Thermal and mass convection - General principles

Heat transfer in moving fluids is called thermal convection. In a rigid material medium, the phenomenon is reduced to thermal conduction, but fluids are not rigid media, and are very rarely immobile, as low-intensity forces are enough to set them in motion.

Thermal convection is therefore the combination of two physical mechanisms: molecular diffusion (thermal conduction) and advection (entrainment by fluid movement). Similar laws govern mass convection, due to concentration gradients in a mixture.

In this field, the engineer's needs mainly concern the calculation of heat (or mass) flows on the walls that delimit the fluid domains, and knowledge of temperature fields (since limit temperatures often have to be respected for materials, and sometimes also for cooling fluids).

This article presents the physical aspects of convection: coupling with conduction, balances of the main extensive quantities (mass, momentum, energy), distinction between forced, natural and mixed convection (in laminar or turbulent regime), wall conditions.

The multiplicity of parameters to be taken into account encourages the use of similarity and dimensionless quantities, which are presented in relation to the source with which they are associated. Particular emphasis is placed on the distinction between similarity criteria, coupling parameters and simple dimensionless groupings, and on the practical use of these various quantities. Finally, a paragraph is devoted to fluids whose thermal diffusivity is much greater than their kinematic viscosity, such as liquid metals.