Study On Fluid Flow And Heat Transfer Of Flow Separation And Reattachment Over A Step In Transitional Flow

The heat transfer enhancement technology is one of the effective measurements to improve the energy utilization efficiency in heat transfer equipment.This technology is most widely used in the convection heat transfer.The flow instability in the convention is usually enhanced by changing the channel section area,changing the wall surface shape,inserting turbulent structure or using extra force.The enhanced flow instability can destroy the wall boundary layer,which has a positive effect on heat transfer enhancement.The step flow is a benchmark model for changing the channel section aera.It can enhance the flow instability and heat transfer in the channel.This flow instability shows periodic and quasi-periodic characteristic in the transitional flow.Thus,the study on the transient behaviors of flow instability and its effects on heat transfer are significant.Now,the study in the transitional flow over a step is not much,the characteristics of vortex motion in different step models are not clear.In addition,the relationship between the transient flow and transient heat transfer also needs to be clarified.In order to solve the above problems and reveal the mechanism of heat transfer enhancement in the step flow,experimental platform is set up for the backward facing step flow and the numerical heat transfer models are established for the backward facing step flow,forward facing step flow and combined step flow.The flow and heat transfer problems are studied by numerical simulation and PIV experiment.Firstly,in order to discuss the effect of Reynolds number,the fluid flow and heat transfer characteristics are studied for 400?Re?1000.In the backward facing step flow,the heat transfer of the bottom wall is enhanced with the increase of Reynolds number,especially in the region downstream of the primary recirculation zone.In the forward facing step flow,the flow instability does not appear and the heat transfer increases evenly with the increase of Reynolds number.In the combined step flow,the heat transfer along the bottom wall increases with the increase of Reynolds number.The heat transfer in the primary recirculation zone and the reattachment region is better than that in the backward facing step.In addition,the geometric factor effects in different step models are studied in the transitional flow.The flow instability and heat transfer downstream of the reattachment region increase with the decrease of expansion ratio in the backward facing step flow.Due to the acceleration effect downsteam of the forward facing step,the heat transfer in local area of the downstream wall increases with the decrease of contraction ratio.In the combined step flow,with the increase of bottom wall length(8h?L_b?14h),the irregular flow fluctuation turned into periodic fluctuation and then the flow fluctuation loses its fundamental frequency.The periodic flow fluctuation can increase the local time-mean Nusselt number by more than 10%.With the increase of step height,the fluctuating velocity increases and the fluctuating frequency decreases.The increased fluctuating velocity can increase the local time-mean Nusselt number by up to 100%.Furthermore,based on the above results,the heat transfer enhancement mechanism of the separation and reattachment flow over a typical step in the transitional flow is analysed.In the backward facing step flow and combined step flow,the clockwise rotating vortices appear in the vicinity of bottom wall by the impact of reattachment flow.The periodic vortex motions destroy the wall boundary layer and bring the cold fluid toward the bottom wall,which causes the heat transfer enhancement of bottom wall.In order to clarify the dissimilarity between transient momentum transfer and transient heat transfer,its formation reason is investigated.A series of vortices appear along the channel walls due to the flow reattachment,which increases the flow instability in the channel.The interaction of these vortices causes the temperature fluctuation.The temperature fluctuation leads to the mix of cold fluid and hot fluid,which increase the heat transfer instability.The combined effects of flow instability and heat transfer instability cause the dissimilarity between transient momentum transfer and transient heat transfer.Finally,according to the flow and heat transfer results in different steps,the comparative analysis is done.In the aspect of flow instability,the flow instability appears earlier in the combined step flow when compared with backward facing step flow.The flow instability map based on Reynolds number and bottom wall length is given in this study.In the aspect of heat transfer,the heat transfer in the backward facing step increases the most from laminar to transitional flow.The heat transfer in the forward facing step is stable.The heat transfer in the combined step flow shows the best performance.In this study,the results of the fluid flow and heat transfer characteristics and the heat transfer enhancement mechanism in the transitional flow over a step can provide theoretical guidance on the design of heat transfer equipment.Thus,these results have some scientific significance and industrial value.