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):
Me neither. These are my results with a large transient.
I still don’t know where the difference between my file and yours is to make that difference (From 136.1W to 49.97W). I have compute Sink temperature (51 Kº) from S2 and checked if S3 was radiating the expected value known it’s surface temperature 273.1 Kº and it is as expected. 284 W/m2 with an overall RFL (-49.97W) according to the formula (861)
Thanks for your contribution.
This shows the result with Elmer. But in Elmer, a sink temperature is not taken into account for a closed cavity, so no matter what value i enter here, it ends up always given the same results:
Using this input file (and only redefined cavity radiation because it has different syntax - I also had to set it to open cavity to define sink temperature) in Abaqus, I get 87.96 as total heat flux for SURFACE-1 and -45.41 for SURFACE-2.
Okay i did now the same in prepomax (open cavity and sinktemp. -1K) and in Elmer (open cavity, external temp 0K, radiation idealized):
prepomax -87.96 / -49.97
elmer -87.96 / -49.97
Now both values are negative.
In Elmer i needed to change the setting from “diffuse grey” to “idealized”. With diffuse grey i still get similar results for both surfaces. But Elmer needed way more iterations to converge (from 6 to 34), I need to get back to this later…
Edit
I should have taken a look at the temperatures first… From what i read, idealized radiation is not meant for heat transfer between bodies but only for heat flux to environment (the temperature ends up for the inner ring at 5401K).
I have built the equivalent problem but with spheres.
Same radius as the cylinders, and same BC and loads.
Here there is no open space in between S2 and S3. Cavity radiation active with negative sink temperature.
Nice, i had the same idea but didn’t get a similar even flux as you with your model. I used only one layer of elements as it didn’t make much difference, so maybe the unevenly shaped elemets (C3D20) in my mesh are a problem.
With this mesh I got lower deviations when I enter 273.15K (1) as sink temperature instead of the recommended negative Kelvin (2):
Your results looks good too.
My spheres have both the same number of elments (Copy + Scale) .9600 . Four layers.
I guess the space inbetween cylinders can’t be considered a cavity and a lot of radiation from the inside cylinder doesn’t find a target surface in the outer one.
I have also tried 2D plane stress to see if ccx computes the view factors considering the radiation is confined in the 2D plane (Space between cylinders would be a cavity) but no lucky.
Aproaching surfaces would deminish the amount of radiation lost through open extremes matching the energy balance too.
I can’t figure out which are the numbers to obtain those 136W.
