Numerical Study On The Effect Of Flow Morphology On Enhanced Heat Transfer In The Microchannel

In recent years,with the application of heat exchanger in high energy demand more widely,the requirements for enhanced heat transfer are becoming higher and higher,especially in the field of aerospace thermal protection,due to the constraints of aircraft structure,heat exchanger design gradually tends to small scale and compact.The research focus of heat transfer devices is gradually transferred from single-phase flow heat transfer in conventional channels to multiphase flow heat transfer in microscale channels.Although the application of multiphase flow is more popular,the research on the characteristics and mechanism of multi-phase flow heat transfer in micro-scale channels is still insufficient.Therefore,this paper mainly carried out numerical simulation studies on the phase-free Taylor flow and boiling flow in the microchannel,and analyzed the influence of different flow forms on the convection heat transfer characteristics in the microchannel in detail.The phase-free Taylor flow(also known as slug flow or plug flow)is a typical two-phase flow pattern in microchannels,which has high heat and mass transfer capacity.This paper for the liquid-liquid Taylor flow in the microchannel,with T-shaped microchannel model,based on the gradient adaptive grid technique was used to study the internal heat transfer characteristics of flow.It analyzes the state of full development the length and thickness of liquid film of liquid under the control mechanism and explores the flow velocity distribution and pressure drop in the process of heat transfer.The prediction models of the length and friction coefficient are established with the average error of 9.9% and 2.7% respectively.The prediction results are good and can provide some reference for engineering application.Compared with single-phase flow and Taylor flow without phase change,the boiling flow in the microchannel can utilize the latent heat of phase change,and its heat transfer ability is better than that of the former.Therefore,based on Taylor flow simulation,phase change heat and mass transfer model was added to investigate the boiling flow characteristics of smooth microchannels and micro-finned tubes respectively.And the effects of heat flux density,mass velocity,saturation temperature,cross-section shape and finned structure on the boiling heat transfer coefficient in the two channels were analyzed in detail.The boiling heat transfer model with the highest prediction accuracy for smooth microchannels and micro-finned tubes was evaluated by comparing the simulation results with the prediction correlation formulas in the literature.Finally,the boiling heat transfer characteristics of smooth microchannels and microfinned tubes are compared.The results show that micro-finned tubes have better boiling heat transfer characteristics than smooth microchannels and are less prone to heat transfer deterioration.This shows that the tube shape has a significant effect on flow boiling characteristics.However,micro-finned tubes also produce local drying under high heat flux,which should be taken into consideration in the optimization design of high-efficiency intensified heat exchangers.