I have an vertical enclosure that consists of 3 shelves, each with its own temperature and are thermally isolated from each other. We will call the shelves A, B, and C with A being the base shelf. The enclosure is surrounded by quiescent air. I have performed natural convection calculations before but they have always been for a wall at one temperature. In this case there are 3 different walls at 3 different temperatures but are all a part of the same vertical surface. The convection coefficient correlations in the books I have read deal with a wall at one temperature and the thermal boundary development starting at the base. In this case, the thermal boundary layer is already developed from shelf A by the time it reaches the base of shelf B (middle shelf). Do the same heat transfer correlations apply or do I have to do some sort of 'composite wall' type problem?