Heat flux (W/m^{2}) is the rate of thermal energy flow per unit surface area of heat transfer surface, e.g., in a heat exchanger.

Heat flux is the main parameter in calculating heat transfer. A generalized classification distinguishes between heat fluxes by *convection*, heat *conduction*, and *radiation*. The heat flux vector is directed towards regions of lower temperature.

Convective heat flux is proportional to the temperature difference between solid, liquid, or gaseous media participating in heat transfer. A heat transfer coefficient serves as the proportionality factor.

Under heat conduction, the heat flux vector is proportional to and usually parallel to the temperature gradient vector. However, in anisotropic bodies the direction of the two vectors may not coincide.

In the case of simultaneous heat and mass transfer the effective heat flux may substantially, by several orders of magnitude, exceed the value due to heat conduction only.

The radiative heat flux is a flux of electromagnetic radiation and, in contrast to convection and heat conduction, may occur without any intervening medium, i.e., it can occur through a vacuum. For an idealized black body the radiation heat flux is described by Planck's law. The actual radiation flux values can be only lower than this idealized value.

Heat & Mass Transfer, and Fluids Engineering