Secondary Gas Injection Effect on Dual-Bell Nozzle Flow Transition

Dual bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient and where side loads may occur. At high altitude the flow attaches to the nozzle wall and the full area ratio is used. The flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations carried out on an axi-symmetric geometry and considering two configurations: one with a wall film cooling and the other without film. Aim of this study is to analyze the eect of the secondary gas injection on the dynamics of the separation point and on the transition time. Results show that film injection may substantially change the characteristic transition time and evolution.