The Size Effect on Critical Back Pressure Ratio for Sonic Nozzles

As an important parameter, the critical back pressure ratio is directly related with the application of sonic nozzle. When the stagnation condition was fixed, the size effect on critical back pressure ratio was investigated. Some numerical simulations were conducted for sonic nozzles with throat diameter within (0.15–5) mm under laminar flow. From the simulation results, it was clear that the critical back pressure ratio decreased with throat diameter decreasing. From the analyses of flow filed for sonic nozzles with different throat diameters, the most possible reason was the strong interaction between core flow and boundary layer near to the wall, which intensified with throat diameter increasing. As a result, the boundary layer effect decreased with the throat diameter increased, which resulted in the increase of critical pressure ratio with the throat diameter increasing.