Internal high-speed schlieren imaging of a rotating detonation engine

This study demonstrates the application of an internal schlieren imaging technique to visualize flow and combustion phenomena within an annular rotating detonation engine (RDE). The system enables high-speed 100 kHz imaging through sapphire windows in the combustor annulus by leveraging a self-aligned, single-sided optical setup inspired by focusing schlieren methods. A simplified geometric optic model is developed and applied to study the effects of window reflections on the single-side schlieren technique. Based on the simulations, it is concluded that a circular aperture is required at the image plane instead of typical knife edges or cutoff grids to mitigate reflections. The applied technique provides full-field visualization of key processes during RDE operation, including fuel–air mixing, ignition onset, and detonation wave propagation—all without requiring through-optical access. All three regimes of the RDE process are investigated: (1) cold-flow fuel–air mixing, (2) the ignition process, and (3) quasi-steady detonation operation. The internal schlieren is applied to the first process to demonstrate cold-flow mixing conditions by assessing hydrogen–air mixing, jet penetration, and asymmetries in fuel–air mixing. Next, the technique is applied to combustor initiation to demonstrate efficacy in assessing ignition methods. As an example, ignition parameters such as induction length are quantified as a function of mass flow rate. Lastly, the quasi-steady detonation wave propagation is assessed by providing imaging of the detonation wave, where dynamic detonation features are demonstrated. This work establishes a robust internal schlieren methodology capable of capturing key stages of RDE operation with minimal perturbation to the flow. The technique provides an integrated diagnostic framework for assessing injector designs, ignition behavior, and detonation structure under varying operating conditions and fuels.