A numerical study of counterflow diffusion flames of methane/air at various pressures

Abstract

A numerical study of the counterflow diffusion flames of methane/air at both subcritical and supercritical pressures, which have very important applications in the air-breathing rocket and advanced gas turbine engines, is conducted to obtain fundamental understanding of the flame characteristics. The analysis is based on a general mathematical formulation and accommodates a unified treatment of general fluids thermodynamics and accurate calculations of thermophysical properties. Results reveal that the maximum flame temperature occurs on the fuel-rich side for low-pressure conditions and shifts toward the stoichiometric position when the pressure increases. The maximum flame temperature increases with an increasing pressure, but decreases with an increasing strain rate. The flame width is inversely proportional to the square root of the product of the pressure and strain rate as ${{\delta \propto 1} \mathord{\left/ {\vphantom {{\delta \propto 1} {\sqrt {p \cdot a} }}} \right. \kern-\nulldelimiterspace} {\sqrt {p \cdot a} }}$. The total heat release rate varies with the pressure and strain rate in a relationship of (Q _release √(p·a)^0.518. An increased pressure leads to a slightly more complete combustion process near the stoichiometric position, but its effect on NO production is minor. Under the test conditions, variations of the strain rate have significant impacts on the formation of major pollutants. An increased strain rate leads to the decreased mole fraction of CO in the fuel-rich region and significantly reduced NO near the stoichiometric position.