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Apparently we don't understand definition of "h"?!

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Old   October 17, 2005, 18:37
Default Apparently we don't understand definition of "h"?!
  #1
Chris Bailey
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OK, this is embarrasing, but a couple of us now are convinced we don't understand what heat transfer coefficient "h" means for an impinging jet on a surface.

We calculate the flux at the surface, and divide that by the difference between the incoming fluid temperature and the surface temperature. Isn't that "h"?

But we are a scientist using Fluent and an engineer using FloWorks, and each time we solve essentially the same problem, and ask the programs to calculate "h", they give us a result different from flux over delta temp. It's different by the same ratio in both environments, and the ratio is a function of both the high and the low temperatures. The ratios can be numbers like 0.6 or 2 or 30. Doing this point by point across the surface gives exactly the same ratio in both Fluent and FloWorks to several decimal places everywhere.

What in the world are we too stupid to get here??? What do we completely misunderstand???
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Old   October 17, 2005, 18:45
Default Re: Apparently we don't understand definition of "
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Jonas Larsson
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Fluent uses a reference temperature set in the reference panel as the "bulk" (ie fluid) temperature. Did you set this to the correct incoming fluid temperature?
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Old   October 17, 2005, 22:52
Default Re: Apparently we don't understand definition of "
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Dong Ye
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The temperature and heat flux are changing on the surface. h is different from point to point. Is there a mean heat flux and a mean temperature? So a general convective coefficient?
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Old   October 18, 2005, 08:33
Default Re: Apparently we don't understand definition of "
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ZubenUbi
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It could also come from your test case. The value of h is correctly defined for a flow aligned with the plate where you compute h. For impinging jet, you may find something odd since the deformation tensor of the flow look strange. I know this have been corrected in some publications, but I can not find them now. May be you can googled it.

Hopes that may help,

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Old   October 18, 2005, 11:38
Default Jonas nailed it
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Chris Bailey
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Jonas,

This looks like it. In Fluent there's a temperature setting "which is used to compute entropy for incompressible flows", according to the docs. I don't see it mentioned in discussions of heat transfer coefficients. But its default setting of 288.16 K or 15.01 C (?!) fits with my most recent experiment through 8 decimal places!

But can this be the explanation in FloWorks? It would have to be the same bizarre value. Is 15.01 C the International Standard for Convected Air, or what? I'll ask the FloWorks user to try this.

Thanks! - Chris
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Old   October 18, 2005, 13:30
Default Re: Jonas nailed it
  #6
Scott Shaw
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I think (haven't got reference materials to hand) 15 degrees C is standard sea-level temperature for an ISA (international standard atmosphere) day.
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Old   October 18, 2005, 13:37
Default 15.01 is standard
  #7
Chris Bailey
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Sort of. I found it, since posting.

15.01 C is a standard atmosphere. I think maybe this odd number reflects a change in the definition of the Celsius scale - 1983, maybe?
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