CFD Online Discussion Forums

CFD Online Discussion Forums (
-   CD-adapco (
-   -   Emptying tank & Multiphase (

Ruggero August 15, 2007 13:00

Emptying tank & Multiphase
I have to simulate an emptying tank with a multiphase liquid-gas inside, in a transient simulation. I'm using a Eulerian Multiphase treatment. My problem is:

1-How to insert a fixed mass flow outlet (eg: exit of 0,1 kg/s)? When i tried an error occurs saying that there is no pressure gradient that allows that mass outflow. But i cannot insert a pressure condition!

2-Isn't possible at all to use a ideal model for the gas phase? It let me use only isobaric model for density.

I have some others questions, but for the moment I think it's enought. Thanks to allo of you for answering.

groundhog August 16, 2007 04:54

Re: Emptying tank & Multiphase

Why not try a negative facing inlet. That can be used to suck fluid out of the domain.

Please check the manual for details of the density options.

Hope this helps.

Ruggero August 16, 2007 06:24

Re: Emptying tank & Multiphase
Ok. I'll try and then i'll be back with the results. I hope.

Ruggero August 16, 2007 06:54

Re: Emptying tank & Multiphase
Here i am. Unluckily it doesn't work. I have back an error:

## error 064: fixed velocity boundary condition outflow exceeds inflow by 5 per cent. Continuity correction not attempted.

What shall i do? The fact is that i need to know the emptying pressure with a fixed mass outflow.

groundhog August 16, 2007 07:07

Re: Emptying tank & Multiphase
I guess you need to think about the boundaries.

You have, I guess, an inflow inlet, and now an outflow inlet. If the density is constant, then the system is overconstrained.

Ruggero August 16, 2007 07:11

Re: Emptying tank & Multiphase
No! I choose an isobaric model for density. And i have no inlet, since the tank is full at the beginning of simulation end shoud empy little by little.

Harry August 16, 2007 13:57

Re: Emptying tank & Multiphase
What other boundaries do you have in the model?

allan August 16, 2007 14:36

Re: Emptying tank & Multiphase can you empty the tank with only one outlet. In reality if you had a full tank with one outlet and it was opened, it would not empty. It needs a breather. If you put a put a pressure boundary on a couple of cells at the top of your tank and set it to zero, you should get outflow. You probably need gravity here as well...or have I mis-interpreted what your trying to do!



Harry August 16, 2007 15:01

Re: Emptying tank & Multiphase
that was what I was wondering when i asked about boundaries and surely vof would be a better choice than e2p

Ruggero August 17, 2007 02:45

Re: Emptying tank & Multiphase
I've got the wall boundaries and the symmetry boundaries and, of course, the outlet. No other kind of boundaries.

Harry August 17, 2007 04:28

Re: Emptying tank & Multiphase
I think you need to think about the physics and not just the CFD, if there is no inlet (or method of fluid ingress) how is it ever going to empty? Get an old beer bottle fill it to the top with water, wrap your lips around it and try and suck out the water, it's never going to happen.

Ruggero August 17, 2007 04:40

Re: Emptying tank & Multiphase
Yeah, but the tank should have a pressure bigger than outside. The problem should be an incompressible gas-phase, but i used an isobarico model for density, so there should not be.

What i have to simulate is how much I can empty such a tank, in which, while emptying, the pressure inside get lower and so the fluid evaporate (so tending to go against the pressure falls). I could use a pressure boundary, but the mass outflow is limited and i cannot impose this.

Ruggero August 17, 2007 04:43

Re: Emptying tank & Multiphase
As I explained in another post, the difference of pressure between inside and outside the tank should let the fluid to exit. But the problem is how to limitate the max mass outflow.

allan August 17, 2007 05:15

Re: Emptying tank & Multiphase
I don't really understand why you're wanting to limit the max mass outflow. The flow area or pressure drop will determine this.


Ruggero August 17, 2007 05:22

Re: Emptying tank & Multiphase
The idea is that this tank should feed an engine, so it should guaranted a determined massflow (no more, no less) for 5 seconds. We are assigned to study the tank with an autopressurization sistem.

allan August 17, 2007 05:30

Re: Emptying tank & Multiphase
in that case then, I would use a suction inlet as and outlet suggested previously, but you will still need a pressure inlet to allow air into the tank to let it drain


Ruggero August 17, 2007 05:36

Re: Emptying tank & Multiphase
ok! But what pressure use? In my simulation pressure should be found not imposed.

Ralphie August 17, 2007 05:43

Re: Emptying tank & Multiphase
I think the problem is similar to an emptying LPG bottle or a lighter. You have a pressurised liquid-gas-mixture inside your tank. When the valve is opened gas is released, the pressure in the tank drops and more liquid fuel evaporates, i.e. the pressure is buffered.

AndyR October 24, 2007 12:23

Re: Emptying tank & Multiphase

Work backward from the engine. The engine wants X gpm, but it also needs Y psi to flow that amount from your supply through the plumbing, carbeurators, injectors etc etc. In otherwords a requirement of just X gpm is incomplete, it needs to be X gpm at Ypsi.

If you cant get that data, make an estimate.

You know have a known fixed pressure that you can apply to the "outlet" boundary of your problem. Then you can adjust the tank pressure iteratively to get the required mass flow rate at the required supply pressure.

The code should run just fine that way. Andy R

All times are GMT -4. The time now is 22:49.