Thread Query

wow, I can't remember either, feature that. Searching...

this one

rapid air one

Parker one

ETB one

could it be one of these?

Thanks for that. Sounds like it wasn't you who posted the link on the earlier thread.

The first 3 links here give pressure drop when flow and pipe size, giving a selected, moderate velocity are known.

ETB one has a section

Air Discharge through Hose - Discharging air volume through hoses at pressures ranging 30 - 90 psi

which is more like what I'm after. Results of my calcs aren't vastly different, which is reassuring. I'd ideally like to be able to enter up and downstream pressures, length and dia and get flow, but this link is the best I've got so far!

Good. Let us know how well things match up, if you get good data?

Had a few more thoughts. Looking at the isothermal case, the equation is

which can be solved directly for mass flowrate m, when p₁ and p₂ known, the case I’m interested in. p₂ atmospheric pressure, 1 bara.

My book says the exit velocity from the pipe cannot exceed Mach No. 1/g = 0.845. I’m looking at a case where using calculated m, and pressure hence gas density at p₂, Mach No. comes to 1.2. The book doesn’t say what to do about that. I could recalculate (by trial) m to give Mach 0.845 at that density, but I think that underestimates the flow. In an orifice at supercritical pressure drop, there’s sonic velocity in the orifice, but the density is something like upstream density, for mass flow calculation. A bit more complicated than that, but it definitely isn’t downstream density or the mass flow would be independent of upstream pressure, whereas it varies directly as u/s pressure.

So I’ve assumed the effective downstream pressure is higher than p₂, and using Mathcad calculated the effective downstream pressure, density and mass flow (which are of course related) to give Mach 0.845.

Still looking at the adiabatic case, but the book says they don’t in practice differ all that much.

I don’t know what you think (if you have an opinion about it ).

I’d have liked to find the online calc I was referring to, for a check, but I’ve tried every keyword I can think of without success, beginning to wonder if I imagined it!

How about searching directly for subsonic and supersonic nozzles?

NASA mass flow choking

This one indicates maximum flow at Mach 1.

Compressible mass flow at a nozzle at Mach 1

And this for isentropic flow at converging or diverging nozzles (can be supersonic on divergent nozzle)

isentropic nozzle flow

Just found this! http://cr4.globalspec.com/thread/112378. So it was you!

It's the thread I thought I remembered, but a lot more recent than I guessed. Haven't had time yet to check if it gives me what I'm after, but it looks promising..

Minor point - should have said cannot ".........exceed Mach No. 1/√γ (gamma) = 0.845."

If I recall, this γ is the compressibility of the air....correct?

No, γ is Cp/Cv, specific heats at constant pressure and volume, 1.4 for air. There's expansion factor Y in orifice and nozzle calcs. Not easy to tell the difference in CR4!

Try orifice and nozzle in your search terms....

http://cr4.globalspec.com/thread/111884/Relationship-Between-Flow-Rate-Pressure-and-Nozzle-Size

Thanks, but orifice or nozzle flow isn't quite the same. That's reasonably straightforward when you've got your head round it. Discharge through a longish pipe is more complicated. I'm looking at a fluid mechanics book but it isn't easy to follow. I think I've got the isothermal case sussed but in practice it's more likely to approach adiabatic, as the velocity will be high.

Unless my memory's gone to pot, the link I referred to allowed you to put in up and downstream pressures and it gave the flow. I hoped to use it to check my calculations.

http://www.pipeflowcalculations.com/

Thanks for the link, unfortunately the calculator won't run on my old 32-bit computer. Also they wanted paying for it, which I wasn't planning to do as it's only for my interest. But it doesn't look expensive, so I might go for it when I upgrade the computer.

I found this link Flow of Gas from Tank through Pipe which might interest. They also charge for it (rather more than yours) and it doesn't give much confidence when it says the Reynolds No. is constant in adiabatic flow, as falling temperature decreases gas viscosity hence increases Reynolds No.

My next question: did you post on someone else's thread, or were you the OP?

I wasn't the OP. (I think even I would be able to trace it if I was !) I don't think I posted on the thread, as I couldn't add anything to others' comments, but I read it with interest.

Thanks anyway, I'll reply to other posts shortly.