The large-scale turbulent structure of nonpremixed planar jet flames

A comparative study was conducted of the large-scale turbulent structure of nonpremixed planar jet flames and nonreacting planar jets of varying densi ty. The diagnostic techniques used included planar laser-induced fluorescen ce of OH and planar laser Mie scattering (PLMS) from alumina particles seed ed into the jet fluid. The PLMS imaging shows that the nonreacting jets at all density ratios studied exhibit large-scale organized motions as observe d in previous studies at a density ratio of unity. Such organized structure s are not nearly as dominant in the jet flames at comparable local Reynolds numbers. To quantitatively investigate the large-scale motions, ensembles of PLMS images were used to compute root-mean-square (RMS) intermittency fi elds. The RMS intermittency profiles (normalized by the local jet width) ar e very similar for all of the nonreacting cases, while the flame profile wa s considerably narrower. Furthermore, plan view PLMS images reveal quasi-tw o-dimensional organization for the nonreacting case, but more random three- dimensional structure for the flame. These differences argue for the substa ntial modification of the large-scale turbulent structure owing to the pres ence of the flame, and do not appear to be due to global Reynolds number ef fects or global density changes resulting from heat release. (C) 1998 by Th e Combustion Institute.