Thrust Control Regimes in a Solid Ramjet Using Variable Air Bypass
| This work investigates variable air bypass as a method for thrust modulation in a solid fuel ramjet. Solid fuel ramjets inherently face challenges in thrust control due to their dependency on predetermined fuel grain geometry and varying flight conditions. To overcome this limitation, an experimental study was conducted to evaluate how active air bypass control affects airflow distribution, fuel regression rate, combustion efficiency, and thrust. Experiments were performed using a experimental setup consisting of a hydrogen-air heater to simulate operational flight conditions and a solid fuel ramjet with an adjustable bypass valve. Two solid fuels, HTPB and HDPE, were tested under a range of conditions varying in air mass flow rates, bypass ratios, combustion chamber pressures, and inlet temperatures Regression rate models were found using time marching algorithm and optimization methods, revealing deviation of up 10% from literature models. Higher than expected fuel regression rate led to fuel rich combustion at the tested conditions. The results also revealed significant deviations between measured bypass flow rate and these expected based on the valve geometry, due to high pressures losses in the bypass duct, limiting the bypass flow. As a result, increasing the bypass flow to obtain fuel lean conditions was not possible and the thrust modulation with the experimental ramjet was limited. To address these limitations, a fuel grain length analysis was conducted, demonstrating that adjusting fuel grain length could effectively shift the ramjet operation to fuel lean conditions, enabling thrust modulation capability through bypass adjustment. |
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