In our discussion of forced convection in Chapters 22 and 23, we noted that the convecting fluid was forced through or past the heat transfer surface by an external mover such as a pump, fan, or blower. In free or natural convection, the fluid is set into motion by density differences between the confining surface and the bulk of the fluid. These density differences give rise to buoyant forces that circulate the convective fluid (either a liquid or a gas) and transfer heat between the surface and the fluid.