Experimental And Numerical Study On Engulfment Regime Of Impinging Jets Reactors

As an effective way to enhance mixing,impinging streams are widely used in many industrial processes.It is significant to study the flow characteristics of impinging streams for further application and development of impinging stream reactor.In this paper,the characteristics of engulfment flow in T-shaped and cross-shaped impinging jets reactors and the influence of boundary conditions on their formation and evolution are systematically studied by means of experiment and numerical simulation.This paper is summarized as follows:1.Flow regimes in the T-jets reactor were studied by planar laser induced fluorescence(PLIF)technique and large-eddy simulation(LES).It was found that the separated flow,the steady engulfment flow,the unsteady engulfment flow and the unsteady symmetric flow emerge in turns with the increase of Reynolds number.In particular,the characteristics of unsteady engulfment regime were investigated by LES and the simulation results are in good agreement with the experimental results.Results show that eddies periodically merged in the impingement plan which causes self-sustained oscillation in the reactor.The pressure,velocity and vorticity in the impact zone also vary periodically,and the periods are consistent with that of eddy merging in unsteady engulfment flow regime.This oscillation of eddy merging and propagation is caused by the periodic variation of velocity and pressure in the impingement plane.2.The flow patterns in impinging stream reactors with different geometrical structures are experimentally and numerically studied.The effects of inlet Reynolds number,headspace,reactor thickness and jet separation on the engulfment flow are investigated.Results show that the confinement of the impact zone is the most important factor affecting the formation and development of the engulfment flow.The height of the headspace and thickness has important influence on the engulfment flow in T-j ets reactor.For Hlh>0.3,the engulfment flow disappears in the reactor.When d/h=2,the merging of eddies disappears.However,the jet separation has little influence on the engulfment flow.3.The flow regimes in a cross-shaped reactor are experimentally and numerically studied by planar laser induced fluorescence(PLIF)and large eddy simulation(LES).It is found that with the increase of inlet Reynolds number,five flow regimes appear in the reactor,namely,separate flow,steady engulfment flow,shaking flow,unsteady engulfment flow and flapping oscillation flow.The experimental and numerical results show that the engulfment flow occurs at Re>50,and the recirculating region occurs at Re>100.The fluid in the recirculating region has much longer residence time than the other.The critical Reynolds number and the flow characteristics in the recirculation region are investigated and the reasons for the forming of recirculating region are revealed.