Model of the Ignition and Flame Development on a Vaporizing Combustible Surface in a Stagnation Point Flow: Ignition by Vapor Fuel Radiation Absorption

Abstract

A numerical analysis is conducted of the initiation and evolution of a combustion reaction generated by fuel vapor absorption of radiation over an evaporating combustible surface in an oxidizer stagnation point flow. The combustible is initially evaporating due to an externally applied irradiance, and is assumed to be in equilibrium vaporization. The transient, stagnation point, gas conservation equations, including one-step Arrhenius type kinetics and fuel vapor absorption of radiation, are solved numerically for the case of PMMA as combustible evaporating in air. Detailed calculations are presented, for a specific case of irradiance and flow velocity, of the evolution of the velocity field, and temperature and species concentration distributions during the ignition of the mixture, and subsequent establishment of a diffusion flame over the combustible surface. Ignition is characterized by thermal run-away of the gas and it is considered to have occurred if, after discontinuing the external irradiance, a self-sustained reaction is present. It is predicted that ignition occurs in the fuel-rich side of the mixing layer formed around the dividing streamlines of the fuel and oxidizer opposing stagnation flows. After ignition a premixed reaction front moves toward the lean side separating the fuel from the oxidizer and leaving behind a diffusion flame which eventually reaches steady conditions. A parametric study is also conducted on the effect of the flow velocity on the minimum irradiance for ignition and on the ignition time. It is predicted that the minimum irradiance for ignition increases approximately linearly with the velocity and that for a given velocity the ignition time decreases approximately inversely with the irradiance.