HiSET Science: Heat Transfer

Heat Transfer
Heat transfer is the flow of thermal energy, which is measured by temperature. Heat will flow from warmer objects to cooler objects until an equilibrium is reached in which both objects are at the same temperature. Because the particles of warmer objects possess a higher kinetic energy than the particles of cooler objects, the particles of the warmer objects are vibrating more quickly and collide more often, transferring energy to the cooler objects in which the particles have less kinetic energy and are moving more slowly. Heat may be transferred by conduction, convection, or radiation. In conduction, heat is transferred by direct contact between two objects. In convection, heat is transferred by moving currents. In radiation, heat is transferred by electromagnetic waves.

Convection
Heat always flows from a region of higher temperature to a region of lower temperature. If two regions are at the same temperature, there is a thermal equilibrium between them and there will be no net heat transfer between them. Convection is a mode of heat transfer in which a surface in contact with a fluid experiences a heat flow. The heat rate for convection is given as q = hAΔT, where h is the convection coefficient. The convection coefficient is dependent on a number of factors, including the configuration of the surface and the nature and velocity of the fluid. For complicated configurations, it often has to be determined experimentally.
Convection may be classified as either free or forced. In free convection, when a surface transfers heat to the surrounding air, the heated air becomes less dense and rises, allowing cooler air to descend and come into contact with the surface. Free convection may also be called natural convection. Forced convection in this example would involve forcibly cycling the air: for instance, with a fan. While this does generally require an additional input of work, the convection coefficient is always greater for forced convection.

Conduction
Conduction is a form of heat transfer that requires contact.
Since heat is a measure of kinetic energy, most commonly vibration, at the atomic level, it may be transferred from one location to another or one object to another by contact. The rate at which heat is transferred is proportional to the material's thermal conductivity k, cross-sectional area A, and temperature gradient dT/dx, q = kA(dT/dx). If two ends of a rod are each held at a constant temperature, the heat transfer through the rod will be given as q = kA(TH – TL)/d, where d is the length of the rod. The heat will flow from the hot end to the cold end. The thermal conductivity is generally given in units of W/m-K. Metals are some of the best conductors, many having a thermal conductivity around 400 W/m-K. The thermal conductivity of wood is very small, generally less than 0.5 W/m-k. Diamond is extremely thermally conductive and may have a conductivity of over 2,000 W/m-k. Although fluids also have thermal conductivity, they will tend to transfer heat primarily through convection.

Radiation
Radiation heat transfer occurs via electromagnetic radiation between two bodies. Unlike conduction and convection, radiation requires no medium in which to take place. Indeed, the heat we receive from the sun is entirely radiation since it must pass through a vacuum to reach us. Every body at a temperature above absolute zero emits heat radiation at a rate given by the equation q = eσAT4, where e is the surface emissivity and σ is the Stefan-Boltzmann constant. The net radiation heat-transfer rate for a body is given by q = eσA(T4 – T04), where T0 is the temperature of the surroundings. Emissivity, which has a value between 0 and 1, is a measure of how well a surface absorbs and emits radiation. Dark-colored surfaces tend to have high emissivity, while shiny or reflective surfaces have low emissivity. In the radiation heat-rate equation, it is important to remember to use absolute temperature units, since the temperature value is being raised to a power.