Study On Particle Flow Behaviors Of Dense Granular Impinging Jets

Dense granular impinging jets are the key scientific issue of the opposed multi-burner gasification,and the results will provide important theoretical basis for the design,scale-up and optimization of the impinging jets gasifier.Present thesis is devoted to studying the liquid-like behavior and instability of dense granular impinging jets,as well as the regulation principle of the flow pattern and instability by means of the high-speed camera,fiber-optic probe(PV6D)and Discrete Element Method(DEM),etc.The main contents and results are summarized as follows:1.The liquid-like granular sheet and diffuse pattern as well as the regulation method of dense granular jet impacting on a target are studied based on the combination of experimental study and numerical simulation.Experimental results show that the diffuse pattern will evolve to the granular sheet as the ratio of granular jet diameter to particle diameter(D/d)and the solid fraction of granular jet(xp)increase.The critical transition parameters and maps of the two patterns are obtained.Simulation results further indicate that increasing xp and D/d both increase the interparticle collision frequencies,which decreases the interparticle collision forces and particle velocity fluctuations,and consequently gives rise to the granular sheet.A method to regulate the flow patterns of dense granular jet impact is proposed by changing the solid fractions of granular jets.2.Liquid-like wave structures on the granular sheet of dense granular jet impact are experimentally and numerically studied.Results show that wave structures form on the granular sheet with increasing granular jet velocities and decreasing solid fractions.Symmetric and asymmetric patterns of granular wave propagation are observed.With the increase of radial positions,the wavelength increases and the wave frequency decreases due to the merging of neighboring waves.As the upright length of nozzle exit is increased or a screen is embedded into the nozzle,the enhanced gas-particle interaction inside the nozzle causes the instability of the granular jet,which further gives rise to the liquid-like wave structures.Simulation results further reveal that distinct wave structures form as the oscillation amplitudes of granular jet are enhanced and Strouhal numbers(St)are in the range of0.2-0.8.3.The images of flow patterns of two dense granular impinging jets are captured using two high-speed cameras from the side and front views,simultaneously.Effects of the solid fraction of granular jet,particle diameter,jet diameter,granular jet velocity and impinging angle on the flow pattern are revealed.Results show that as the solid fraction of the granular jet(xp)increases,three patterns i.e.,the penetrating pattern,diffuse pattern,and liquid-like granular sheet display in turns.DEM simulation results further reveal that the granular sheet primarily results from rapid interparticle collisions at large xp and D/d,Increasing interparticle collision frequencies in the impact region reduce the collision forces and axial particle velocity fluctuations.The flow patterns and particle dispersion are well regulated by changing the solid fractions of granular jets.4.The asymmetric oscillation and wave structures of the granular sheet from dense granular impinging jets are investigated,and the instability of the granular sheet and regulation mechanism are revealed.Results show that the asymmetric oscillation of the granular sheet takes place as the granular jet velocity increases and solid fraction decreases.Time-averaged oscillation frequencies of granular sheet are slightly less than those of disturbed graular jets,and increase linearly with the granular jet velocity but are nearly not affected by the impinging angle.Time-averaged oscillation amplitudes of granular sheet increase with the propagation distance,granular jet velocity and impinging angle.Wave structures form on the granular sheet by the small disturbance of granular jet,and develop to the asymmetric oscillation as the disturbance of granular jet increases.The instability of granular jet will cause the momentum imbalance of the impinging jets at the impact point,and consequently results in the asymmetric oscillation of the granular sheet.The method of changing the solid fraction of granular jet is used to induce the impact instability in the impinging region,which leads to the asymmetric oscillation and further enhances the particle dispersion effectively.