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Giovanni October 10, 2001 11:50

reverse flow
 
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

hvn October 11, 2001 02:52

Re: reverse flow
 
Perhaps, put away your outlet with an extrusion.

Chetan Kadakia October 11, 2001 13:07

Re: reverse flow
 
I'm sorry, I don't understand what is meant by "put away your outlet with an extrusion". Could you clarify that Giovani?


Steven Smith October 11, 2001 14:12

Re: reverse flow
 
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?

hvn October 12, 2001 02:45

Re: reverse flow
 
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.

Giovanni October 12, 2001 03:01

Re: reverse flow
 
Yes Steven,

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

Thank you Giovanni

Giovanni October 12, 2001 03:05

Re: reverse flow
 
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

hvn October 12, 2001 03:37

Re: reverse flow
 
Sorry, I would say outlet pipe.

Alain October 12, 2001 07:18

Re: reverse flow
 
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


Giovanni October 12, 2001 08:40

Re: reverse flow
 
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


stephen October 12, 2001 14:56

Re: reverse flow
 
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.

Giovanni October 15, 2001 03:01

Re: reverse flow
 
Ok, thank you Stephen, I'll try it.

Giovanni


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