This is very complicated problem not something to cut your teeth on. you need to do one of the tutorial problems with multiple flow domains. The try a very small sub section of the fin and tube HX - like 5 fins and symmetry on either side with a short section of tube going through it - put it in a wind tunnel type box with ideal walls would be my suggestion. Then try something with a porous media in it. then try and figure out the heat exchange coefficients to apply to the porous media that give the same over all heat transfer as your little fin and tube model. Then remove the fins from the big model and replace it with the porous media. It is a bit brief but hopefully you get the general idea. It is not really practical to model all the fins directly and get a good result in reasonable time.