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    Discussion in 'The main mechanical design forum' started by AriLiquids, Dec 5, 2013.

    1. AriLiquids

      AriLiquids Member

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      Okay, engineers. Here's a great question. Can you answer it?

      We desperately need an axial fan that:
      1. Puts out as much cfm as an off-the-shelf 16" stand fan or box fan
      2. Does it at only 400 to 600 rpm! (ordinary fans from China run at 1,000 rpm or a little more.)
      I mean, why not? I don't see why it could not be done using a slighter larger diameter blade than normal, together with a lot more fan blade surface than regular, and maybe a heavy tilt to the blade...possibly it would have to be boxed in with a tube around it, I don't know...

      We need the smallest diameter blade possible, just for esthetic reasons.

      Anyway...I'm not an aeronautical engineer, just a product designer in a little over his head.

      Can anyone apply standard fan engineering and give me rough specs for such a thing? Thank you ever so!
       
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    3. Dana

      Dana Well-Known Member

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      To get the same air velocity at half the rpm, you need twice the pitch (blade angle). But if there's too much pitch, the blades will stall. High pitch will also impart more of a swirling motion to the air rather than pushing it through. One possibility is two fans with a stator between them to reduce the swirl.
       
    4. Roy_Kliffen

      Roy_Kliffen Member

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      Simply changing the pitch won't be enough I'm afraid.
      Most fans use very simple blade profiles, which aren't the most effective. Change the blades into higher aspect variants, using more (beware of draft effects from one blade to the next), adapt the blade profiles (maybe the type gliders use) for higher efficiency, use a ring connecting the tips of the blades to reduce tip-vortices i.e. losses, use an inlet stator to change the incidence angle of the airflow for maximum efficiency, us an outlet stator to remove the swirling effect, etc .... in short: build an electrically driven jet engine :)
      You might consider a Venturi effect to increase the flow (similar to the Dyson blade-less fans)
      A lot of changes are possible; it's all a question of budget.

      Most fans I know either have metal fans with curved sheet metal blades, or plastic fans with blades of more-or-less equal thickness, required for a dependable moulding process. As the suggested glider wing profiles have a much larger variance in thickness it will complicate the moulding process, while the metal variant will require milling in stead of simple bending.

      When designing a new type of fan, it might be advisable to invest in a flow simulation application such as Flow-Works (only one I know, probably a lot of others around)
       
    5. AriLiquids

      AriLiquids Member

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      Again, amazingly high quality answers from this forum. Thank both of you so much, Dana and Roy Kliffen!

      It looks like we've bumped up against the wall on this one. No way to make the product quick & dirty cheap.

      I find this time and again in product design: you think you can make anything you want, but then you find that physics has invisible walls built around the way most products are, and you are not going to break into new territory very easily. If it were easy, it would have been done already.

      Thanks again and all the best!
      Ari
       

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