Unsteady fluid-combustion interactions in optical axisymmetric scramjet models are experimentally investigated in Mach 4.5 flows. The axisymmetric scramjet reveals generic fluid and combustion dynamics without the corner boundary-layer effects that can distort flame propagation in rectangular geometries. Fluid and combustion dynamics are characterized by pressure measurement on the combustor wall along the entire flowpath. High-speed flame chemiluminescence and luminosity imaging are used to visualize the unsteady shock and flame propagations driven by fluid-combustion interactions. It is shown that the initial choking location of the scramjet combustors depends on the competing effect of combustion heat addition and combustor area relief. For the 2° combustor, the combustion-induced flow choking occurs inside the diverging section of the combustor, while the 5° combustor cannot be thermally choked due to the excessive area expansion. The unsteady pseudo shock propagates upstream until it reaches the cavity flameholder. A shock train is established in the isolator to balance the pressure rise downstream. Once the shock train is disgorged out of the inlet, a normal shock is formed in front of the inlet, i.e., inlet unstart. Inlet buzz phenomenon is observed at unstarted conditions due to the fluid-combustion interaction. The buzz frequency is correlated to the acoustic wave propagation in the subsonic flow regime established between the inlet and the flow choked location. Nevertheless, the resonance mode for a duct with an open and a closed-end is applicable only to the case with an initial choking location at the cavity ramp.