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For a school project, we are making a wind tunnel which is still in conception. The problem is that I looked at a lot of other wind tunnel project and a lot of them seems to have a lot of blades on their fan. So I was wondering how the number of blades affect the flow of the air for a wind tunnel.

Thanks a lot,

PS: I hope I'm asking this at the right place... Im new here...

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    $\begingroup$ when you increase number of blades , it's create more drag in air and then fan circulates decrease.that why wind turbine have only 2-3 blades $\endgroup$ Feb 6 '20 at 1:07
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The number of blades depends on the disk loading, i.e. how much power is available to drive the fan relative to its size. Use as few blades as you can get away with, but not fewer. NASA has a page on that and they use the term "activity factor" which indicates how much of the fan disk is covered by the blades.

If your fan has a low power loading, you might want to use Mark Drela's XROTOR for checking how well your fan design matches the available power.

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A propeller fan with a small number of blades (like 2 or 3) will impart regular pulsations to the air flow through it with each revolution of the propeller. Those pulsations will affect the accuracy of the measurements taken in the tunnel.

More blades raises the frequency of the pulsations, making them easier to filter out of the airflow upstream of the test section in the tunnel, but this will also introduce more "swirl" into the airflow, as the air tends to get dragged around in a spiral pattern with the rotating fan disc. This can seriously affect the airflow uniformity in the test section of the tunnel.

The usual filtration system is a series of closely-spaced wire mesh window screens just downstream of the fan, followed by a section of the tunnel which diverges into a very large cross-section, then converges again just before the test section.

Another way to minimize the effect of the fan on airflow dynamics through the test section is to place the fan on the exhaust end of the tunnel, leaving the window screens in the intake end of the tunnel.

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It is probably best to put the fan downstream of the test zone, you will have enough problems reducing turbulence as it is.

In that situation, once you know the volumetric flow, the faster you can draw the air through the smaller your fan can be, but the more blades you will need to get that extra speed. The rotation speed of the fan motor may also be a consideration.

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