Experimental and Numerical Study of Autoignition/Deflagration Transition Limit in a Flat Piston Optical Rapid Compression Machine

This work investigates the transition limit between autoignition front and deflagration in hot and premixed conditions. In particular, the objective is to analyze the sensitivity of the combustion regime to the temperature gradient, mixture composition and thermodynamical conditions, which is first analyzed numerically. The results are then compared to experimental data obtained in an optical RCM, fitted with a flat piston and a multi-zone heating system. Temperature gradient after compression is measured in the hot core region using two thin wire thermocouples of about 7.6 µm diameter. The chemiluminescence records confirm that the dynamics of autoignition fronts is strongly dependent on (i) the gaseous mixture composition and the thermodynamical conditions, and (ii) on thermal gradients in the unburned mixture. In particular, the authors evidence experimental conditions for which a steeper gradient or a change in mixture composition leads to a transition from autoignition to deflagration.