Prior research into Ultra Compact Combustors (UCC) showed an axial length savings compared to traditional gas-turbine combustors. This savings is achieved by swirling the reactants circumferentially in a recessed cavity around the outside diameter of the engine. A similar circumferential combustor is envisioned for a new engine configuration that positions the combustor outboard of a radial compressor and an inflow turbine. This configuration will offer an axial length savings for the entire engine, not just the combustor. The new engine configuration will not utilize a core flow path and thus requires all engine air from the compressor to pass through, or around, the combustor cavity. This report characterizes the cavity flow behavior as the core flow quantity was reduced from 80% of the total engine mass flow rate down to zero, representing the new engine configuration, while maintaining constant cavity mass flow rates. Velocity profiles were obtained with particle-shadow image velocimetry (PSV) in cold flow and with particle streak emission velocimetry (PSEV) in reacting flow experiments. The cold flow results showed that the core flow produced a suction and removed fluid from the circumferential cavity resulting in a lower cavity mass flow rate. This behavior resulted in lower circumferential velocities at higher core flow percentages and the fastest cavity velocity with zero core flow. Reacting flows produced a similar result with the fastest cavity velocities achieved at reduced, but non-zero core flows, and the slowest velocities at the highest and zero core flows. Overall, it was found that there was no negative impact on performance from the removal of the core flow that would prohibit development of the new engine.

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