Reaction zone structure of non-premixed turbulent flames in the “intensely wrinkled” regime

Abstract

Simultaneous images of the CH and OH reaction zones are reported for “Intensely Wrinkled” nonpremixed flames, to determine whether reaction zones retain their thin “laminar flamelet” structure or become “distributed reaction zones”. Intensely Wrinkled Flames (IWFs) were achieved by using a special burner with large coflow air velocities to obtain a normalized turbulence intensity ( u′/Ū ) of 3.6, which is 10 times greater than the turbulence intensity within jet flames. The images were used to measure profiles of the flame surface density (Σ) and the average CH layer thickness ( δ¯ - CH ) it is argued that these parameters are the ones that should be used to assess new large eddy simulations (LESs), rather than insensitive parameters such as mean concentrations. In the regime of IWFs, the CH reaction zones remained as thin as those measured in laminar jet flames (i.e., less than 1 mm thick) and had the appearance of flamelets. These thin reaction zones were extinguished before they became thickned by intense turbulence, which provides experimental evidence to support laminar flamelet modeling concepts. “Shredded flames” occurred, within which the reaction zones were short, discontinuous segments, and the degree of flame wrinkling was significantly larger than in jet flames. Shredded flames have not been observed previously. There is no evidence of small-scale wrinkling of the reaction zones at scales less than half the integral scale. The images showed where the instantaneous stoichiometric contour is located, since it exists at the boundary between the CH and OH layers. Flame surface densities were typically 0.3 mm −1 .