Varmeoverføringsydelse

Varmeoverføringsydelse is a Danish term used in thermodynamics and engineering to denote the rate at which thermal energy is transferred between bodies or inside a system due to a temperature difference. The quantity is commonly measured in watts (W) and is often denoted by Q̇. The magnitude of varmeoverføringsydelse depends on the temperature difference between the surroundings and the body, the geometry and surface area, and the thermal properties of the involved materials and media.

In steady-state one-dimensional systems it can be described by different transfer mechanisms: conduction, convection, and radiation. Conduction follows Fourier's law: Q̇ = k A ΔT / L, where k is the thermal conductivity. Convection is described by Newton's law of cooling: Q̇ = h A ΔT, with h the convective heat transfer coefficient. Thermal radiation follows the Stefan-Boltzmann law: Q̇ = ε σ A (T^4_s - T^4_env), with ε emissivity and σ the Stefan-Boltzmann constant. In practical calculations, these mechanisms are often combined into an overall heat transfer coefficient U, giving Q̇ = U A ΔT_lm for a system with temperature difference across a surface.

Varmeoverføringsydelse is a central concept in designing thermal systems, such as heat exchangers, building envelopes, electronic cooling, and HVAC installations. It informs assessments of energy efficiency, safety, and thermal performance.