To explore the combustion performance of non-rectangular type supersonic combustors, the flow and combustion characteristics in round and round-to-elliptic shape-transition (RdEST) supersonic combustors under the same configurations of flight Mach number and fuel equivalence ratio were compared based on modeling results. The fuel equivalence ratio is maintained the same as 0.8, while two inlet Mach numbers of 2.5 and 3.0 both corresponding to a real flight Mach number of 6.5 are tested. To alleviate the strict requirements on wall-normal and -parallel grid spacing, Improved Delayed Detached Eddy Simulation (IDDES) is employed in this study to enable an automatic choice of RANS or LES mode depending on the local boundary layer thickness and turbulent viscosity. To reduce the computational cost of stiff kerosene oxidation chemistry, a total of four versions of skeletal mechanisms (respectively 48s/197r, 39s/153r, 28s/92r and current 19s/54r) have been developed based on the detailed 2815s/8217r Dagaut mechanism by using a highly efficient and reliable directed relation graph with error propagation and sensitivity analysis (DRGEPSA) method together with manual path analysis. Although the mechanism size has been significantly reduced, key kinetic properties such as adiabatic flame temperature, heat release rate, ignition delay and laminar flame speed all agree well with the original detailed mechanism. The static pressure along streamwise direction is compared with the measurement to validate the modeling results. Two key aspects are well predicted, i.e. the pressure ratio and the initial pressure rise location, indicating that the flame anchoring location and the distribution of wave structures inside the combustor are close to the actual situation. Then the aerodynamic fields are analyzed for the round and elliptic combustors to compare their flow, mixing and combustion related flow structures. The three-dimensional wave structures inside the elliptic combustor are firstly shown to reveal the influence of non-axisymmetric cross-section on the shock train and Mach field. Especially the time evolution of the flame region is analyzed, and dominant flame modes are extracted by the aid of proper orthogonal decomposition (POD) method. 漏 2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.