Fan Selection for air cooling of a computer

[mech-eng]

I 90% understand the question and solution but unfortunately I have some hesitations about the formal solution in the book.

First of all, when calculating P4-P3, choosing lines and points on them, why do book assume velocities of 3 and 4 to be the same?

http://opencourses.emu.edu.tr/pluginfile.php/4500/mod_resource/content/1/Chapter5.pdf

Thank you.

 

[CivilSigma]

I'm not familiar with the equations, but at point 3 the air will be sucked in at some velocity 'v' and just at 4 it will be pushed out at the same velocity 'v' excluding any energy losses.

 

[rcgldr]

why does book assume velocities of 3 and 4 to be the same?

Because the tube diameter is constant between 3 and 4, and assuming in-compressible fluid, the density does not change. Mass flow is constant or else mass would be accumulating. So from point 4 to the end of the exhaust tube, velocity and pressure ideally remain constant. Afterwards, the pressure would normally decrease and velocity would increase, affected by viscous reaction with the surrounding air.

 

[mech-eng]

Can we say as one consequence that when fans sucking airs, they just change their pressures at first by fan work?

Thank you.

 

[rcgldr]

Can we say as one consequence that when fans sucking airs, they just change their pressures at first by fan work?

Looking at the diagram, the funnel shaped tube intake would correspond to accelerating air. Within the cylindrical part of the tube, there would be no net acceleration in the direction of the tube.

 

[mech-eng]

Looking at the diagram, the funnel shaped tube intake would correspond to accelerating air. Within the cylindrical part of the tube, there would be no next acceleration in the direction of the tube.

Yes, this can be another result from directly looking the scheme but what about deriving a result for pressures?

Thank you.