Heat transfer

When a physical body (object, fluid, etc.) is at a different temperature than its surroundings or another body, transfer of thermal energy, also known as heat transfer, occurs in such a way that the body and the surroundings reach thermal equilibrium. Thus, heat transfer always occurs from a hot material to a cold one (this is due to the second law of thermodynamics). Transfer of thermal energy occurs mainly through conduction, convection or radiation.

Conduction

Conduction is the transfer of thermal energy through free electron diffusion or particle vibration, without a flow of the material medium. In other words, heat is transfered by conduction when adjacent atoms vibrate against one another, or as electrons move from atom to atom. Conduction occurs mainly in solids, as in liquids and gases the particles are further apart, hence the chances of particles colliding and passing on thermal energy is lowered.

The best conductors of thermal energy are metals. This is due to the way that metals are chemically bonded. metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons and a crystalline structure, greatly aiding in the transfer of thermal energy.

Convection

Convection is the transfer of thermal energy by circulation of the heated material medium. That is to say that convection occurs from or to a fluid. When a fluid is heated, it becomes less dense and rises. The fluid which is cooler then sinks as it is denser. It is then reheated and the process continues forming a convection current. Convection occures mainly in fluids as it requires the bulk movement of particles, which is not possible in solids, where particles are tightly packed.

Radiation

Radiation is transfer of heat through electromagnetic radiation in the heat spectrum. Hot or cold, all objects radiate heat—unless they are at absolute zero, which is believed to be unattainable. No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum. Indeed, the radiative heat of the Sun, necessary for life on earth, travels through the vacuum of space before warming the earth.

Reflective, shiny materials typically reflect radiant heat, just as they reflect visible light; dark materials typically absorb heat, just as they absorb visible light. Indeed, light is another a form of electromagnetic radiation with a shorter wavelength and higher frequency. The difference between visible light and radiant heat is small: they are simply different "colors" of electromagnetic radiation.

Heat transfer in education

Heat transfer is typically studied as part of a general chemical engineering or mechanical engineering curriculum. Typically, thermodynamics is a prerequisite to undertaking a course in heat transfer, as the laws of thermodynamics are essential in understanding the mechanism of heat transfer. Other courses related to heat transfer include energy conversion, thermofluids and mass transfer.

Heat transfer methodologies are used in the following disciplines, among others:

Sources

Welty, J., Wicks, Charles, E. & Wilson, R. (1984). Fundamentals of Momentum, Heat, and Mass Transfer. New York: John Wiley & Sons. ISBN 0471874973

External links

Heat Transfer Basics - Overview