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This work presents a novel approach for the modeling of the entrainment in the numerical simulation of the internal ballistics of hybrid rocket engines with paraffin-based fuels. This model, coupled with a more sophisticated gas–surface interaction treatment, is an improvement of the model previously developed by some of the authors, which was based on some oversimplifying assumptions. Indeed, the old entrainment model was performed in a close range of averaged oxidizer mass flux, and it made the overall numerical model not scalable on different motor sizes. Therefore, a new correlation is introduced, which is based on the Reynolds number and takes into account the dependence of the entrained fraction on the shear stress exerted by the gas flow and the tube diameter. Firstly, the new and the old numerical models are compared in order to highlight the improvement obtained by the current efforts. Then, the model has been validated on experimental tests involving two different thrust class motors. Finally, the effect of the motor size on the fuel consumption is shown, thus revealing the crucial influence of the recirculating zone extension due to the oxidizer axial injection.
Gallo et al. (Mon,) studied this question.