Hello,
I have done a few thermo-electric simulations for power distribution equipment. My advice is to learn about conjugate CFD models. Where the body temperature affects fluid properties and flow field etc...
Also you assumptions on your transformer heat generation will be important for a good cooling model.
Some good places to read up on are:
cfd-online.com/books
you might also want to look for info on the finite volume method which is CFD method for computing flow field.