Shock reflection and local re-initiation in achieving spray detonation: A distinct mechanism from gaseous combustion

The practical application of liquid-fueled detonation engines is hindered by a critical challenge: large-molecule liquid hydrocarbon fuels are notoriously difficult to achieve deflagration-to-detonation transition (DDT) under conventional conditions. This study systematically investigates the dynamic evolution of DDT in n -heptane spray/air mixtures within an obstructed channel employing the Eulerian–Lagrangian method combined with GPU-accelerated high-precision numerical simulations. For the first time, the unique recurrent phenomenon of “initiation-quenching-reinitiation” cycle of local detonation waves in spray DDT has been revealed, which is distinct from stable gas-phase DDT. It is attributed to the extremely non-uniform equivalence ratio (ER) field (coexistence of fuel-rich zones with local ER > 10 and fuel-lean vortex cores with ER ≈ 0.1) caused by the lagging droplet evaporation. And the spray detonation wave quickly decouples and quenches after bypassing obstacles, while gaseous detonation can propagate stably. Therefore, appropriately increasing the proportion of gaseous fuel can effectively promote the self-sustaining propagation of spray detonation after initiation. Enhancing droplet evaporation can also supply more combustible gas fuel. But counterintuitively, faster evaporation may not necessarily make DDT easier to achieve. The classic droplet breakup model was adopted to accelerate the evaporation of liquid fuel, the results showed that the introduction of the droplet breakup model actually prolonged the ignition time and distance. Its dual suppression mechanism lies in the following: droplet breakup absorbs energy, weakening the intensity of the reflected shock wave, while accelerated evaporation intensifies gas-liquid heat exchange, and reduces the temperature of the flow field, thereby slowing down flame acceleration. Novelty and significance statement Due to the inherent complexity of the two-phase reaction systems and the limitations of existing diagnostic techniques, our understanding of spray DDT is very limited. This study reveals a recurrent quenching-reinitiation cycle in spray DDT distinct from stably gaseous detonation, caused by extreme equivalence-ratio non-uniformity from delayed evaporation. Although a suitable pre-evaporation rate is necessary for the initiation of spray DDT, faster evaporation does not necessarily mean faster DDT realization. It is found that droplet breakup and enhanced evaporation attenuate shocks and cool flow, thereby delaying ignition, challenging the intuitive judgment that faster evaporation favors DDT. These findings provide design guidelines emphasizing spray control as crucial as chemical.