Experimental and numerical study on the ignition of fuel/air mixtures at laser heated silicon nitride particles

Abstract

Mechanical sparks as ignition sources play an important role in safety-related assessments of the ignition of combustible mixtures. In this study, we use laser heated, small bearing balls made from silicon nitride as a model of non-combustible mechanical sparks. Fuel/air mixtures made from methane, ethane, propane, n-butane, ethylene, acetylene, hydrogen and diethyl ether were investigated over a wide range of equivalence ratios. The particle surface temperatures required to ignite the gas phase varies widely between the different fuels and in most cases increases towards the limits of ignition. Numerical simulations of the ignition process yielded good agreement with the experiment for stoichiometric gas-mixtures only, but failed to reproduce the ignition temperatures close to the ignition limits. The discrepancy might be due to not well validated kinetic parameters as we approach the ignition limits, since the ignition process at hot particles is mostly governed by chemical kinetics, rather than species transport or heat conduction. We provide an empirical correlation between the particle temperatures at ignition and the laminar flame speed.