Three-dimensional structure of shear-layer and thermally choked combustion in a dual-combustion ramjet

Dual-combustion ramjets can exhibit distinct combustion regimes under identical inflow conditions, including a shear-layer-anchored supersonic mode and a thermally choked mode accompanied by strong pressure rise and flow deceleration. This study examines how the constant-area length of the main combustor governs regime-dependent flow organization and combustion characteristics using three-dimensional simulations. Two configurations are considered at the same operating condition: an immediate expansion case (L c /R = 0) producing a shear-layer combustion mode and an extended constant-area case (L c /R = 50) producing a thermally choked mode. Comparisons based on common reaction-layer reference surfaces, global thermochemical fields, and mode-dependent wave structures show that the thermally choked mode concentrates heat-release upstream and forms an extended high-pressure region in the mid-combustor, whereas the shear-layer mode maintains downstream-persistent reaction along the turbulent mixing layer with a predominantly supersonic core. Integrated metrics from time-averaged plane integrals indicate a clear trade-off: the thermally choked mode increases combustion efficiency but reduces total-pressure recovery. When performance is assessed using a momentum-flux proxy normalized by mass flow rate, the two modes exhibit only a marginal difference under the present conditions. These results provide a physically grounded separation between shear-layer and thermally choked combustion and clarify how thermal choking reorganizes coupled compressible flow and finite-rate chemistry in dual-combustion ramjets.