Fan Thought Experiment

[arkie87]

Apologies in advance if this question is not clear; I will revise based on feedback to make it more clear.

I have a thought experiment:

(1) consider a centrifugal fan with the inlet in the central hub (at radius=0) and the outlet at the outer radius. If the fan blades spin, the centrifugal/centripetal forces causes a radial pressure gradient. The pressure at the outer radius is higher than the inner radius. Since the outlet is venting to ambient pressure, flow is created.

(2) Now imagine you weld the fan blades to the housing. If you spun the housing at the same RPM, you would still get flow, since the outlet pressure is still above ambient.

(3) Now imagine you welded a duct connecting the outlet to the inlet hub. The system is now fully contained. If you spun the device, would you get flow? 

My inclination is no, since in the return duct, there is the same centripetal acceleration that causes  the same pressure gradient in the radial direction. Thus, i cant think of a reason the flow would go one way vs. the other.

Thoughts? Clarifications? 

TIAA

[Zeiss Ikon]

You are correct.  You'd get a pressure gradient due to the centripetal acceleration, both in the fan housing and in the return duct, but you'd get no flow beyond startup and shutdown transients as the "spin gravity" rises and falls.

You'd probably also tear up your mounting system, because the duct would imbalance your erstwhile squirrel cage blower beyond the ability of the bearings and struts to hold it in place.

[arkie87]

13 hours ago, Zeiss Ikon said:

You are correct.  You'd get a pressure gradient due to the centripetal acceleration, both in the fan housing and in the return duct, but you'd get no flow beyond startup and shutdown transients as the "spin gravity" rises and falls.

Ok, thanks!

13 hours ago, Zeiss Ikon said:

You'd probably also tear up your mounting system, because the duct would imbalance your erstwhile squirrel cage blower beyond the ability of the bearings and struts to hold it in place.

Sounds like something a Kerbal would do.