Varmeflux

Varmeflux, or heat flux, is the rate at which thermal energy crosses a surface per unit area. It is expressed in watts per square meter (W/m^2). The direction of heat flux follows the physical path of energy transfer, with positive values typically representing heat flowing from regions of higher temperature to lower temperature.

Heat flux can arise from conduction, convection, or radiation. In one-dimensional conduction, Fourier's law gives q" = -k dT/dx, where k is the thermal conductivity and dT/dx is the temperature gradient. For convection, the local heat flux is q" = h (T_s - T∞), with h the convective heat transfer coefficient and T_s the surface temperature. For thermal radiation, q" = ε σ (T^4_s - T^4_sur), where ε is emissivity and σ is the Stefan–Boltzmann constant.

Measuring heat flux involves sensors such as thermopile or heat-flux meters, or experimental setups like guarded hot plates and heat-flow meters. This quantity is central to evaluating building envelope performance, insulation effectiveness, and thermal management in electronics and industrial processes.

In engineering analyses, the total heat transfer rate q is the product of heat flux and area, q = q" A, and Fourier’s law for conduction leads to q" = k ΔT / L for a slab of thickness L with a linear temperature difference ΔT. Varmeflux is thus a key parameter in designing energy-efficient systems and assessing thermal safety.