Modeling gas expansion in vacuum

cryonole · August 19, 2014, 11:46

I am new to FLUENT and want to know if gas expansion in a vacuum tube can be modeled using it. The geometry is similar to a shock-tube but with a state of vacuum in the 'driven' section and finite pressure in the 'driver' section. I want to determine the transient pressure/density/velocity field in the tube when the diaphragm separating the finite pressure and vacuum sides is suddenly ruptured.

RodriguezFatz · August 21, 2014, 04:41

Did you think about whether CFD is capable of doing this at all? I mean, if you have a (nearly-) vacuum on one side the Knudsen number will be pretty high there. Maybe CFD equations aren't valid there...

pakk · August 21, 2014, 10:19

I think you can model it, and the results will be reasonably reliable.
RodriguezFatz is correct, if it is really vacuum on one end, then the continuum hypothesis fails there, and your equations will be incorrect. But because the majority of the mass will be in the region where the continuum hypothesis is valid, your results will generally be OK-ish. How good it is, depends on what you want to get out of your model. If you want to know the shockwaves that go back into your high-density part, you are safe, I think. If you want to see exactly what happens on the front of the shockwave into the vacuum part, you will not get a good answer. The order of magnitude of the shockwave velocity will be correct, but can be off by 50%.

Some parts of your transient pressure/density/velocity field will be accurate, some will not. In general, the parts in the vacuum will be less accurate.

If you want to simulate this in Fluent, make sure to use the density-based solver.

August 19, 2014, 11:46	Modeling gas expansion in vacuum	#1
cryonole New Member rcd Join Date: Aug 2014 Posts: 1 Rep Power: 0	I am new to FLUENT and want to know if gas expansion in a vacuum tube can be modeled using it. The geometry is similar to a shock-tube but with a state of vacuum in the 'driven' section and finite pressure in the 'driver' section. I want to determine the transient pressure/density/velocity field in the tube when the diaphragm separating the finite pressure and vacuum sides is suddenly ruptured.

August 21, 2014, 04:41		#2
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26	Did you think about whether CFD is capable of doing this at all? I mean, if you have a (nearly-) vacuum on one side the Knudsen number will be pretty high there. Maybe CFD equations aren't valid there... __________________ The skeleton ran out of shampoo in the shower.

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August 21, 2014, 10:19		#3
pakk Senior Member Join Date: Nov 2013 Posts: 1,965 Rep Power: 26	I think you can model it, and the results will be reasonably reliable. RodriguezFatz is correct, if it is really vacuum on one end, then the continuum hypothesis fails there, and your equations will be incorrect. But because the majority of the mass will be in the region where the continuum hypothesis is valid, your results will generally be OK-ish. How good it is, depends on what you want to get out of your model. If you want to know the shockwaves that go back into your high-density part, you are safe, I think. If you want to see exactly what happens on the front of the shockwave into the vacuum part, you will not get a good answer. The order of magnitude of the shockwave velocity will be correct, but can be off by 50%. Some parts of your transient pressure/density/velocity field will be accurate, some will not. In general, the parts in the vacuum will be less accurate. If you want to simulate this in Fluent, make sure to use the density-based solver.