Structural properties of lifted hydrogen jet flames measured by laser spectroscopic techniques

The region near the lift-off height of several turbulent H-2/air and H-2/N- 2/air diffusion flames with Reynolds numbers between 3600 and 17,300 was in vestigated to study the effects of chemical composition, large-scale struct ures, and gradients on the flame stabilization process. Using Raman and Ray leigh scattering, quantitative single-pulse one-dimensional profiles of 911 major species concentrations and temperature have been measured with high accuracy and good spatial resolution. The local mixture fraction has been d etermined from these images; postprocessing of the data allowed the identif ication of large-scale structures, the accurate determination of the positi on of reaction zones, and of regions with high scalar dissipation or large temperature gradients. Double-pulse experiments allowed the direct determin ation of the local heat release. This is illustrated by individual examples . The interpretation of the data, in view of current flame stabilization th eories, suggests an extended analysis with respect to statistical criteria. Evaluation of all images shows that fuel and air at the lift-off height ar e generally mixed over a region that is several mm wide and that these mixt ures have stoichiometries well within the H-2 flammability limits. The majo rity of images exhibiting a distinct high-temperature zone also show the pr esence of large-scale structures, which appear to be related to the flame s tabilization process. The scalar dissipation at the lift-off height is one order of magnitude lower than the critical value for flame extinction A sta tistical analysis of several thousands of images shows that maxima in the s calar dissipation rate are not correlated to temperature gradients or to th e position of the instantaneous flame front. The observed structural featur es and their statistical relevance are discussed in the context of recent a dvances in flame stabilization theories. (C) 2000 by The Combustion Institu te.