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Mass flow sink (source point) --Absolute pressure keeps dropping

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Old   November 18, 2013, 15:35
Default Mass flow sink (source point) --Absolute pressure keeps dropping
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Hi, there,

I am using source point to simulate the mass flow sink on the wall.
I set the option of the continuity as "Total Fluid Mass Source", the "Total Source" set to a negative value.

The problem is that the absolute pressure at the source point keep dropping to non-physical value, which causes the CFD unstable, and can't converge.

Anyone knows why? And what should I change the source point setup?

I have used "pressure coefficient". Since it is mass flow sink, I have a positive number there.

If I turn on the heat transfer, what else should I pay attention to?

Thanks in advance.

Nan
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Old   November 18, 2013, 17:47
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The answer is pretty obvious - a mass sink point is not physically possible (this side of the nearest black hole anyway), so of course it will lead to non-physical results.

I would replace the point with a source volume and then you can spread the mass sink out over a larger volume and reduce the pressure spike and hopefully get convergence.
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Old   November 18, 2013, 23:59
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Glenn,

Thanks for the reply. I did quick search, can't find the way to create volume source. If you can give me some instructions or point me to some references. I really appreciate it. I am using cfx 14.0 now.

Thanks again.

Nan
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Old   November 19, 2013, 00:21
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It is very simple - you just make a mesh volume and assign a volume source to it. An example is the Heat transfer in a heating coil tutorial. (Actually I see in V14.5 that the heating coil example has been changed to use the electric potential model rather than a heat source)
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Old   February 27, 2017, 07:53
Default Volume sourse term
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Quote:
Originally Posted by ghorrocks View Post
It is very simple - you just make a mesh volume and assign a volume source to it. An example is the Heat transfer in a heating coil tutorial. (Actually I see in V14.5 that the heating coil example has been changed to use the electric potential model rather than a heat source)
Hello, I am very new to the world of CFD amd faced a similar problem with the mass sink term. I looked up what you suggested Ghorrocks , but i cant seem to find any option to add volume source term. I only find mass sources under continuity tab .
Using CFX 14.5 to model the drainage of water droplets inside a coalescence filter. I am trying to get rid of the dispersed water phase from the bottom (due to gravity) without affecting the air. So kind of like degassing but instead take dispersed liquid out the the system. Tried to do this using both subdomains and boundary sink term (mass source under continuity tab), but the simulation is crashing.
Any help is much appriciated.

Last edited by Jayotpaul; February 27, 2017 at 09:19.
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Old   February 27, 2017, 18:34
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A clarification to my very old post: You apply a heat source to a volume. The phrase "volume source" is ambiguous. Source terms are added to the fundamental equations, and the fundamental equations are momentum, mass, heat, volume/mass fractions and turbulence. There is no "volume" fundamental equation.

So yes, you will find mass sources under the continuity option in subdomains as you say, but you apply it to a volume region.

But reading your description of what you want to do it sounds like you want a volume fraction source term, not a mass source term. You do this under the "fluid sources" term, where you can specify a mass source which applies to one phase only.
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Old   April 13, 2017, 05:19
Default DPM ; Sink Term and Mass Transfer
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Quote:
Originally Posted by ghorrocks View Post
A clarification to my very old post: You apply a heat source to a volume. The phrase "volume source" is ambiguous. Source terms are added to the fundamental equations, and the fundamental equations are momentum, mass, heat, volume/mass fractions and turbulence. There is no "volume" fundamental equation.

So yes, you will find mass sources under the continuity option in subdomains as you say, but you apply it to a volume region.

But reading your description of what you want to do it sounds like you want a volume fraction source term, not a mass source term. You do this under the "fluid sources" term, where you can specify a mass source which applies to one phase only.
Sir,

I need you guidance + utmost help .

I am simulating a venturi sccrubber for cleaning of SO2 from air. For this i have have taken SO2+ air mixture in eularian and water droplets as discrete phase using DPM.
I need to know that
1) Is it possible to transfer So2 from continous phase to discrete phase?
2)Can we use the sink term for SO2?
3)If So2 absorbs into water droplet then how i would know that it has been absorbed ,Is there some kinda of udf to be written also for the droplet of what?

Kindly guide and help.
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Old   April 13, 2017, 19:09
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This all sounds possible. But getting all working properly is not straight forward.

A side question - this model may be easier if you use eularian particles for the water droplets rather than Lagrangian. Then all your multiphase models are Eularian and that may be simpler.
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Old   April 14, 2017, 08:17
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Originally Posted by ghorrocks View Post
This all sounds possible. But getting all working properly is not straight forward.

A side question - this model may be easier if you use eularian particles for the water droplets rather than Lagrangian. Then all your multiphase models are Eularian and that may be simpler.
Sir thank you for reply.

You are suggesting the use of Eularian model with 2 phases and water as secondary phase. If I have to achieve the inter phase mass transfer , i need to turn on the interfacial area concentration.

This also suggests the breakage and coalescence along with the minimum and maximum diameter.

Furthermore I don,t want to see the nucleation so will it be a good choice to leave the nucleation rate option under the secondary phase tab as none.

Regards;

Atta
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