Question regarding converging-diverging nozzles

[Amaelle]

TL;DR Summary

Good day!

I have a question regarding the converging-diverging nozzle (look at the image)

Normally, in order to expand the mass flow rate, we reduce the downstream pressure

my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?

many thanks in advance

Good day!
I have a question regarding the converging-diverging nozzle (look at the image)
Normally, in order to expand the mass flow rate, we reduce the downstream pressure
my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?
many thanks in advance

 

[boneh3ad]

In short: you can't. If you don't have the required pressure differential, you don't have the appropriate motive force to accelerate the flow that much.

 

[Arjan82]

To be picky, if you have a considerable pressure downstream, you need a higher pressure upstream to achieve the choking condition. But the downstream pressure will always have to be the critical pressure (either by lowering downstream pressure or increasing upstream pressure) in order to get choking.