Learning Forum

[mmoataz]

I am using Fluent DO radiation model to simulate an electric baking oven containing:nAn electric heater (radiation source)nA reflector (specular from one side and diffuse from the other with no coating)nA baking plate (opaque diffuse wall with no coating)nOven walls (opaque diffuse wall with no coating)nIt is not clear to me how should radiative properties be set on the wall and the wall-shadow:nShould both of them have the same emissivity and diffuse-fraction or different?nIf they should be the same, does that mean the energy from radiation will be absorbed/reflected/transmitted by both? nCould it happen for example that rays keep reflecting between the wall and its shadow? (which is not wanted here)nIf they should be different then how to know which wall to set because sometimes I find the momentum tab enabled for the wall and some other times it will be enabled for the wall-shadow, which again confuses me which one is the solid side and which one is the fluid side?nShould I make the fluid side transparent and the solid side opaque? if yes then how?nPlease advise.nThank you.n

[Rob]

It depends. nWall and wall:shadow pairs allow you to set diffusivity and roughness to suit the neighbouring fluid (or solid) zone. If you give the coupled wall a thickness (and turn on shell conduction & transparency) then it'll behave like glass in that light can bounce around or be reflected off the glass to fluid surface having passed through the glass. nThere is a methodology behind which side is wall and which is shadow. The sanatoriums are full of those who've tried to figure it out: just click on the wall and check neighbouring zone. Or in the newer builds open the Mesh display and have a look in the Adjacency tool. n

[mmoataz]

Thanks for your fast reply Rob and for saving me from entering a sanatorium . Now I know for sure which side is which, however, still I am not sure if both the wall and its shadow should be set the same emissivity and diffuse-fraction?nFor example, in my case the polished steel reflector (mirror) has ~0.1 internal-emissivity and ~0.25 diffuse-fraction. nShould these values be set for both the wall and the shadow?nOr should I set the fluid-side to be transparent non absorbing (emissivity=0) and only set these values for the solid-side?n

[Rob]

Is the wall solid? Ie opaque? If so you need to set the emissivity etc to suit the surface(s) facing the fluid. The values can be different, the extreme being a wall which is white on one side and black on the other: that's why the wall & shadow pair exist. n

[mmoataz]

OK so based on your answer if we have a solid uncoated opaque wall, the emissivity and diffuse-fraction values shall be set on the wall adjacent to the fluid while the wall (or shadow) adjacent to the solid can be set to black (emissivity=1).nbecause if the wall adjacent to the solid is set a value of emissivity <1 it will mean that the total emissivity of the coupled system = emissivity of wall * emissivity of shadow. Is that correct?nI believe the only applications where emissivity for the wall adjacent to the solid can be <1 is when antireflective coatings (or similar) are used. Is that also correct?n

[Rob]

If the solid is opaque then surface emissivity on that side shouldn't make a difference. On the wet side you set the values to suit the material finish. The shadow (solid in this case) side will only affect the region it's attached to:it has no bearing on the other side. n

[mmoataz]

OK thank you very muchn