The fluid and combustion dynamics were experimentally studied in Mach 4.5 high-enthalpy flows using constant area scramjet models with the axisymmetric and the perisymmetric cross-sectional shapes. The perisymmetric shape is analogous to the axisymmetric, inspired to enhance the fuel injection and mixing efficiency of the axisymmetric scramjet. High-speed direct chemiluminescence imaging along with the synchronized wall pressure measurement were performed. The ethylene fuel was auto-ignited within the flowpaths, and three different operational modes (scramjet mode, ramjet mode, and unstart) were simulated by altering the global equivalence ratio. Time-averaged data revealed that the mode transitions in the perisymmetric model in terms of the equivalence ratio occurred earlier than the axisymmetric model, and the inlet unstart of the perisymmetric model was triggered at an equivalence ratio 20% lower than that of the axisymmetric model. The analysis on the instantaneous data showed the transient phenomena such as upstream flame propagation, reverse flame propagation into the isolators, emergence of the pseudo shock within the isolator, and the inlet unstart. Moreover, reverse flame propagation into the isolator was closely analyzed by computing the standard deviations of the high-speed direct chemiluminescence images. The result confirmed the axisymmetric distribution of large standard deviations for the axisymmetric isolator and the asymmetric distributions of strong standard deviations along the top and the bottom walls of the perisymmetric isolator. It is speculated that the perisymmetric cross-section could be beneficial in enhancing the fuel penetration into the core flow and the fuel-air mixing.