Improving Detonability in a Small-Scale Rotating Detonation Engine Using Partial Premixing

Small-scale rotating detonation engines (RDEs) with low mass flows and high frequency have potential as small thrusters, robust flame holders, and research tools that can be quickly and cheaply modified. However, the necessarily small channel geometry and low mass flow force the detonation wave to propagate near the limits of detonability. Previous success in a small-scale, 28-mm-diameter micro-RDE, operating on nitrous oxide (N2O) and ethylene (C2H4), exhibited unstable wave behavior with propagation velocities between 47 and 56% the Chapman-Jouguet speed. These results were observed at mass flows from 25 to 75 g/s using an in-chamber jets-in-crossflow (JIC) injection scheme with spark plug initiation. Building upon this work, the current investigation looked to improve the detonation behavior in the micro-RDE without changing the channel geometry or resultant operability and better understand the nature of mechanisms governing the design limitations in RDEs. To improve the wave behavior, a new partially premixed JIC injection scheme was used along with a predetonator ignition device. With these changes, tighter windows of operable mass flows (m) and equivalence ratios (phi) were observed, between 35 and 68 g/s and 0.6 and 1.3, respectively. Maps of m versus phi before and after detonation showed that the space of conditions conducive to ignition was not isomorphic with the space of conditions that sustained operation. Increases to the injection pressure to chamber pressure ratios improved the probability of detonation but resulted in greater pressure losses than were seen in the original JIC injection scheme. Single wave detonation was achieved for the first time in this RDE and had a peak frequency of 16.6 kHz, or 66% Chapman-Jouguet speed. Increases in fill height drove increases in wave frequency. Decreased fill height estimates compared to the previous design showed that partial premixing decreased the required fill time. These results support that the limits of detonability in small RDEs can be expanded without significantly compromising the characteristic low-mass-flow and high-frequency operation inherent to their geometry.