To explore the combustion performance of nonrectangular supersonic combustors, the flow and combustion characteristics in the round and round-to-elliptic shape-transition scramjet combustors were compared, based on the measurements and the improved delayed detached-eddy simulation modelings. The global equivalence ratio was kept at 0.8, whereas two inlet Mach numbers of 2.5 and 3.0 were tested. To reduce the computational cost, four versions of skeletal mechanisms (respectively, 48 s/197 r, 39 s/153 r, 28 s/92 r, and 19 s/54 r) have been developed for China RP-3 kerosene; and the latest one was used. The predicted static pressure profiles along the streamwise direction were first validated against the measurements. Then, the aerodynamic fields were analyzed for the two combustors to compare their flow, mixing, and combustion performances. The three-dimensional wave structures inside the nonaxisymmetric elliptic combustor were revealed for the first time to show the influence of shape transition. Specifically, the time evolution of the flame structures was analyzed, and dominant modes were extracted by the aid of proper orthogonal decomposition.