Numerical Investigation of Combustion Characteristics in a Dual-Combustion Scramjet Engine

A three-dimensional large-eddy simulation (LES) with high spatial resolution was performed to investigate mode-dependent combustion characteristics in a dual-combustion scramjet combustor. A CO/H2–air system was considered under identical inflow and fueling conditions. Two combustor geometries were compared, and they differ only in the length of the constant-area section. In the first configuration, which has no constant-area section (Lc/R = 0), a supersonic shear-layer flame develops and the reacting core remains predominantly supersonic. This configuration corresponds to scramjet mode operation. In the second configuration, which has a long constant-area section (Lc/R = 50), strong shock–boundary-layer interaction occurs. Thermal choking appears and a subsonic core is formed, so the combustor operates in a ramjet mode. Velocity, temperature, scalar dissipation rate, and streamwise distributions of species mass fractions were analyzed. In both configurations, H2 is almost completely consumed. However, CO oxidation remains incomplete in the scramjet mode, while it is significantly promoted in the thermally choked ramjet mode. As a result, the outlet CO mass fraction is lower and the H2O level is higher in the thermally choked case. These results show that combustor geometry is a key factor that selects between scramjet and ramjet operation in a dual-combustion system. They also reveal a basic trade-off. Scramjet mode operation preserves a supersonic core and limits total-pressure loss, whereas thermally choked ramjet mode operation achieves higher combustion completeness and a larger release of chemical energy.