Air Flow Formula Help

How condescending of you.

When measuring the pressure upstream of the orifice, where the speed of the flow is negligible, yes.

Not truly isentropic, no.

Still, the calculations here

make use of isentropic relationships and apply a correction factor to adjust for the deviation from isentropic behavior.

OK.

It is not the shock wave that chokes the flow. A shock wave might occur in the diverging section of a converging-diverging nozzle. The flow is choked at the throat of the nozzle regardless of where or whether a shock wave occurs.

Within the throat of an isentropic nozzle with choked flow, the pressure is 87 psia if the upstream air pressure is 165 psia.

Actually, I doubt that there will be a shock wave in the situation which is the subject of this thread.

So I'll amend my description of the pressure profile through OP's hole:

1. 165 psi in the reservoir upstream from the hole
2. A smooth but short transition to the critical pressure, 87 psia within the hole
3. A longer, messier transition (downstream from the hole) from 87 psia to atmospheric pressure.

In looking again at Figure 1 here

Shock waves transition from a supersonic condition. There is no diverging section in the OP's apparatus where supersonic flow can develop. And note that the pressure _increases_ across a shock wave.

But whether a shock wave occurs is irrelevant to what happens in the throat of the nozzle at choked flow.

The 28 psia is calculated by assuming that the flow is isentropic. That may be at odds with the earlier statement, "we can be reasonably sure it is not isentropic". But we can adjust for non-isentropic behavior with a correction factor, as mentioned above.

Another assumption in calculating the 28 psia is that the velocity is negligible at the location where the pressure is 28 psia. That is not the case in the aperture's channel, where the flow is at the speed of sound -- the velocity in the channel is far from negligible.

No.

The pressure at the end of the aperture channel is 87 psia (or, perhaps better, in the neighborhood of 87 psia -- because the flow isn't truly isentropic).

For the case of compressible nozzle flow, the pressure, mass flow rate and velocity have the relationship described below. ... 3. Choked Flow Choked Flow P2 = Pc > P3

That is, the pressure across the entire exit plane (illustrated in the link above) for choked flow though a converging nozzle (with no diverging section) is the critical pressure -- which may exceed atmospheric pressure.

The exit pressure is not necessarily the same as the threshold _reservoir_ pressure (28 psia) needed to create sonic flow at atmospheric pressure (15 psia).

Other on-line sources say the same:

Since the nozzle does not have a diverging section, further reduction in back pressure PB will not accelerate the flow to supersonic condition. As a result, the exit pressure PE shall continue to remain at P* even though PB is lowered further.

Consider first a convergent nozzle ... When PB is decreased below the critical pressure, ... PE remains constant at a value equal to the critical pressure ... The drop in pressure from PE to PB takes place outside the nozzle exit.

[Before someone else brings it up -- the engsoft.co.kr link above contains the following statement which is given credence elsewhere: "In case of orifice, actually there is no critical pressure." But I don't think that the implications of that statement affect the conclusion below.]

The pressure 28 psia does not exist anywhere within the OP's apparatus when it is operating at 150 psi. Rather, the pressure 28 psia exists somewhere in the jet of gas that is expanding after it exits the hole.

OK?

Adam

(not directed at one person), I am truely amazed that this group would continue to argue over a post that provided insufficient information.

I provided a suggested Google, which directed to: (as the first selection) a table of pressure vs flow, as a function of orfice diameter.

Some have argued about the composition of the gas, the F'ing top (first) post says "AIR".

And while supersonic or subsonic will influence the issue, and the outcome.

YOU DON'T HAVE A CLUE.

So, STFU, until the OP provides complete inforation.

And yet, you gave him a numerical answer anyway.

I doubt that it precipitated some dire result. I just note irony of the complaint.

The first hit on Google here and now is: Results 1 - 10 of about 2,480 for "air flow orifice"

I don't see the table.

The first hit was probably something else when you posted. Three of the first 10 hits point to this thread now.

Actually, the subject line of _every_ post in this thread says that. The body of the OP's post does not.

The layout of my newsreader makes it easy to overlook the subject line after a message is opened or I am focused on messages listed later in the thread.

Some similar behavior in your newsreader is suggested by your mention that the _first_ post says "air".

After I saw the OP's older post about argon, I overlooked the subject line of the current thread. My bad. Contributory badness by the OP for not saying "air" in the body.

What information do you think is lacking? Have you asked for it?

The OP asked for a number. He got one. He seems to be done with the issue.

Others seem to be done impugning my abilities after smelling blood when I made mistake.

Newsgroups have a serious purpose. They are also a form of entertainment. And they are places to hone one's skills while hammering out ideas -- preferably with a minimum of pompousness or acrimony.

Stick around. Good stuff happens here.

Adam