Thermodynamic cycle analysis of ramjet engines using magnesium-based fuel

To predict cyclic performance and propulsive performance, and further instructs the integral engine design, an ideal thermodynamic cycle model for a magnesium-based fuel ramjet engine employed in underwater vehicles is established. Especially, the magnesium-based fuel grain consisting of most magnesium, small ammonium perchlorate, and some binder is carried on-board, while water as the main oxidizer and working medium is injected into the chamber through different pipelines. Thus, the ideal thermodynamic cycle model is regarded as a combination of Brayton cycle and quasi-Rankine cycle due to phase-transition characteristics of water. Based on the thermodynamic cycle model and thermodynamic calculation model, thermal efficiency, propulsive efficiency, as well as specific impulse are predicted theoretically in the present study. The results show that thermal efficiency is dominantly influenced by water/fuel ratio and fuel composition distribution when fixing navigational depth of ramjet, instead, the chamber pressure has the slight influence especially under a higher pressure environment. Otherwise, thermal efficiency relies on navigational depth rather than the fuel composition distributions while fixing the water/fuel ratio. As a heat engine and thruster, the rules of propulsive efficiency for this water ramjet also provide qualitative guidance for improving the energy conversion between kinetic energy and propulsive energy. Furthermore, toward rules of specific impulse versus water/fuel ratio under different chamber pressure, the occurrence of peak specific impulse of order 5000 N s kg−1 on each curve is observed.