Curvature and Orientation Statistics of Turbulent Premixed Flame Fronts

Abstract

The curvature of turbulent premixed flame fronts is an important spatial property that needs to be quantified over a range of turbulence conditions. In this study, measurements of flamelet curvature along with orientation statistics for u'/ SL = 1.42-5.71 are obtained from OH planar laser-induced fluorescence images of the flame boundary by applying a curve-tracing difference formula with interval length of the order of the inner cutoff scale. Use of this particular interval length is essential for accurate tracing of the flame boundary with implicit filtering of extraneous noise that can introduce significant errors in the curvature measurements. The distributions of flamelet curvature are found to be symmetric with respect to the zero mean, while the variance increases with increasing u'/S_L. These distributions can be approximated by Gaussian distribution functions. The positive and negative mean curvatures show a nearly square-root dependence on u'/S_L whereas the mean flamelet radius of curvature is approximately a factor of two larger than the Taylor scale of turbulence in the approach flow. The effect of Lewis number on flamelet curvature is evidenced by a 20% increase in mean cuvatures which is attributed to the unstable flame fronts at Lewis number less than unity. The evolution of flamelet orientation with increasing u'/S_L shows a trend toward isotropy, which is estimated to prevail when u' becomes an order of magnitude larger than S_L. For unstable flame fronts (Le < 1), flamelets are more randomly orientated and thus isotropy may be achieved for somewhat smaller ratio of u'/S_L.