Impact of middle strut invasion depth on combustion characteristics of multi-mode shared combustor in an innovative ramjet combined cycle engine

Fan Augmented Air-Breathing Ramjet Engine (FABRE) is identified a promising candidate to propel next-generation reusable aerospace transportation vehicles. However, the combustor encounters significant challenges associated with achieving high-efficiency combustion under the constraints of multi-mode, wide-speed-range operation, and small length-to-diameter ratio. To address the issue, this study develops a combustion organization scheme including a set of middle strut flame stabilizers with adjustable invasion depth. Numerical simulation is performed then to investigate the impacts of invasion depth of middle struts (IDMS) on the flow field structures, thermodynamic characters and combustion performances of the combustor. The results indicate that distinct IDMS can effectively alter both flow field and fuel distribution states within the combustor; thus, reasonable IDMS adjustment enables multi-mode shared combustors to achieve superior combustion performance under complex operating conditions. In turbocharged mode at Mach 0 ∼ 3, increasing IDMS from 0 % to 83 % improves mixing uniformity. A better mixing state boost heat release rates, and finally, the combustion efficiency rises notably about 5 %, while the total pressure recovery decreases steadily due to the enlarges low-speed recirculation zone behind the middle strut. In ramjet mode at Mach 3 ∼ 4, a greater IDMS decreases the mixing uniformity, leading to a reduction in total temperature, and the average combustion efficiency declined about 4 %, while the total pressure recovery remains unaffected. At Mach 5 ∼ 6, optimal mixing uniformity occurs at an IDMS of 44 %, resulting in a 5 % average boost in combustion efficiency versus the 0 % IDMS case. However, the total pressure recovery remains stable. These findings offer valuable insights into the combustion organization of wide-speed-range ramjet combined-cycle engines.