Heat Transfer Coefficient not changing with fluid temperature/ Wall temperature

seshu1353 · November 30, 2016, 00:24

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

LuckyTran · November 30, 2016, 09:45

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.

oozcan · November 30, 2016, 13:08

Quote:

Originally Posted by seshu1353

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

Kushal Puri · December 1, 2016, 06:02

Quote:

Originally Posted by seshu1353

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.

LuckyTran · December 1, 2016, 10:02

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.

November 30, 2016, 00:24	Heat Transfer Coefficient not changing with fluid temperature/ Wall temperature	#1
seshu1353 New Member cfd_msc Join Date: Sep 2016 Posts: 1 Rep Power: 0	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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November 30, 2016, 09:45		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	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.

December 1, 2016, 10:02		#5
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	The Nusselt number is a similarity variable and depends only on Reynolds number and Prandtl number. Nu = fn(Re,Pr) From Nu=hL_ref/k_ref Heat transfer coefficient is thereby h = Nuk/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.