Measurement of flame propagation through step changes in mixture composition

Abstract

Flame speed is a central metric in the field of combustion. While particular mixtures have characteristic flame speeds, it has also been shown that in compositionally stratified mixtures, flames exhibit a path dependency, or memory effect. The goal of the experimental work presented in this thesis was to investigate the behavior of flame speed over step changes in equivalence ratios. This sharp stratification was achieved using a soap bubble blown at the center of a combustion bomb. A laser was used to ignite the mixture from the center. Flame speed was calculated from both a pressure trace analysis and from measuring the movement of the flame front through high speed Schlieren imaging. Both methods demonstrated good correlation with the literature for homogeneous charges. However, the analysis necessarily assumes a spherical flame, but the Schlieren video showed that the laser ignition system induced a significant protrusion in the flame front. This protrusion smooths the transition from the flame speed of the inner mixture to that of the outer. Therefore, it was demonstrated that this setup is not suitable for the measurement of flame speed transitional behavior over step-changes in equivalence ratio.