Swirl Injection in Hybrid Rocket Engines

Hybrid Rocket Engines (HRE) are a class of thrusters that have experienced considerable development in recent years due to their safety and cost-effectiveness. However, they have disadvantages intrinsically linked to their combustion mechanism: most notable among them are their low regression rate, and consequently lower performance than purely liquid or solid propellant rockets, and the presence of a characteristic low-frequency instability (LFI). A CFD code was developed at Politecnico di Torino for the analysis of paraffin-fueled HREs, a fuel that has been shown to carry a significant increase in regression rate due to the entrainment phenomenon. This thesis focuses on the study of swirl injection in HREs and the consequent modification of the thermal model: the presence of a tangential velocity component in fact leads to an improvement in the regression rate due to the increase in wall heat flux, as well as a reduction in the LFI. Initially, generalities on swirl-injection and the CFD code are presented, focusing on the characteristic equations and the chemical model with low computational cost neural network derived from CEA. For the development of the thermal model, several experimental testcases are analyzed and a multiplicative factor of the regression rate in the presence of swirl is determined, related to the injector geometry alone and a model of exponential decay of the swirl itself; then, corrections are introduced on the calculation of the wall friction coefficient and entrainment factor, calibrated appropriately based on the required regression rate. After verifying the stability of the code, a parametric analysis is carried out on the effects of swirl injection on combustion chamber thermodynamics, comparing the trends with those expected from experimental testcases. Finally, by introducing a delay term in the boundary layer dynamics suitably modified with the new expressions of the friction coefficient and velocity field, the LFI at various degrees of swirl is studied by looking at the phase coupling between the fluctuations of heat and pressure release by the Rayleigh criterion.