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Category# Heat transfer physics

Summary

Heat transfer physics describes the kinetics of energy storage, transport, and energy transformation by principal energy carriers: phonons (lattice vibration waves), electrons, fluid particles, and photons. Heat is energy stored in temperature-dependent motion of particles including electrons, atomic nuclei, individual atoms, and molecules. Heat is transferred to and from matter by the principal energy carriers. The state of energy stored within matter, or transported by the carriers, is described by a combination of classical and quantum statistical mechanics. The energy is different made (converted) among various carriers.
The heat transfer processes (or kinetics) are governed by the rates at which various related physical phenomena occur, such as (for example) the rate of particle collisions in classical mechanics. These various states and kinetics determine the heat transfer, i.e., the net rate of energy storage or transport. Governing these process from the atomic level (atom or molecule length scale) to macroscale are the laws of thermodynamics, including conservation of energy.
Heat is thermal energy associated with temperature-dependent motion of particles. The macroscopic energy equation for infinitesimal volume used in heat transfer analysis is
where q is heat flux vector, −ρcp(∂T/∂t) is temporal change of internal energy (ρ is density, cp is specific heat capacity at constant pressure, T is temperature and t is time), and is the energy conversion to and from thermal energy (i and j are for principal energy carriers). So, the terms represent energy transport, storage and transformation. Heat flux vector q is composed of three macroscopic fundamental modes, which are conduction (qk = −k∇T, k: thermal conductivity), convection (qu = ρcpuT, u: velocity), and radiation (, ω: angular frequency, θ: polar angle, Iph,ω: spectral, directional radiation intensity, s: unit vector), i.e., q = qk + qu + qr.
Once states and kinetics of the energy conversion and thermophysical properties are known, the fate of heat transfer is described by the above equation.

Official source

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