Cross flow fans

[karlbamforth]

Hi,
A quick question,
Can anybody explain how a crossflow fan works? I have seen an entry on another site that they can be used as an air pump to considerable increase pressure and or flow. So far I have been unable to find anyone or a website to explain how it works.
My first impression is that it would tke air in around the circumference tube and expel it at the ends. I already know this not to be true, as implied by its name the air crosses the fan, but what denotes intake and exhaust? Is it possible to get high flow or pressures from a crossflow fan?

Karl.

 

[electricpete]

in my limited experience, cross-flow is a term applied to heat exchangers, radial and axial are terms applied to fans.

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[electricpete]

I googled and saw several results for cross-flow fans. From what I can tell they look like radial / squirrel cage fans.

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[25362]

When fans are classified according to the direction of flow through the impeller, four distinct types can be mentioned: axial-flow, radial-flow, mixed-flow and cross-flow fans.

In cross-flow fans, aka transverse-flow fans, the flow enters the impeller at one section of the periphery, proceeds radially inward and exits radially outward through the blades at a section different from the inlet section. Perforce their housings are elongated to match wheel inlets and outlets.

 

[electricpete]

What makes air flow radially inward?

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[cbrn]

As far as I know, 25362's explanation is the right one. The conformation of the stator duct, the rotation direction and the shape / LE angle / TE angle of the blades, as well as the radius difference btw LE and TE (note that at the blades, each edge becomes alternatively a LE and then a TE, and then a LE again etc...) all dictate the flow direction, and the balance between flowrate and pressure rise.
I can not be more precise since the info I have about this kind of runners is extremely limited. Though they have a wide-spread application field (for example, all the air conditioners' evaporators fans are of this type nowadays), the flow theory underlying is complicated and the manufacturers hide their know-how about these in a very jealous way.

Regards

 

[25362]

Imagine the curved blades arranged in a ring type cylindrical structure. It appears that after the flow passes through the blades towards the center it is acted upon by some vortex-producing device which enables it to proceed radially outward through the blades at a different section.

Google offers several (commercial) sites just by asking crossflow fans.

 

[karlbamforth]

Hi all,
Thanks for your comments, it is apparently quite difficult to explain how cross-flow fans work. The link below is for a 2D animation if anyone is interested.

http://www.fluent.com/solutions/examples/img/x144i1_lg.gif

It would appear that the comments about a vortex in the centre is correct, but it has to be displaced off-centre to encourage the air to flow out. The displacement of the vortex and the inlet airflow is controlled by the ducting.
Still not sure I fully understand it.

Thanks for your help and comments.
Karl.

 

[electricpete]

Pretty funky. Where is the air intake... the whole upper half from 10:00 around to 4:00?

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[25362]

For an example visit

http://lionball.en.alibaba.com/group/50043279/Fans.html

and then enlarge the picture.

 

[cbrn]

Hi,

looking at the animation on fluent site, it can be seen that:
1- the colorscale can not refer to pressure, otherwise the extrados of the blades would be in overpressure with respect to the intrados, which is nonsense. So, I argue the plot refers to velocities.
2- the rotating device works to create an energy increment between the interior side of the blades and the peripheral. This is because of the conservation of the angular momentum (or however it is called in English...)
3- the depression created by the rotating blades recalls air from the intake. This is made possible by the shape difference between the intake and the exhaust duct. In fact, the whole thing works like the "extremization" of a normal centrifugal-machine volute, where the volute attack angle is greater than 90° (normally in fans and compressors it doesn't exceed 30° for efficiency reasons, or 50-60° when compactness is absolutely necessary)
4- In order for the thing to work with an acceptable efficiency, I argue that care must be taken in choosing the correct coupling between exhaust duct volute angle and blades' trailing angle (I mean the "exterior" one). I also suppose that, on the "interior" side, the blade angle should be dictated by the velocity triangles of the fluid, though in this case the whole thing is complicated by the non-uniform distribution of the velocities.

Interesting...

Regards