Premature ignition in scramjets with intake injection: a preliminary laminar mixing layer simulation

Abstract

Injecting fuel on the intake of scramjet engines is one strategy that might be used to minimize the required length of scramjet combustion chambers. Premature ignition must be avoided for the strategy to be viable. Premature ignition is not normally observed in shock tunnel experiments with compression ignition scramjet configurations even though local regions of elevated temperatures sufficient to support combustion would have been present on the model scramjet intakes. However, for full scale flight vehicles, we cannot conclude that ignition will generally be delayed until the combustion chamber based on limited empirical results from shock tunnel ground-testing. Reliable intake/injection design correlations for premature ignition avoidance in a flight scramjet are yet to be developed. Numerical simulation offers an approach for the investigation and identification of premature ignition regimes which should be avoided in compression-ignition scramjets. A particular case of hydrogen injection in the presence of a laminar boundary layer is simulated numerically. The location of the stoichiometric mass fraction of hydrogen occurs very close to the peak mixing layer temperature which is also within the lowest speed region of the mixing layer. An ignition delay correlation is used to demonstrate that ignition will almost certainly occur. This case is offered as an example to highlight the potential problem and perhaps stimulate further study in the area of premature ignition with intake injection.