Everything about Heat Flow totally explained
In
thermal physics,
heat transfer is the passage of
thermal energy from a hot to a colder body. When a physical body, for example an object or fluid, is at a different
temperature than its
surroundings or another body,
transfer of thermal energy, also known as heat transfer, or
heat exchange, 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. Where there's a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed down.
Overview
Classical transfer of thermal energy occurs only through
conduction,
convection,
radiation or any combination of these. Heat transfer associated with carriage of the heat of phase change by a substance (such as steam which carries the heat of boiling) can be fundamentally treated as a variation of convection heat transfer. In each case, the driving force for heat transfer is a difference of temperature. There are 3 types of heat transfer known as convection, conduction and radiation.
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
chemical,
electrical and
mechanical engineering curricula.
- Heat — a transfer of thermal energy, (for example, 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).
- Phase change — transfer of heat by the potential energy associated with the heat of phase change, such as boiling, condensation, or freezing.
- 1-D Application Using Thermal Circuits — application of heat transfer analysis using the concept of thermal circuits.
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.
Heat conduction is directly analogous to diffusion of particles into a fluid, in the situation where there are no fluid currents. This type of heat diffusion differs from mass diffusion in behaviour, only in as much as it can occur in solids, whereas mass diffusion is limited to fluids.
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.
As
density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature but only slightly with pressure near and above atmospheric. Conduction doesn't 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's primarily dependent on the medium's
phase, temperature, density, and molecular bonding.
A
heat pipe is a passive device that's constructed in such a way that it acts as though it has extremely high thermal conductivity. Note that conduction always takes place from higher to lower temperature.
Convection
Convection is a combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium. Unlike the case of pure conduction, now
currents in fluids are additionally involved in convection. This movement occurs into a
fluid or within a fluid, and can't happen in solids. In solids, molecules keep their relative position to such an extent that
bulk movement or flow is prohibited, and therefore convection doesn't occur.
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 any type of acceleration is present in the system.
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.
Radiation
Radiation is the 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 Planck’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 micrometres. Its emissivity, however, at a temperature of about -5C, peak energy wavelength of about 12 micrometres, 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 a factor of about 20 in frequency and wavelength; the two kinds of emission are simply different "colors" of electromagnetic radiation.
Newton's law of cooling
A related principle,
Newton's law of cooling, states that
the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, or environment.
The law is
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