Strange 80mm Tube Axial Fan Curve - Flowrate vs RPM

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Discussion Overview

The discussion revolves around the flowrate versus RPM characteristics of an 80mm tube axial fan, particularly focusing on an observed curve that shows multiple flowrates at the same RPM within a specific range. Participants explore the implications of this behavior, questioning the underlying physics and testing conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion over the curve showing the same RPM for different flowrates, questioning how this can occur in a fixed geometry fan.
  • Others suggest that the fan's behavior may not align with that of a constant displacement pump, indicating that varying static pressures can lead to different flow characteristics.
  • One participant introduces the concept of hysteresis, proposing that multiple stable solutions could exist for the same RPM, influenced by the fan's operational history.
  • Another participant speculates that the fan blades may experience stall or surge conditions, which could explain the observed flowrate discrepancies.
  • There is a discussion about the testing methodology, with some participants clarifying that tests are conducted under varying resistance to generate the static pressure versus flowrate curve.
  • Participants inquire about the specific devices used to measure pressure and flowrate, indicating a desire for deeper understanding of the fan curve analysis.
  • One participant mentions the potential for oscillations in flowrate at certain RPMs, which could lead to physical damage to the fan.

Areas of Agreement / Disagreement

Participants generally agree that the observed behavior of the fan curve is unusual and warrants further investigation. However, multiple competing explanations are presented, and no consensus is reached regarding the exact cause of the phenomena.

Contextual Notes

Participants note that the testing conditions may not be as fixed as initially assumed, with variations in resistance affecting the results. There are also mentions of potential limitations in the fidelity of the flow data presented in the fan curve.

Who May Find This Useful

This discussion may be of interest to those studying fluid dynamics, fan design, or anyone involved in the testing and analysis of airflow systems.

  • #31
The fan will be in as much of a steady state as can be controlled. When you approach surge you start to get rapid pressure excursions and thus large rpm changes due to the fan loading. You have to understand that, in these curves, you can get different flows at the same speed because one is changing the downstream conditions.
 
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  • #32
So in my scouring the internet for information about fan laws and curves, I came across a source (a snipping of a textbook it appeared) in which the author stated
...the fan laws can be readily identified by doing a vector analysis of a fan wheel ...
But he doesn't actually bother to do this. So if anyone knows of a source where they have seen this done, please let me know.

Also:

1 How do they keep the fan running at constant speed during the fan mapping? I assume they monitor the speed and control the voltage accordingly?

2 This is not generally how a fan operates in an actual application right? The RPM's vary with pressure. For example: when I have my window fan cranking along in the summer, when I close the door to the room, I can distinctly hear the fan slow down.
 
  • #33
I have never heard of the vector deal with the affinity laws. I was looking through the pump handbook and they don't mention that. I'll have to check out my fan reference at home.

1) Yes. They will monitor the voltage and probably have some form of speed sensor on the test article. It's very easy to do from a magnetic prox probe or even a simple strobe light.

2) This is true. I would question Russ as to how many times he has seen a previously designed system change to the point it would affect a fan. I would imagine it would have to be a rather large change to cause an appreciable change in operation.

Are you sure your fan slows down? I know my fans will change pitch because of, what I perceive as lower flow or such. I can't say it sounds like they slow down, but maybe they do. I'll have to see if I can take a strobe home and do a quick test.
 
  • #34
FredGarvin said:
I have never heard of the vector deal with the affinity laws. I was looking through the pump handbook and they don't mention that. I'll have to check out my fan reference at home.

1) Yes. They will monitor the voltage and probably have some form of speed sensor on the test article. It's very easy to do from a magnetic prox probe or even a simple strobe light.

2) This is true. I would question Russ as to how many times he has seen a previously designed system change to the point it would affect a fan. I would imagine it would have to be a rather large change to cause an appreciable change in operation.

Are you sure your fan slows down? I know my fans will change pitch because of, what I perceive as lower flow or such. I can't say it sounds like they slow down, but maybe they do. I'll have to see if I can take a strobe home and do a quick test.

I am not sure that I understand how 'lower flow' causes a pitch change. I am not doubting that it can, but the lower fan speed was the more intuitive explanation for me. It never even occurred to me that lower flow might cause a change in pitch.

Now I am curious.
 

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