Influences of steps in hybrid rocket engines: Simulation and validation on simplified geometries

In this study, we investigate the effect of single steps inside the fuel grains of typical hybrid rocket motors numerically. Steps can be a promising candidate to increase the regression rate of hybrid engines. We show that simulations on simplified geometries yield surprisingly close numerical regression rate profiles when compared to experimental data. Moreover, there is no necessity to calculate the progression of the fuel port at different time steps, as a single simulation on the average diameter can match the experiments sufficiently well. This observation allows for larger parametric studies on optimal positioning of the steps while limiting computational resources. The potential to increase the average regression rate through steps can be predicted numerically, and the simulations are following the same trend as the experiments. The augmented mixing and heat transfer inside the turbulent boundary layer can be predicted. It is concluded that backward facing steps increase the regression rate proportional to the step height up to 10 mm, and are preferably utilized upstream. For forward facing steps, the regression rate augmentation is saturated starting with 5 mm step height, and the position should be further downstream.