Complex Flow Regime And Mixing Mechanism In Impinging Jets Reactors

Impinging jets can enhance mixing performance,which has been applied successfully for increasing fields,e.g.chemical engineering,energy,environment,biology and pharmacy.It is important to investigate and understand the flow regime and mixing mechanism for the design,development,and optimization of impinging jets reactors.Current paper investigates the flow dynamics and mixing mechanism in impinging jets reactors using experimental method and numerical simulation.The main contents and results are summarized as follows:1.The flow regime and mixing mechanism in T-jet reactor are studied using planar laser induced fluorescence(PLIF)and numerical simulation.The formation and evolution of three-dimensional vortices in the reactor chamber are investigated.The influence of engulfment flow on mixing performance is analyzed,and the flow mechanism of engulfment flow is revealed.Results show that,segregated flow,steady engulfment flow,unsteady engulfment flow,and flapping oscillation appear in turn with the increase of Reynolds number.With the appearance of steady engulfment flow at Re?105,the impingement plane is tilted,on which two vortices are formed.Two streams intertwine each other,and the mixing degree increases.With the appearance of unsteady engulfment flow at Re?190,the periodic self-sustained oscillation of vortex merging appears,which induces the folding and stretching of the interface,and the mixing effect is further increased.Results also show that,a pair of vortices appears at the top of the reactor and two pairs of symmetric Dean vortices are formed on the impingement plane.As Reynolds number increases,the tilt of the top vortices leads to the instability of Dean vortices and the occurance of steady engulfment flow.With the further increase of Reynolds number,vortices merge on the impingement plane,and the velocity and pressure of impinging region transform periodically,which induces the oscillation of unsteady engulfment flow.2.The influence of headspace height,outlet aspect ratio,chamber depth,and inlet stagger degree on the engulfment flow is investigated.The control method of engulfment flow and mixing performance is studied in T-jet reactors with different geometrical parameter.Results show that,the tilt angle of impingement plane decreases as the headspace height H/h>0.2,or the chamber depth w/h?2 or w/h?0.5,which causes the decrease of the interwinement of two inlet streams and the disappearance of unsteady engulfment flow.Two vortices rotates around the center axis of the reactor chamber as the outlet aspect ratio W/w<2.A non-aligned T-jet reactor is proposed.As the stagger degree of inlet jets increases,the critical Reynolds numbers of steady and unsteady engulfment flows decrease significantly.Therefore,the interwinement of two streams is enhanced and the mixing degree is improved.3.The flow regime and mixing mechanism in cross-shaped reactors are studied by experimental method and numerical simulation.The three-dimensional structure of vortex formed in engulfment flow is investigated and its formation mechanism is revealed.Results show that,the impingement plane is tilted and a vortex is formed in the center of the chamber as steady engulfment flow occurs at Re>45.The periodic oscillation of vortex merging and breakup appears for unsteady engulfment flow at Re?300,which causes the enhancement of mixing degree.Simulation results reveal that,the tilt of impingement plane is caused by the chamber confinement and the center vortex is formed by the effect of vortex stretching for steady engulfment flow.For unsteady engulfment flow,the pressure difference between impingement plane and the center vortex results in the vortex breakup,which induces the periodic self-sustained oscillation in the reactor chamber.4.A novel experimental method is designed and a recirculation region where there is almost no fluid transport with outside is firstly depicted.And the particle trapping effect in recirculation region is discovered.The influence of geometrical parameter on the recirculation region is investigated and the formation mechanism of recirculation reigon is revealed by simulation method.The fluid inside the recirculation region recirculates in this region,and the fluid outside the recirculation region flow spirally toward the outlet.The simulation results reveal that the vortex breakdown of center vortex is the formation mechanism of recirculation region.As the reactor depth is w/h?0.5,recirculation region disappears.As w/h?1,the recirculation region appears and its scale increases with w/h.The particle trapping effect in recirculation region is discovered and the efficiency of particle trapping increases with the increase of particle diameter and decreases with the increase of particle density.