Statistical behaviour of vorticity and enstrophy transport in head-on quenching of turbulent premixed flames

Artikel i vetenskaplig tidskrift, 2017

The transport of vorticity and enstrophy in the near-wall region for head-on quenching of turbulent combustion by an isothermal inert wall has been analysed using three-dimensional Direct Numerical Simulation (DNS) data of statistically planar turbulent premixed flames characterised by various global Lewis numbers Le (ranging from 0.8 to 1.2) and turbulence intensities. In all cases the vorticity magnitude shows its maximum value at the wall and the vorticity magnitude drops significantly from the unburned to the burned gas side of the flame-brush. Moreover, the vorticity magnitude shows an increasing trend with decreasing Le, and increasing turbulence intensity. A significant amount of anisotropy has been observed between the vorticity components within the flame-brush and this anisotropy increases as the wall is approached. The baroclinic torque term has been found to be principally responsible for this anisotropic behaviour. The vortex-stretching and viscous dissipation terms remain the leading order contributors to the vorticity and enstrophy transport for all cases when the flame is away from the wall, but as flame approach the wall, the baroclinic torque begins to play an increasingly important role. The combined molecular diffusion and dissipation contribution to the enstrophy transport remains negative away from the wall but it changes its sign near the wall due to the torque arising from dilatation rate gradient. Detailed physical explanations have been provided for the observed influences of flame and wall on the statistical behaviours of vorticity and enstrophy and the various terms of their transport equations.