different operating conditions(inlet quality range 0.01～0.07,inlet mass flow rate range 300～500kg/(m~2·s))are analyzed.At the same time,the effect of the transition of the flow pattern on the distribution characteristics of the two-phase flow is evaluated.Open CV image processing function library was used to identify the gas-liquid interface and extract the contour,and some key characteristic parameters such as average void fraction in the channel were obtained.On the other hand,combined with the data of the mass flow rate,pressure gradient and average void fraction of the working fluid in each channel,the mechanism of gas-liquid phase distribution in microchannel heat exchanger is explored,and the reason of uneven phase separation in different channels is tried to explained.It is found that,compared with the middle inlet mode and the upper inlet mode,the lower inlet mode can obtain better gas-liquid two-phase distribution,because gravity can weaken the influence of inlet inertial force on liquid momentum flux to a certain extent in this inlet mode.When the inlet quality is large,compared with the low inlet quality,the momentum flux of the inlet gas phase cannot be ignored compared with the liquid phase.The effect of the interfacial shear force between the two phases increases the momentum flux of the liquid phase,and the liquid phase tends to concentrate on the last branch pipe,which makes the overall liquid separation uniformity worse.In the range of experimental inlet mass flow rate,the influence of interphase force is small,and the momentum flux of inlet liquid phase is greater under the condition of high inlet mass flow rate,so the liquid phase is easy to concentrate in the last branch pipe,which leads to poor overall phase separation uniformity.In addition,the experimental results also show that under slug flow,stratified flow or annular flow patterns,the average void fraction in each channel is positively correlated with the flow resistance.However,with the increase of gas-liquid velocity difference,the gas momentum flux will also affect the distribution of average void fraction in the channel.The research results can provide reference for the optimization design of two-phase flow cold plates.