Introduction to Engineering Heat Transfer

Chapters 7 through 9 discuss forced convection problems. In a forced convection problem the fluid is driven externally over a surface (for example by a fan or a pump). Free (or natural) convection refers to a problem where, in the absence of a temperature difference between the surface and the fluid, the fluid would be completely quiescent. However, because the density of most fluids depends at least weakly on temperature, the heating or cooling of the fluid leads to density gradients and an imbalance in the buoyancy forces (i.e., forces related to the action of gravity) that may cause fluid motion. The fluid motion in a free convection situation is fundamentally driven by density gradients that are induced in the fluid as it is heated or cooled due to the presence of a surface. The velocities induced by these density gradients are typically small and therefore the absolute magnitude of natural convection heat transfer coefficients is also small compared to forced convection values.