A physical simulation model for turbulent mixing layer combustion

Abstract

A previously proposed discrete vortex model is applied to reveal the underlying physics of diffusion combustion in vortical flow fields generated by pairing vortices. The model accounts for the finite size of each localized vorticity region as a circulation monopole plus a vortex quadrupole, and describes a self-rotating elliptic vortex deformable in a stretching flow due to the presence of other vortices and flame. The coalescence of pairing vortices can be recognized as a resonance phenomenon which takes place, through interaction between a pair of quadrupoles, when the magnitude of the stretching rate in the vorticity regions exceeds that of the self-rotating angular velocity. The Shvab-Zeldovich formulation is used for combustion modeling. Results of simulation performed for a temporally evolving mixing layer are in fairly good agreement with existing numerical calculation results. The effect of thermal expansion is incorporated in the form of volume source at the flame front, so that changes from constant density case attribute to the expanding flow from the flame. The burning rate is reduced since the flow opposes the diffusive fuel and oxidizer fluxes toward the flame. The distance of separation between flame segments is enlarged, resulting in an rapid increase in the flame length and mixing layer thickness. This has another important significance in the light of the present model because the effect of vortex quadrupole decays faster than that of circulation monopole away from each vortex core and the preservation of localized vorticity regions through the repeated processes of vortex pairing and coalescence is inhibited. In addition to the commonly mentioned increased viscosity due to temperature rises, this mechanism is proposed to provide a physical explanation why turbulent free shear layer with combustion assumes a simpler structure in a shorter time.