![]() In all simulations, an axisymmetric approach with a periodic boundary condition ensuring the tangential symmetry of the flow has been employed. The numerical simulations have been performed solving the Reynolds-averaged Navier–Stokes (RANS) equations, with submodels accounting for the effects of turbulence, chemistry, gas–surface interaction and thermal radiation. 4 the main features of hybrid rockets with swirl injection are investigated and the regression rate prediction capabilities of the numerical approach are validated though the rebuilding of small-scale firing tests. 2), then it is validated through the rebuilding of firing tests with axial injection (Sect. The manuscript is organized as follows: first, the numerical model is briefly described (Sect. All simulations use an axisymmetric approach, to reduce the computational time and to be used as a quick design tool. In this work, RANS simulations of HREs burning gaseous oxygen and paraffin–wax, employing both axial and swirl injection, are performed. Most works also neglect the effects of radiation, which is instead known to give an important contribution to the total heat flux. Several firing tests have shown how such injection results in a regression rate increase up to 5 times with respect to axial injection.ĬFD analysis of HREs with swirl injection has been the subject of many works in the past years, mostly employing 3D or LES simulations, wich have high computational cost, or making use of simplified injection approaches. In fact, the resulting vortex motion increases the convective heat flux to the grain and the regression rate, and improves the mixing between the propellants, which may lead to an increase in combustion efficiency. The performance of paraffin-based HREs may be further improved by the use of swirl injection. In addition, paraffin-based fuels are nonhazardous, nontoxic, and easy to handle. High regression rates allow one to design high-volumetric-loading single-port combustion chambers, avoiding complex and inefficient multiport grains. ![]() ![]() Regression rates up to three to four times higher than the conventional values were first observed in laboratory-scale motors and then confirmed in scale-up tests with different oxidizers. This mechanism allows for a continuous spray of fuel along the port, leading to an additional mass transfer toward the melt layer and the flame front, with most of the fuel vaporization occurring around the droplets. In fact, contrary to conventional polymeric fuels, which pyrolyze before burning, paraffin-based grains in HREs exhibit a liquid or supercritical fluid layer, depending on the operating pressure, leading to the entrainment of droplets into the gaseous mixture stream. ![]() However, the most promising one to date is the use of paraffin-based fuels. This shortcoming can be mitigated with the application of different techniques, such as multiport grains or the introduction of fuel additives. However, one of the most important drawbacks of conventional HREs using classical pyrolyzing fuels such as hydroxyl-terminated poly-butadiene is the low regression rate of the grain, which entails low thrust levels especially in combination with high-performance oxidizers, such as oxygen, which require lower O/F for maximum efficiency. HREs are therefore considered one of the potentially preferred options for specific future-generation propulsion systems as they have already shown some promising results with the successful flight of SpaceShipOne and SpaceShipTwo, and with the ongoing work on the SL1 launcher by Hympulse. On the other hand, with respect to liquid rocket engines, they are much simpler and cheaper to build, more reliable, and have higher average propellant density. They allow throttling, shutdown, and restart capabilities, and they present less ambient temperature sensitivity, higher crack robustness of fuel grain, and higher specific impulse. If compared to solid rocket engines, HREs are safer during fabrication, storage, and operations. Hybrid rocket engines (HREs) are propulsion devices usually employing a solid fuel and a gaseous or liquid oxidizer, stored physically separated from each other.
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