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.