Reaction zone structure in turbulent nonpremixed jet flames - From CH-OHPLIF images

It is shown that simultaneous images of the CH and On concentration fields can be obtained throughout a high-Reynolds-number (18,600) turbulent nonpre mixed, nonsooting jet flame, and that the CH-OH boundary is a useful marker of the instantaneous stoichiometric contour. Previous CH-OH imaging was co nfined to the flame base. The structure of the fuel-decomposition zone-iden tified by the CH images-includes the following regions: those with high-cur vature cusps; those with low CH concentration; and those where the flame "p inches" 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 rip of the high -Reynolds-number flame, CH layers in the turbulent flame are not thicker th an the CH layers in the laminar jet flame at the same sin location. In fact , CH layer thickness is relatively insensitive to Reynolds number and the l evel of turbulence. This implies that turbulence does not broaden the CH re action zone, and that flamelet concepts are justified in modeling the react ion zone associated with fuel decomposition. The CH layers become thicker i n the streamwise (x) direction, which is expected because scalar gradients and the dissipation rate are expected to decrease in the streamwise directi on. Imaging the CH layer makes it possible to measure the flame surface den sity (Sigma), which has a typical value of 0.2 mm(-1). Surface density is s hown to be related to the turbulent blush thickness and the degree of wrink ling. (C) 2000 by The Combustion Institute.