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If I have a fan with these attributes

Category Value
Air flow 108.2 CFM (3.03 $m^3/min$)
RPM 3100
Dimensions LxHxW (mm) 120x120x25

Is it correct to calculate the thrust force at max rpm like this:

Fan air flow = 108.2 CFM = 0.0511 $m^3/s$

Air density at room temperature = 1.2 $kg/m^3$

Fan speed = 3100 rpm = 19.48 $m/s$ given the radius is $\frac{\frac{120}{2}}{1000} = 0.06$m

$F_{thrust}$ = Fan air flow * Air density * Fan speed = 0.0511 * 1.2 * 19.48 = 1.19N

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  • $\begingroup$ Use a pitot tube and measure the velocity profile across the fan output. Integrate under the curve and calculate the thrust. Makes a neat lab experiment. $\endgroup$
    – Solar Mike
    Mar 10, 2022 at 6:55

1 Answer 1

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If I remember correctly thrust of a stream of gas is:

$$F=\dot m *v + (P_e-P_0)A_e$$

Assuming the pressure before and after the fan is equal to 1atm:

$$F= \dot m *v$$

So you need to find out the stream's flow speed not the fan speed. (The rotation of the fan is not directly related to the flow speed. One needs to calculate the blade's angle of attack and aspect ratio and even then it will be an approximation.)

Therefore

$$V= \dot Q/A=\frac{0.051}{\pi 0.06^2}$$

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  • $\begingroup$ If im also given the static pressure of the fan as 69.7 Pa can I use that instead to estimate the thrust? If looked a lot online and there are a lot of conflicting formulas and one of them said thrust force = pressure * area of duct $\endgroup$
    – MisterM
    Mar 9, 2022 at 21:05
  • $\begingroup$ Then just go ahead and plug it in the parentheses. the $\dot m*v, $ is essential, it's the part of thrust that works in vacuum. $\endgroup$
    – kamran
    Mar 9, 2022 at 22:54

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