Laser-induced ignition using a conical cavity in CH4-air mixtures

A method that confines all the available energy in the vicinity of the ignition point during a laser-induced ignition process is proposed. It utilizes multiple reflection by a conical cavity surface when a small-diameter laser beam is directed into the cavity. Shadowgraphs of the early stages of the combustion process for quiescent methane/air mixtures show a hot gas jet to emerge from the cavity. During subsequent flame propagation, both similarities with and differences from conventional spark ignition processes are observed, depending on the cavity size and the concentration of mixtures. With laser cavity ignition, the chamber pressure increases relatively rapidly and higher maximum pressure can be achieved. As a result, the combustion duration for laser cavity ignition is decreased relative to laser-induced spark ignition. A model, which simulates flame kernel development and the subsequent combustion process, is tested numerically using the KIVA-II code. The associated flow, pressure, and temperature profiles are evaluated and satisfactory agreement achieved between the experiment and calculated results. (C) 1999 by The Combustion Institute.