Measurements of baroclinic enstrophy production in turbulent premixed flames

Reynolds averaged enstrophy transport budgets, conditioned on mean progress variable, are measured in a variety of turbulent swirl flames using tomographic particle image velocimetry (TPIV) and CH2O planar laser induced fluorescence (PLIF) across the Karlovitz number range Ka = 10-45. The combined contributions of baroclinic torque and dissipation to enstrophy transport are evaluated by subtracting the vortex stretching and dilatation terms from the Lagrangian derivatives. Enstrophy production through baroclinic torque dominated over dissipation towards the product sides of the flame brushes. Indeed, the net effect of baroclinic torque and dissipation in these regions was enstrophy production at a rate that generally exceeded production due to vortex stretching. In contrast to direct numerical simulations of flames in homogeneous isotropic turbulence, the significance of baroclinic torque relative to vortex stretching increased with increasing Karlovitz number due to effects of a realistic configuration. The observed flame-scale turbulence generation towards the combustion products has implications both for turbulence and combustion closure models.