Fluid passing through a convergent-divergent nozzle

[russ_watters]

Especially when the velocity at the throat is choked and inlet pressure is sufficient, right?

Right.

 

[boneh3ad]

Note that he said divergent only this time, so for this to be true you have to assume there's still/also a convergent section and a large enough reservoir feeding it. And the 5bar is the pressure at the throat, not the reservoir.

No you don't. If the throat is choked, how it got that way is effectively irrelevant to what happens downstream of the choke point. You can just have a hole in the wall of a pressure vessel and attach a divergent nozzle to it to create supersonic flow. Now, you likely will have separation and therefore a different effective area ratio than the actual physical nozzle, but the fundamental principles remain the same.

 

[russ_watters]

No you don't. If the throat is choked, how it got that way is effectively irrelevant to what happens downstream of the choke point.

The OP didn't say the throat flow was choked in the scenario given in post #21 (or even the original scenario).

You can just have a hole in the wall of a pressure vessel and attach a divergent nozzle to it to create supersonic flow.

I'm aware; a hole in the wall of a pressure vessel is effectively a poor efficiency converging nozzle.

 

[boneh3ad]

The OP didn't say the throat flow was choked in the scenario given in post #21 (or even the original scenario).

I'm aware; a hole in the wall of a pressure vessel is effectively a poor efficiency converging nozzle.

My point is that literally anything can be used as an inefficient converging section.

And the OP's implication was that the flow was choked since his goal was to continue accelerating the flow. The problem is that he doesn't seem to understand the physical principles involved.

 

[russ_watters]

My point is that literally anything can be used as an inefficient converging section.

Agreed.

And the OP's implication was that the flow was choked since his goal was to continue accelerating the flow.

Maybe, but when I pressed on what was most important to him in the original scenario, he said his specified inlet velocity, which would yield a very, very subsonic throat velocity. Given this member's history, it's entirely possible he really thinks you can get a velocity increase on both sides of the throat, even in fully subsonic flow. After all, that's what he predicted his original scenario would do.

The problem is that he doesn't seem to understand the physical principles involved.

Agreed, which is why I caution against making assumptions that make sense about what he wants in the scenario. It's entirely possible (and often reality) that he is envisioning something that doesn't make sense. I often try to let go or correct and move on from little errors in the setup of a scenario, but for this user those little issues often contain landmines.

 

[T C]

Now, you likely will have separation and therefore a different effective area ratio than the actual physical nozzle, but the fundamental principles remain the same.

How to determine the effective area for such pressure level?

 

[boneh3ad]

How to determine the effective area for such pressure level?

Pressure has little to do with it, and at any rate, you aren't going to find a simple way to calculate the effects of separation accurately without resorting to CFD.