insulated container divided into equal volumes by a partition

Alain_aftch · April 5, 2011, 11:20

Hi,

my goal is to model the following problem:
A gas is confined to one side of a rigid, insulated container divided into equal volumes by a partition. The other side is initially evacuated. The partition is removed, and the gas expands to fill the entire container. Assuming ideal gas behavior, determine the final equilibrium pressure.

I tried two ways to do this: first using two domains of fluid with an fluid-fluid interface, then using cell expression language to do a global initialization for a single fluid domain. But my results until now have been unsuccessful.

Anybody knows how to simulate this kind of problem?

stumpy · April 5, 2011, 16:27

I guess the first question is why not just use the back of an envelope rather than a CFD solver! I assume you have your reasons?
Did you run this as steady-state? In steady-state mass is not going to get conversed during convergence, so I think you would need to run this as a transient case.
Unfortunately that would be difficult to converge due to the high mach numbers in the evacuated space. Perhaps there's an expert parameter for the steady state case - anybody?

ghorrocks · April 5, 2011, 18:51

There are no expert parameters which will help. Stumpy's comment is correct - sounds like a first year uni thermodynamics question which can be solved in a few minutes with the ideal gas laws.... So why bother with CFD?

It will be a tricky CFD simulation with all sorts of nasty initial transients.

stumpy · April 6, 2011, 08:44

Not sure if this would work, but if you set it up as a multiphase case with initial conditions set so the volume fraction of gas is 1, then you may be able to run steady-state. For steady-state multiphase cases in a closed system a global correction is applied to converse the mass of each phase.

Alain_aftch · April 10, 2011, 02:35

Quote:

Originally Posted by stumpy

Not sure if this would work, but if you set it up as a multiphase case with initial conditions set so the volume fraction of gas is 1, then you may be able to run steady-state. For steady-state multiphase cases in a closed system a global correction is applied to converse the mass of each phase.

the main idea is to simulate a burst disc

Alain_aftch · April 10, 2011, 02:40

Quote:

Originally Posted by Alain_aftch

the main idea is to simulate a burst disc

e.g.:
http://www.drthfrank.de/publications...ensation_b.pdf

ghorrocks · April 10, 2011, 06:49

What are you trying to get out of the analysis? Your first post only mentioned the final pressure, but now you are talking about a burst disc.

Alain_aftch · April 10, 2011, 15:51

Quote:

Originally Posted by ghorrocks

What are you trying to get out of the analysis? Your first post only mentioned the final pressure, but now you are talking about a burst disc.

i'm trying to get the simulation of a pressure vessel failure and the release/dispersion of fluid into a low pressure chamber. Imagine that the fail is due to a burst disc. For this reason, I started this work with the first problem, which I know the theoretical answers.

ghorrocks · April 10, 2011, 22:21

I assume you mean the final steady state mixed result for the "first problem".

So is your question how can you make the opening suddenly appear?

Alain_aftch · April 10, 2011, 23:23

Quote:

Originally Posted by ghorrocks

I assume you mean the final steady state mixed result for the "first problem".

So is your question how can you make the opening suddenly appear?

that´s the stuff

ghorrocks · April 11, 2011, 06:28

Lots of ways.

1) Define an initial condition with different conditions on each side. This is the easiest way.
2) Do a simulation to define the initial conditions where they are not connected (maybe a wall in the gap). Then use this as an initial condition on a simulation with the gap connected.
3) Put a source term, probably a momentum source, in the gap which initially stops flow (or at least greatly slows it down) and then dissappears.
4) Use a GGI and a sliding interface to slide it open.
5) Use moving mesh to open a connection up.

I could probably think of more if I could be bothered. But option 1 is simplest so go with that if it is suitable.

April 5, 2011, 11:20	insulated container divided into equal volumes by a partition	#1
Alain_aftch New Member Alain Join Date: Apr 2011 Posts: 5 Rep Power: 16	Hi, my goal is to model the following problem: A gas is confined to one side of a rigid, insulated container divided into equal volumes by a partition. The other side is initially evacuated. The partition is removed, and the gas expands to fill the entire container. Assuming ideal gas behavior, determine the final equilibrium pressure. I tried two ways to do this: first using two domains of fluid with an fluid-fluid interface, then using cell expression language to do a global initialization for a single fluid domain. But my results until now have been unsuccessful. Anybody knows how to simulate this kind of problem? Last edited by Alain_aftch; April 10, 2011 at 23:32.

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April 5, 2011, 16:27		#2
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 22	I guess the first question is why not just use the back of an envelope rather than a CFD solver! I assume you have your reasons? Did you run this as steady-state? In steady-state mass is not going to get conversed during convergence, so I think you would need to run this as a transient case. Unfortunately that would be difficult to converge due to the high mach numbers in the evacuated space. Perhaps there's an expert parameter for the steady state case - anybody?

April 5, 2011, 18:51		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,934 Rep Power: 145	There are no expert parameters which will help. Stumpy's comment is correct - sounds like a first year uni thermodynamics question which can be solved in a few minutes with the ideal gas laws.... So why bother with CFD? It will be a tricky CFD simulation with all sorts of nasty initial transients.

April 6, 2011, 08:44		#4
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 22	Not sure if this would work, but if you set it up as a multiphase case with initial conditions set so the volume fraction of gas is 1, then you may be able to run steady-state. For steady-state multiphase cases in a closed system a global correction is applied to converse the mass of each phase.

April 10, 2011, 06:49		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,934 Rep Power: 145	What are you trying to get out of the analysis? Your first post only mentioned the final pressure, but now you are talking about a burst disc.

April 10, 2011, 22:21		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,934 Rep Power: 145	I assume you mean the final steady state mixed result for the "first problem". So is your question how can you make the opening suddenly appear?

April 11, 2011, 06:28		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,934 Rep Power: 145	Lots of ways. 1) Define an initial condition with different conditions on each side. This is the easiest way. 2) Do a simulation to define the initial conditions where they are not connected (maybe a wall in the gap). Then use this as an initial condition on a simulation with the gap connected. 3) Put a source term, probably a momentum source, in the gap which initially stops flow (or at least greatly slows it down) and then dissappears. 4) Use a GGI and a sliding interface to slide it open. 5) Use moving mesh to open a connection up. I could probably think of more if I could be bothered. But option 1 is simplest so go with that if it is suitable.