Flame Dynamics in an Optically Accessible Solid Fuel Ramjet Combustor

Flow-flame interactions were investigated in an optically accessible solid fuel ramjet combustor. Experiments were performed with a single hydroxyl-terminated polybutadiene fuel slab located downstream of a backward-facing step in a rectangular chamber. To emulate flight-relevant combustor conditions, unvitiated heated air was directed through the combustion chamber with an inlet temperature of similar to 655 K, chamber pressures of 450-690 kPa, and port Reynolds number of similar to 500,000. To characterize the heat-release distribution and velocity field, 20 kHz OH*-chemiluminescence and 10 kHz particle imaging velocimetry measurements were used. Comparison between the mean OH* chemiluminescence images acquired at three flow conditions indicates reduction in flame height above the grain with increasing air mass flow rate. Dominant heat-release coherent structures in the statistically stationary flow are identified using the spectral proper orthogonal decomposition technique implemented on time series of instantaneous images. The spatial mode shapes of the chemiluminescence and velocity field measurements indicated that the flow-flame interactions were dominated by vortex shedding generated at the backward-facing step in the combustor, at Strouhal numbers of 0.06-0.10. The frequency corresponding to these modes is shown to be invariant of air mass flux, indicating that system dynamics are primarily dependent on the backward-facing step geometry and the bulk velocity in the combustor.