Performance Enhancement of Solid Fuel Ramjets via Aft-Chamber Oxidizer Injection

Air-breathing solid-fuel ramjets (SFRJs) often experience incomplete combustion in the aft-chamber due to spatial segregation between fuel-rich pyrolysis gases and the incoming oxidizer. This study investigates secondary air injection as a strategy to enhance fuel–oxidizer mixing and improve thrust performance, using high-fidelity large-eddy simulations of unsteady turbulent combustion. A computational framework was developed in OpenFOAM and validated to capture the coupled dynamics of compressible reacting flows and solid-fuel pyrolysis kinetics. Baseline simulations show that oxygen-depleted channels form near the fuel surface, allowing unreacted pyrolysis products to pass through the aft-chamber and preventing the engine from achieving its design thrust. Seven secondary-injection locations were evaluated to determine an optimal configuration. Results show that mid-point side-wall injections provide the best overall performance, yielding approximately 60% increase in mean thrust while maintaining oscillation amplitudes within acceptable limits. This injection strategy effectively disrupts the oxygen-depleted channel and promotes distributed combustion throughout the aft-chamber, enabling more complete oxidation of the pyrolyzed fuel and significantly enhancing propulsion performance.