Author Topic: Heat transfer  (Read 4034 times)

jack

  • Guest
Heat transfer
« on: September 28, 2007, 02:07:57 AM »
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. Heat transfer can never be stopped; it can only be slowed down.

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) is sometimes treated as a variety of convection heat transfer.

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).
Phase change potential -- transfer of heat by the potential energy associated with the heat of phase change, such as boiling or freezing.

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 behavior, only inasmuch 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.

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
Main articles: convection and convective heat transfer
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 cannot happen in solids. In solids, molecules keep their relative position to such an extent that bulk movement or flow is prohibited, and therefore convection does not 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 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 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 small: they are simply different "colours" of electromagnetic radiation.


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 exchanger flows include parallel flow, counter flow, and cross flow. In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions and in cross flow the fluids move at right angles to each other. The common constructions for heat exchanger include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate. More information on heat exchanger flows and arrangements can be found in the heat exchanger article.

When engineers calculate the theoretical heat transfer in a heat exchanger, they must contend with the fact that the driving temperature difference between the two fluids varies with position. To account for this in simple systems, the log mean temperature difference (LMTD) is often used as an 'average' temperature. In more complex systems, direct knowledge of the LMTD is not available and the number of transfer units (NTU) method can be used instead.

Boiling heat transfer

Heat transfer in boiling fluids is complex but of considerable technical importance. It is characterised by an s-shaped curve relating heat flux to surface temperature difference (see say Kay & Nedderman 'Fluid Mechanics & Transfer Processes', CUP, 1985, p529).

At low driving temperatures, no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapour bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is sub-cooled nucleate boiling and is a very efficient heat transfer mechanism. At high bubble generation rates the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the departure from nucleate boiling DNB). At higher temperatures still, a maximum in the heat flux is reached (the critical heat flux). The regime of falling heat transfer which follows is not easy to study but is believed to be characterised by alternate periods of nucleate and film boiling.

At higher temperatures still, the hydrodynamically quieter regime of film boiling is reached. Heat fluxes across the stable vapour layers are low, but rise slowly with temperature. Any contact between fluid and the surface which may be seen probably leads to the extremely rapid nucleation of a fresh vapour layer ('spontaneous nucleation').


Engineer Forum

Heat transfer
« on: September 28, 2007, 02:07:57 AM »

Jose

  • Newbie
  • *
  • Posts: 2
    • View Profile
Re: Heat transfer
« Reply #1 on: November 08, 2008, 06:52:22 PM »
Well, wondering about heat transfer (which is something I find very intriguing)... There's this idea that I've been having lately about how to take advantage of this heat "lost" or "released" by electrical devices... such that something like a sensor could absorb or attract the heat and then convert it back into electrical energy, that way, the energy used would be the same everytime.
Mind you, I am turning 16 in a month.. so I have no kind of knowledge in Engineering, so I might be completely wrong in this.. but I am passionate about it and that's why I'm here, to learn! :)

engineer

  • Jr. Member
  • **
  • Posts: 88
    • View Profile
Re: Heat transfer
« Reply #2 on: November 09, 2008, 12:37:18 PM »
I was woundering the same thing. If it is possible then we can use energy more efficient.

Jose

  • Newbie
  • *
  • Posts: 2
    • View Profile
Re: Heat transfer
« Reply #3 on: November 09, 2008, 01:57:22 PM »
Yes, as in, theoretically speaking, we have a cell phone that uses up 20 J, and some 5 J will go unused (they'll be released as heat, disadvantage energy) and 15 will be used (as light energy, sound energy, and some advantage heat that makes some small machines operate inside the phone... advantage energy).
so all in all, the phone will be using up 20 J.

So is there a way, for a device, to attract or absorb that energy (mainly heat, useful for some other devices that give a lot of heat more than a cell phone) then convert it back into the input state (electrical energy).
or is it possible to attract ALL the output energy (light, sound, heat, ...) by one device? (what is common in the output energies? anything that we might build up on, anything related to their state, electron movement, movement of the atoms, whatever!)

mansour

  • Newbie
  • *
  • Posts: 3
    • View Profile
Re: Heat transfer
« Reply #4 on: February 23, 2009, 09:42:38 AM »
THANK YOU

Regards


ENG/ MANSOUR