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Blocking Back Of Fan Creates Vacuum In Front?

  1. Aug 20, 2015 #1
    Hi there,

    I have recently observed a strange effect regarding a fan and was wondering if someone could explain what's going on.

    I have a 22cm computer case fan which I am using in a project. For my purposes, I had to partially tape over the back of the fan. I also put a large plastic jar lid on the back with a hole on the side in which I instered a long plastic tube. Essentially, the entire back of the fan is blocked except for the space created within the plastic jar lid (which is about 2cm in height and 15 cm in diameter). I had thought this would be enough to create a kind of vacuum in the tube.

    Well, when I turned the fan on, I did observe a vacuum effect, but not much in the tube.... The front of the fan suddenly started sucking things toward it! I tested this, once I took the jar lid off the back, the fan returned to blowing things away, but as soon as I put the lid back, it started sucking things toward it. And, disappointingly, the vacuum effect in the tube is minimal at best.

    Can anyone explain what's going on here?

  2. jcsd
  3. Aug 20, 2015 #2


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    With the back blocked, the fan is probably acting a bit like a centrifugal pump, drawing air towards the innner part of the fan, and blowing air radially away from the outer edges of the fan blades. Check for the flow near the outer edges of the fan blades.
  4. Aug 20, 2015 #3
    Something else I considered is that the fan is lowering the air pressure quickly in the enclosed space I created and therefore air should come rushing in to replace it. The only problem with this is, like I said, there is very little vacuum effect in the tube which I attached to the back, which I found surprising.

    Your explanation makes sense. I did notice there was some air moving around the edges, but in the center there was a strong force drawing air toward the fan. It just seems very counter intuitive.
  5. Aug 20, 2015 #4


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    Yes. Even is the fan is an axial type, there is still a cengtrifugal aspect to the air flow - even when it has to come in from the front. The fan speed will probably have increased noticeably when the inlet is blocked.
  6. Aug 20, 2015 #5


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    Getting back to the vacuum in the tube issue, try removing the tube and place your finger near or over the hole to see if you can get any sense of air flowing into the hole, which it should be. If the fan is acting like a centrifugal pump, then the pressure near the axis will be lower than ambient, and the pressure near the blade edges will be greater than ambient, but the overall average pressure may not be that much different than ambient. It might be possible to orient the tube so that it's inner end is near the axis of the fan, but there's the risk of the tube getting sucked into the fan.

    If the goal is to produce a vacuum in a tube, you could consider buying an actual pump of small size.

    Another alternative is to have the open end of the tube perpendicular to a flow of air (driven by a fan). Cutting the end at an angle and facing the angled surface away from the flow should help. A vortice wlll be formed over the end of the tube, producing a vacuum. This is how some atomizers operate. It's enough to draw fluid, but I'm not sure how much vacuuum effect you get versus the speed of the air flow.
  7. Aug 20, 2015 #6
    The ultimate goal is to put the fan in a somewhat enclosed space, but to have it maintain it's maximum amount of airflow. The area directly behind the fan will be only about 4cm deep, but there will be plenty of space parallel to the fan.

    The fan is 12cm in diameter, and about 4 cm deep. The total space I would like to place it in is about 8cm deep, which means there would only be about 4cm directly behind the fan. Originally, I didn't acount for centrifugal forces and thought it would only be an issue of airflow/air pressure, and to this end I placed a plastic disc about 10cm in diameter and about 2cm deep directly behind the fan with a plastic tube about 2cm wide coming out the "bottom" of the disc. I thought this would be enough to provide airflow to the fan, and thus create a strong vacuum effect in the tube as it drew in the air.

    But, as I said, what happened instead was centrifugal forces took over and the fan completely lost all forward airflow and in fact the airflow reveresed, pulling objects toward the center of the fan. I then tried making a wire mesh backing for the fan which was again about 2cm raised away from the fan back. I left the sides completely opened but taped over the area directly behind the fan. This again almost totally killed all forward airflow, despite my leaving the sides completely unobstructed.

    I am considering using a large plastic tube about 8cm in diameter, bending it and leading it out of the enclosed space to act as a "vent" of sorts. Again, it would only be able to extend about 2-3 cm behind the fan, but I'm thinking because it would lead to a direct air outlet, it should prevent the blocking action and the centrifugal forces from taking over.

    Any other ideas would be appreciated.
  8. Aug 21, 2015 #7


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    See if you can locate the fan as far as possible away from the wall behind it. Computer fans are sometimes labeled (marketed) as high pressure versus high flow. Usually the high pressure fans have obstructed flow on the output side, like cpu cooler fans mounted onto the cooling fins. I don't know if there are high pressure fans 12 cm in diameter.
  9. Aug 21, 2015 #8
    That's very interesting, I didn't realize they had high pressure fans specifically for situations where airflow is restricted. I guess now the issue becomes finding high pressure fan which can also move a decent amount of air.

    I did come up with a solution to my problem with the fan I have, it was, as you said, all about distance. Creating a system whereby the fan cannot physically get close to the back wall has worked well. Airflow is still less than 100% unrestricted, but it's an acceptable tradeoff.

    But I will for sure look into some high pressure fans and see if they can help with my situation.

    Thanks, I think this topic has been resolved.
  10. Aug 21, 2015 #9


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    @GoBot Could you just go over the actual purpose of this fan and a bit more context? It is not clear to me why you are using this particular layout to achieve air flow. Is it a cooling problem?
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