Numerical Study on the Effect of the Nozzle Pressure Ratio on the Starting Characteristics of the Axisymmetric Divergent Dual Throat Nozzle

The effect of nozzle pressure ratio (NPR) on the starting characteristics of an axisymmetric divergent dual throat nozzle is investigated numerically. The steady and unsteady numerical simulation methods are used to study the internal flow field of the nozzle and variation in the related performance parameters as a function of the nozzle pressure ratio. The results show that for change in the total pressure of the nozzle inlet flow or back pressure under the same nozzle pressure ratio, the flow field structures in the nozzle cavity remains basically the same, and there is a little difference between the unsteady numerical simulation results and the corresponding time-independent numerical simulation results. In addition, the discharge coefficient of the nozzle increases rapidly with the increase of the nozzle pressure ratio, and then changes only slightly when the nozzle pressure ratio reaches a certain value. With increase in the nozzle pressure ratio, the thrust coefficient of the nozzle will oscillate in the initial stage, then gradually decrease, and then slowly increase, and suddenly decrease near the critical nozzle pressure ratio (NPR_cr), and then gradually increase. The typical flow field structures in the cavity under the starting and non-starting conditions are presented before and after reaching the critical nozzle pressure ratio at which the thrust coefficient of the nozzle suddenly drops.