Modeling Combustion Reactions in a Diffusion Flame: Comparison between the Detailed-chemistry and Equilibrium Models

Abstract

This paper discusses the validity of the mixture-fraction model with the assumption of chemical equilibrium, which is widely used to simulate a diffusion flame. Two equilibrium models were tested: (1) a single-step equilibrium model that considers only the fuel, oxygen, nitrogen, carbon dioxide, and water vapor; (2) a multi-step equilibrium model that considers 53 intermediate species. These equilibrium models were compared with the results of a detailed-chemistry model that considers the rates of 325 elementary reactions. The detailed-chemistry model was applied to one-dimensional counterflow diffusion flames. Temperature and mass fractions computed by the detailed-chemistry model were generally between the values given by the two equilibrium models. Depending on the condition and the range of mixture fraction, either a single-step equilibrium model or a multi-step equilibrium model better approximated the detailed-chemistry model. When the flame temperature was increased by using pure oxygen as the oxidizer, the multi-step equilibrium model tended to be more accurate than the single-step equilibrium model.