Differential turbulent supersonic combustion of hydrogen, methane, and ethylene, without assisted ignition

Although hydrogen has the desired ignition properties for supersonic combustion in a scramjet, it has the serious disadvantage of low energy density; thereby motivating the investigation of alternate, mostly hydrocarbon, fuels, such as methane, ethylene, and kerosene. Because the hydrocarbon fuels do not ignite easily, their use in scramjet combustion, where the strain rate is large and flame stability is difficult to maintain, depends on assisted ignition – for example, spark discharge or pulse detonation. However, a comparative evaluation of the combustion characteristics of these fuels under realistic turbulent supersonic conditions in a scramjet engine, and within the framework of advanced and highly efficient combustion modeling with detailed chemical mechanism, has not received enough attention. This study has the objective of addressing this particular issue. That is, we set out to investigate the baseline of supersonic combustion using hydrogen, methane, and ethylene as the fuels in a model air-breathing hypersonic flight environment. The effects of superposing ignition assistance can be clearly delineated subsequent to the non-assisted baseline calculations. Specifically, three questions are answered in this paper: How do the combustion characteristics of the fuels differ under laminar flow conditions, particularly as functions of pressure? What are the differential behaviors under supersonic combustion conditions? What are the supersonic combustion modeling implications for these fuels; for example, how does the modeling of the progress variable differentially affect the accuracy of the supersonic combustion calculations for the fuels? The various analyses leading to the answers to these questions are presented.