Large Eddy Simulation Of Transverse Jet Mixing And Combustion In Supersonic Crossflow

The fast mixing process and high combustion efficiency are the key issues in scramjet combustor design.Transverse jet in supersonic crossflow(JISCF),as a basic structure of relatively efficient scramjet combustor,has been studied by many researchers with its very high fuel/air mixing efficiency.A new solver,based on the computational code in OpenFOAM,was used to study the flow field,mixing and combustion process in the transverse jet in supersonic crossflow.The main work and conclusions are as follows.Firstly,large eddy simulations have been performed to investigate the the influence of different jet to cross-flow momentum flux ratios on coherent structures and mixing characteristics in a typical transverse hydrogen jet scramjet combustor,which has a compression ramp at inlet to generate shock train.The numerical results show that higher jet to cross-flow momentum flux ratio results in stronger bow shock,which reduces the streamwise velocity and increases vertical velocity of the cross-flow downstream of the bow shock allowing a higher penetration of the jet and the Mach disk.The jet penetration bends down due to the effect of up wall and reflected shock,and it is found that the bending down location is near the place where reflected shock interacts with the jet shear layer related to the jet to cross-flow momentum flux ratios.The vorticity analysis is conducted to understand the turbulent mixing mechanism,and it is found that the shock-induced baroclinic torque plays an important role on the generation of the vorticity in the near field of the fuel jet and the place where the reflected shock interacts with the jet plume.Comparing the mixing efficiency and mixed burnable hydrogen flow rate,higher jet to cross-flow momentum flux ratio leads to higher burnable hydrogen mass flow rate,but it does not improve the mixing efficiency.Secondly,large eddy simulations have been performed to investigate the flow field and mixing characteristics in transverse single/double hydrogen jet in supersonic crossflow.The numerical results show that turbulence characteristics in the windward mixing layer of secondary jet are changed due to the interactions between the shear layer vortices of primary and secondary jets.Results of the two-dimensional and three-dimensional streamlines show that the trailing counter-rotating vortex pairs(TCVP),secondary TCVP of primary jet and the horseshoe vortex would merge and form a new horseshoe vortex.Three counter-rotating vortex pairs(CVP)are formed in the downstream of secondary jet,the CVP-B due to interactions between the supersonic crossflow and secondary jet,the CVP-C due to interactions between the supersonic crossflow,primary and secondary jets,and the CVP-D due to interactions between the supersonic crossflow and primary jet.Analysis of the mixing characteristics shows that penetration and mixing efficiency of transverse double jet in supersonic crossflow are significantly higher than single jet.Thirdly,large eddy simulations with Partially Stirred Reactor(PaSR)sub-grid combustion model have been performed to investigate mixing and combustion field in the high-enthalpy hydrogen JISCF combustor.Self-ignition flame is observed after the interaction of the oblique shock and the jet mixing layer for the jet to crossflow momentum flux ratio J = 0.71.However,the self-ignition takes place at jet orifice,and the interactions of the oblique shock and the jet mixing layer enhance the combustion and stabilize the flame for J=2.11 and J=4.00.A pair of symmetrical recirculation zone,which contributes to the flame anchoring of the supersonic transverse jet combustion,is observed in the region of 10<x/D<20 near the wall.The results of combustion mode analysis show that the combustion heat release with lower jet to crossflow momentum flux ratio J = 0.71 is dominated by premixed combustion,however,that with higher jet to crossflow momentum flux ratios J=2.11 and J=4.00 are dominated by non-premixed combustion.Chemical explosive mode analysis(CEMA)is employed to analyze the numerical results.High temperature auto-ignition is identified as the dominant stabilization mechanism in JISCF.Flame along the horseshoe vortex region in the near field of jet orifice of J=2.11 and J=4.00 exhibits distributed reaction like mode,which is leaded by auto-ignition.Finally,three-dimensional unsteady Reynolds average numerical simulations(URANS)have been performed to investigate the mixing characteristics in transverse pulsed jet in supersonic crossflow for the mean jet to crossflow momentum flux ratio J = 2.11.The pulsed jet frequency of optimum mixing efficiency is about 50 kHz under the condition of constant pulse amplitude,and higher pulsed frequency will lead to the jet plume tending to the one of steady jet in the downstream.When pulsed jet frequency is constant,the mixing efficiency increases with the increase of the pulsed jet amplitude,but the attenuation rate of the maximum mass fraction does not change in the downstream of jet orifice.Moreover,large eddy simulations with Partially Stirred Reactor(PaSR)sub-grid combustion model have been performed to investigate the combustion process in transverse pulsed jet in supersonic crossflow with the optimum pulsed jet frequency.As the pulsed jet issuing into supersonic crossflow,the large-scale structure in the flow field can be enlarged and always appear periodically at 1/4T0,accompanied by the shear vortex structures of the original steady jet.The non-premixed combustion heat release is changed by the pulsed jet,which has little effect on the premixed combustion heat release.The dominant Strouhal numbers relevant to unsteadiness of Bow shock and jet shear layer vortices for steady jet are 0.04 and 0.27,respectively.However,the dominant Strouhal numbers for pulsed jet in supersonic crossflow are 0.05,which is consistent with the pulsed jet Strouhal number.