Compressible Turbulence Combustion Modeling For Supersonic Combustion

Scramjet has tended to be a research hotspot due to its applicability for hypersonic propulsion (5<Ma<15) in recent decades. As the extremely complicated measurement and costly experiment, computational fluid dynamics (CFD) has attracted increasing attention. Because of its substantial complexity interactions of shock/boundary and shock/flame as well as self-ignition, local extinction, re-ignition and flame stabilization, it is critical to implement accurate numerical methods and combustion models for understanding combustion characteristics in Scramjet. This paper mainly focus on the following areas:(1) Compressible turbulence combustion models based on thermo-chemistry tabulation approaches are developed. (2) The compressible flamelet/progress variable approach has been performed to investigate the combustion charcteristics in strut based DLR scramjet using RANS and LES. (3) Large eddy simulations combined with modified compressible auto-ignition turbulent combustion model are performed to investigate mixing and combustion characteristic in the Gamba combustor with transverse jet in high-enthalpy supersonic cross-flow.In supersonic reacting flows, the variations of pressure, temperature and density resulting from the compressions and expansions which have effects on the important chemistry reactions. Therefore, when coupling low-Mach thermo-chemistry tabulation models into compressible supersonic flows, the effects of fluid compressibility need to be considered. In this paper, temperature-dependence and pressure dependence modification methods for thermo-chemistry tabulation models have been deeply studied. Temperature is obtained by solving filtered energy transport equation, which could consider the partial compressibility. The reaction rate of the progress variable is rescaled by pressure. Pressure correction coefficient for the source term of reaction progress variable is introduced by analyzing laminar thermo-chemistry tables under different pressures. This method has the advantages of taking into account the pressure variations in supersonic flow without increasing of the size the thermo-chemistry tabulation, and could be applied to the flamelet model in which the reaction progress variable is one of the tabulation parameter. For the simulation of the supersonic transverse flow, a modified initial temperature is introduced to the auto-ignition model based on the pressure and temperature modification. What’s more, this modified method could take into account the non-uniform temperature distribution in the near-field arise from the compression/expansion.Compressibility flamelet/progress variable approach has been performed to investigate the three-dimensional flow and combustion characteristics in strut based DLR scramjet using RANS and LES method. And LES exhibits better accuracy than RANS, on the results for pressure, wave distribution and velocity in non-reacting flow as well as for wave distribution, velocity and temperature in reacting flow. LES can provide large scale coherent structures and unsteady phenomena of flow and combustion, and has the ability to capture the interactions between shock and boundary, turbulent mixing, extinction/re-ignition and flame stabilization. The results of LES show that an unstable flame with extinction and ignition develops in the shear layers of bluff body and a fuel-rich partially premixed flame stabilizes in the central recirculation bubble. In the LES framework, two different PDFs, delta-function and beta-function, are applied to model the reaction progress variable. It’s found that the results of beta-function PDFs are in good agreement with experimental data, whereas, delta-function PDFs over-estimate the mean temperature due to ignoring the effects of the SGS variance of reaction progress variable. It is concluded that the SGS variance of reaction progress variable plays an important role in the simulation of supersonic combustion.Large eddy simulations are performed to investigate the large scale coherent structures and mixing characteristics in the Gamba combustor with transverse jet in high-enthalpy supersonic cross-flow. The probability density functions (PDFs) are introduced to study the mixing characteristic in the near field, far field and the recirculation regions in the upstream of the jet. The effect of the jet-to-crossflow momentum ratios on flow structure, jet penetration, scalar distribution and mixing efficiency are also concerned. Owing to the strong interactions between CVP and wall boundary layer, the fuel concentration tends to rapidly spreads at span-wise direction in the low jet-to-crossfiow momentum ratio case, leading to a high mixing efficiency. Considering the high temperature in the supersonic freestream with above the auto-ignition temperature, the modified auto-ignition model is performed to investigate the flow structure, jet penetration and combustion characteristics in the scramjet with transverse hydrogen jet in high-enthalpy supersonic cross-flows. Due to the increasement of flow viscosity caused by combustion, the size of CVP and TCVP structure increases, jet trajectory penetration improves in reacting flow. Furthermore, There are three reaction zone can be observed:a reactive shear layer on the windward side of the jet, a reacting recirculation region upstream of the jet wraps around the jet and near-wall burning region in the wake of the jet. The flame stabilization the near wall region associated with the TCVP structure. The numerical results are in good agreement with the experimental observations.