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Predicting Choked Flow

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#1 FireEng

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Posted 28 May 2010 - 08:40 AM

I'm designing a system flowing nitrogen gas, and I want the maximum flow I can get through an orifice plate given choked flow conditions (since I also want max pressure drop). I know the upstream pressure (200 bar), and I know what the downstream pressure will be (~97 bar), but I don't know how to find the orifice at which choked flow begins to occur. For example, a pinhole orifice would surely choke the flow, and a wide open pipe would surely not choke it. How do I find the orifice in between that borders between choked and non choked? I know how to do this experimentally, but I was hoping to get a good idea theoretically before I start setting up tests.

Thanks!

#2 Art Montemayor

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Posted 28 May 2010 - 10:58 AM

Fireeng:

First – and foremost – you have to organize the algorithm or the sequence of resolving the problem.

If you know the concept of choked flow, then you know that at a certain pressure drop, where you have approximately a 1.6 to 1.9 ratio of P1 to P2, you develop choked flow. I usually use a ratio of 2 to make a quick judgment. I recommend you go to Milton Beychok´s website: http://www.air-dispe...m/feature2.html and then also to his citizendium websites where he specifically deals with the subject of choked flow and how to meter it.

#3 FireEng

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Posted 28 May 2010 - 11:16 AM

I understand the concept of choked flow, and I know how to calculate the flow rate in a system given that choked flow is occurring (including what P2 is). My question is how can I be sure that a system I'm designing has choked flow with a particular orifice size without first experimenting? As I mentioned in my initial post, a pinhole orifice would definitely choke the flow, so I can predict that flow rate with confidence. However, let's say I use an orifice that's half the size of my pipe diameter. In that case, I'm unsure if the flow is really choked or not without physically measuring it, so I can't be sure of my flow rate calculation if I'm using a choked flow model. When it all comes down to it, I'm wondering how to determine the largest orifice that still chokes the flow.

#4 fallah

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Posted 29 May 2010 - 11:38 AM

First, the pressure unit (barg or bara) should have been specified.

Second ,seems the degree of freedom in your described system would be such that one can not submit a reasonable response.

Size of line, flowrate, size of orifice, probably downstream pressure are variables inreasing degree of freedom.

#5 chemsac2

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Posted 30 May 2010 - 11:58 PM

FireEng,

I am not sure if choked flow depends on orifice size. For example consider a case where upstream pressure is 100 barg and downstream pressure is atmospheric. Choked flow would occur in this case irrespective of orifice size. In fact choked flow would occur even when orifice is not installed in this pipe.

In your case, upstream pressure is 200 bar and downstream pressure is 97 bar (I assume these to be at pressure control systems upstream and downstream). Here is how you can check if choked flow occurs in your system:

- Assume mass flowrate in the system
- Knowing source pressure of 200 bar, calculate P1 (i.e. orifice upstream pressure) considering losses
- Knowing sink pressure of 97 bar, do backward calculation to ascertain P2 (i.e. orifice downstream pressure)
- If P1/P2 > 0.5 (exact value of this ratio depends on fluid k value), choked flow would occur for this flowrate and you can calculate corresponding orifice size with formula for choked flow
- If P1/P2 < 0.5, flow would be subsonic. Repeat calculations with higher flowrate.

You can refer to an excellent article by Trey Walters in Chemical Engineering, 2000, "Gas flow calculations: Don't choke". I think a simple google search would be enough to get this article.

Regards,

Sachin

#6 sheiko

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Posted 02 October 2010 - 11:56 AM

In fact choked flow would occur even when orifice is not installed in this pipe.
...
- If P1/P2 > 0.5 (exact value of this ratio depends on fluid k value), choked flow would occur for this flowrate and you can calculate corresponding orifice size with formula for choked flow
- If P1/P2 < 0.5, flow would be subsonic. Repeat calculations with higher flowrate.

You can refer to an excellent article by Trey Walters in Chemical Engineering, 2000, "Gas flow calculations: Don't choke". I think a simple google search would be enough to get this article.

Sorry for coming back on that topic but i have read some misconceptions that i would like to correct:
First, choked flow in a pipe is different than through an orifice. In the pipe case, you can't use the above mentionned magic ratio. This is explained in the article mentionned in the quote.
Second, the criteria for choked flow in an orifice is: P1/P2 > 2 (as Mr Montemayor have showed) or P2/P1<0.5 (not P1/P2 > 0.5 as written in the quote).

Regards.

Edited by sheiko, 02 October 2010 - 12:01 PM.