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

Abstract

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 investigated to study the effects of chemical composition, large-scale structures, and gradients on the flame stabilization process. Using Raman and Rayleigh scattering, quantitative single-pulse one-dimensional profiles of all major species concentrations and temperature have been measured with high accuracy and good spatial resolution. The local mixture fraction has been determined from these images; postprocessing of the data allowed the identification of large-scale structures, the accurate determination of the position of reaction zones, and of regions with high scalar dissipation or large temperature gradients. Double-pulse experiments allowed the direct determination of the local heat release. This is illustrated by individual examples. The interpretation of the data, in view of current flame stabilization theories, suggests an extended analysis with respect to statistical criteria. Evaluation of all images shows that fuel and air at the lift-off height are generally mixed over a region that is several mm wide and that these mixtures have stoichiometries well within the H 2 flammability limits. The majority of images exhibiting a distinct high-temperature zone also show the presence of large-scale structures, which appear to be related to the flame stabilization process. The scalar dissipation at the lift-off height is one order of magnitude lower than the critical value for flame extinction. A statistical analysis of several thousands of images shows that maxima in the scalar dissipation rate are not correlated to temperature gradients or to the position of the instantaneous flame front. The observed structural features and their statistical relevance are discussed in the context of recent advances in flame stabilization theories.