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This experimental work reports on detonation behaviors in a curved chamber without inner wall after diffraction of a Chapman-Jouguet (CJ) detonation from a straight channel tangent to the chamber outer wall. The upper and lower faces of the chamber receive either soot foils for recording the history of the transmission dynamics, or optical windows for schlieren high-speed visualizations. Tests were carried out with the stoichiometric propane-oxygen mixture at initial temperature 1 T 0 = 288 K and initial pressures ranging between 8 kPa and 15 kPa. The primary observation is the existence of an initial pressure range (8 kPa-12 kPa) for which, after diffraction transients, a Mach detonation can rotate normal to the outer wall with a constant angular velocity such that the normal front velocity at the wall is larger than the CJ value. The height of the Mach front decreases and its tangential velocity increases with increasing initial pressure. This over-driven detonation results from the irregular reflection of the initially-oblique front as the outer wall tilts with respect to the front, similarly to shock propagation in a continuouslyconverging channel. The Mach triple-point moves away from the wall and stabilizes its trajectory parallel to the wall. This Mach front shows detonation cells parallel to the wall with a constant mean width very small compared to that on the initial CJ front. A detonation can thus smoothly propagate in a curved chamber without center body, though the Mach front does not sweep the entire equivalent cross-section of the chamber. These observations are consistent with experimental results for RDE chambers without center body, or with linearly-increasing cross sections, that show improved properties of rotating detonation, compared to chambers with constant cross sections. This supports the interest of further studies on the hollow-chamber configuration of RDEs.

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