Reaction zone structure in turbulent nonpremixed jet flames—from CH-OH PLIF images

Abstract

It is shown that simultaneous images of the CH and OH concentration fields can be obtained throughout a high-Reynolds-number (18,600) turbulent nonpremixed, nonsooting jet flame, and that the CH-OH boundary is a useful marker of the instantaneous stoichiometric contour. Previous CH-OH imaging was confined to the flame base. The structure of the fuel-decomposition zone—identified by the CH images—includes the following regions: those with high-curvature cusps; those with low CH concentration; and those where the flame “pinches” due to oxidizer being entrained to the centerline. It is found that the reaction zone that is associated with fuel decomposition (i.e., the CH layer) remains thin and rarely exceeds 1 mm, even near the tip of the high-Reynolds-number flame. CH layers in the turbulent flame are not thicker than the CH layers in the laminar jet flame at the same x/ d location. In fact, CH layer thickness is relatively insensitive to Reynolds number and the level of turbulence. This implies that turbulence does not broaden the CH reaction zone, and that flamelet concepts are justified in modeling the reaction zone associated with fuel decomposition. The CH layers become thicker in the streamwise ( x) direction, which is expected because scalar gradients and the dissipation rate are expected to decrease in the streamwise direction. Imaging the CH layer makes it possible to measure the flame surface density (Σ), which has a typical value of 0.2 mm −1. Surface density is shown to be related to the turbulent brush thickness and the degree of wrinkling.