anyone seen this new dyson fan- and its potential for marine use?

Discussion in 'Propulsion' started by tugboat, Jun 24, 2012.

  1. b430
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    b430 New Member

    Does anyone make an optimized conventional fan? Meaning efficiently shaped blades in a tight fitting properly contoured duct? Normal home fans are pretty terrible from an aerodynamic standpoint. I would be one designed properly could move more air with less power and noise than the dyson, and could even be made to look pretty cool, something like this:
    [​IMG]
     
  2. Jeremy Harris
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    Jeremy Harris Senior Member

    The hovercraft people are pretty good at getting the best from fans. There are a lot of commercial fan blade systems that are adjustable pitch and in which the blades can be trimmed to fit inside a nice duct, with tight tip spacing for high efficiency.

    The downside with putting an efficient duct around a fan like this, at least for use as a desk top fan, is that air stream from it would be fairly narrow and have a fairly significant swirl. You could add flow straighteners to remove the swirl, at the expense of a bit extra power loss, and then add a diffuser, or decelerating duct section, to widen and slow down the output air stream, but by the time you've done that you're probably back to similar efficiency and output flow characteristics to an ordinary open propeller type desk fan.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Ducted fans have been around for a while and they do improve efficiency a bit at lower speeds, though as pointed out also focus the flow stream. On a boat, this has some good benefits; thrust is moved directly aft, prop protection, weed protection, much less environmental impact on bottom growth, smaller wakes, etc. It can still swallow a line, but not nearly as easily. On the down side the duct must be really close to the blade tips, so maintenance goes way up. At higher speeds, drag becomes a major issue. It can increase draft slightly too, so as with everything else in yacht design, there's several things to consider, both good and bad.
     
  4. tugboat

    tugboat Previous Member

    i think you guys are talking about the same principle as the kort nozzle- blade tips must be kept close to the duct. blade tips cut off to be almost square, with milimeters to spare between the prop and the shroud(nozzle). how the nozzles dont get grooved out slightly from any prop whip is a mystery to me...but they are 20% more efficient and the same should apply to a fan...but the tunnel or nozzle would have to be lengthened a bit more in air if im not mistaken to make it more efficient?.
     
  5. Jeremy Harris
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    Jeremy Harris Senior Member

    Kort (or their more efficient cousins, Rice) nozzles are indeed just ducts that act to reduce tip losses, by partially suppressing span-wise flow across the blades, reducing tip vortex shedding and allowing blades to operate at a higher pressure ratio before encountering cavitation (because the intake dynamic pressure is increased at the blade by the increased flow velocity in the forward part of the duct).

    They do give a useful efficiency gain at low boat speeds, I tried one on my electric boat, as a way of maintaining efficiency whilst allowing a reduced prop diameter to reduce draught. At 4 kts the ducted prop/nozzle worked at better than 80% efficiency. The snags are principally that the efficiency gain soon disappears as boat speed increases. By the time you get up around 8 to 12 kts nozzles are getting to be worse than an open prop. The other problem I found was that they are very susceptible to fouling. Mine worked like a very effective vacuum cleaner, sucking in weed and debris and fouling the prop, which, because of the small tip clearance stayed fouled (there was no where for weed etc to be flung off from).
     
  6. tugboat

    tugboat Previous Member

    if one wanted to design nozzles-would they need to be shaped like an aerofoil?
    i thought of using a shroud with minimal blade clearances- but i think dave gerr mentions some special shape to the shroud when designed....
    and ive seen nozzles on tugs that are almost a round airplane wing shape?:confused:
     
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    Yes, that's why the Rice nozzles out perform the original Kort nozzles. The Rice design uses an aerofoil section to improve performance over the fairly flat Kort design.

    A small tip clearance is key to getting good efficiency, but also the biggest problem when it comes to dealing with weed and debris ingestion, at least on a small boat with limited power. Once something gets sucked in to the nozzle, then the chance are it'll get jammed around the tips of the prop or just wound around it.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Ducted fans or props can have cutters installed, just like any other prop, though as I and Jeremy mentioned, the tip clearance tightness doesn't leave much room for any type of debris, after it's shredded.

    The biggest issue I see with any duct, is the flow limitations imposed by the duct shape. A reasonable designer will shape the duct, for a target speed/flow rate. Below this and it's not so bad, but above and they quickly "out grow" the duct shapes selected. Variable ducting would be the answer, possably vectored as well, for more efficiency gains, but it would need to be computer controlled to keep up with changes in flow rate. I suppose it could be mechanically attenuated, based on speed and/or flow rate, but either will be an interesting engineering exercise to say the least.

    On displacement speed craft, particularly those that work in sensitive, shoal environments, ducting can offer some real benefits. On craft that need more flexibility in speed range, not so much.
     
  9. joco
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    joco Junior Member


  10. troy2000
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    troy2000 Senior Member

    I agree: worked OK, but not great. I could see having a small one on my desk, if they didn't cost so much.
     
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