Fluid flow limit for given orifice
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Re: Fluid flow limit for given orifice
Re: Fluid flow limit for given orifice -- SJB	Post Reply	Top of thread	Forum

Posted by: Raph0007 ® 08/12/2005, 16:07:34 Author Profile Mail author Edit

I think you are looking for "choking". Although it has been a long time since I studied this myself.. A good refresher course can be found at the NASA site: https://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html look for equation #19 Hope this helps Raph

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Re: Re: Fluid flow limit for given orifice
Re: Re: Fluid flow limit for given orifice -- Raph0007	Post Reply	Top of thread	Forum

Posted by: mbeychok ® 09/15/2005, 01:37:48 Author Profile Mail author Edit

Raph0007: The velocity of gas flowing through an orifice becomes choked (and is also referred to as sonic velocity) when the ratio of the absolute upstream pressure to the absolute downstream pressure is equal to or greater than [ ( k + 1 ) / 2 ] k / ( k - 1 ), where k is the specific heat ratio of the discharged gas.  For many gases, k ranges from about 1.09 to about 1.41, and thus [ ( k + 1 ) / 2 ] k / ( k - 1 ) ranges from 1.7 to about 1.9 ... which means that choked velocity usually occurs when the absolute upstream pressure is at least 1.7 to 1.9 times as high as the absolute downstream pressure. In the customary units used in the USA, when the gas velocity is choked, the equation for the mass flow rate is: Q = C A [g k d P]1/2[2/(k+1)](k+1)/(2k-2) or this equivalent form: Q = C A P[g k M/(R T)]1/2[2/(k+1)](k+1)/(2k-2) It is important to note that although the gas velocity reaches a maximum and becomes choked, the mass flow rate is not choked. The mass flow rate can still be increased if the source pressure is increased. Q = mass flow rate, lb/s C = discharge coefficient (dimensionless, usually about 0.72) A = discharge hole area, ft2 g = gravitational acceleration of 32.17 ft/s2 k = gas cp/cv = (specific heat at constant pressure)/(specific heat at constant volume) d = gas density, lb/ft3, at upstream pressure P = absolute upstream pressure, lb/ft2 M = gas molecular weight R = the Universal Gas Law Constant = 1545.3 (ft-lb)/(lbmol)(°R ) T = gas temperature, °R Milton Beychok Modified by mbeychok at Fri, Sep 16, 2005, 12:20:09

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Re: Fluid flow limit for given orifice
Re: Re: Fluid flow limit for given orifice -- Raph0007	Post Reply	Top of thread	Forum

Posted by: SJB ® 08/12/2005, 19:19:39 Author Profile Mail author Edit

Many thanks, Raph. Most useful, and the information contained in your link lead to further information that was similarly so. Choking was indeed the subject.

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