Investigation of dynamic mixing combustion characteristics in variable thrust hybrid rocket motors

The dynamic mixing combustion characteristics in variable thrust hybrid rocket motors (HRMs) have been investigated by using a newly developed two-dimensional transient numerical model. This model takes into account the heat transfer process in the solid fuel grain coupled with dynamic combustion flow in HRMs. Fire experiments were conducted to validate the developed model by using an optical-access slab burner and a lab-scaled HRM. It shows that the model developed in the present work can predict accurately the regression behavior of fuel grains in HRMs. Based on this model, simulations of two variable thrust processes with different thrust ratio were performed. The results show that the recirculation zones located at pre- and post-combustion chamber directly affects the inhomogeneity of the temperature distribution of the inner surface of the fuel grain, leading to a fuel regression rate difference along axial direction. In thrust variation process, the fluctuation duration of the fuel regression rate is longer than the regulation time of oxidizer flow. The fluctuation range of regression rate is influenced by the amplitude and proportion in oxidizer flow regulation. In addition, a decrease in the characteristic velocity and combustion efficiency (from 97.1% to 72.63% and 70.64%, respectively) was observed during the thrust variation process. A slight thrust loss was observed at all stages of the variable thrust process and reaches to maximum value at an oxidizer flow rate of 20 g/s for the lab-scaled HRM studied in the present work.