Effect of inlet air distribution on the performance of a boron-fueled triple-combustion ramjet

A solid-fuel triple-combustion ramjet (TCR) provides a staged-combustion approach for boron-fueled hypersonic propulsion, but its performance depends strongly on air allocation between the subsonic and supersonic combustors. An Euler–Lagrange CFD framework is used to investigate a TCR operating at Mach 7 and an altitude of 27 km. The gas phase is described by a 19-species/64-step mechanism, and boron ignition and combustion are modeled using an 11-step multiphase mechanism. Four matched cases are examined at k _p = 2:1, 1:1, 1:2, and 1:3, where k _p denotes the supersonic-to-subsonic inlet-air mass-flow ratio; the cases are established by varying the supersonic-inlet capture area at an overall equivalence ratio of 0.7. Decreasing k _p produces a more uniform exit Mach-number field and a sandwich-like oxygen distribution, with oxygen-rich near-axis and near-wall regions surrounding the principal reaction zone. This structure increases fuel–oxidizer contact and promotes gas–particle mixing and combustion. As k _p decreases from 2:1 to 1:2, the overall boron mixing efficiency increases from 63.1% to 95.1%, and the boron combustion efficiency increases from 60.5% to 94.3%; further decreasing k _p to 1:3 provides only marginal improvement. Carbon conversion exceeds 99.5% at the subsonic-combustor exit in all cases. The global combustion efficiency and specific impulse reach their highest values at k _p = 1:2, namely 97.1% and 605.3 s, representing increases of 24.6 percentage points and 137.5 s relative to k _p = 2:1. Under the present configuration and operating condition, k _p = 1:2 is therefore the best-performing case among the four ratios examined.