Numerical Simulation On Heat Transfer During Flow Boiling And Bubble Dynamics In Mini-channel Filled With Metal Foam

With the development of technology and production of actual requirement,the focus of human cognition has changed from macro-scale to mini-and micro-level.Due to its unique porous medium skeleton structure,metal foam has high void ratio and high thermal conductivity.At present,the research of enhanced boiling heat transfer in metal foam channel mainly focuses on the experimental observation of pool boiling and small-scale channel flow boiling.Through experimental data and high-speed camera capture,the theoretical calculation formula and bubble motion behavior are obtained.However,due to the complex structure of metal foam,the enhanced heat transfer mechanism and the bubble dynamics behavior in the channel filled with metal foam during the flow boiling in mini-channel are still unclear.Based on the research of enhanced boiling heat transfer,the heat transfer mechanism,bubble dynamic behavior and statistics during flow boiling in mini-channel filled with metal foam are studied.The relationship between vortex and turbulent kinetic energy and heat transfer performance is explained.The research contents are as follows.(1)A numerical model of mini-channel filled with metal foam was established.The metal foam skeleton structure was composed of the tetradecahedral unit cell.The central two-dimensional section was taken for simulation calculation,and the mesh independency study was performed using three different grids.The simulation results show that the boiling heat transfer coefficient was in good agreement with the experiment,and the error was cntrolled within ±20%.(2)The Fluent was used to simulate the process of R141 b gas-liquid phase transition during flow boiling in conventional empty mini-channel and filled metal foam mini-channel.The CLSVOF(Couple Level-Set and Volume of Fluid)multihphase flow method and user-defined function(UDF)based on evaporative condensation phase transformation model proposed by Lee was used to simulate flow boiling.The range of heat temperature was 350 K – 450 K,and the velocity was 0.01m/s – 0.10m/s.(3)The boiling heat transfer phenomena in two channels under different conditions were observed.The number of bubbles and the average bubbles area at different times were counted.The effects of bubble dynamics,vortex and turbulent kinetic energy on heat transfer enhancement were analyzed.The simulation results show that compaerd with empty channel,the bubble motion range and vortices area was smeller,and more vorteices in channel filled with foam metal,which leads to the mixing frequency increasing and heat transfer enhancement.With the increase of temperature,the bubbles number increases gradually in mini-channel filled with metal foam,but that of the increases and then decreases in conventioal empty channel.As the speed increases,the bubbles number increases gradually in mini-channel filled with metal foam,but the trend of change is not obvious in conventioal empty channel.Meanwhile,the heat transfer coefficient is inversely proportional to the average bubbles area but proportional to the bubbles number.Turbulent kinetic energy is an important indicator to measure mixing ability of turbulence.Its change with time reflects the growth and weakness tendency of vortex.The relationship between the turbulent kinetic energy first order differential k' and the heat transfer coefficient is compared.It is found that the tendencies of the turbulent kinetic energy first order differential k' and the heat transfer coefficient are at equal pace,k' can be used to predict the change trend of the heat transfer coefficient of the foam metal channel.