In order to design hybrid rocket engines, we developed a numerical prediction method to the internal ballistics, such as the characteristic of fuel regression rate. Our model includes quasi-one-dimensional flowfield and one-dimensional thermal conduction into the solid fuel. Besides, the energy-flux balance equation at the solid fuel surface is solved to determine the regression rate. In our previous method, Karabeyoglu's model was used when evaluating convective heat flux, and only the radiation from gas was considered when evaluating radiative energy flux. In this paper, the model for convective heat transfer is modified considering the velocity-profile peak at the flame location, and soot is also considered as a radiation source. We employ two method; (1) the method where the original convective-heat-transfer model is used and where radiative heat transfer is ignored, (2) the method where the modified convective-heat-transfer model is used and where radiative heat transfer from gas and soot is considered. The calculation results are compared with the experimental data in an open literature. As the results, it is confirmed that the order of magnitude of estimated regression rate is the same order of the experimental data. Next, the parametric studies for hybrid rocket design parameters are demonstrated. The three design parameters, which are chamber scale, initial grain temperature and nozzle throat diameter, are employed in the parametric studies. Consequently, we conclude that this method is useful for estimating hybrid rocket internal ballistics.
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