Heat flux

Heat flux
Heat flux
	Heat flux through a surface.
Common symbols
SI unit	W/m2
Other units	Btu/(h⋅ft2)
In SI base units	kg⋅s−3
Dimension

In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density^[1], heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m²). It has both a direction and a magnitude, and so it is a vector quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

Heat flux is often denoted ${\vec {\phi }}_{\mathrm {q} }$ , the subscript $q$ specifying heat flux, as opposed to mass or momentum flux. Fourier's law is an important application of these concepts.

^ The word "flux" is used in most physical disciplines to refer to the flow of a quantity (mass, heat, momentum, etc.) across a surface per unit time per unit area, with the primary exception being in electromagnetism, where it refers to the integral of a vector quantity through a surface. Refer to the Flux article for more detail.

[1] The word "flux" is used in most physical disciplines to refer to the flow of a quantity (mass, heat, momentum, etc.) across a surface per unit time per unit area, with the primary exception being in electromagnetism, where it refers to the integral of a vector quantity through a surface. Refer to the Flux article for more detail.

[1]

Heat flux
Heat flux ${\vec {\phi }}_{\mathrm {q} }$ through a surface.
Common symbols	${\vec {\phi }}_{\mathrm {q} }$
SI unit	W/m²
Other units	Btu/(h⋅ft²)
In SI base units	kg⋅s⁻³
Dimension	${\mathsf {M}}{\mathsf {T}}^{-3}$