Effect of the Expansion Ratio on Combustion Oscillations in Solid-Fuel Ramjets: An Experimental and Numerical Study

This study investigates the impact of the expansion ratio on combustion oscillation phenomena and their driving mechanisms in a solid-fuel ramjet (SFRJ) through experimental and numerical simulations. By analyzing flow characteristics, combustion oscillations at varying expansion ratios, heat release patterns, and species distribution within the engine, the following key conclusions are drawn: Ground tests revealed first-order pressure oscillations with a dominant frequency of approximately 600 Hz, exhibiting a half-wavelength oscillation pattern. As the expansion ratio increased from 1.75 to 2.25, the amplitude of the first-order oscillation increased by 21%, from 1.89 kPa, while the dominant frequency initially rose and then decreased, peaking at an expansion ratio of 2. The shear-induced downstream of the step was identified as a critical factor influencing heat release and acoustic pressure oscillations in the combustion chamber, which led to periodic unstable heat release that amplified combustion oscillation amplitudes. The numerical results show that the periodic motion of vortices induced by flow shear downstream of the sudden expansion step causes oscillations in the temperature distribution on the propellant surface, which is one of the key factors contributing to combustion instability and pressure oscillations. When the expansion ratio is increased to 2.0 and 2.25, the dominant frequency of pressure oscillations increases by 15.2% and 13.2%, respectively, while the amplitude rises by 7.9% and 30%, respectively. The vortice development blurs the oxygen-rich and fuel-rich zones, enhancing the mixing of oxygen and fuel-rich gases and exacerbating the instability of heat release. Larger expansion ratios further extended the axial range of unstable species distribution, increasing nonuniformity within the combustion region.