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Strange Fan Curve

  1. Nov 3, 2009 #1
    I am looking at fan curves here at work and I stumbled upon this one. Its for an 80mm diameter tube axial fan.

    What's up with the Flowrate vs RPM curve (the dashed line)?
    In the flowrate interval between 12 - 23 CFM there are instances of the same RPM for different flowrates.

    That doesn't make sense to me :confused: For a fixed geometry and a fixed RPM how on earth can we have different flowrates?

    Unless it is accounting for blade deformation?

    Any thoughts?

    fancurveHAB51.jpg
     
  2. jcsd
  3. Nov 3, 2009 #2

    Mech_Engineer

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    That seems atrange to me as well. It seems to me that generally the flow rate should increase with fan RPM so either something is wrong with the test, or something unexpected is going on with their fan.
     
  4. Nov 3, 2009 #3
    The fan is not a constant displacement "pump" like a water or hydraulic pump. With different static pressures, there is different "leakage".
    Bob S
     
  5. Nov 3, 2009 #4
    I am not sure that I follow you BoB S. Could you elaborate? I am under the
    impression that these tests are done under fixed conditions. Some sort of tunnel with
    a fixed resistance.

    What are these leakages? And how can they cause the RPM to Flowrate relationship
    to not have a 'one-to-one' correspondance?

    Thank you,
    Casey
     
  6. Nov 3, 2009 #5
    From a physics background this looks like hysteresis, where there is more than one stable solution to your conditions (eg at 3100 RPM you can have two different flow rates), and the certain state that you are presently is dependant on history. I can't think of a physical explanation for it, though.
     
  7. Nov 3, 2009 #6

    minger

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    Maybe at those conditions the blades were in some sort of stall or surge condition.
     
  8. Nov 3, 2009 #7
    Ah. I think that could make sense. So I've seen other fan curves that explcitly
    show a 'stall region.' This one does not. Is there a way to get that info
    from this plot? Or do I need pstag to know that (or even more than that)?
     
  9. Nov 3, 2009 #8

    russ_watters

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    No, the test is done under a varying resistance in order to generate the static pressure vs cfm curve. You use a damper in a duct and as you throttle it back, you measure the cfm, static pressure and rpm.
    Yes, what that curve is telling you is that the fan at its nominal 12v can maintain a roughly constant 3,000 rpm, but that changes in the airflow and static pressure will cause various aerodynamic effects such as stalling and surging. Note that the difference in rpm across the operating range is only about +-5%. That's a pretty small variation.
     
  10. Nov 3, 2009 #9

    russ_watters

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    That's the "surging" that minger was referring to. Under some conditions a fan can indeed have two different flow rates at the same rpm. The flow rate will actually jump back and forth between the two and that oscillation will make the duct rumble like there is a freight train running through it and can physically damage a fan.

    Here's a long article on various causes of flow instabilities:

    http://www.achrnews.com/Articles/Feature_Article/ac51bcf96e75a010VgnVCM100000f932a8c0____
     
  11. Nov 3, 2009 #10
    Hydraulic pumps with pistons might pump 50 or 100 cc's per revolution, independent of RPM. Air compressors (piston type) will take a specific amount of air at 1 atm per revolution and compress it. These are constant displacement pumps. A fan blade is not like that. It depends on the viscosity (Reynolds number) of air. Air has a very low viscosity, and flows around the fan blade. If you mount the fan on a closed box and run it, the air pressure inside will rise, but the CFM (air flow) will equal zero.
    Bob S
     
  12. Nov 4, 2009 #11
    Interesting. That makes sense. Russ, does this particular test have a name? I would like to look into it some more to get a better understanding of fan curves in general. If I knew the specific name of the test, it would make my researching a little easier. Thanks.
     
  13. Nov 4, 2009 #12

    FredGarvin

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    Usually we just refer to these tests as fan mapping tests. I don't know of an "official" name for them. Usually, you will see a "whoopy-doo" in the flow line in the same area as the rpm increase. That is where the stall region is. It doesn't really show in that plot which adds to the confusion. Perhaps they don't have the fidelity in the flow data to pick it up.
     
  14. Nov 4, 2009 #13
    :rofl: Whoopy-doo. Awesome.

    Alright so, I have been googling the hell out of fan curves, since I have never seen one before this internship. Let me see if I am catching on, or if I am totally lost.

    On the abscissa we have the volume flow rate Q
    On the ordinate we can have the total pressure drop or the static pressure drop, depending on the fan manufacturer.

    Either way, ΔP is defined as ΔP=Pin - Pout

    I gather from the fan curve and Russ' explanation of the test, Pin is fixed. By varying the damper, both Pout and Q will change accordingly.

    I assume that the 'damper' is just some sort of 'mesh' screen of sorts that allows one to constrict and loosen the openings; one extreme being 'completely open' and the other being 'completely closed.'

    Questions:

    What kind of devices do we use to measure the Pout and Q?

    I am also currently baffled by fan curves with positive slopes, but we'll get to that later. I'm sure it has more to do with stalling. :smile:


    EDIT

    Also, I am still confused by the fan curve in post #1. Is it in this 'sloppy' region over the entire fan curve? Why wouldn't they adjust the power input such that the fan runs at a nominal RPM < 3000?
     
    Last edited: Nov 4, 2009
  15. Nov 4, 2009 #14

    minger

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  16. Nov 4, 2009 #15
    I am not sure what you are asking. Surely there is a difference in the total pressure at the inlet of the duct and outlet of the duct (before fan, after fan).

    So said it yourself, the fan does work on the fluid. How can there not be a drop?
     
    Last edited: Nov 4, 2009
  17. Nov 4, 2009 #16

    FredGarvin

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    They are measuring the pressure rise across the fan. It should make sense where you have a fan in a duct and you close off the duct, you should get the highest static pressure rise. At the other end it is similar to having the fan outside the duct in that there is no resistance to flow so you get the highest flow from the fan.

    I attached an article from a fan manufacturer that should help.
     

    Attached Files:

  18. Nov 4, 2009 #17
    Well, I thought that I demonstrated that I understood that in my post#13 . My question was to minger wrt post #14.

    He seems to imply that there is no total pressure drop, which I do not understand or I am interpreting him incorrectly.

    This is how I picture the test:

    faninduct.jpg

    where the the fraction of 'free' airflow out of the duct at 2 can be adjusted by messing with the damper.
     
    Last edited: Nov 4, 2009
  19. Nov 5, 2009 #18
    Sorry. Still lost. If fan laws say that Flowrate scales with RPM, then why does this graph imply that for roughly constant RPM of 3000 there are flow rates all over the map?

    Q1/Q2 = RPM1/RPM2
     
  20. Nov 5, 2009 #19

    FredGarvin

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    They are, essentially, doing a single speed line from a compressor map. The fan is held at a constant speed (as close as you can) and the outlet of the fan is varied from wide open (highest flow, lowest pressure rise) to fully closed (zero flow and highest pressure rise). The flow and static pressure is measured on the outlet of the fan.

    The affinity laws are used in the "if everything else remains constant" scenario. In this case, the outlet conditions of the fan change so the affinity laws don't apply.
     
  21. Nov 5, 2009 #20
    Okay. Thanks for that explanation FredGarvin. I just don't understand what other fan curves are doing now. For example, in this curve, how did they do the mapping? Did they hold RPM constant or not? Is it assumed that they did? I just don't see how one would know if they do not publish that detail.

    That's why I tried Googling 'fan mapping test' so I could better understand the test. But I did not get any returns on 'fan mapping test.'


    Normfancurve.jpg
     
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