Swirl Injection Modeling for Paraffin-Based Hybrid Rocket Engines Combustion Instabilities

Hybrid Rocket Engines have gained more and more attention in recent years due to their appealing safety and cost-effectiveness features, but they present some disadvantages, most notably a low regression rate. Swirl injection offers a promising solution, enhancing regression rate and stabilizing combustion by means of increased wall heat flux. This paper introduces a novel swirl model for regression rate evaluation consisting in two integrated sub-models: the first one involves a corrective factor linked to the geometric swirl number of the injector, while the other accounts for the naturally occurring swirl decay. The validity of the model was assessed across a range of different test cases, varying geometric parameters, engine scales, injection, and feeding conditions, and the results shows that the accuracy of local and mean regression rate prediction is improved. Corrections for the wall friction coefficient and the entrainment factor were introduced in an in-house code for paraffin-based fuels. A parametric analysis, carried out considering different swirl levels, revealed combustion chamber thermodynamics consistent with the expected trends observed in firing tests. Finally, an effective reduction in the delay time characteristic of the boundary layer was observed; moreover, the Rayleigh index results showed a marked decrease, indicating a decoupling of pressure fluctuations and heat release which tends to stabilize combustion as the swirl increases.