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 Post subject: Choked Flow of Gasses
 Post Posted: Sat Nov 27, 2010 2:29 pm 
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Location: Florida
Found this doing some browsing, interesting info that can be applied to racing.

http://okcc.com/PDF/Choked%20Flow%20of% ... 0pg.48.pdf


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 Post Posted: Thu Dec 02, 2010 9:55 pm 
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Interesting reading, but choked flow through an orfice, and choked flow in a port would be 2 different things.
The shape before, and after a restriction will affect the recovered pressure.
If you placed an orfice in something shaped like a carb venturi, would it choke at the same flow rate, as it would with open space above, and below the orfice?

The studying of this type of thing is always a plus in my very limited mind.

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 Post Posted: Thu Dec 02, 2010 10:39 pm 
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randy355 wrote:
Interesting reading, but choked flow through an orfice, and choked flow in a port would be 2 different things.

Choked flow in a port is not because of sonic velocity, its because of the air speed causing the flow to detach from the walls. Once the flow becomes turbulent, (as it does when it separates) the total flow efficiency reduces but the flow volume can still be increased if you suck on it harder. Port flow is about pressure to mass ratios.
randy355 wrote:
The shape before, and after a restriction will affect the recovered pressure.

Pressure recovery angle on the exit of an orifice affects the mass that can be flowed through an orifice. Entry angle has minimal effect exit angle has major effect.
randy355 wrote:
If you placed an orfice in something shaped like a carb venturi, would it choke at the same flow rate, as it would with open space above, and below the orfice?

I think your referring to a sharp edge orifice. Sharp edge orifice flow less mass per unit of pressure differential compared to a venturi with a correct recovery angle.
The choke point for every orifice or venturi or pipe or port etc is the same Mach number. The Mach choke number is independent of shape.


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 Post Posted: Thu Dec 02, 2010 11:54 pm 
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There isn't any place in the induction system that has enough dP to cause real "choked flow". The exhaust port probably has enough dP to go "sonic" or "choked" at low lift.

Rick


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