Can a simple fan create a vortex without a screen obstruction?

[wil3]

Hello. I am working on a project that requires me to make air move in a vortex. From research, it appears that one of the most common ways of doing this is to rotate a cylindrical screen, which imparts curl to the airflow and thus creates a vortex coaxial and concentric to the center of the cylinder.

I would like to make a vortex without a screen obstructing my view of it. Do simple electrical fans, like computer fans, actually cause air to "swirl" effectively? I know that this seems like a basic question, but it is surprisingly difficult to find a good answer online anywhere.

 

[my_wan]

I would mount a large squirrel cage fan above the floor, with the top end of the cage blocked, so that it drew air from below into the region inside the cage. Add a partition at the bottom to separate the intake air from below from the output air above. This may not by itself impart sufficient rotation in the incoming air below. To increase this rotation, add stationary adjustable vanes at the intake area below the cage fan, directing air into the same rotation as the fan. Moving it closer to the floor also reduces the air the rotational equipartition is distributed to, increasing the rotational effect.

Never tries it, but that's what I would try.

 

[AJ Bentley]

I can't see why a glass (or acrylic) cylinder wouldn't work.

 

[wil3]

The reason why the screen is used is because the experimenter actually rotates the mesh on a lazy susan or something. The spinning screen forces the air that the flame sucks into adopt a vortex shape. A plastic or glass cylinder would not allow sufficient air intake, and it would not impart any rotation to the motion of the air molecules.

 

[rcgldr]

If you want a vortex, and a small size is OK, put a flat object oriented nearly perpendicular to the air flow. An alternative is to place a small wing into the air flow, so that the tips create a vortex. You'll need a fairly strong source for the air flow, perhaps a leaf blower. I'm not sure what you should use to inject smoke or vaport into the air flow so you can observe it though.

 

[AJ Bentley]

If you want a vortex, and a small size is OK, put a flat object oriented nearly perpendicular to the air flow. An alternative is to place a small wing into the air flow, so that the tips create a vortex. You'll need a fairly strong source for the air flow, perhaps a leaf blower. I'm not sure what you should use to inject smoke or vaport into the air flow so you can observe it though.

Wouldn't work - same reason my idea of a glass cylinder is wrong.
The air has to enter tangentially and (presumably) exit axially - is that right?

 

[Phrak]

Wouldn't work - same reason my idea of a glass cylinder is wrong.
The air has to enter tangentially and (presumably) exit axially - is that right?

As a mass of air, rotating as a solid, is not the goal, there need be a mechanism to generate shear between the inside and outside layers of air. A smooth, tube of rotating glass would do it, but not as well as a rough surface, provided there is a means to impart different angular velocity to the central core of air.

 

[AJ Bentley]

there need be a mechanism to generate shear between the inside and outside layers of air.

Hmmm... There seems to be more than one concept of what constitutes a vortex.
I'm thinking of something like a tornado, where there is no separation of incoming air from the rotating column but rather a gradual increase in angular momentum as the layers 'drag' on each other. - I can imagine an open screen would help to induce such a state.

OTOH a rotating cylinder would give you spinning air with a radial change of density but little else...

 

[wil3]

I'll be honest- I'm trying to make a small fire devil. I just didn't want the mods to go crazy. Do we know if a fan has the capability of making a tornado-like vortex?

My idea was to use a fan oriented straight up with a steady fuel source feeding a volatile liquid from underneath. The fan would help atomize the liquid and, if the above assumption is correct, guide the burning liquid into a vortex.

Skip to the end to see the screen effect:
http://www.youtube.com/watch?v=pkCNP83Dtbw&feature=related

 

[Phrak]

I'll be honest- I'm trying to make a small fire devil. I just didn't want the mods to go crazy. Do we know if a fan has the capability of making a tornado-like vortex?

My idea was to use a fan oriented straight up with a steady fuel source feeding a volatile liquid from underneath. The fan would help atomize the liquid and, if the above assumption is correct, guide the burning liquid into a vortex.

Skip to the end to see the screen effect:
http://www.youtube.com/watch?v=pkCNP83Dtbw&feature=related

I like the idea. It could be even better, esthetically, if the vortex were free standing instead of confined to a cylinder. However I don't know if you have other purposes in making one.

You might start out simple, to get oriented, and use propane or natural gas instead of a liquid fuel. If you had a metal pipe directed properly into a hole in the bottom side of a steel soup can, you might get some ideas.