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Pressure system

Pressure system

Pressure system

(OP)
Hi,

I'm carrying out an experiment in a wind tunnel where I am to supply pressurized air through a tube of 7 mm of ID. I have a bottle connected up to a regulator and my two questions are as follows:

- Do I use Bernouilli for calculating the velocity at the exit of the tube? So say for example my bottle has a pressure of 50 bar, then regulated by the regulator to 2. Then is this correct: 2 bar = Patm + 0.5*rho*Vexit^2

- How do I calculate the amount of time for the bottle pressure to drop down to 2 bar if I am to use a constant supply of 2 bar?

I know these might be really silly questions but I've never been good with this problems, I'm more of a structural person!

Many thanks!

RE: Pressure system

Q1 x P1 = Q2 x P2

P1 and P2 are I assume gage pressures, so convert to absolute
P1a = 50 + atm = 51 BarA
P2a = P2 + atm = 1 BarA

For any assumed flowrate Q2 at the outlet to atmosphere, you can calculate the flowrate from the bottle using 51 barA, or from the regulator using 3 BarA

I'd assume its a flow across an orifice, if the length of 7mm tubing is relatively short, with 3 BarA on one side and 1 BarA on the outlet.
 http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm

You should correct the flowrate from the bottle using the compressibility factor of air at 50 BarG, then you'll know how long the bottle will last.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Pressure system

I don't think so.  With 50 bar on your tank and atmospheric pressure in the wind tunnel, the flow is choked so you have to calculate sonic velocity and then convert that to a mass flow rate upstream of the nozzle (the change in density is far too great for the "constant density" assumption in Bernoulli to be valid).

David

RE: Pressure system

David, there's a 2 barg pinhole-regulator between the 50 B and atmos.  Maybe there'll be sonic flow immediately across the regulator, but that's about it, no?

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Pressure system

I know my scuba reg doesn't noticeably go sonic, even if take a deep breath, and that's almost the same thing, except the regulator outlet is in MY mouth.   

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Pressure system

It gets weird as the hole gets smaller and smaller relative to the "pipe length".  If the length of the pinhole is a significant fraction of the diameter (e.g., a 1/64 hole in a 3/8 pipe wall) then friction effects within the hole have to be considered and the arithmetic gets pretty nasty.  That is how your scuba tank keeps from blowing the back of your head off (the flow path through the regulator is really tortuous).

He didn't say what the length of his nozzle is, but with 7 mm (0.27 in) if it is shorter than about 3 meters then he'll certainly have choked flow.  The big problem (that requires an iterative solution) is figuring the pressure at the exit plane.  If his pressure drop in the 7 mm tube is less than around 48 bar, then he'll have choked flow at the exit.  The mass flow rate will be considerably more at 50 barg than at 2 barg, but the velocity will still be sonic.

David

RE: Pressure system

Then he should use a scuba regulator.  If you push the clear button, that gets a lot of flow, still w/o going sonic.  A little bubble gum is all you need sometimes.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

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