On burning rate enhancement in spherically expanding turbulent flame

The mechanism responsible for the enhancement of the burning rate in turbulent premixed flames is an active topic of research. The dependence of the burning rate on the turbulence intensity is well documented through a large number of experiments, although the scatter is high even at low turbulence levels. In the present study, we aim to explain this dependence with data from three Direct Numerical Simulations (DNS) of spherically expanding turbulent flames at different Reynolds numbers. First, the ratio of turbulent surface area to that corresponding to the mean radius is decomposed into factors involving the flame brush thickness and the surface density function. Then a detailed governing equation for the flame brush thickness is derived systematically starting from the surface density transport equation. It is observed that the flame brush evolves due to four different mechanisms, one of which is the turbulent transport. However, the gradients in the mean velocity field, dependence of propagation velocity on mean curvature, and the flame stretch term also contribute to the evolution of the brush. Our analysis proves that the burning rate enhancement in turbulent flames depends to a large extent on the configuration, which may explain the large scatter in experimental data.