swirling vanes

[siretb]

We are trying to design a set of swirling vanes that would force a swirling motion of air in a duct of 3.1 m in diameter.
I would like to know how to calculate the optimum number of vanes for this task.
The whole idea is to have something that would behave like a uniflow cyclone, for the removal of very large droplets of liquid (more than 100 microns)

 

[RayTrowsdale]

I am also interested in the answer to this question. I would also be interested in how to design the vanes themselves as well as the number. My application is a much smaller scale - more like 6 inch diameter.

 

[MikeHalloran]

A cyclone typically comprises a tangential entry to the large end of a cone, a small axial flow out of the small end, and a large flow axially out of a cylinder concentric with the cone. It removes particles by accelerated settling toward the cone surface as the flow's rotation rate increases and the radius decreases while the fluid is traveling down toward the cone tip.

Somewhere I found equations relating the proportions of a cyclone to flow rates and particle size, but I can't recall them right now. The point is, the math exists.

The flow exiting the apparatus through the cylinder is highly rotational. I have appended reverse cyclones in several configurations to the exit cylinder of a liquid cyclone, but they were not able to recover a substantial fraction of that rotational energy. I.e., from outside, a cyclone acts pretty much like a centrifugal pump in reverse, except you don't get any shaft power out.

I have since been exposed to swirl vanes in gas flows in tubes around 6" diameter and somewhat larger. I remain unimpressed by their efficacy. Even relatively deeply curled vanes (like a STOL aircraft wing with full flaps) really don't deflect the flow enough to call it 'rotational'. At best, each vane acts to redirect some of the gas passing near it in a direction that is mildly skewed to the tube axis. The flow impinges on the downstream tube wall at a low angle of incidence, and _that_ causes the overall flow to swirl, slowly.

I'm guesstimating that you'd need multiple rows of vanes, each nearly filling the cross section of the tube, in order to get enough rotational speed to affect accelerated settling.

There might be a way to make a uniflow cyclone as you wish. You might even find it.

Before you get real pumped up about the idea, I suggest you read up on erosion of fan blades. It should be covered fairly well in the literature for helicopters, where the air flow is more likely to carry abrasive dust.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[siretb]

Thanks, I have settled for a rather large number 12-20 of overlapping vanes. single stage.
One thing is that I do not seek a high efficiency, although published papers are rather exiting when one compares reverse-flow cyclones and swirl tubes (with no flow reversal). I am convinced that, because a lot of pressure drop is used during the reversal of flow, the relationship pressure drop - cut size (or Euler Number - Stokes Number) is better with swirl tubes.
My real concern is the size.
If length is too short, collection is not efficient; if it is too longn some rotational momentum is lost through friction and viscosity. That's why is is important to have initial vanes as good as possible. For a large diameter.
What do "STOL" wing-vanes look like?

 

[MikeHalloran]

Thick airfoil, lots of camber. Google "STOL" and look at wing cross- sections.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA