Numerical investigation of flow phenomena in a scramjet combustor has been performed for different geometric and operating parameters. The present investigation aims to find the optimal geometric parameters for a better fuel injection system with maximum combustion efficiency. Two-dimensional unsteady equations governing the compressible, turbulent reacting flow are solved using a commercial CFD solver. A combination of eddy dissipation (ED) and finite rate chemistry (FRC) models are used to model the combustion. The effect of divergence angles and scaling on the performance of a scramjet combustor is reported here. The effect of shocks generated by the strut and inlet conditions of a scramjet combustor on combustion efficiency has been reported. Present results show that the divergence angle and inlet conditions of combustors have significant effects on performance. A multiple-strut combustor has shown higher efficiency than a single-strut combustor. The modifications that improved the combustion efficiency of a DLR combustor are reported. The present simulation results match well with the experimental results available from the literature. © 2014 American Society of Civil Engineers.