PDF modeling of lean premixed combustion using in situ tabulated chemistry

The velocity-composition probability density function (pdf) model coupled with a k-epsilon-based mean flow computational fluid dynamics (CFD) model was used to describe the turbulent fluid flow, heat transfer, chemistry, and their interactions in a bluff-body, lean, premixed, methane-air combustor. Measured data [1, 2] including velocity, temperature, and chemical species concentrations were used to evaluate the model. The chemistry calculations were performed with an in situ look-up tabulation method [3]. A reduced, 5-step chemical mechanism [4] for describing fuel oxidation, CO, and NO chemistry was used in the model. NO formation from thermal, N2O-intermediate, and prompt pathways was included in the 5-step mechanism. An axisymmetric, unstructured grid was used for solving the Eulerian, mean flow equations and the vertices were used to store mean statistics for solving the Lagrangian, fluid particle equations. Predicted velocity and composition mean statistics were compared to measurements in the bluff-body combustor for a lean equivalence ratio of 0.59. The predictions of major species matched measured and calculated equilibrium values in the recirculation zone. Comparisons of mean CO throughout the combustor were always within an order of magnitude and showed marked improvements over past predictions. Maximum discrepancies between measured and predicted NO concentrations were between 5 and 7 ppm (similar to 50%). The accessed composition space in this turbulent combustion simulation represented the values of species mole fraction and enthalpy for each fluid particle at each time step and was found to lie in a relatively small, uniquely shaped region that was dictated by the mixing, reaction, and heat transfer in the combustor. This accessed composition region was obtained in situ and required about 35 megabytes of storage once a steady state was reached. This memory requirement was more than three orders of magnitude less than would be needed in a standard, a priori table. The in situ tabulation approach allowed for technically correct and efficient chemical kinetic calculations using the 5-step mechanism in this pdf-method-based, multidimensional combustor model. (C) 1999 by The Combustion Institute.