PDF modeling of lean premixed combustion using in situ tabulated chemistry

The velocity-composition probability density function (pdf) model coupled w ith a k-epsilon-based mean flow computational fluid dynamics (CFD) model wa s 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 chemi stry was used in the model. NO formation from thermal, N2O-intermediate, an d prompt pathways was included in the 5-step mechanism. An axisymmetric, un structured 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 statisti cs were compared to measurements in the bluff-body combustor for a lean equ ivalence ratio of 0.59. The predictions of major species matched measured a nd calculated equilibrium values in the recirculation zone. Comparisons of mean CO throughout the combustor were always within an order of magnitude a nd 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 combustio n simulation represented the values of species mole fraction and enthalpy f or each fluid particle at each time step and was found to lie in a relative ly small, uniquely shaped region that was dictated by the mixing, reaction, and heat transfer in the combustor. This accessed composition region was o btained in situ and required about 35 megabytes of storage once a steady st ate was reached. This memory requirement was more than three orders of magn itude 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, m ultidimensional combustor model. (C) 1999 by The Combustion Institute.