Correlation and Analysis of Blowout Limits of Flames in High-Speed Airflows

Abstract

This work identifies a scaling parameter (which is a modified Damkohler number) that is found to correlate the flame blowout limits that were measured in six previous studies of nonpremixed flames which were stabilized in high-speed airflows by wall cavities, bluff bodies, and struts.Understanding the scaling of the combustor is needed to select the correct height of a cavity or step flameholder for ramjets, scramjets, or afterburners.This work focuses on nonpremixed conditions that occur when fuel is injected directly into a wall cavity or behind a strut, as is done with new designs.Thus, the Damkohler number that is identified is different from of that of Zukoski and Ozawa, who considered the different case of premixed flames in afterburners.Nonpremixed conditions introduce a new parameter that is not relevant for premixed conditions: the location of the fuel injector with respect to the recirculation zone.An analysis of a shear layer was performed in order to derive equations for the appropriate Damkohler number.Using this result, approximately 100 measured values of blowout limits from six previous studies were plotted, and a best-fit correlation curve that has a rich limit branch and a lean limit branch was determined.Although this correlation result provides a general estimate of blowout limits, it also indicates that additional research is needed to reduce the scatter in the correlation by improving the model of entrainment into the recirculation zone and by including unsteady effects.The results show that a reasonable correlation is achieved using the concept that the propagation speed of the flame in the shear layer is matched to the velocity of the local incident gas flow.Hot products in the recirculation zone preheat the shear-layer gases and increase the propagation speed of the flame.The analysis avoids an assumption that has been used previously-that the residence time of reactants in a "well-stirred" homogeneous reaction zone is matched to a global chemical reaction time.Experimental justification of the present approach is presented.