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April 8, 2015, 14:41 |
Mass/continuity sink question
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#1 |
New Member
Leonardo
Join Date: Nov 2014
Posts: 27
Rep Power: 11 |
Hello there,
I'm currently modelling a fuel cell where I have a solution (water+ethanol) that, once in contact with a certain surface (catalyst), reacts. Both water and ethanol are consumed, so I'm working on the sink/source terms as of now. Water is the constraint in my fluid, so my question is: how can I set up a sink for the consumption of water at the catalyst surface? I've seen people suggesting adding a bogus component with a sufficiently low mass fraction so that it doesn't affect the composition, and set it as constraint. Is there any better solution? Also, if I set up a continuity sink, is it a sink for the whole fluid in the case of a multicomponent fluid such as mine? Thanks in advance. |
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April 8, 2015, 18:08 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,700
Rep Power: 143 |
I don't know this off the top of my head and don't have time to test anything, but my suggestions are:
* I have no idea what you mean about a bogus mass fraction component and set it as a constraint. In my experience bogus work-arounds create more trouble than they are worth. Model what is happening as it is intended, so ethanol with water mixture as a constraint mass fraction. * Have you read the documentation about source terms? There are a lot of options for multicomponent mixtures. You may find one of the options is what you are looking for. |
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April 10, 2015, 09:30 |
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#3 |
New Member
Leonardo
Join Date: Nov 2014
Posts: 27
Rep Power: 11 |
The idea behind adding the third 'bogus' component is that I can't seem to find a way to add a sink for the water, if water is set as a constraint for the mixture. I need two separate sinks for both water and ethanol in my model.
Yes, I've read the documentation before posting this, and my answer doesn't seem to be there. The closest thing I found that could lead to something are the 'continuity sinks', which I couldn't figure out how exactly they work. |
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April 12, 2015, 07:58 |
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#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,700
Rep Power: 143 |
You should look closer into the documentation. I think you will find what you want is already implemented.
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April 13, 2015, 14:29 |
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#5 |
Senior Member
Join Date: Jun 2009
Posts: 1,803
Rep Power: 32 |
Not sure I understand your concerns. Because mass is conserved, your sinks must also satisfy continuity, i.e.
rate of change of mass in the volume = mass in - mass out where mass in = mass through inlet, opening (mdot > 0) and sources of species/components mass out = mass through outlet/opening (mdot <0) and sinks of species/components. Do you know the fluid mass flux for the sink ? If you are trying to add both sinks, I think you do since the fluid mass flux for the sink is the summation of the sinks. The software should take care of the rest. Recall for a sink, you cannot specify the composition, but the fluid mass flux. For a source, the composition is defined by the user. |
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April 13, 2015, 14:39 |
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#6 | |
New Member
Leonardo
Join Date: Nov 2014
Posts: 27
Rep Power: 11 |
Quote:
My question is: since water is the constraint in my two-component fluid, I cannot set a sink specifically for water, just the way I can for ethanol. My options for sinks/sources, from the CFX-Pre interface are (as shown in image attached): Ethanol.mf and continuity. So how do I properly set a sink for water, since water is also consumed at the catalyst surface? From what you told me, would it work if I set things up this way? Ethanol.mf sink: (ethanol mass flux) Continuity sink: (ethanol mass flux+water mass flux) |
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February 11, 2021, 13:26 |
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#7 |
New Member
Join Date: Feb 2016
Posts: 20
Rep Power: 10 |
For those of you who are facing the same problem, here is the solution:
Suppose you have two fluids 1 and 2 in a homogenous Variable Composition Mixture. You want to add a mass flux of fluid 1 to the mixture and at the same time extract a mass flux of fluid 2 from the mixture: Provided, that fluid 2 is the constraint component:
Comparing eqs. (1) and (2) leads to: Please note, that is smaller than zero, since for an overall mass sink, the mass source of fluid 1 is smaller than the mass sink of fluid 2. However, this is no problem for CFX. Then you have to set the Continuity Source as follows: Option: Fluid Mass FluxFlux: MCF/Energy Sink Option: Spec. Mass Frac and Loc. Temp Fluid 1.mf: Attention: No further mass fraction source (Fluid 1.mf) is needed! Regards, DaveD! |
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