Strange 80mm Tube Axial Fan Curve - Flowrate vs RPM

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SUMMARY

The discussion centers on the flowrate versus RPM characteristics of an 80mm tube axial fan, highlighting anomalies in the fan curve where identical RPMs correspond to different flowrates. Participants identified potential causes, including hysteresis effects and aerodynamic phenomena such as stalling and surging. The conversation also clarified that fan mapping tests involve varying resistance to generate static pressure versus CFM curves, rather than maintaining fixed conditions. The importance of understanding these dynamics is crucial for accurate fan performance analysis.

PREREQUISITES
  • Understanding of fan mapping tests and their methodology
  • Knowledge of aerodynamic principles, including stalling and surging
  • Familiarity with static pressure and flowrate measurement techniques
  • Basic grasp of fan laws and their application in performance analysis
NEXT STEPS
  • Research "fan mapping tests" for detailed methodologies and applications
  • Study the effects of hysteresis in fluid dynamics
  • Learn about measuring static pressure and flowrate using devices like Pitot tubes
  • Explore the implications of stalling and surging in fan performance
USEFUL FOR

Engineers, HVAC professionals, and anyone involved in the design or analysis of fan systems will benefit from this discussion, particularly those focused on optimizing airflow and understanding fan performance characteristics.

  • #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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