Characterization and Sensitivity Analysis of a Turbulent Diffusion Flame in Diluted Hot Coflow

Large-eddy simulation of a jet flame in a hot diluted coflow has been performed. The burner under consideration was operated in the moderate and intense low oxygen dilution (MILD) combustion regime, and utilizes a three-stream feed system to supply fuel, diluted coflow, and air to the burner (Dally, B. B., Karpetis, A. N., and Barlow, R. S., Proc. Combust. Inst., Vol. 29, 2002, pp. 1147–1154). The reduced oxygen concentration in the coflow decreases the reactivity of the mixture, and, in turn, increases the sensitivity of the flame to variations in mixture composition and inflow conditions at the burner inlet. This work addresses both issues, namely the modeling of a three-stream burner system and the characterization of the sensitivity of the burner to variations in inflow and scalar boundary conditions. In the first part of this paper, a flamelet-based combustion model is extended to account for variations in the composition of the oxidizer stream. To this end, a scalar quantity, representing the oxidizer split, is introduced, and all thermochemical quantities are represented in terms of two conserved scalars and a reaction progress variable. The second part of this paper investigates the sensitivity of the flame structure to variations in scalar inflow boundary conditions. LES calculations with nominal boundary conditions and boundary conditions determined from experimental data are prescribed for all scalar quantities. Results show that the selection of the inflow conditions not only affects the nozzle-near region of the flame, but also leads to significant variations in the upper part of the flame. Reasons for this sensitivity are attributed to the population of the flamelet state-space and are further analyzed in the paper.