Air discharge from a tank using nozzle.

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Av.E
Posts: 2
Joined: Sat Dec 22, 2012 12:56 am

Air discharge from a tank using nozzle.

Post by Av.E »

Hey all,

I really need help with this one:

I have a tank in volume V which contains air, the pressure in the tank is X Kpa. from the tank there's a pipe, not sure what diameter yet, lets call it D.
the pipe goes into a converging-diverging nozzle, and this nozzle is located inside a bigger pipe, so the air eventually flows in the bigger pipe. I assume the pressure there in the beginning is 1atm.

I need to calculate the discharge (flow rate) of the air from the tank, from the beginning untill the tank emptys from air.

I tried the calculators but I wasnt sure if I did it right and got a correct answer. I would be glad to know the equations I can use to solve my problem.
Honestly, I would spend much more time before I'd bother you with this, and would look much better for an answer in the calculators or the rest of the web, but unfortunately it is pretty urgent and I dont have the time to do it. I would really appreciate any help, especially if it comes fast ,since its urgent-as I said before.

thanks a lot, and have a pleasant weekend.
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Air discharge from a tank using nozzle.

Post by admin »

Calculators are not good to be used for this purpose because you have unsteady flow from tank to atmosphere.
I heve tried to explain how it can be done but it is not fully solved:
viewtopic.php?f=3&t=10&p=23&hilit=empty#p23

I am not sure what do you really need to calculate? If it is only flow rate you can use discharge flow rate calculator and calculate flow rate for different pressure in the tank. As pressure in the tank goes down, also will the flow rate and you can calculate flow rate for different values of pressure difference, until pressure is equaled in tank and in the pipe after.

Also, if you have converging-diverging nozzle than it looks like the system for flow velocities higher than speed of sound. Is this true?
Pipe flow calculations - since 2000
Av.E
Posts: 2
Joined: Sat Dec 22, 2012 12:56 am

Re: Air discharge from a tank using nozzle.

Post by Av.E »

Actually I'm trying to calculate how long it would take to empty this presurrized air tank through a pipe with con-div nozzle in the end of it, and yes I expect a fast velocity out of the nozzle, but I dont think it'll be higher than the speed of sound. If i'll have the discharge I can divide the volume by the discharge and get the time it takes.

I can calculate the discharge for every pressure level in the tank- lets say once for 3.5 atm, second for 3.4 atm, then 3.3 atm, and all the way down to 1 atm.
but what will I do with all the discharges I'll get? And does it mattar that I have a nozzle in the end of the pipe? If I'd use the calculator, what should I write as a diameter?

Thank you very much.

P.S- if you could offer a way just to find the average discharge so I can find the average discharge and the average time to empty the tank- that would be great.
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Air discharge from a tank using nozzle.

Post by admin »

In the link above I tried to set the steps and equations required for time calculation to empty tank. It is for liquid but it can be adopted to be for air as well. Instead of liquid level in tank pressure is in tank is used. As flow drops during time, flow rate change is not linear I can not tell the average flow. If air exits reservoir without conv-div nozzle maximum flow rate is choked flow rate.
You can see some theory here:
http://www.pipeflowcalculations.com/pip ... -and-pipes
If you have De Laval nozzle on the outlet you will have speeds and flow rate higher than in choked flow. It is possible if sonic (critical) conditions are met in nozzle throat.
Pipe flow calculations - since 2000
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