Choked Flow in Orifice 1

[choack]

If I have an orifice with a size corresponding to a fixed beta value... and I have a two inch line.


When using the choked flow formula found in wikipedia... why is it the discharge area (2 inch area) was making the 'choking' and not the orifice itself?

Christopher Kenneth Choa

 

[zdas04]

"The equation in Wikipedia" really doesn't convey much information. A link would have made it clearer what the heck you were talking about. Take a look at faq798-1196. This is a discussion of Choked flow that has been extensively peer reviewed (which is not always the case with Wikipedia). I didn't dig through the 8 versions of choked flow that are in Wikipedia to try to ferret out the one that confused you.

In real life, the flow reaches Mach 1.0 at the minimum of the vena contracta which happens just slightly after the orifice and is just slightly smaller than the orifice. That is the limiting flow area for choked flow, not the carrier pipe upstream of the choke. The mass flow rate through that flow area is a function of the density of the fluid upstream of the choke, and the flow velocity is a function of the temperature upstream of the choke.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[choack]

Thanks David for the post. It really helped me in my calculations.

By the way, i'm really interested about the 8 versions of choked flow... can you give a good link/reference to that? Thanks..

Christopher Kenneth Choa

 

[zdas04]

nope, I'm on my way out the door.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TD2K]

What is the orifice diameter, how much 2" pipe do you have, is the orifice at the pipe inlet or outlet?

If you have a copy of Crane it shows how much pressure drop you can take before you begin to choke in a piping system, the higher the overall K, the more dP you can take. You can use the charts in Crane to estimate where in the piping system you are beginning to choke and compare that to the cummulative K as you move down your piping system.