Monitor Heat Flux in CFD-Pre

cltormar · November 14, 2014, 07:18

Hi,

I am running conjugate heat transfer transient CFX calculation of an air cooling system where the heat load to cool down is applied only during a short amount of time.

I want to monitor the heat flux in the outlet of the cooling system. So I can obtain afterwards a curve of total Power/heat removed through the outlet as a function of time, and check how efficiently I am removing the heat.

The problem is that, when I define the expression I want to monitor in CFX-Pre, which would be something that:

areaInt(Heat Flux)@Outlet

I got a cell error, as it seems Heat Flux is not a CFX-Pre variable. If you look at it in the variables tag of CFX-Pre you will not find it.

This is very surprising, as Heat Flux it does appear as a variable in CFD-post.

I tried using "Wall Heat Flux" which appears in CFX-Pre, but when I check in CFD-post "Heat Flux" and "Wall Heat Flux" do not give the same values at all for the case of an outlet.

Do you know any reason why this is happening? Propose any solution?

Thanks in advance

Claudio Torregrosa
CERN
EN-STI-TCD
Bld 865/R-D16
F-01631 CERN Cedex
France

Mobile: +41 76 7337 983
Office: +41 22 76 62663

Opaque · November 14, 2014, 11:01

Unfortunately, you are correct. "Heat Flux" is a CFD-Post only variable at the moment. Wall Heat Flux is a similar variable available from CFX-Pre->CFX-Solver and CFD-Post as well.

The main differences between Wall Heat Flux and Heat Flux are boundary edge effects when averaging from the faces values (integration points) to the vertices of the faces (a vertex is shared by several faces).

If what you need is the energy flow through an inlet/outlet, you can always write the pieces that account for it. For a non-radiation case, the energy flow can be computed as

Thermal Energy --> massFlowInt(Static Enthalpy)@Inlet

Total Energy --> massFlowInt(Total Enthalpy)@Inlet

Such calculation should be fairly close to areaInt(Heat Flux)@Inlet.

Hope the above helps

carla menale · November 18, 2014, 07:44

Hello! I have a problem to monitor the heat flux on line constructed with the option Surface->line/rake

It's always zero, but the temperature is changing.
I'm using: monitor->Surface->total Surface heat flux

evcelica · November 19, 2014, 10:52

Heat Flux is a wall boundary variable, you can't just place a line in space or at in inlet/outlet and get a variable for heat flux.

carla menale · November 20, 2014, 05:17

I'm not sure this is the problem. If I make a line in correspondence of a wall created with gambit, there is the heat flux.

I plotted the wall made with gambit and the line made in corrispondence of this wall and the flux is the same.

The problem is with other lines made inside the domain.

Opaque · November 20, 2014, 12:45

Perhaps not worded properly, but "boundary only variables" only contain values at boundaries; therefore, their values is not defined in the interior. Any derived locator could contain section where those variables are not defined.

In your case, if the lines crossed a boundary, you will see the value to be undefined everywhere except at the crossing point.

In ANSYS CFX, any variable prefixed by Wall XXX is a boundary only quantity. Examples include Heat Flux, Wall Shear, Wall Irradiation, etc.

Hope the above helps,

November 14, 2014, 07:18	Monitor Heat Flux in CFD-Pre	#1
cltormar New Member Claudio Torregrosa Join Date: Sep 2013 Posts: 3 Rep Power: 13	Hi, I am running conjugate heat transfer transient CFX calculation of an air cooling system where the heat load to cool down is applied only during a short amount of time. I want to monitor the heat flux in the outlet of the cooling system. So I can obtain afterwards a curve of total Power/heat removed through the outlet as a function of time, and check how efficiently I am removing the heat. The problem is that, when I define the expression I want to monitor in CFX-Pre, which would be something that: areaInt(Heat Flux)@Outlet I got a cell error, as it seems Heat Flux is not a CFX-Pre variable. If you look at it in the variables tag of CFX-Pre you will not find it. This is very surprising, as Heat Flux it does appear as a variable in CFD-post. I tried using "Wall Heat Flux" which appears in CFX-Pre, but when I check in CFD-post "Heat Flux" and "Wall Heat Flux" do not give the same values at all for the case of an outlet. Do you know any reason why this is happening? Propose any solution? Thanks in advance Claudio Torregrosa CERN EN-STI-TCD Bld 865/R-D16 F-01631 CERN Cedex France Mobile: +41 76 7337 983 Office: +41 22 76 62663

November 18, 2014, 07:44	monito heat flux on lines	#3
carla menale New Member carla menale Join Date: Nov 2014 Posts: 3 Rep Power: 12	Hello! I have a problem to monitor the heat flux on line constructed with the option Surface->line/rake It's always zero, but the temperature is changing. I'm using: monitor->Surface->total Surface heat flux

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November 14, 2014, 11:01		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Unfortunately, you are correct. "Heat Flux" is a CFD-Post only variable at the moment. Wall Heat Flux is a similar variable available from CFX-Pre->CFX-Solver and CFD-Post as well. The main differences between Wall Heat Flux and Heat Flux are boundary edge effects when averaging from the faces values (integration points) to the vertices of the faces (a vertex is shared by several faces). If what you need is the energy flow through an inlet/outlet, you can always write the pieces that account for it. For a non-radiation case, the energy flow can be computed as Thermal Energy --> massFlowInt(Static Enthalpy)@Inlet Total Energy --> massFlowInt(Total Enthalpy)@Inlet Such calculation should be fairly close to areaInt(Heat Flux)@Inlet. Hope the above helps

November 19, 2014, 10:52		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	Heat Flux is a wall boundary variable, you can't just place a line in space or at in inlet/outlet and get a variable for heat flux.

November 20, 2014, 05:17		#5
carla menale New Member carla menale Join Date: Nov 2014 Posts: 3 Rep Power: 12	I'm not sure this is the problem. If I make a line in correspondence of a wall created with gambit, there is the heat flux. I plotted the wall made with gambit and the line made in corrispondence of this wall and the flux is the same. The problem is with other lines made inside the domain.

November 20, 2014, 12:45		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Perhaps not worded properly, but "boundary only variables" only contain values at boundaries; therefore, their values is not defined in the interior. Any derived locator could contain section where those variables are not defined. In your case, if the lines crossed a boundary, you will see the value to be undefined everywhere except at the crossing point. In ANSYS CFX, any variable prefixed by Wall XXX is a boundary only quantity. Examples include Heat Flux, Wall Shear, Wall Irradiation, etc. Hope the above helps,