Heat transfer

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(Redirected from Heat flow)

In thermal science, heat transfer is the passage of thermal energy from a hot to a cold body. When a physical body, e.g. an object or fluid, 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. Heat transfer always occurs from a hot body to a cold one, a result of the second law of thermodynamics. Transfer of thermal energy occurs mainly through conduction, convection or radiation. Heat transfer can never be stopped; it can only be slowed down.

Heat transfer is of particular interest to engineers, who attempt to understand and control the flow of heat through the use of thermal insulation, heat exchangers, and other devices. Heat transfer is typically taught as an undergraduate subject in both chemical and mechanical engineering curriculums.

1 Terminology
2 Radiation
3 Insulation and radiant barriers
4 Heat exchangers
5 Heat transfer in education
6 See also
7 Other fundamental engineering topics
8 References
9 Related Journals
10 External links

[edit] Terminology

Heat - a transfer of thermal energy (i.e., of energy and entropy) from hotter material to cooler material. Heat transfer may change the internal energy of materials.
Internal Energy — the internal vibrational energy that the molecules or electrons composing all materials contain (except at absolute zero)
Conduction — transfer of heat by electron diffusion or phonon vibrations (see below)
Convection — transfer of heat by conduction in a moving medium, such as a fluid (see below)
Radiation — transfer of heat by electromagnetic radiation or, equivalently, by photons(see below).

[edit] Radiation

Main article: Thermal 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 unattainable. No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum. A prime example of this is the energy of the Sun, which travels through the vacuum of space before warming the earth.

Shiny materials typically reflect radiant heat, just as they reflect visible light; dark materials typically absorb heat, just as they absorb visible light. In actuality, light is another a form of electromagnetic radiation with a shorter wavelength (and therefore a higher frequency) than heat radiation. The difference between visible light and radiant heat is small: they are simply different "colors" of electromagnetic radiation.

[edit] Insulation and radiant barriers

Main articles: Insulation and Radiant barrier

Thermal insulators are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both. Radiant barriers are materials which reflect radiation and therefore reduce the flow of heat from radiation sources. Good insulators are not necessarily good radiant barriers, and vice versa. Metal, for instance, is an excellent reflector and poor insulator.

The effectiveness of an insulator is indicated by its R- (resistance) value. The R-value of a material is the inverse of the conduction coefficient (k) multiplied by the thickness (d) of the insulator. The units of resistance value are in SI units: (K·m²/W)

Rigid fiberglass, a common insulation material, has an R-value of 4 per inch, while poured concrete, a poor insulator, has an R-value of 0.08 per inch.<ref>Two websites: E-star and Coloradoenergy</ref>

The effectiveness of a radiant barrier is indicated by its reflectivity, which is the fraction of radiation reflected. A material with a high reflectivity has a low emissivity, and vice versa (reflectivity = 1 - emissivity). An ideal radiant barrier would have a reflectivity of 1 and would therefore reflect 100% of incoming radiation.

[edit] Heat exchangers

Main article: Heat exchanger

A Heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. Heat exchangers are widely used in refrigeration, air conditioning, space heating, power production, and chemical processing. One common example of a heat exchanger is the radiator in a car, in which the hot radiator fluid is cooled by the flow of air over the radiator surface. It should be noted, however, that the term 'radiator,' when used in reference to an automotive application is a misnomer, as automotive 'radiators' function through convection and to a much lesser extent, conduction.

Common types of heat exchangers include parallel flow, counter flow, cross flow, shell and tube, and plate heat exchangers.

[edit] 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:

[edit] See also

[edit] Other fundamental engineering topics

[edit] References

Additional information from:

Welty, J., Wicks, Charles, E. & Wilson, R. (1984). Fundamentals of Momentum, Heat, and Mass Transfer. New York: John Wiley & Sons. ISBN 0-471-87497-3