Modeling and CFD Simulation of Regression Rate in Hybrid Rocket Motors

As the research on hybrid rocket motors advances, more accurate tools are needed to estimate the performance of the system by determining its fundamental parameters. One of them is certainly the regression rate of the solid fuel. Unfortunately, it depends on many complex physical phenomena and interactions which vary with time, space and scale, making the task of predicting its evolution very difficult. To address this issue, Computational Fluid Dynamics (CFD) was employed to investigate the inner workings of a hybrid rocket motor and develop a useful tool to help the design process and contribute to the physical understanding of the problem. By implementing a User-Defined Function (UDF) in a commercial CFD software, it has been possible to simulate the regression rate as a function of heat flux at the fuel surface. The calculation is performed by solving the energy balance at the solid-fluid interface coupled with the pyrolysis Arrhenius equation. Validation has been performed using literature data from Carmicino and Sorge. The results generally agree with the experimental regression rates within 10% of error for HDPE and 20% for HTPB. A significant discrepancy in the regression rates of these two fuels not accounted for by the classical theory was exposed.