Analysis of Gaseous Fuel and Air Mixing in Flames and Flame Quenching

Abstract

A model for fuel ‐air mixing in diffusion e ames is presented and applied to study the mixing and quenching of methane‐air e ames. The model is based on the ideal gas law, the energy equation, the equation of continuity, and Arrhenius form of rate equation and is, therefore, strictly valid for mixtures having low density, that is, for lowpressurecombustors.Intheabsenceofpreferentialdiffusion,chemicalreactionscauseanunbalancedconsumption offuelandoxygeninnonstoichiometrice ames.Untilthedesiredequivalenceratioisachieved,enhancedpreferential diffusion ofoxygen orfuel isrequired in fuel-rich orfuel-lean e ames, respectively. Aftera desiredequivalenceratio is achieved, preferential diffusion of oxygen or fuel should bereduced to the exact level required to compensate for the unbalanced consumption of fuel and air. In the absence of these conditions, e ame chemistry cannot be strictly controlled. In addition, unless the desired equivalence ratio is at a position of stable equilibrium over an extended range of operational conditions, the e ame may be quenched. Net transport of fuel or oxygen due to diffusion is correlated with distributions of pressure,temperature, velocity, mass fraction of species,and heat transfer through radiation and conduction. Results show that negative rates of pressure (or positive rates of temperature ) and positive rates of pressure (or negative rates of temperature ) can enhance preferential diffusion of oxygen and fuel, respectively. Negative velocity divergence also enhances the diffusion of oxygen, whereas positive velocity divergenceenhances thediffusion offuel. Recirculation of burntgasesimproves thestability ofall e ames. Forrates of pressure of less than 1 atm/s, heat addition through conduction or radiation can provide a position of stable equilibrium for fuel-rich e ames. A position of stable equilibrium can be provided for both fuel-rich and fuel-lean e ames by combining a positive rate of temperature with positive velocity divergence, for rates of pressure of up to 25 atm/s. At higher rates of pressure or temperature, increased initial pressure or temperature, respectively, also assists in e ame stabilization.