Flow And Combustion Characteristics Of Non-isothermal Supersonic-subsonic Mixing Layer

The non-isothermal supersonic-subsonic mixing layer,which has the properties of large velocity gradient,complex flow structure,rapid transportation of momentum,heat and species,is commonly seen in the combustor of air-breathing high-speed vehicle,playing an insignificant role on mixing and combustion.In this paper,flow and kerosene spray characteristics of the mixing layer companied with combustion characteristics under supersonic-subsonic shear flow condition were investigated systematically and thoroughly by experiments and numerical simulations.Flow characteristics and effects of flow and thermal parameters were obtained using experimental and numerical methods.A method for predicting the flow and thermal parameters in the mixing layer was proposed.The transient flow field shows the development of mixing layer includes three regions,laminar structure and instability forming region,instability increasing and large-scale structure motion region and turbulent region.The large-scale structure develops with rolling up,splitting,pairing and merging process while there is no obvious coherent characteristic.The flow parameters fluctuate a lot in the mixing layer.The non-dimensional time-averaged velocity,total pressure and total temperature of mixing layer profiles show self-similar characteristic and can be expressed in Gauss function form.Meanwhile,the turbulent intensity profiles can also be expressed in self-similar form,as a single-humped distribution with peak tending to supersonic side.The velocity-and temperature mixing layer both develop as a wedge in the streamwise direction,whereas the temperature mixing layer grows faster.The mixing layer prefers to entrain more supersonic side fluid.Studies of effects of flow and thermal parameters show that the variation of subsonic Mach number affects total pressure distribution and mixing layer development remarkably.The mixing layer distributes as a wave in streamwise direction accompanied with the decreases of the non-dimensional total pressure,layer thickness and percentage of entrained supersonic side stream with increasing pressure ratio p₁/p₂.Raising total temperature ratio T_1t/T_2tleads to decreases of turbulent intensity and percentage of entrained supersonic side stream as well as an increase of dimensionless static temperature.The changing of pressure p_ehas no impact on flow field.The peak of turbulent intensity and non-dimensional growth rate decrease as increasing the convection Mach number.The prediction method proposed calculates the spatial location,velocity and total temperature of mixing layer precisely.Investigations of kerosene spray characteristics including spatial and drop diameter distributions in the non-isothermal supersonic-subsonic mixing layer and influences of flow,thermal and fuel supplying parameters were accomplished.The spatial distribution of the mist is comprised of two regions four stages,namely,cross-jet region including cross-jet breakup stage and entraining diverting stage and mixing layer region which covers shear breakup stage and droplet tracing stage in the streamwise direction.The mist distribution in the droplet tracing stage is related to the large-scale structure motion.The cross-jet breakup stage and shear breakup stage are the main two stages reducing the SMD.The large velocity gradient of the supersonic-subsonic mixing layer is beneficial to the droplet breakup,especially for larger diameter droplets.The variations of the flow,thermal and fuel supplying parameters have little influences on the characteristic of kerosene mist spatial distribution in the mixing layer and mainly affect the initial location of shear breakup stage,responsible for variation of the SMD in this stage and mixing layer region.The residence time of droplets in the mixing layer is very short resulting in the fuel vaporization difficultly.Changing the fuel-or flow-temperature has a certain influence on the vaporization.Improving the temperature of fuel or supersonic stream enhances the vaporization significantly.The vaporization in the cross-jet region is promoted when the subsonic stream temperature increases.The overall vaporization capacity,however,decreases.Combustion characteristics under supersonic-subsonic shear flow condition were investigated experimentally and numerically.The flame characteristics and reaction region forming mechanism were analyzed while the flame holder was on duty or not.The ignition process and flame structure were compared between subsonic flow and supersonic-subsonic shear flow conditions.Results show while the flame holder is off duty,an orange flame generates in the mixing layer and reaction exhibits a diffusion combustion feature when supplying the fuel by the carry of the supersonic side stream;and the reaction region initially forms in the mixing layer along the supersonic side border,subsequently,across the mixing layer developing along subsonic side border of mixing layer,which is in the nature of diffusion and premixed combustion,when the fuel is fed into the mixing layer directly from the tail end of splitter plate.Both reaction regions are ignited by the mechanism of auto-ignition and locate where the Mach number is smaller than unity in the mixing layer.The ignition time becomes larger,and the flame shifts from subsonic stream to supersonic one with a shorter flame length under supersonic-subsonic shear flow condition as compared to the subsonic flow one.There is a matching relationship between reaction region and flow field.When the flame holder is on duty,flocculent flame or striped flame locates at regions in the mixing layer near subsonic border,the reaction region forming mechanism in common with that in the marginal shear layer of flame holder.In summary,this paper innovatively reveals the following characteristics of the non-isothermal supersonic-subsonic mixing layer:characteristics of“three regions”of flow development of mixing layer in streamwise direction and self-similar distribution of flow parameters of mixing layer;characteristics of“two regions four stages”spatial distribution of kerosene spray in the mixing layer and promotion of droplets breakup due to large velocity gradient of mixing layer;characteristics of matching relationship between reaction and flow field and two mechanisms of reaction region formation by high temperature combustion products support and thermal auto-ignition,respectively.The results obtained here are of a significant reference value to have a deeper insight into the non-isothermal supersonic-subsonic mixing layer and would be instructive to the combustor design of the air-breathing high-speed vehicle.