Computational and experimental studies of laser-induced thermal ignition in premixed ethylene-oxidizer mixtures

A one-dimensional, transient model is developed for solving the system of coupled, nonlinear, partial differential equations governing the fields in our experimental investigations of laser-induced heating and subsequent thermal ignition in an open, premixed ethylene-oxidizer system. The model includes complex chemistry and transport processes, and features a realistic and experimentally consistent laser/absorption submodel for the ignition source. The model is used to explain the character of the temperature-time history prior to ignition, above and beyond traditional heat transfer theories, and shows that perturbations in the laserʹs output characteristics can explain the range of delay times observed experimentally. Furthermore, the laser/absorption/ignition model is used to explain the experimentally observed phenomenon of ignition occurring beyond the focus of the laser. Ignition chemistry is shown to be dominated by the same reactions, in terms of rates of progress and rate coefficient sensitivity analysis, in spatially dependent ignition events as in purely kinetic studies, with sensitivity of ignition delay virtually the same. Lastly, a theory is developed and used to quantify the effect that species diffusion has on retarding ignition in a spatially dependent system. This retarding effect may contribute more or less than kinetic processes to the ignition delay period. This retarding effect is shown to be not only due to suppressed radical concentrations, but also to diffusion of fuel back towards the ignition source.