An Experimental And Numerical Investigation Of Mixing Enhancement And Combusting Flow Field Of Dual Combustor Ramjet

Under the background of Dual Combustor Ramjet(DCR),an experimental and numerical investigation of reacting and non-reacting mixing layer are launched,which are focused on the mixing enhancement method of applying splitters with different geometric configurations.In order to check whether those splitters have the effect of mixing enhancement and how the effect is, an schlieren and NPLS experiment are launched.In the experiment with schlieren photograph,it is discovered that the thickness of mixing layer in the flow field with splitters in which a cavity is mounted grows as the ratio of the length to depth of the cavity becomes larger;The schilieren photograph also shows that splitters with step,jet and jet combined with cavity have no obvious effect of mixing enhancement;It is also discovered that the thickness of the mixing layer is thinner when a splitter with an aft angle is applied;Splitters with staggered distributed trail show great effect of mixing enhancement,one of which with up-and-down-toward staggered trail making the flow field highly unsteady and the mixing layer's thickness inverse-proportional to the shockwave span;The phenomenon of refraction is discovered when the shockwave penetrates through the mixing layer;In the NPLS experiment,phenomena are consistent with the schlieren.The steady RANS numerical simulation of non reacting mixing layer is launched to investigate the mechanism of influence of splitters on the mixing layer under steady state condition.It is demonstrated that the mixing-enhancement effect of splitters with cavity is due to the cavity shear layer's reattachment to the trail of the splitter,that is to say,the thicker the reattaching shear layer is,the thicker the mixing layer is.Actually,splitters that have no obvious mixing-enhancement effect can also be attributed to the thinner attaching layer(splitters with aft angle due to the narrow reversed flow area);Beside the refraction as discovered in the schlieren experiment,reflection is also obsersed when shockwave penetrates through the mixing layer;Splitters with staggered distributed trail equip the flow field with highly three-dimensional characteristics,and they make the fluid undertaking such great shear stress that it induces large-scale streamwise vortexs which play an important role in the instability of the mixing layer。The influence of the cavity self-oscillation on the mixing layer is investigated through LES simulation.Under the hypothesis of periodic motion of vortexs in a mixing layer with a flat splitter,it is demonstrated that the frequency of the pressure at a spatial point just equals that of the vortexs which occur there.That means high frequency according to small vortex and low frequency according to large vortex.Furthermore,the scale of vortexs occurring at one point during a period can be estimated given the plot of frequency-power spectral density;The scale of vortexs in a cavity-mounted-splitter mixing layer is much larger than that in a flat-splitter mixing layer and the frequency of pressure becomes near that of the cavity self-oscillation.The higher the oscillation's power is,the nearer the frequency is.As the back ramp angle of the cavity decreases,the power of oscillation becomes weak,which consequently reduces the scale of vortexs and weakens the mixing enhancement.In order to test the effect of enhancing the combusting efficiency of cavity-mounted splitters and staggered-distributed-trail splitters, the experiment of OH spontaneous radiation photograph and OH PLIF on combusting flow field of mixing layer are launched.It is discovered that the flame distribution of cavity-mounted-splitter mixing layer is no much different with flat splitter;The flame area of staggered-distributed-splitters mixing layer is relatively wide and the concentration of OH is relatively high.However, this difference is not so distinguish compared to non-reacting mixing layer,which demonstrates that in the combusting flow field of mixing layer,those splitters'effect of enhancing combusting efficiency through enhancing mixing efficiency is certainly depressed. In the numerical simulation,two chemical reacting models of laminar finite-rate and finite-rate/eddy dissipation are compared and the consequence shows that there is no distinguish difference between them,that means,the chemical rate controls the reacting process and the eddy dissipation has little influence;The reacting flow field with cavity-mounted splitters is compared to that with a flat splitter.The consequence displays that the effect of mixing enhancement of the former does being depressed in reacting flow field,which is consistent with the experiment.