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