heattransfers

Heat transfer is the movement of thermal energy between objects or substances due to a temperature difference. It occurs from regions of higher temperature to lower temperature and does not require bulk material movement, though convection involves fluid flow that enhances transfer. The rate of heat transfer is measured in watts.

Heat can be transferred by three primary mechanisms. Conduction occurs through solids (and still air) by molecular interaction and lattice vibrations. Fourier's law describes the heat flux q" = -k dT/dx, where k is the material’s thermal conductivity. Convection involves heat transfer between a surface and a moving fluid; the rate is q" = h (T_surface - T_fluid), with h as the heat transfer coefficient. Convection can be natural or forced depending on fluid motion. Radiation transfers energy via electromagnetic waves; the rate is q" = ε σ (T_surface^4 - T_sur^4), where ε is emissivity and σ is the Stefan–Boltzmann constant.

In engineering practice, heat transfer is analyzed with quantities such as heat transfer rate Qdot, total heat Q, thermal resistance, and the overall heat transfer coefficient U. For steady-state conduction through a slab, Qdot can be expressed using q" and area A. In heat exchangers, the log mean temperature difference (LMTD) and efficiency depend on ΔT_lm. Transient behavior involves thermal mass and time constants, while conduction in solids relates to characteristic times L^2/α, where α is thermal diffusivity.

Applications span building insulation, electronic device cooling, automotive radiators, cooking, refrigeration, and industrial processing. Understanding heat transfer helps design systems that manage temperature, improve energy efficiency, and protect materials from thermal damage.