# reverse flow

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 October 10, 2001, 12:50 reverse flow #1 Giovanni Guest   Posts: n/a 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

 October 11, 2001, 03:52 Re: reverse flow #2 hvn Guest   Posts: n/a Perhaps, put away your outlet with an extrusion.

 October 11, 2001, 14:07 Re: reverse flow #3 Chetan Kadakia Guest   Posts: n/a I'm sorry, I don't understand what is meant by "put away your outlet with an extrusion". Could you clarify that Giovani?

 October 11, 2001, 15:12 Re: reverse flow #4 Steven Smith Guest   Posts: n/a 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?

 October 12, 2001, 03:45 Re: reverse flow #5 hvn Guest   Posts: n/a 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.

 October 12, 2001, 04:01 Re: reverse flow #6 Giovanni Guest   Posts: n/a Yes Steven, this is my doubt. Infact the reverse flow is at the centre of the pressure outlet surface. Thank you Giovanni

 October 12, 2001, 04:05 Re: reverse flow #7 Giovanni Guest   Posts: n/a 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

 October 12, 2001, 04:37 Re: reverse flow #8 hvn Guest   Posts: n/a Sorry, I would say outlet pipe.

 October 12, 2001, 08:18 Re: reverse flow #9 Alain Guest   Posts: n/a 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

 October 12, 2001, 09:40 Re: reverse flow #10 Giovanni Guest   Posts: n/a 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

 October 12, 2001, 15:56 Re: reverse flow #11 stephen Guest   Posts: n/a 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.

 October 15, 2001, 04:01 Re: reverse flow #12 Giovanni Guest   Posts: n/a Ok, thank you Stephen, I'll try it. Giovanni

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