Varmeoverføringskoefficienten

Varmeoverføring, or heat transfer, is the study of the movement of thermal energy between systems or within a material due to a temperature difference. It occurs through three fundamental mechanisms: conduction, convection, and radiation.

Conduction is energy transfer within a substance or between substances in direct contact, driven by temperature gradients. In solids it proceeds mainly through lattice vibrations and, in metals, free electrons. The local heat flux is described by Fourier's law q = -k ∇T, where k is thermal conductivity.

Convection involves fluid motion that carries energy. It can be natural, driven by buoyancy, or forced, produced by fans or pumps. The heat transfer rate at a surface is often described by q = hA(Ts - T∞), where h is the convective heat-transfer coefficient and Ts is the surface temperature. Radiation is transfer by electromagnetic waves and occurs independently of a medium. All bodies emit thermal radiation proportional to T^4, described by the Stefan-Boltzmann law q = εσA(T^4s - T^4∞), with ε the emissivity and σ the Stefan-Boltzmann constant.

In engineering practice, overall heat transfer combines these mechanisms and depends on material properties, geometry, and boundary conditions. The concept of thermal resistance helps quantify losses or gains, and analyses may use analytical solutions for simple cases or numerical methods such as finite difference or finite element methods. Applications include building insulation, cooling of electronic devices, and heat exchangers in industry. Advances in materials, including phase-change materials and aerogels, influence efficiency and safety in thermal design.