Bigger fan on armature shaft - output air temperature higher or lower?

[BrianE22]

We're going to start evaluating different fan designs on our small (1 HP) motors. The fan is a centrifugal type mounted on the armature shaft. I thought a thermocouple mounted in the output air stream would tell us if the newer designs were better (lower armature surface temperature). I thought more air flow would lower the temperature of the output air. My colleague disagrees.

Looking at the convective heat transfer equation I find that it's not obvious if the output temperature increases or decreases with increased air flow. A higher flow fan would increase the heat transferred from the armature to the air but the surface temperature of the armature would drop. Also, I'm sure the convective heat transfer coefficient (h) changes.

So to save me from having to actually do real heat transfer equations (it's been 35 years since college), can any of you offer your opinion: For increased air flow, would the output air temperature go up or down?

 

[MintJulep]

More airflow = more heat transfer.

However, more airflow also = more fan power used --> more heat produced.

 

[1gibson]

Air temp would go down with higher flow, whether the motor stayed cooler or not. If you measured ambient air temperature, outlet air temperature, and airflow, you could correlate the data and get meaningful information.

 

[IRstuff]

To the first order, the air temperature will decrease, assuming that your heat transfer isn't already maxed out, but that might have little or no impact on the motor temperature. The specifics on the motor will depend on its ability to move heat to the external convective transfer surface, and what the delta temperatures are between the motor core, its convective transfer surface, the exit temperature and the ambient air temperature. for example, say the air temp is 25C, the exit temperature is 30C the surface temp at 40C, and the core temp at 100C. That would give you a different answer than if the core temp was 45C.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!