The production of premixed flame surface area in turbulent shear flow

Abstract

In the flamelet theory for turbulent premixed combustion, the modeling of the mean reaction rate is based primarily on a statistical description of the wrinkling of the flame surface due to the turbulent motions. The amount of wrinkling is quantified by the flame surface density, Σ, and the flamelet theory produces an exact evolution equation for Σ. The Σ-equation has been studied in previous work in simple basic configurations like one-dimensional flames in freely evolving, isotropic turbulent flow. In practical systems, however, the combustion zone features more complex flow geometries with mean velocity gradients. In the present study, we use three-dimensional direct numerical simulation of premixed flames to characterize the effect of a mean shear motion, parallel to the flame, on flame surface production. The shear is uniform in the unburned gas and simulations are performed for different values of the mean shear rate, S. The database is then used to estimate and compare the different terms appearing in the Σ-equation as a function of S. The analysis gives in particular the relative weightings of the mean flow and turbulent flow components of flame stretch, κ^. The mean flow component accounts for possible, initial, rapid distortion effects, whereas the turbulent flow component measures the progressive adjustment of the turbulence to the applied deformation. The main results are that (1) the direct straining of the flame surface by the mean velocity gradients remains negligible, even at high shear rates, and the dominant effect of the applied shear on flame surface production is to increase the turbulent flow component in the expression of κ^ (2) the scaling of κ^ does not seem to follow standard isotropic turbulence behaviors.