Heat transfer
From Wikipedia, the free encyclopedia
In thermal physics, 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 only through conduction, convection, radiation or any combination of these. 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 undergraduate and graduate subjects in both chemical and mechanical engineering curricula.
- 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).
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[edit] Conduction
Conduction is the transfer of thermal energy from a region of higher temperature to a region of lower temperature through direct molecular communication within a medium or between mediums in direct physical contact without a flow of the material medium. The transfer of energy could be primarily by elastic impact as in fluids or by free electron diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. In liquids (except liquid metals) and gases, the molecules are usually further apart, giving a lower chance of molecules colliding and passing on thermal energy.
Metals(eg. copper) are usually the best conductors of thermal energy. 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 form a crystalline structure, greatly aiding in the transfer of thermal energy.
Fluids (liquids (except liquid metals) and gasses) are not typically good conductors. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. As density decreases so does conduction. Conductivity of gases increases with temperature but only slightly with pressure near and above atmospheric. Conduction does not occur at all in a perfect vacuum.
To quantify the ease with which a particular medium conducts, engineers employ the thermal conductivity, also known as the conductivity constant or conduction coefficient, k. The main article on thermal conductivity defines k as "the quantity of heat, Q, transmitted in time t through a thickness L, in a direction normal to a surface of area A, due to a temperature difference ΔT [...]." Thermal conductivity is a material property that is primarily dependent on the medium's phase, temperature, density, and molecular bonding.
A heat pipe is a passive device that is constructed in such a way that it acts as though it has extremely high thermal conductivity.
[edit] Convection
Convection is a combination of conduction and the transfer of thermal energy by circulation or movement of the hot particles to cooler areas in a material medium. This movement occurs from or to a fluid or within a fluid. In solids, molecules keep their relative position to such an extent that bulk movement or flow is prohibited.
Convection occurs in two forms: natural and forced convection.
In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. The driving force for natural convection is buoyancy, a result of differences in fluid density when gravity or another body force is present.
Forced convection, by contrast, occurs when pumps, fans or other means are used to propel the fluid and create an artificially induced convection current. Forced heat convection is sometimes referred to as heat advection, or sometimes simply advection for short. But advection is a more general process, and in heat advection, the substance being "advected" in the fluid field is simply heat (rather than mass, which is the other natural component in such situations, as mass transfer and heat transfer share generally the same equations).
In some heat transfer systems, both natural and forced convection contribute significantly to the rate of heat transfer.
To calculate the rate of convection between an object and the surrounding fluid, engineers employ the heat transfer coefficient, h. Unlike the thermal conductivity, the heat transfer coefficient is not a material property. The heat transfer coefficient depends upon the geometry, fluid, temperature, velocity, and other characteristics of the system in which convection occurs. Therefore, the heat transfer coefficient must be derived or found experimentally for every system analyzed. Formulae and correlations are available in many references to calculate heat transfer coefficients for typical configurations and fluids.
[edit] Radiation
Radiation is transfer of heat through electromagnetic radiation. Hot or cold, all objects radiate energy at a rate equal to their emissivity times the rate at which energy would radiate from them if they were a black body. No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum. The energy from the Sun, travels through the vacuum of space before warming the earth. Also, the only way that energy can leave earth is by being radiated to space.
Both reflectivity and emissivity of all bodies is wavelength dependent. The temperature determines the wavelength distribution of the electromagnetic radiation as limited in intensity by Plank’s law of black-body radiation. For any body the reflectivity depends on the wavelength distribution of incoming electromagnetic radiation and therefore the temperature of the source of the radiation while the emissivity depends on the wave length distribution and therefore the temperature of the body itself. For example, fresh snow, which is highly reflective to visible light, (reflectivity about 0.90) appears white due to reflecting sunlight with a peak energy wavelength of about 0.5 micron. Its emissivity, however, at a temperature of about -5C, peak energy wavelength of about 12 microns, is 0.99.
Gases absorb and emit energy in characteristic wavelength patterns that are different for each gas.
Visible light is simply another form of electromagnetic radiation with a shorter wavelength (and therefore a higher frequency) than infrared radiation. The difference between visible light and the radiation from objects at conventional temperatures is small: they are simply different "colors" of electromagnetic radiation.
[edit] Insulation and radiant barriers
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.[1]
The effectiveness of a radiant barrier is indicated by its reflectivity, which is the fraction of radiation reflected. A material with a high reflectivity (at a given wavelength) has a low emissivity (at that same wavelength), and vice versa (at any specific wavelength, reflectivity = 1 - emissivity). An ideal radiant barrier would have a reflectivity of 1 and would therefore reflect 100% of incoming radiation. Vacuum bottles (Dewars) are 'silvered' to approach this. In space vacuum, satellites use multi-layer insulation which consists of many layers of aluminized (shiny) mylar to greatly reduce radiation heat transfer and control satellite temperature.
[edit] Heat exchangers
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.
Common types of heat exchangers include parallel flow, counter flow, cross flow, shell and tube, and plate heat exchangers.
the example of heat exchanger in which fluids are directly contacted is cooling tower widley find application in process industry. the types of heat exchanger are: 1. shell and tube type 2. double pipe type. 3. fin fan cooler. 4. spiral type. 5. u-tube type
its very good to see these exchangers working in process plant for heating plus sometime cooling purpose also.
[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:
- Automotive engineering
- Thermal management of electronic devices and systems
- HVAC
- Insulation
- Materials processing
- Power plant engineering
[edit] See also
- Heat
- Thermal contact conductance
- Thermal insulation
- Thermal physics
- Thermal science
- Heat transfer mechanisms
[edit] Other fundamental engineering topics
[edit] References
- ^ Two websites: E-star and Coloradoenergy
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
[edit] Related Journals
- Heat Transfer Engineering[1]
- Experimental Heat Transfer[2]
- International Journal of Heat and Mass Transfer[3]
- ASME Journal of Heat Transfer[4]
- Numerical Heat Transfer Part A[5]
- Numerical Heat Transfer Part B[6]
- Nanoscale and Microscale Thermophysical Engineering[7]
[edit] External links
- Heat Transfer Podcast - Arun Majumdar - Department of Mechanical Engineering - University of California, Berkeley
- Heat Transfer Basics - Overview
- A Heat Transfer Textbook - Downloadable textbook (free)
- Hyperphysics Article on Heat Transfer - Overview
- Heat transfer fundamentals
