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Old   October 10, 2001, 11:50
Default reverse flow
  #1
Giovanni
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Hello,

I'm analyzing a particular blower. The fluid is air and it is ideal-gas. The inflow section and the outflow section have the same size. The fluid flow presents a swirl at outflow due to the particular blower. Velocity near the delivery is greater than velocity at the in-flow. Boundary conditions are: mass-flow-inlet and pressure outlet.

I've already elongated the delivery piping, but there is a reverse flow the same at the centre of outflow surface.

What do I have to do?

Thank you Giovanni
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Old   October 11, 2001, 02:52
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  #2
hvn
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Perhaps, put away your outlet with an extrusion.
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Old   October 11, 2001, 13:07
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Chetan Kadakia
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I'm sorry, I don't understand what is meant by "put away your outlet with an extrusion". Could you clarify that Giovani?

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Old   October 11, 2001, 14:12
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Steven Smith
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I'm not familiar with blowers, so my question might be off-base. Have you considered the possibility that given your swirling outflow, that a reversed flow through your pressure outlet might be real?
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Old   October 12, 2001, 02:45
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  #5
hvn
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I mean that maybe the outlet is to close to the particle blower. So you must increase your calculation domain. For that, you need to increase the length of your inlet pipe. One other solution is to increase the mesh density at the outlet because the mesh is too coarse at this location.
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Old   October 12, 2001, 03:01
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  #6
Giovanni
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Yes Steven,

this is my doubt. Infact the reverse flow is at the centre of the pressure outlet surface.

Thank you Giovanni
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Old   October 12, 2001, 03:05
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  #7
Giovanni
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Why if the outlet is to close to the particle blower I have to increase the length of inlet pipe? What are the changes?

Thank you Giovanni
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Old   October 12, 2001, 03:37
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  #8
hvn
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Sorry, I would say outlet pipe.
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Old   October 12, 2001, 07:18
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  #9
Alain
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As a matter of fact reverse flow at outlet are often seen in blower simulation.

You are perfectly right when you doubt of the physical or numerical origin for this feature (I saw both).

First of all, about the physical cause, it depend of the kind of blower you have. if it is an axial one : if your rotation speed is very high you can have a very strong pressure gradient due to swirl which cause a "vortex breakdown" and then a reverse flow.

among the possible numerical causes of this feature you have :

Too coarse mesh (flow in a blower generally need a fine mesh with a good resolution at wall)

Ill posed boundary condition

An outlet too close to the fan wheel.

etc...

For example, you can try Total pressure inlet/static pressure outlet instead of massflow (beside, I think that the massflow inlet in fluent is suitable for compressible gaz only and can give convergence trouble).

You can also try to refine your mesh.

Best regards

alain

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Old   October 12, 2001, 08:40
Default Re: reverse flow
  #10
Giovanni
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In my particular case, the flow turns around a core before to go out of the blower and this causes the swirl.

The compression ratio is 1.21.

May you explain "vortex breakdown"?

Thanks a lot Giovanni

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Old   October 12, 2001, 14:56
Default Re: reverse flow
  #11
stephen
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In order to know if the real flow is also reverse flow,maybe you'd better calcuate the off-design points,i.e.different compression ratio and mass flow at the same rotating speed. For lower pressure ratio and larger mass flow, the flow will be less possible reverse and perhaps reverse flow will disappear.
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Old   October 15, 2001, 03:01
Default Re: reverse flow
  #12
Giovanni
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Ok, thank you Stephen, I'll try it.

Giovanni
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