Mixed convection refers to conditions when forced and natural (buoyancy-driven) effects are both important and neither one can be neglected. Situations in which forced and buoyancy-driven convection terms are of similar orders of magnitude obviously fall in the mixed-convection flow category. However, in many applications we deal with either a predominantly forced convective flow in which buoyancy-driven effects are small but considerable or a predominantly buoyancy-driven flow in which a nonnegligible forced-flow contribution is also present.

Mixed convection is relatively common in nature. In more recent applications, it occurs in rotating flow loops and in the cooling minichannels in the blades of modern gas turbines. In these flow loops, Coriolis centripetal forces arise because of the rotation. When the fluid is compressible, secondary flow caused by the centripetal effect contributes to the wall–fluid heat transfer.

Mixed-convection effects are not always undesirable. In some applications we may intentionally seek buoyancy effect in order to augment heat transfer. Some recent applications of supercritical fluids are examples to this point. The very large compressibility of these fluids, which is achieved without a phase change (although a pseudo–phase change does occur for near-critical fluids) is very useful.

In situations that are predominantly forced flow, buoyancy-driven effects have four types of impact on the overall flow field:

1. They contribute (assist, resist, or do both at different parts of the flow field) to the forced-flow velocity field.

2. They cause secondary flows. The secondary flows can enhance or reduce the heat transfer rate.

3. […]