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Old 03-04-10, 09:58 AM   #15
AC_Hacker
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Default Laminar Flow & Turbulent Flow

Quote:
Originally Posted by Daox View Post
Yeah the fan speed thing is odd. I'd tend to think that slower would be better as Xringer said, but their findings show otherwise. In any case, it definitely looks like larger is the way to go.
In Fluid Mechanics, there is the issue of laminar vs. turbulent flow.


This graphic illustrates a couple of aspects of the difference:

In laminar flow mode, the friction of the surface of the pipe or in our case, the surface of the heat exchanger, causes the flow rate to become slower at the boundary between pipe and fluid. The maximum flow rate is at the center of the pipe and it decreases toward the pipe surface. This means that if there is a temperature difference between the fluid and the pipe, the slower moving layers near the pipe edge change temperature more readily than the layers near the center.

If the flow rate is increased, there comes a point where the fluid no longer flows in regular stratas, but changes flow mode and the flow becomes chaotic, the fluid tumbles as it flows.

In turbulent flow mode, the layers are continuously and chaotically changing, The graphic suggests that some of the fluid flows backward, which it does not, but some flows forward more slowly.

Since heat transfer happens as a function of temperature difference, the turbulent flow mode is more efficient than laminar flow, because the 'particles' of fluid get exposed to particles of pipe and are more subject to temperature change.

Also, the idea that slowing down the the fluid flow rate will result in increased heat transfer doesn't hold up either. While it is true that if the fluid flows more slowly, it's temperature will be higher when it exits, there is less total heat exiting with a lower speed.

To appeal to intuition, if you had been working in the hot sun and had gotten dangerously overheated and needed to cool down, would you rather sit by a window through which the air was barely moving, or would you rather sit in front of a window where there was a fan turned to high speed?



That's the difference that increased flow rate, and turbulent flow makes.

Regards,

-AC_Hacker

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