Scalar Pdf Modeling of Turbulent Nonpremixed Methanol-Air Flames

Abstract

A numerical modeling of turbulent methanol-air nonpremixed jet flames with a five-step and a four-step reduced reaction mechanisms is presented. The model calculates the scalar properties by a joint pdf (probability density function) approach and a second-order moment closure for the turbulent velocity field. Although the flame is considered adiabatic, mixture enthalpy has been included as a passive scalar to properly account for the pilot flame which has a flame temperature higher than the adiabatic flame temperature of the stoichiometric mixture. A total of seven scalars, including two passive scalars and five reactive scalars, are used to describe the thermo-chemical states of the reactive mixtures. The evolution of the joint pdf of these seven scalars was simulated by the Monte Carlo technique. An extensive analysis of the predicted results in comparison with experimental data is presented. The predicted mass fractions of most reactive species are in good agreement with the experimental data, and the probability of local flame extinction is reasonably predicted as well. Carbon monoxide is the only species for which a substantial discrepancy exists between the predictions and the measurements. The noted discrepancy is similar to that observed in a previous numerical study of turbulent methane-air nonpremixed jet flames by Chen et al.(1989).Although several possible causes are investigated, no satisfactory reasons were found to explain the noted discrepancies.