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Heat Transfer Coefficient not changing with fluid temperature/ Wall temperature

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Old   November 29, 2016, 23:24
Default Heat Transfer Coefficient not changing with fluid temperature/ Wall temperature
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Hello All,

I was trying to change fluid temperature and wall temperature and find out Heat transfer coefficient. But I'm getting a constant HTC.

note : Im using nanofluid properties
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Old   November 30, 2016, 08:45
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Sounds correct! Heat transfer coefficient is a property of the flow and it shouldn't change with temperature, except in reality there are non-linearities.
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Old   November 30, 2016, 12:08
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Quote:
Originally Posted by seshu1353 View Post
Hello All,

I was trying to change fluid temperature and wall temperature and find out Heat transfer coefficient. But I'm getting a constant HTC.

note : Im using nanofluid properties
heat transfer coefficient is actually convective heat transfer coefficient (w/m2K) and it is not flluid property. As Lucky train said, it only changes with flow model you have specifed
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Old   December 1, 2016, 05:02
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Originally Posted by seshu1353 View Post
Hello All,

I was trying to change fluid temperature and wall temperature and find out Heat transfer coefficient. But I'm getting a constant HTC.

note : Im using nanofluid properties
Can you explain the complete model. how you are modelling.

If the fluid flow rate is same and temperature is increasing or decreasing then htc has to be change. As there will be change in temperature difference.
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Old   December 1, 2016, 09:02
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The Nusselt number is a similarity variable and depends only on Reynolds number and Prandtl number.

Nu = fn(Re,Pr)

From
Nu=h*L_ref/k_ref

Heat transfer coefficient is thereby
h = Nu*k/L

There's no temperature!

Newton's law of cooling defines heat transfer coefficient as:
q" = h*(T-Tref)
Which means that if the temperature changes, the q" changes but not the heat transfer coefficient

The only reason heat transfer coefficient should change, is due to non-linearities. i.e. Reynolds number, Prandtl number, k_ref, L_ref are temperature dependent quantities. If the heat transfer coefficient did change, it would have to be for one of those reasons. However, if you tell me that you changed the wall temperature and heat transfer coefficient did not change, then I will automatically respond with it's not supposed to.
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