Hello Kremella Thank you very much for your response.
You are right. Convection is indeed a surface phenomenon. However, there is both conduction and convection happening in this setup. There is conduction from the constant heat source at the bottom, to the fin as well as the fluid, and also convection between the source and the fluid and the fin and the fluid.
Hence, I am going for a volume-averaged heat transfer coefficient. I will be calculating h_fins and h_fluid and will take a ratio and would plot it cell-wise. Then I have to do the same for the 8x8 and compare these plots.
Because this h_solid/h_fluid ratio is important for me, is why I need to know how I can find it cell-wise. Please let me know if it is possible to do that.
Thank you very much for your time!