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. Increasing 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, under 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 Damköhler number (Da) was established, revealing that the average Da 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 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 particles.

Powder-Fueled Water Ramjet Engine (PFWRE) is the most promising powerplant in underwater high-speed propulsion. However, the effect of powder injection mode on its performance and the mechanism of this effect are not well understood. In this paper, a computational framework for multiphase combustion flow is developed and validated. Further, the effects of different injection schemes on flow combustion characteristics and engine performance are evaluated via simulation. Our findings indicate that the dominant recirculation zone in front of the primary water inlet delivers water vapor to the combustor head, providing the necessary oxidant for the ignition and combustion of Al particles. Changing the injection parameters directly affects the flame zone distribution and the ability of the recirculation zone to deliver water vapor, leading to variations in particle ignition delay. The engine combustion efficiency and specific impulse efficiency exhibit a negative correlation with injection height, peaking before declining with increased injection angle. It is shown that particle mixing degree and particle dispersion degree are closely related to engine performance. Enhanced particle mixing in front of the primary water inlet and particle dispersion behind the secondary water inlet are considered favorable approaches to improve engine performance, which promotes the particle combustion process and improves the heat-work conversion efficiency.

Powder-Fueled Water Ramjet Engine (PFWRE) is of great attraction for high-speed and long-voyage underwater propulsion, as well as air–water trans-media navigation applications due to its high energy density and thrust adjustability. However, the complex multiphase combustion process in the combustor significantly affects engine performance. In this study, a detailed model for aluminum particle combustion in water vapor is developed and validated via literature data as well as the ground direct-connected test we conducted. Thereafter, the numerical study on the multiphase combustion process inside the aluminum-based PFWRE combustor is carried out within the Euler–Lagrange framework using the developed model. Results show that a reverse rotating vortex pair before the primary water injection causes particles to flow back towards the combustor head and leads to product deposition. Aluminum particles external to the powder jet have shorter preheating time than internal particles and burn out in advance. The analysis of the particle combustion process indicates that the flame structure inside the combustor consists of the particle preheating zone, the surface combustion heat release zone, the gas-phase combustion heat release zone, and the post-flame zone. In the present configuration, as the particle size increases from 10 μm to 20 μm, the preheating zone length increases from 35 mm to 85 mm. Meanwhile, heat release from gas-phase combustion decreases, and the average temperature of the combustor head first increases and then decreases. This study not only provides insight into the multiphase combustion characteristics of the aluminum-based PFWRE combustor but also offers guidance for the design of the combustion organization schemes and engine structure optimization.