Rotating detonation wave mechanics through ethylene-air mixtures in hollow combustors, and implications to high frequency combustion instabilities

Abstract

Recent investigations into the rotating detonation phenomenon have involved its inception and sustenance in hollow combustors, in contrast to the traditional annular rotating detonation combustor (RDC) designs. Despite this proof-of-concept, the mechanism of propagation of detonation waves in hollow combustors is unclear. On the other hand, the decades-old issue of high frequency combustion instabilities, especially in rocket engines, has been known to produce distinct shock waves that are in-sync with regions of intense combustion, the reason for which is widely attributed to the Rayleigh criterion. In this paper, we argue that there is a considerable overlap in the physics behind the reported rotating detonations in hollow RDCs and the high frequency tangential combustion instabilities that are known to wreak havoc on engines. To support this notion, an atmospheric hollow combustor is experimentally tested to attain the baseline performance. It is then ‘transformed’ into a hollow RDC by the use of a flow-turning obstacle that diverts the combustible ethylene-air mixture towards the outer wall. Two distinct mechanisms are found to cause rotating detonations in a hollow combustor, and subsequently predicate its stability. The observed modes are analogous to the behavior exhibited by planar detonations at the near-limit. This explains not only the widely observed velocity and pressure deficits in rotating detonations, but also the “steep-fronted”, “detonation-like” behavior noted in high frequency combustion instabilities.