Inverstigations On Mixing And Combustion Mechanism Of Supersonic Mixing Layers

The present research studied the flow/combustion mechanisms and theenhancement methods of the supersonic mixing layer, using different experimentalinvestigations and numerical simulation.In order to determine the discipline of the development of the supersonic mixinglayer, hybrid LES/RANS method is applied to simulate the influence of the clapboardthickness, the pressure-nonmatching degree and fuel species. The results suggest: withina certain range, the mixing efficiency，the total-pressure loss and the combustionefficiency grow with an increased clapboard thickness. In the nonreacting mixing layer,the higher the pressure-nonmatching degree is, the higher the mixing efficiency and thelower the total-pressure loss are; the pressure-nonmatching degree is nonlinear to themixing efficiency and the total-pressure loss. The trend of mixing and combustionefficiency in reacting mixing layer is consistent with no-reacting mixing layer. Themixing efficiency and the total-pressure loss increase when the reaction is getting closeto the chemical stoichiometry.Supersonic mixing layer is investigated by using passive and activemixing-enhancement methods. It is found that as the length-depth ratio of the cavityincreases,the vortexs in the mixing layer are rolled up at an earlier time,leading to ashorter distance during which the flow is laminar.Besides,the vortexs assume strongerstability and their dissipation rate decrease.As the aft-angle of the cavity increases,thespace between vortexs become shorter in the downstream regime of the flow field andthe vortexs also assume stronger stability and their dissipation rate decrease.Theup-and-down staggered clapboard makes the steamwise vortexs develop rapidly,theirentrainment increases the contact surface area between the fuel and the air,facilitatingmixing during a short distance.In the left-and-right staggered clapboard case,thestreamwise vortexs assume strong stability before a distance of40mm from theclapboard,while they are rapidly dissipated during a distance of40-60mm.Both of thetwo types of staggered clapboards have mixing-enhancement effect,with the formerbetter than the latter.The scales of vortexs with disturbance in the flow field areobviously larger than those without disturbance, and high-frequency disturbance assumebetter effect than that of low-frequency.Based on the schlieren photographs,with different clapboards, the unsteady featureof the incidence and reflection locations of the shock wave, the mixing layers’sincreasing thickness and boundary structures are investigated. The location wherevortexs are rolled up and vortexs’ moving speed are investigated. The flame propagatingmechanism during ignition period of the supersonic reacting mixing layer and theunsteadyness of the flame are investigated. By comparing OH radicals distribution under different clapboards cases, it is found that the combustion area basically is thesame as the mixing area, the flame structure is mainly influenced by flow and mixing.Besides, the effect of mixing-enhancement methods is suppressed more or lesscompared to the nonreacting case. The combustion intensity upstream does influencethat of the downstream and the flow field assumes three-dimensional feature, severelyinfluenced by spanwise disturbance.