A difference in temperature between the surface of a body and the surrounding, unconfined fluid produces a gradient in density, which in turn generates fluid motion. This motion increases the rate of heat transfer between the body and the fluid over that corresponding to pure thermal conduction. The process of motion and heat transfer due to such motion is called free convection.

A difference in composition between the surface of the body and the surrounding fluid may also produce a gradient in density, hence fluid motion and enhanced transfer of species (mass transfer). Insofar as the net transfer of mass from the surface is small relative to the mass rate of flow, the rate of transfer of species can be inferred from the results herein for heat transfer. When a difference in temperature and a difference in composition both occur, the rates of heat and species transfer are affected by both differences.

Free convection may also occur as a result of other potential differences, such as surface tension and magnetic fields, but such special processes will not be considered here. Combined free and forced convection is discussed in Section 176 and Section 177.

A well established theory has been developed for free convection in the laminar boundary-layer regime. It provides a priori predictions and a fundamental structure for the correlation of experimental results. The development of computing facilities and techniques has led to numerical solutions for even a wider range of flow and conditions within the laminar regime. Even so, many problems of intrinsic and practical interest remain unresolved.

The theory of turbulent free convection is less well established. Numerical solutions based on eddy diffusivities for momentum and heat transfer are currently at a critical stage of development, and results of increasing reliability and extent are to be expected.