Effect of strut-lobe combination on combustion characteristics in multimode combustor of innovative ramjet combined-cycle engine

The Fan Augmented Air-Breathing Ramjet Combined Engine (FABRE), integrating an air-turbo-rocket and a ramjet, emerges as a promising propulsion system for two-stage-to-orbit vehicles and hypersonic aircraft. Central to FABRE is the multi-mode combustor, which should work efficiently in both turbocharged and ramjet modes across the Mach 0-6 range. However, the broad operation conditions and lower length-to-diameter ratio requirements with large scales make it remain a significant challenge for multi-mode combustor. To this end, this study performs a systematic numerical evaluation of several representative combustion organization schemes with different numbers of struts and struts-lobes combinations to explore their wide-operation-range behavior on reactants mixing, thermodynamic, and combustion performances under lean, stoichiometric, and rich conditions. Results indicate that for the strut-only schemes, attributed to improved fuel penetration, an increasing number of struts enhances the mixing efficiency by 8% at Mach 2 and 22% at Mach 6. This enhancement in mixing elevates the average temperature, boosting combustion efficiency by 18% at Mach 2 and 9% at Mach 6, albeit with a slight reduction in total pressure recovery. For the strut/lobe combination scheme, it shows that introducing lobes increases the average mixing efficiency of reactants by approximately 5.5%. This improved mixing environment fosters greater fuel heat release, raising total temperature and enhancing the combustion efficiency by 6% at Mach 2 and 1.3% at Mach 6, respectively, despite a minor decrease in total pressure recovery. These findings offer valuable insights into combustion organization strategies for multi-mode combustors, advancing the development of air-breathing ramjet combined cycle engines.