Numerical investigation of the ignition of dilute fuel sprays including detailed chemistry

Abstract

One-dimensional unsteady numerical calculations have been performed to study the ignition of dilute fuel sprays. Recent investigations in this field employ global kinetic rate expressions to model the chemical reactions occuring in the gas phase. In the present paper a detailed kinetic mechanism is employed to describe the ignition process of dilute methanol sprays in air, the mechanism for methanol comprises 23 species and 168 elementary reactions. The present investigation accounts for unsteady droplet heating and vaporization using a distillation-limit model; temperature dependence of liquid phase properties is included. The study investigates uniform sprays; both monodisperse and bidisperse sprays are considered. The quiescent mixture of fuel and air is ignited by assuming hot air and cold fuel. The present study concerns ignition of a fuel spray in a closed volume. Effects of varying parameters such as initial gas temperature, initial fuel-vapor concentration, pressure, droplet size in both monodisperse and bidisperse sprays as well as equivalence ratio on both ignition-delay time and on spray lifetime are investigated. It is shown that there is a minimum ignition-delay time in dependence of both equivalence ratio and of initial droplet size for various conditions. Ignition-delay time of a bidisperse spray is dominated by the characteristics of the smaller droplets. There is a two-stage heating for small droplets caused by gas-phase characteristics. Fuel-vapor accumulation leads to delayed ignition and to higher gas temperatures for small droplets. The results of the calculations give profiles of radical concentrations such as OH as well as concentration of pollutants.