Effect of volumetric heat loss on triple-flame propagation

Abstract

We present a numerical study of the effect of volumetric heat loss on the propagation of triple flames in the strained mixing layer formed between two opposed streams of fuel and air. The propagation speed of the triple flame is computed for a wide range of values of two non-dimensional parameters: a normalized flame thickness ε , proportional to the square root of the strain rate, and a heat-loss parameter κ . It is shown that, for relatively small values of κ , the propagation speed U is decreased by heat loss, and its dependence on ε is similar to the adiabatic case, known in the literature: in particular, a monotonic decrease in the speed from positive to negative values is observed as ε is increased. However, for κ larger than a critical value, this monotonic behavior is lost. It is shown that the more complex behavior obtained is mainly associated with the fact that, in the presence of heat loss, the trailing planar diffusion flame is extinguished both for sufficiently large and sufficiently small values of the strain rate. Moreover, for sufficiently small values of ε , the dependence of U on κ is similar to that of the non-adiabatic planar premixed flame, with total extinction occurring for a finite positive value of U . On the other hand, for larger values of ε , negative speeds, corresponding to extinction fronts, appear before total extinction is brought about by an increase in κ . A summary of the main results is provided by delimiting the different combustion regimes observed in the κ-ε plane.