Mild Detonation Initiation in Rotating Detonation Engines: An Experimental Study of the Deflagration-to-Detonation Transition in a Semiconfined Flat Slit Combustor with Separate Supplies of Fuel and Oxidizer

Rotating detonation engines (RDEs) are considered to be promising thrusters for aerospace propulsion. Detonation initiation in RDEs can be accompanied by a destructive explosion of an excess volume of the fuel mixture in the combustor. To exclude this phenomenon, a "mild" rather than "strong" initiation of detonation is required. For the mild initiation of detonation in RDEs, it is necessary to ignite a mixture of a certain minimum volume sufficient for deflagration-to-detonation transition (DDT). In this study, the critical conditions for detonation initiation through DDT in a semiconfined slit combustor simulating the RDE combustor with a separate supply of ethylene and oxygen diluted with nitrogen (from 0 to 40%) were obtained experimentally. It turned out that for the mild initiation of detonation, it is necessary to ignite the mixture upon reaching the critical (minimum) height of the combustible mixture layer. Thus, for the mild initiation of detonation in the undiluted C2H4 + 3O(2) mixture filling such a slit combustor, the height of the mixture layer must exceed the slit width by approximately a factor of 12. In terms of the transverse size of the detonation cell lambda the minimum layer height of such mixtures in experiments is similar to 150 lambda. Compared to the experiments with the premixed composition, the critical height of the layer is 20% larger, which is explained by the finite rate of mixing. As the degree of oxygen dilution with nitrogen increases, the critical height of the layer increases, and the role of finite rate mixing decreases: the results no longer depend on the method of combustible mixture formation.