radiation module, no accuracy result

Young

I'm modeling a radiation problem. In my model, I have some lamps hanging above a substrate. I give a pre-determined temperature to the lamps and want to see the radiation distribution of these lamps on my substrate. I use gambit to set out the model and mesh the models. I mesh the volume with Tet/Hybrid Tgrid type and set the interval size to 3. After setting my boundary conditions, I get a very large mesh file, about several hundred MBytes.

I use DO radiation model in fluent, and set theta divisions and phi divisions to 10 and use parallel mode. After a very long time calculation (3days), I get my results. However, the results which is the radiation along a side of substrate are not smooth at all. And I find the result is very different from that when I set the volume mesh size to 6 (which requires a shorter time but shows rougher data). Could you tell me why this happens and give me some advices for meshing my model?

Thanks very much for your help.