Choking phenomena of sonic nozzles at low Reynolds numbers

The choking phenomena of sonic nozzles were investigated for the Reynolds n umber 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, wi th different characteristics for different nozzle shapes. In ISO-type toroi dal-throat Venturi nozzles, the minimum Reynolds number satisfying the chok ing condition is about 40 and the critical back pressure ratio is only abou t 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 t he local maximum value is around Re-th=10 000 due to the change in characte ristics of the boundary layer in the diffuser. On the other hand, the criti cal back pressure ratio in quadrant nozzles decreases monotonically with de creasing Reynolds number, unlike the former nozzle, and the minimum Reynold s number necessary for choking is estimated to be approximately the same as that in the Venturi nozzle. (C) 2000 Elsevier Science Ltd. All rights rese rved.