Laser Measurements on Nonpremixed & Air Flames for Assessment of Turbulent Combustion Models

L Introduction M practical combustion devices are turbulent nonpremixed flames. The development of appropriate computer models" for practical combustors holds the potential for their computer-aided design with lower development costs, higher fuel efficiencies, lower emissions, and wider fuel specifications. However, to be reliable over a wide range of conditions, such models should be based upon a fundamental, quantitative understanding of fuel/air mixing and chemical reactions. Considerable progress has been made in modeling fuel/air mixing processes (particularly in larboratory-scale turbulent nonpremixed flames) using probability density functions (pdf) of a conserved scalar variable to account for turbulent fluctuations" and Favre averaging to account for density variations. However, the empirical correlations used in the model still rely on comparisons with experiments in nonreacting flows. A better understanding of the analogies between reacting and nonreacting flows and their deviations can result from detailed experiment/modeling comparisons in turbulent nonpremixed flames. Measurements of at least some of the key model variables (such as pdf's of conserved and reactive scalars, Favre and conventional averages and fluctuations, inter mitt ency, extent of reaction, scalar fluxes, gradients, and correlations) are useful to directly test the model assumptions. This level of understanding is required to model turbulent chemistry interactions realistically or to predict in a fundamental way the stability limits (blowoff and relight) or the formation and emission of pollutants such as carbon monoxide and nitric oxide. Because of the problems in making detailed measurements in practical combustors and the difficulties in modeling their complex geometries and flow patterns, the development and testing of quantitative combustion models is based upon detailed comparisons with quantitative experimental data in well-controlled, laboratory-scale, nonpremixed flames operated over a range of known initial conditions. However, few comprehensive measurements have been attempted even in laboratory-scale nonpremixed flames because of the difficulties in obtaining data having sufficient temporal and spatial resolution, without significantly perturbing the flame under study. Fortunately, recent advances in laser measurement techniques and, in particular, in pulsed Raman scattering, laser velocimetry, and fluorescence imaging have greatly expanded the experimental capabilities to make nonperturbing quantitative measurements in nonpremixed reacting flows. This paper summarizes detailed experiments reported previously'" and presents new experimental results characterizing four H2-air jet diffusion flames. Admittedly, H2 jet diffusion flames do not possess some elements of practical combustor systems (e.g., swirl, recirculation, corn-