
Predicting Choked Flow
#1
Posted 28 May 2010  08:40 AM
Thanks!
#2
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.airdispe...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
Posted 28 May 2010  11:16 AM
#4
Posted 29 May 2010  11:38 AM
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
Posted 30 May 2010  11:58 PM
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
Posted 02 October 2010  11:56 AM
Sorry for coming back on that topic but i have read some misconceptions that i would like to correct: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.
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.
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