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At first, I will try to describe my problem.

I have a compressed air at 0.4 MPa (4 bars) that goes trough a control valve and a flow meter into two 5 mm pipes.

Flow meter shows the value of 200 normalized liters/min (100 l/min into each pipe). According to a simple equation Q=Area*velocity this should mean that the velocity of the air coming from the pipe should be around 82 m/s (maybe I am incorrect and it doesn't work like this).

My problem is that the speed is nowhere near this value. I didn't measure it with a proper tool but I know what It should feel like to have outlet speed around 80 m/s. I guess the speed is no more than 20 m/s (the air speed is similar to the speed of air you are able to blow from your mouth with pursed lips).

My cfd simulation also showed the speed around 80-90 m/s with inlet flow of 100 l/min. So where can be the problem? I tested two different flowmeters, both with similar results. Is my calculation wrong or is there something wrong with the flowmeters? Flowmeter were just ordinary cheaper ones.

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    $\begingroup$ How do you know what it should feel like? Sticking your hand out the car window is a very different feel from feeling what is coming out a 5mm pipe. $\endgroup$ – Jon Custer Sep 25 at 12:33
  • $\begingroup$ Are your flow meters measuring standard liters/min or actual liters/min? Should you be using a pressure correction on the flow meters? Did you use the the diameter instead of the radius to calculate the exit pipe cross-sectional area (this would produce an error of 4x)? $\endgroup$ – David White Sep 25 at 23:24
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I think it is possible that you are neglecting compressible effects. Air that is flowing from a reservoir/compressor at 4 bar and exhausting through a pipe to atmospheric pressure will almost certainly be choked. I.e. it will be at sonic velocity (~330 m/s) at the outlet of the pipe. Appearances can be deceptive - it is very possible for an air flow to be sonic at the end of a small pipe, but not 'feel' like it is going anywhere near that fast, when you touch it with your bare hand.

However, in this case, you also state that you have a control valve. So, it is possible that the flow may be choking through the control valve and not at the end of the pipe. Is your flow meter located straight after the control valve?

In short, you may be over-simplifying the situation, because flow of air through valves/piping cannot always be assumed to be incompressible. If compressible effects are relevant, then you cannot rely on $Q=Av$, because the density is not constant. In that case, your calculations and CFD would both need to account for the effects of compressibility.

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  • $\begingroup$ Thank you for the information. It really helps meunderstand the problematic since I don't have much experiences in this area. If I fully open the control valve I can see that the pressure gauge on the control valve show only 1.5 bar but I suppose it is ok. The flow meter is connected directly after the control valve. Can the flow be still chocked if the control valve is fully opened? $\endgroup$ – Matej Petráš Sep 26 at 9:09
  • $\begingroup$ @MatejPetráš yes, it can. Imagine if you have a tank at 4 bar, simply exhausting to atmosphere through a pipe, with no valve in between. The pressure drop there is around 80%, so the flow will definitely be choked at the end of the pipe. $\endgroup$ – Time4Tea Sep 26 at 12:47

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