Choking phenomena of sonic nozzles at low Reynolds numbers

The choking phenomena of sonic nozzles were investigated for the Reynolds number range from 40 to 30 000 for nitrogen gas. The results showed that the critical back pressure ratio is a function of the Reynolds number only, with different characteristics for different nozzle shapes. In ISO-type toroidal-throat Venturi nozzles, the minimum Reynolds number satisfying the choking condition is about 40 and the critical back pressure ratio is only about 0.05 at this minimum Reynolds number. It was also found that the critical back pressure ratio has a local minimum value around Re_th=4000 and that the local maximum value is around Re_th=10 000 due to the change in characteristics of the boundary layer in the diffuser. On the other hand, the critical back pressure ratio in quadrant nozzles decreases monotonically with decreasing Reynolds number, unlike the former nozzle, and the minimum Reynolds number necessary for choking is estimated to be approximately the same as that in the Venturi nozzle.