Mechanics of Combustion Mode Transition in a Direct-Connect Ramjet–Scramjet Experiment

Abstract

Combustion mode transitions triggered through active fuel actuation and passive wall heating have been examined in a direct-connect dual-mode combustor experiment. Nonallowable flow configurations are identified through the comparison of the experimental results with an isolator impulse flow theory. These nonallowable configurations consist of three types: those that are facility loss limited, those that are associated with negative entropy generation, and those that fall outside the bounds of the normal shock compression limit. A second law of thermodynamics analysis of the isolator impulse theory and the experimental observations support these results, with the three sets of nonallowed flow configurations presented in terms of both the impulse theory and an experimentally determined operating point analysis of the device. The unstable transition from scramjet to ramjet operation was also noted during the wall heating triggered combustion mode transitions, for particular fueling and inlet flow conditions. A driving mechanism is proposed for this instability, which begins in scramjet operation and creates the rapid, and repeated, transition between combustion modes before stabilizing under ramjet operation.