# Radiation test case

**URL:** <https://prepomax.discourse.group/t/radiation-test-case/1979>\
**Category:** General Questions\
**Created:** [September 3, 2024, 7:31am UTC](https://prepomax.discourse.group/t/radiation-test-case/1979 "2024-09-03T07:31:02Z")\
**Posts on this page:** 1\
**Showing post:** 41

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**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [September 15, 2024, 3:39pm UTC](https://prepomax.discourse.group/t/radiation-test-case/1979/41 "2024-09-15T15:39:58Z")

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Hi,

I am testing another case found here and need now help interpreting the results:  
[https://www.elmerfem.org/forum/viewtopic.php?t=5998](https://www.elmerfem.org/forum/viewtopic.php?t=5998)  
 ![sk](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/7/7ec6c5aa300478cf5d021ea49cafb1fbd93f6bdb.png)

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

 ![bc](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/1/172a88d86cd7e1b714490a094f1c316e8feeb6f0.png)

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](https://prepomax.discourse.group/uploads/short-url/lJroZAujD1oEwKjkC4lz1dM1vv1.pmx) (109.7 KB)

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