An investigation on flow field characteristics of trapped-vortex flameholder under different modes of tandem TBCC engines

Tandem turbine-based combined-cycle (TBCC) engines must operate during afterburner, transitional, and ramjet-related operating conditions. These changes can modify the flow organization around the trapped-vortex cavity flameholder (TVCF), whereas the corresponding flow-field evolution remains insufficiently understood. In this paper, “mode” denotes the bypass-flow state set by a fixed RVABI angle. This study experimentally investigates the cold-flow field downstream of a TVCF under four rear variable-area bypass injector (RVABI) angle settings, α = 0°, 5°, 14°, and 23°, using time-averaged particle image velocimetry (PIV). The inlet temperature was 300 K for both bypass streams, and the Mach number downstream of the RVABI was 0.12. A persistent mean trapped-vortex structure was observed inside the cavity at all four RVABI angles, together with a large-scale recirculation zone downstream of the radial strut. In the α = 0° baseline case, the strut-wake recirculation zone extended approximately from x = 10 to 90 mm and y = 90 to 135 mm in the measured plane, with a maximum streamwise width of approximately 80 mm. The α = 5° case caused the most pronounced deformation and displacement of the primary cavity vortex. At α ≥ 14°, the mainstream was markedly deflected, and the low-velocity region downstream of the strut expanded. The results indicate that RVABI-angle variation reorganizes the coupled momentum distribution among the cavity fore-inlet flow, the mainstream, and the strut wake. This study provides spatially resolved experimental evidence of RVABI-angle-dependent TVCF cold-flow changes and identifies flow-topology features that should be considered when designing flameholder geometry and cavity-inlet allocation. Because the present measurements were conducted under non-reacting, fixed-angle conditions, their implications for ignition and combustion stability require further reacting-flow and unsteady-flow validation.