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Is it possible to increase an air column's diameter over a short distance? Let me explain:

Imagine a small 10 cm electric computer cooling fan that's pushing air into a cone-shaped duct 20 cm long. If the exit side of the cone-shaped duct has a diameter of 5 cm, I would think the diameter of the air "column" at the exit of the duct will be halved, and the pressure to be double of what it is at the beginning of the duct.

Now, if the diameter of the cone-shaped duct is instead increased to 20 cm on the exit side, will the air column have double the diameter and half the air pressure than at the beginning of the cone-shaped duct?

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  • $\begingroup$ Not with axial flow, but you can do it with a centrifical fan just fine. $\endgroup$
    – Phil Sweet
    Mar 17, 2019 at 4:12

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probably not until the flow has traveled several pipe diameters downstream of the transition. To speed up the establishment of uniform flow you'll have to put in several fine mesh screens (like a window screen) right at the end of the transition section, spaced about 2" apart; this is a common trick used to smooth out flow fields in old-school wind tunnels.

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Make the changes of diameter gradual to minimize the losses : about 6 degrees is best...

You can do the analysis to prove it... It was an analysis done in class by our professor... Of course if space is an issue then 6 degrees may not be appropriate.

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Actually Bernuoulli law says that when you decrease the flow diameter the pressure goes down, while speed goes up.

$$\frac{v^2}{2}+gz+\frac{P}{\rho}=\text{constant}$$ Assuming the fluid incomprehensible, which at these low Mach numbers it applies.

So when the fan outlet is decreased the fluid is accelerated, by leaching energy from the pressure.

So your assumption is not right.

And inversely when you increase the duct diameter the flow speed goes down giving back some energy to pressure, causing it to go up.

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  • $\begingroup$ I think autocorrect nailed you on "uncompressable" $\endgroup$
    – K H
    Mar 18, 2019 at 21:49
  • $\begingroup$ @KH, thank you, right. Appreciate reading carefully. $\endgroup$
    – kamran
    Mar 18, 2019 at 22:39

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