Hi,
I am testing another case found here and need now help interpreting the results:
https://www.elmerfem.org/forum/viewtopic.php?t=5998
While the original case worked well, I slightly changed it by thicken the surface mesh to 0.2m to have a 3D model and replaced the temperature on surface S1 by a Surface flux of 1000W/m^2. The surface S1 is now 0.0879m^2.
r1 = 0.07m ; r2 = 0.08m ; r3 = 0.14m ; r4 = 0.15m
emissivity is 0.9 for both surfaces (S2/S3) with sink Temp. -1K
Flux S1 = 1000W/m^2 ; Temp. S4 = 273.15K
My question is, when I look at the results for the TOTAL heatflux, it gives me 87.9W for S2 (what I expected) and 136.1W for S3, which i don’t understand. Shouldn’t it be nearly the same for both surfaces?
P=1000W/m^2 x 0.0879m^2=87.9W
If I increase the extrusion to 1, I get almost the same value for both surfaces (1,7% deviation).
Listed results for different extrusion lengths (all with the same mesh and one element layer thickness) + results via section print (q):
L=0.02m / S1=0.00879 m^2
S2 / S3
RFL=-8.796 / 12.2 (+38.7% dev.)
total surface flux (q) = 8.794497E+00 / -1.220213E+01
L=0.2m / S1=0.0879 m^2
S2 / S3
RFL=-87.96 / 136.1 (+54% dev.)
total surface flux (q) = 8.794497E+01 / -1.360838E+02
L=1m / S1=0.4396 m^2
S2 / S3
RFL=-439.8 / RFL= 432.3 (-1.7% dev.)
total surface flux (q) = 4.397248E+02 / -4.322784E+02
L=2m / S1=0.8793 m^2
S2 / S3
RFL=-879.6 / RFL= 786.8 (-10.6% dev.)
total surface flux (q) = 8.794497E+02 / -7.867447E+02
I guess I’m missing something fundamental 
pmx file:
radiation_3D.pmx (109.7 KB)