# What is the gas pressure after it comes out of the nozzle?

There is one thing that bothers me a lot. Recently our lab ordered a Nitrogen cylinder from AirGas. When I am using it, I understand that when the nitrogen comes out, the pressure equals the delivery pressure, which is set by me as 65psi. However, when the nitrogen comes out of it, the pressure will get smaller until it reaches equilibrium with the ambient pressure (1atm). How do I know the pressure distribution after nitrogen comes out? Can I see it equals 1atm in a very very short distance?

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Bystander
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How do I know the pressure distribution after nitrogen comes out?
the pressure will get smaller until it reaches equilibrium with the ambient pressure
What do you know about the location of the outlet that we don't? Is it into a gas sparger immersed in water? Expanding freely into the room? Filling a balloon? Depends on what the gas stream has to work against before it reaches ambient.

What do you know about the location of the outlet that we don't? Is it into a gas sparger immersed in water? Expanding freely into the room? Filling a balloon? Depends on what the gas stream has to work against before it reaches ambient.

"Expanding freely into the room" is the thing that I am interested in. Do you have any ideas on that?

Bystander
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A pressure/velocity/density profile of flow from the outlet is going to depend on the geometry of the outlet, and give you smoke rings, mach diamonds, any number of flow phenomena before final equilibration with ambient conditions.

boneh3ad
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While it will be dependent on the geometry, you can at least rest assured that the flow is going to be choked here and will likely expand to some low supersonic velocity, creating some shock pattern. It will probably persist for a nontrivial distance past the outlet (i.e. you can't just assume that the pressure is immediately at equilibrium). Exactly how that all occurs will depend on the geometry, though, as Bystander said.

A pressure/velocity/density profile of flow from the outlet is going to depend on the geometry of the outlet, and give you smoke rings, mach diamonds, any number of flow phenomena before final equilibration with ambient conditions.
Thanks for your ideas. For "Expanding freely into the room", do you know what kind of books/website that might be useful for doing such analysis? I want to know the pressure drop 10cm away from the nozzle, for example. Do you know how I can calculate this? If you can provide some direction, it will be great!

boneh3ad
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Any good book on compressible flow would be a good start. You can start with a cheap but good one, https://www.amazon.com/dp/0486419630/?tag=pfamazon01-20 by Liepmann and Roshko.

If you have the proper equipment, you could always just get one of the cylinders, open the valve to start venting gas and set up a shadowgraph to see how far the shock structure propagates. Once it is completely gone you can pretty effectively say the pressures are equal (or very nearly).

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Bystander
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Schleiren photography (Schlieren?) would be my first inclination for looking at such an effect. I've never actually played with it, and it might be more involved than it looks, but it would at least give you a picture of density profile.

And boneh3ad beat me to the Scheiren.

boneh3ad
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Shadowgraph images (shadowgrams) are generally less complicated to produce than schlieren images are. Schlieren images require a collimated light source (usually using lenses or parabolic mirrors) and a lens to focus the light onto a knife edge. A shadow graph just needs the collimated light shown through the region of interest.

They are both fairly easy to do if you have the right equipment. Shadowgraphy just requires less equipment.