solver radiative and convective heat fluxes in CFD-Post

user · July 29, 2010, 11:41

Hi,

is there any possibility to get the exact radiative and convective heat flux in CFD-Post? I mean the values given in the solver output file under section "H-Energy" and "I-Radiation".

In my case there are big differences (300 W vs. 960W) between "wall heat flux" and "heat flux" in CFD-Post. From the manual xmod.pdf at page 63/64 I know the difference between these two values. "Heat flux" is the value written by the solver, "wall heat flux" is calculated in CFD-Post. The "wall radiative heat flux" ist about 230 W and the "wall convective heat flux" is 70 W but as the manual states these values are affected by edge values of adjacent adiabatic walls.

In the output file are "correct" values of 230 W (radiative) and 730 W (convective) which sums nicely to 960 W. Is it possible to follow this balance in CFD-Post? Maybe there ist some expert parameter to export the radiative and convective heat fluxes directly to the results file?

stumpy · July 29, 2010, 16:46

I'd check the doc again. "wall heat flux" is not calculated by Post, it's the total heat flux on a wall, including radiaiton I think. It doesn't make sense to look at "heat flux" on a wall; you would look at that on an inlet for example to see the heat "flow".

user · July 30, 2010, 02:34

Thanks, you're right! Sorry, I misunderstood something.

From manual:
*************
Wall Heat Flux

This is the total heat flux into the domain, including convective and radiative contributions. When running the inhomogeneous heat transfer model, this variable is written per fluid.

Heat Flux
This is the total convective heat flux into the domain. This variable is different from the Wall Heat Flux in several ways:
- It includes the convective part of the heat flux as well as the radiative heat transfer, if it is included in the calculation.
- It can be plotted local to a specific boundary condition. Wall Heat Flux contains contributions from adjacent boundary conditions. For example, edge values of Heat Flux on a heat-transfer coefficient boundary are not affected by being adjacent to an adiabatic wall.
- It is computed on boundary vertices by the post processor directly from the convective energy flows written to the results file by the flow solver. This is sometimes advantageous over the Wall Heat Flux variable in that it eliminates the arithmetic averaging procedure used by the flow solver to compute Wall Heat Flux at each boundary vertex from the faces adjacent to each vertex.

****************

My question is: why is areaInt("Heat Flux","Face") the same as in solver output file (sum of I-Radiation and H-Energy), but areaInt("Wall Heat Flux","Face") is not? From the statement above I got, that "Heat Flux" is computed from convective energy flows, why is then the radiative energy flow included?
All I want is to follow the energy balance in CFD-Post, including radiative and convective heat fluxes - for this "Wall Heat Flux" is not working because of the above mentioned disadvantages...any Ideas?

user · August 3, 2010, 08:21

I just had a talk with CFX-support: up to now there is no differention of heat flux in radiative and convective part. "wall radiative heat flux" and "wall convective heat flux" will everytime sum to "wall heat flux" but not necessarily sum to "heat flux", which is the exact value for (im)balance-calculations. Therefore everybody should check that "wall heat flux" is same "heat flux", which is the same as in output file (sum of I-Radiation and H-Energy of this boundary).

A feature-request is started to write out exact values to result-files (maybe as "radiative heat flux" and "convective heat flux")...

July 29, 2010, 11:41	solver radiative and convective heat fluxes in CFD-Post	#1
user New Member Join Date: Apr 2009 Posts: 12 Rep Power: 16	Hi, is there any possibility to get the exact radiative and convective heat flux in CFD-Post? I mean the values given in the solver output file under section "H-Energy" and "I-Radiation". In my case there are big differences (300 W vs. 960W) between "wall heat flux" and "heat flux" in CFD-Post. From the manual xmod.pdf at page 63/64 I know the difference between these two values. "Heat flux" is the value written by the solver, "wall heat flux" is calculated in CFD-Post. The "wall radiative heat flux" ist about 230 W and the "wall convective heat flux" is 70 W but as the manual states these values are affected by edge values of adjacent adiabatic walls. In the output file are "correct" values of 230 W (radiative) and 730 W (convective) which sums nicely to 960 W. Is it possible to follow this balance in CFD-Post? Maybe there ist some expert parameter to export the radiative and convective heat fluxes directly to the results file?

July 30, 2010, 02:34		#3
user New Member Join Date: Apr 2009 Posts: 12 Rep Power: 16	Thanks, you're right! Sorry, I misunderstood something. From manual: *********** Wall Heat Flux This is the total heat flux into the domain, including convective and radiative contributions. When running the inhomogeneous heat transfer model, this variable is written per fluid. Heat Flux This is the total convective heat flux into the domain. This variable is different from the Wall Heat Flux in several ways: - It includes the convective part of the heat flux as well as the radiative heat transfer, if it is included in the calculation. - It can be plotted local to a specific boundary condition. Wall Heat Flux contains contributions from adjacent boundary conditions. For example, edge values of Heat Flux on a heat-transfer coefficient boundary are not affected by being adjacent to an adiabatic wall. - It is computed on boundary vertices by the post processor directly from the convective energy flows written to the results file by the flow solver. This is sometimes advantageous over the Wall Heat Flux variable in that it eliminates the arithmetic averaging procedure used by the flow solver to compute Wall Heat Flux at each boundary vertex from the faces adjacent to each vertex. ************** My question is: why is areaInt("Heat Flux","Face") the same as in solver output file (sum of I-Radiation and H-Energy), but areaInt("Wall Heat Flux","Face") is not? From the statement above I got, that "Heat Flux" is computed from convective energy flows, why is then the radiative energy flow included? All I want is to follow the energy balance in CFD-Post, including radiative and convective heat fluxes - for this "Wall Heat Flux" is not working because of the above mentioned disadvantages...any Ideas? chek321 likes this.

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July 29, 2010, 16:46		#2
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	I'd check the doc again. "wall heat flux" is not calculated by Post, it's the total heat flux on a wall, including radiaiton I think. It doesn't make sense to look at "heat flux" on a wall; you would look at that on an inlet for example to see the heat "flow".

August 3, 2010, 08:21		#4
user New Member Join Date: Apr 2009 Posts: 12 Rep Power: 16	I just had a talk with CFX-support: up to now there is no differention of heat flux in radiative and convective part. "wall radiative heat flux" and "wall convective heat flux" will everytime sum to "wall heat flux" but not necessarily sum to "heat flux", which is the exact value for (im)balance-calculations. Therefore everybody should check that "wall heat flux" is same "heat flux", which is the same as in output file (sum of I-Radiation and H-Energy of this boundary). A feature-request is started to write out exact values to result-files (maybe as "radiative heat flux" and "convective heat flux")...