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How to fix difference in mass flow?

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Old   May 23, 2017, 14:07
Smile How to fix difference in mass flow?
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Hello All,

I have a simulation where I have hot helium flowing into air through a 2 mm diameter nozzle. I have run reports to compare the mass flow of the inlet (which is set as a velocity inlet) and all the boundaries that are just air (each is set as a pressure outlet). The mass flow of the inlet was -8.8e-7 kg/s while the total mass flow for the pressure outlets was 8.2e-7 kg/s. The models I am using include segregated flow, temperature, and species; ideal gases; steady state; and turbulent. The velocity of the helium is around 42 m/s coming through the nozzle. In addition, I believe something must be wrong due to the fact that the velocity component of the gas that is parallel with the nozzle decreases as it moves toward the pressure outlet; however, the flow of helium does not get much broader over the length of the simulation. I would assume that that component of the velocity would fall to a constant value fairly quickly. Does anyone have some suggestions as to what I might change to fix these errors? It may be that different models would be better. I just don't have the experience to know which models might fit the situation more ideally.
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Old   May 24, 2017, 20:09
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Jets have many similarity solutions, I would recommend you look up one and compare it with your case.

Without seeing your geometry and mesh this is difficult to give you much help with.
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Old   May 25, 2017, 16:47
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I am attaching pictures of the geometry and the mesh. It is axisymmetric with the bottom face being the axis. The left hand side is where the helium velocity inlet it. The boundaries in the narrow region are all walls. The boundaries on the right hand side and top are pressure outlets. The boundary in the top left is a pressure outlet for the first half and then a wall for the rest of the way down.

I was able to get the mass flow fixed by using a laminar steady model with segregated fluid enthalpy. However, the velocity still decreases along the axis more than I think it should due to the lack of radial spreading. I have also attached images of the mole fraction of helium and the axial velocity. Any suggestions would be appreciated.
Geometry.PNG

Mesh.jpg

Helium.PNG

Velocity.PNG
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Old   May 25, 2017, 22:06
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Why does your mesh have so much refinement near the walls and then in the important jet wake you let it coarsen a massive amount? This is probably the issue.

Your flow is moving pretty much in a unified direction. A tet/triangular mesh is not the best choice for this. This geometry is simple and should be easily meshed with a structured hex mesh.
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Old   May 29, 2017, 09:39
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Quote:
Originally Posted by wadeellis View Post
I was able to get the mass flow fixed by using a laminar steady model with segregated fluid enthalpy. However, the velocity still decreases along the axis more than I think it should due to the lack of radial spreading. I have also attached images of the mole fraction of helium and the axial velocity. Any suggestions would be appreciated.
I would use as suggested a different mesh option (I did some jet studies with polyhedral meshes)
One more thing, why is your domain "so small"? the outlet in the axis of the jet should be further away in my opinion
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Old   May 30, 2017, 12:15
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This model is a first step in a different simulation. I export the flow data I get from Star-CCM to another program so that I can model the chemistry of a plasma discharge. The are I am modeling with Star-CCM is the area I am interested in (ie, where the interesting chemistry happens). I suppose I could model a larger area in Star-CCM and then only use the area I want to use in the other part. Would extending the length of the simulation make a significant difference in the results I get from Star-CCM?
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Old   May 30, 2017, 12:18
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I would say how fast you coarsen your cells into the domain is the bigger problem.

Again, I would not use a tet mesh for this. This geometry begs for a hex mesh.
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