Heat flux

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In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, 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 ϕ → q {\displaystyle {\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.

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