Discussion Forum

Hi

in radiation exchange, of complex geoemetries, it's correct that the detailed derivation of the view factors is primordial, bit also particularly challenging, few programmes do it correctly.

I havent studied enough the radiation exchange part of COMSOL to clearly state what is being done and in which cases. So I can only recomment to test it on simple cases to ensure one understands clearly what is included, such to complement when one have a prolem to solve that needs additional tweaking

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Good luck
Ivar

thanks Ivar

I could not find any tool or option for field of view. it is different than view factor I think.

view factor means that your face can get whatever radiation comes from the field of view of about 180 degree but when you talk about field of view it means that you suppose your opbjet can just see whatever is in its for example 80 degree of view.

could I say my mean? like human filed of view is 180 degree but a bird can see all around and has a 360 degree filed of view so it does not have any thing to do with the relation of two object and is just for each.

I searched and think comsol does not have it so just wanted to be sure!

Thanks,
Navid

Hi,

the main points you have to know are that Comsol assumes gray and diffuse radiation and that the Hemicube method takes care of shadowing. Out of these facts, you can tell how radiation exchange is managed.

Diffuse:
The radiation of a surface has a 'field of view' of 180°. According to Lambert's law, the power of radiation per (fixed) area into a specific direction tends to zero, while the angle of the direction tends to 90° (angle betweeen normal vetor of surface and direction). But the the power of radiation per projected area (from a virtual POV) is constant, because the decrease of the power of radiation per area is exactly compensated by the increase of the projected area.

You have (I guess so) no access to specify this behavior in Comsol.

Gray:
The total power per area (including all wave lengths and directions) is calculated by Stefan-Boltzmann-law (black body) multiplied with the Total Emissivity e (0 <= e <= 1)

You have access by the total emissivity.


Maybe, in a few next releases, Comsol inlcudes more radiation models like Multiband or Multigray/Weighted Sum of Gray Gases or some Monte Carlo stuff. Then it would be much more flexible for radiation exchange.


best regards

Thank you both


Ivar do you know which models should I combine to consider either a two phase fluid flow woth level set method and heat transfer in fluid?

I do the named models but it looks like it does not consider the heat transfer with fluid flow and just considers the heat like diffusing in solid!!

do you know any proper example.

Hi again Ivar

here is a two phase model, level set method combined with heat transfer in fluid.

first of all as you see the front is not good enough when it goes further and expands and does not show a good front for fluid and it is like a range than a front. I changed gamma and epsilon (coefficients of level set function in COMSOL a lot but when I want to decrease epsilon which is the thickness of front, smaller values than 4e-4 it says failed to find consistent initial values :( ) so the results with this range of expansion for the front are not satisfactory. overall front is not satisfactory.


next, as you see there is another block connected to the fluid flow way I want to couple it to exchange heat with fluid pass way and be in connection with the fluid wall but whatever I do (define heat transfer in solid and boundaries and run) either it does not consider that part in thermal contours or says failed to calculate Jacobian matrix :(. (which parts should be considered as fluid, heat transfer in fluid, heat transfer in solid??what happens in real physics does not answer)

could you please help me!

Thanks,