Forced ignition modeling of methane-ethylene mixtures in scramjet combustors

Abstract

To propose a design guideline for a scramjet combustor at low cost, an ignition model for a scramjet combustor is required. Therefore, the overall aim of this study was to propose an ignition model that could be used to estimate the forced ignition limits within a cavity. In this study, we developed two forced ignition models (FIMs): the forced ignition model in the recirculating zone (FIMR) and the forced ignition model in the shear layer (FIMS). We evaluated the forced ignition limits using a micro-rocket torch within a scramjet combustor. The validated results of the FIMs indicated that the modified FIMR incorporating cavity refresh time (defined as the time required for complete replacement of all gases in the cavity) was the most appropriate method for calculating the forced ignition limit within the recirculation zone. However, in the case of Φ2.0 + 50 % He torch, the modified FIMR could not precisely estimate the forced ignition limit, showing that further modification to the present prediction model was necessary. The forced ignition process in the cavity was investigated using OH* chemiluminescence images, which indicated that the fuel mixture ignited in the shear layer; therefore, it was necessary to investigate ignition not only within the recirculation zone but also in the shear layer. In the case of Φ2.0 + 50 % He torch, the forced ignition limit could not be predicted by the FIMR, but it could be predicted by the FIMS. These results demonstrate that the FIMS provides improved prediction accuracy for the forced ignition limit in the scramjet combustors.