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Why the shape of fan blad very different?

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Old   April 9, 1999, 17:50
Default Why the shape of fan blad very different?
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Can any one explain me that why a small fan (like computer cooling fan) the chord at root it small than that at tip of blad. while for biger fan is opposit to that. Why? Is the Reynolds number effect?

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Old   April 13, 1999, 14:23
Default Re: Why the shape of fan blad very different?
John C. Chien
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(1). It is not an easy task to model the flow through the fan. It is really a difficult task. (2). So, I don't think it is related to the Reynolds number at all. (3). The lift force acting on the blade is proportional to the blade surface area. At the blade tip, you have more area to use( bigger diameter), so the chord is longer there. On the other hand, at the root ( or the hub), the diameter is smaller and thus the available area is smaller. (4). As the fan blade gets longer, the structure problem becomes more critical. Larger tip area will generate higher lift, and thus higher bending moment. Because the tip speed is normally higher than the hub speed, the simple way to reduce the bending moment is to reduce the tip surface area in order to reduce the lift force there. Therefore, the tip blade chord is normally smaller than the hub chord. ( especially for the tall fan blades). (5). When the blade becomes something like a Boeing 747 aircraft wing, the chord of the wing has to be much shorter and thinner at the tip than at the root section to avoid structure failure. (6). The irregular fan blade shapes sometimes were the results of noise reduction design in order to reduce the relative velocity and the wake formation.
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Old   April 21, 1999, 16:23
Default Re: Why the shape of fan blad very different?
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Thank you for your kind reply. Is there any place on net discuss low noise blade or some thing like this?


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