View Factors

ferbf · June 26, 2019, 06:50

Hi everybody,

I'm new in ANSYS CFX, so sorry if I don't express my ideas properly.

Could someone tell me how to calculate view factors in CFX?

I have read in other posts that ANSYS CFX don't use view factors for calculations (shape factor), but i think that it is possible to calculate this factors using Monte Carlo method, but i don't know how.

Thanks in advance.

ghorrocks · June 26, 2019, 07:55

The Monte Carlo method does not calculate view factor as it works our radiative transfer by the Monte Carlo method (obviously), which does ray tracing on each photon path so does not need a view factor. Instead it requires lots of rays to be traced so that the sum total approximates the view factor.

The method which comes closest to using a view factor is the Discrete transfer model. Investigate this model to see if you can get view factor out of it.

It has been a while since I did radiation modelling but I think that is correct

ferbf · June 26, 2019, 08:48

Thank you, ghorrocks.

Do you know where could I do it? I've been trying to find it in "Variable Selector" in CFD-Post, but it isn't there.

Opaque · June 26, 2019, 10:24

Perhaps going back to the definition of view factor will help to figure out the approach

View Factor is the ratio between the energy leaving a surface, and the amount arriving at another by any possible means.

If you are interested in a specific view factor between surface A, and surface B. You can setup a problem with all boundaries being blackbodies, i.e. Emissivity of 1, all boundaries temperatures set to 0 [K], and set the temperature of A to whatever you want, say 1000 [K].

You now deactivate all the models but radiation, and solve the problem. The view factor between A and any of the other surfaces is the ratio

areaInt(Wall Irradiation Flux)@SurfaceOfInterest / Total Emission atSurface A

It is a costly process if you want the ratio between other surfaces.

Good luck

ferbf · June 26, 2019, 10:45

Thank you Opaque for your answer.

I will try using this method, which seems to be reasonable and intelligent.

Thank both you for your help

June 26, 2019, 06:50	View Factors	#1
ferbf New Member Join Date: Jun 2019 Posts: 3 Rep Power: 7	Hi everybody, I'm new in ANSYS CFX, so sorry if I don't express my ideas properly. Could someone tell me how to calculate view factors in CFX? I have read in other posts that ANSYS CFX don't use view factors for calculations (shape factor), but i think that it is possible to calculate this factors using Monte Carlo method, but i don't know how. Thanks in advance. Last edited by ferbf; June 26, 2019 at 08:42.

June 26, 2019, 07:55		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The Monte Carlo method does not calculate view factor as it works our radiative transfer by the Monte Carlo method (obviously), which does ray tracing on each photon path so does not need a view factor. Instead it requires lots of rays to be traced so that the sum total approximates the view factor. The method which comes closest to using a view factor is the Discrete transfer model. Investigate this model to see if you can get view factor out of it. It has been a while since I did radiation modelling but I think that is correct __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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June 26, 2019, 08:48		#3
ferbf New Member Join Date: Jun 2019 Posts: 3 Rep Power: 7	Thank you, ghorrocks. Do you know where could I do it? I've been trying to find it in "Variable Selector" in CFD-Post, but it isn't there.

June 26, 2019, 10:24		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Perhaps going back to the definition of view factor will help to figure out the approach View Factor is the ratio between the energy leaving a surface, and the amount arriving at another by any possible means. If you are interested in a specific view factor between surface A, and surface B. You can setup a problem with all boundaries being blackbodies, i.e. Emissivity of 1, all boundaries temperatures set to 0 [K], and set the temperature of A to whatever you want, say 1000 [K]. You now deactivate all the models but radiation, and solve the problem. The view factor between A and any of the other surfaces is the ratio areaInt(Wall Irradiation Flux)@SurfaceOfInterest / Total Emission atSurface A It is a costly process if you want the ratio between other surfaces. Good luck

June 26, 2019, 10:45		#5
ferbf New Member Join Date: Jun 2019 Posts: 3 Rep Power: 7	Thank you Opaque for your answer. I will try using this method, which seems to be reasonable and intelligent. Thank both you for your help