| Outline of Annual Research Achievements |
A computational model of a hybrid rocket motor has been developed for the purpose of simulation of internal ballistics and transient behavior such as combustion instabilities. The numerical model is validated against experiments for prediction of steady state regression rates. In the unsteady time-dependent simulation, the unsteady convective heat flux is modelled by the application of a temporal boundary layer delay of the steady state form of wall heat flux to the changes in the regression rate. With analogy to a simple non-linear oscillating system, it is shown that such a delay can result in negative damping in the system causing self-excited oscillations. Upon this modelling, an unstable region ensues. At first an oscillating periodic increase in the regression rate and chamber pressure is observed (linear regime), which then proceeds into a non-linear limit cycle. A positive DC shift in the chamber pressure is also observed. The frequencies of different natural modes (including the intrinsic hybrid oscillation mode) predicted by the model are found to be in good agreement with theoretical prediction. Parametric analyses have been carried out with different motor configurations. Their effect on the predicted values of DC shift and RMS amplitude is reported. Comparison against the prediction of non-linear limit cycle amplitude is performed for experimental data in literature, showing good agreement. It is concluded that using such a novel approach, a hybrid rocket motor designer can study the stability characteristics of the motor being designed.
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