Abstract: Aluminum powder-fueled water ramjet engines demonstrate promising application potential in high-speed underwater propulsion and air-water trans-medium vehicles. However, excessively high powder injection velocities may prevent flame stabilization in the combustion chamber and even lead to blow-off. This study investigates the flame blow-off characteristics of such engines through experimental and numerical methods, aiming to delineate the stable ignition operating range and provide guidance for the design of powder injection velocities. The results indicate that the near-blow-off limit under the current experimental conditions is 48 m/s. In-creasing the injection velocity reduces the mixing efficiency between aluminum particles and surrounding steam as well as the local oxygen-fuel ratio. Under the low-velocity blow-off condition of 50 m/s, a transient flame is established ahead of the primary water inlet before eventual blow-off occurs. In contrast, at the high-velocity blow-off condition of 60 m/s, the flame is directly expelled from the combustor once formed. A particle ignition analysis method based on the Damk?hler number (Da) was established, reveal-ing that the average Da number decreases with increasing injection velocity. The ignition potential remains above 60% under stable flame conditions but drops to 43.6% near the blow-off limit. A characteristic “end-cluster distribution pattern” of Da numbers is observed near blow-off condition, manifested by a significant increase in low-Da particles, a sharp decrease in high-Da particles, and relatively slight variation in the mid-range Da values.

Key words: aluminum powder fuel, water ramjet engine, injection velocity, flame blow-off, Damk?hler number