Combustion organization of boron-based powder fuel in a Mach 3 scramjet with variable geometry combustor

To extend the low-Mach-number operating envelope of powder-fueled scramjets, this study investigates the combustion organization of boron-based powder fuel at Mach 3 using a variable-geometry combustor as the baseline configuration. The effects of combustor structure and powder injection strategy on the ignition, stable combustion, and self-sustained combustion performance of boron-based powder are analyzed. Through pressure spectrum analysis, combustion efficiency evaluation, and multidimensional characterization of combustion products, the effectiveness of the proposed combustion organization scheme is systematically assessed. The results show that the introduction of a cavity enables reliable ignition, while the multi-orifice jet scheme significantly enhances particle mixing and prolongs residence time in the high-temperature region. Under the condition, the combustion efficiency of boron-based powder reaches 60.5%, which is 27% higher than that of the strut injection scheme, and stable self-sustained combustion is successfully achieved. The pressure oscillation amplitude under all operating conditions remains below 5% of the mean combustor pressure, indicating stable engine operation. XRD analysis of the condensed-phase products shows that the dominant product under the multi-orifice jet condition is B₂O₃, the fully oxidized product of boron, whereas the strut injection scheme mainly produces B₆O, a low-valence intermediate oxide associated with incomplete combustion. SEM and EDS analyses further confirm the improved combustion completeness of boron-based powder under the multi-orifice jet condition from the perspectives of microstructure and elemental composition. These findings provide guidance for optimizing combustion organization in powder-fueled scramjets. Novelty and significance statement: This study improves low-Mach-number combustion in powder-fueled scramjets using an “ethylene pilot flame + multi-orifice jet” strategy. A cavity enables stable ignition of boron-based powder fuel in low-total-temperature flows, while the multi-orifice jet enhances particle mixing and residence time, achieving a combustion efficiency of 60.5%. SEM, EDS, and XRD analyses confirm improved combustion completeness and favorable combustion products. The proposed scheme addresses key challenges related to unstable ignition and low combustion efficiency and provides guidance for the wide-speed-range application of powder-fueled scramjets.