Heat Transfer Cofficient

Nadaaaa22 · July 24, 2019, 07:35

Hey

I'm working on a heat sink with applied heat source at the bottom, I want to ask how I can get the heat transfer coefficient?. The more I read about computing heat transfer coefficient in ANSYS the more I get lost. what is the reference temperature and why it can make the results wrong?. I'm using a turbulent model. I have also another question I read that most of the replies about getting heat transfer coefficient suggested writing an expression using the bulk temperature. what is the bulk temperature and why I'm using the bulk temperature not the surface temperature in getting heat transfer coefficient?. I got lost please help me with this

LuckyTran · July 24, 2019, 09:09

First, you need to go back to the very basics to Newton's laws of cooling and learn the definition of heat transfer coefficient.

Heat transfer coefficient involves the surface heat flux, the surface temperature, and always a reference temperature. So of course you have to provide a reference temperature. In principle, the reference temperature can be anything. But if you don't use the same reference temperature as somebody else, then you cannot do an apples to apples comparison. If you use your own reference temperature, then you calculate a different type of heat transfer coefficient and there is no point in comparing.

The bulk temperature is popular (by convention) reference temperature to use in internal flows and duct flows.

Nadaaaa22 · July 24, 2019, 12:39

Is the reference temperature the same as the ambient temperature?.

LuckyTran · July 24, 2019, 12:51

It depends on the problem....

There is no ambient in a pipe flow for example.

July 24, 2019, 07:35	Heat Transfer Cofficient	#1
Nadaaaa22 Member Join Date: Nov 2018 Posts: 41 Rep Power: 7	Hey I'm working on a heat sink with applied heat source at the bottom, I want to ask how I can get the heat transfer coefficient?. The more I read about computing heat transfer coefficient in ANSYS the more I get lost. what is the reference temperature and why it can make the results wrong?. I'm using a turbulent model. I have also another question I read that most of the replies about getting heat transfer coefficient suggested writing an expression using the bulk temperature. what is the bulk temperature and why I'm using the bulk temperature not the surface temperature in getting heat transfer coefficient?. I got lost please help me with this

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July 24, 2019, 09:09		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	First, you need to go back to the very basics to Newton's laws of cooling and learn the definition of heat transfer coefficient. Heat transfer coefficient involves the surface heat flux, the surface temperature, and always a reference temperature. So of course you have to provide a reference temperature. In principle, the reference temperature can be anything. But if you don't use the same reference temperature as somebody else, then you cannot do an apples to apples comparison. If you use your own reference temperature, then you calculate a different type of heat transfer coefficient and there is no point in comparing. The bulk temperature is popular (by convention) reference temperature to use in internal flows and duct flows.

July 24, 2019, 12:39		#3
Nadaaaa22 Member Join Date: Nov 2018 Posts: 41 Rep Power: 7	Is the reference temperature the same as the ambient temperature?.

July 24, 2019, 12:51		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	It depends on the problem.... There is no ambient in a pipe flow for example.