Evaluate bulk temperature as an expression

mullenc525 · January 4, 2011, 15:14

I am evaluating heat transfer between different duct geometries. I want to be able to calculate HTC as h=q/(twall-tbulk). Then I want to take that htc and average it over an area.

I am having difficulty doing this symbolically, versus at a point. For example, to get bulk temp, I need to take an area average of temperature over a location, like ave(Temperature)@Plane 1. The problem is, I want the location to vary, instead of an xy plane at point z, I want z to vary. This way, I would get tbulk as a function of z.

Does anybody know how to do this?

I do not want to use expert parameters to set tbulk constant. This would cause h to decrease in the streamwise direction as the fluid heats, and would make it a pain for me to compare different geometry.

singer1812 · January 4, 2011, 16:49

How many z's do you have? If not that many, just create new expressions at different z's.

If alot, use power syntax.

mullenc525 · January 7, 2011, 16:58

It turns out the easiest way to do this is to divide the wall heat flux by the area average of wall temperature minus the volume average of temperature.

Ade · May 11, 2011, 02:01

Do you mean bulk temperature is defined as volume average of temperature? The bulk temperature would be constant in a domain. Can we use the temperature at far from wall as bulk temperature?

singer1812 · May 11, 2011, 10:14

You can use what ever you want for bulk temperature (as it applies to the h). For a fixed q, changing the bulk temperature will change the h, but you still get the same q.

Now, if you are comparing to literature or empirical expression, you need to define the bulk temperature the same as was done in the literature.

Ade · May 11, 2011, 22:04

It is true that only wall heat flux is accurate and we need manually define Tb value and calculate HTC.
In my view, Tb is not arbitrary value but has certain physical meaning.
Tb=massFlowAve(T)@section.
Because it is not easy to define the section in software, CFX choose most simple method to deal this problem.

But as end user, why not we use more logical and accurate definition of Tb such as the temperature at main stream?

singer1812 · May 12, 2011, 10:17

Entirely up to you. You can use whatever you want for Tb in CFX and it will give you the appropriate h.

Not all problems have an easily defined free stream temperature.

It is up to the end user to make the choice, based on their problem.

January 4, 2011, 15:14	Evaluate bulk temperature as an expression	#1
mullenc525 New Member Join Date: Dec 2010 Posts: 26 Rep Power: 16	I am evaluating heat transfer between different duct geometries. I want to be able to calculate HTC as h=q/(twall-tbulk). Then I want to take that htc and average it over an area. I am having difficulty doing this symbolically, versus at a point. For example, to get bulk temp, I need to take an area average of temperature over a location, like ave(Temperature)@Plane 1. The problem is, I want the location to vary, instead of an xy plane at point z, I want z to vary. This way, I would get tbulk as a function of z. Does anybody know how to do this? I do not want to use expert parameters to set tbulk constant. This would cause h to decrease in the streamwise direction as the fluid heats, and would make it a pain for me to compare different geometry.

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January 4, 2011, 16:49		#2
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	How many z's do you have? If not that many, just create new expressions at different z's. If alot, use power syntax.

January 7, 2011, 16:58		#3
mullenc525 New Member Join Date: Dec 2010 Posts: 26 Rep Power: 16	It turns out the easiest way to do this is to divide the wall heat flux by the area average of wall temperature minus the volume average of temperature.

May 11, 2011, 02:01		#4
Ade New Member Ade Join Date: May 2011 Posts: 2 Rep Power: 0	Do you mean bulk temperature is defined as volume average of temperature? The bulk temperature would be constant in a domain. Can we use the temperature at far from wall as bulk temperature?

May 11, 2011, 10:14		#5
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	You can use what ever you want for bulk temperature (as it applies to the h). For a fixed q, changing the bulk temperature will change the h, but you still get the same q. Now, if you are comparing to literature or empirical expression, you need to define the bulk temperature the same as was done in the literature.

May 11, 2011, 22:04		#6
Ade New Member Ade Join Date: May 2011 Posts: 2 Rep Power: 0	It is true that only wall heat flux is accurate and we need manually define Tb value and calculate HTC. In my view, Tb is not arbitrary value but has certain physical meaning. Tb=massFlowAve(T)@section. Because it is not easy to define the section in software, CFX choose most simple method to deal this problem. But as end user, why not we use more logical and accurate definition of Tb such as the temperature at main stream?

May 12, 2011, 10:17		#7
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Entirely up to you. You can use whatever you want for Tb in CFX and it will give you the appropriate h. Not all problems have an easily defined free stream temperature. It is up to the end user to make the choice, based on their problem.