Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species (mass transfer in the form of advection), either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

Heat conduction, also called diffusion, is the direct microscopic exchanges of kinetic energy of particles (such as molecules) or quasiparticles (such as lattice waves) through the boundary between two systems. When an object is at a different temperature from another body or its surroundings, heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described in the second law of thermodynamics.

Heat convection occurs when the bulk flow of a fluid (gas or liquid) carries its heat through the fluid. All convective processes also move heat partly by diffusion, as well. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". The former process is often called "forced convection." In this case, the fluid is forced to flow by use of a pump, fan, or other mechanical means.

Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid or gas). It is the transfer of energy by means of photons or electromagnetic waves governed by the same laws.^[1]

$\phi _{q}$ is (W/m²),

heat flux

$\rho$ is density (kg/m³),

$c_{p}$ is heat capacity at constant pressure (J/kg·K),

$\Delta T$ is the difference in temperature (K),

$v$ is velocity (m/s).

– Deposition, freezing and solid to solid transformation.

Solid

– Condensation and melting / fusion.

Liquid

– Boiling / evaporation, recombination/ deionization, and sublimation.

Gas

– Ionization.

Plasma

Applications[edit]

Architecture[edit]

Efficient energy use is the goal to reduce the amount of energy required in heating or cooling. In architecture, condensation and air currents can cause cosmetic or structural damage. An energy audit can help to assess the implementation of recommended corrective procedures. For instance, insulation improvements, air sealing of structural leaks or the addition of energy-efficient windows and doors.^[37]

Combined forced and natural convection

Heat capacity

Heat transfer enhancement

Heat transfer physics

Stefan–Boltzmann law

Thermal contact conductance

Thermal physics

Thermal resistance

Martin, Thomas (1951). . Bulletin of the British Society for the History of Science. 1 (6): 144–158). doi:10.1017/S0950563600000567. JSTOR 4024834.

"The Experimental Researches of Benjamin Thompson, Count Rumford"

Thompson, Benjamin (1 January 1786). Philosophical Transactions of the Royal Society of London. 76: 273–304. doi:10.1098/rstl.1786.0014.

"XIV. New experiments upon heat. By Colonel Sir Benjamin Thompson, Knt. F. R. S. In a letter to Sir Joseph Banks, Bart. P. R. S."

- (free download).

A Heat Transfer Textbook

- An online thermal fluids encyclopedia.

Thermal-FluidsPedia

- Overview

Hyperphysics Article on Heat Transfer

- a practical example of how heat transfer is used to heat buildings without burning fossil fuels.