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We have a system involving a fan blowing air through a box which contains a small (but constant) heat source and a temperature sensor. The fan speed is controlled with PWM.

As PWM (and hence fan speed) is increased, the temperature is initially observed to decrease as expected. However above a certain PWM value, increasing fan speed appears to actually have the opposite effect and the temperature is observed to increase. Does anyone have any intuition for what could be causing this?

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Most likely the fan is getting into the stall regime and the flow then drops or the exhaust port is causing a pressure drop that the fan cannot overcome after a given speed.

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    $\begingroup$ Cheers for the reply! I'm a bit of a newbie when it comes to this stuff, what do you mean by the stall region? $\endgroup$ Apr 4, 2022 at 14:55
  • $\begingroup$ @JagoAartsen perhaps this will make good reading: en.wikipedia.org/wiki/Axial_fan_design $\endgroup$
    – Solar Mike
    Apr 4, 2022 at 15:06
  • $\begingroup$ thanks I'll take a look $\endgroup$ Apr 4, 2022 at 15:09

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