Студопедия

Главная страница Случайная страница

Разделы сайта

АвтомобилиАстрономияБиологияГеографияДом и садДругие языкиДругоеИнформатикаИсторияКультураЛитератураЛогикаМатематикаМедицинаМеталлургияМеханикаОбразованиеОхрана трудаПедагогикаПолитикаПравоПсихологияРелигияРиторикаСоциологияСпортСтроительствоТехнологияТуризмФизикаФилософияФинансыХимияЧерчениеЭкологияЭкономикаЭлектроника






Heat transfer






Heat transfer is the transition of thermal energy from a hotter object to a cooler object. When an object or fluid is at a different temperature than its surroundings or another object, heat transfer occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed. Heat can be transferred by different types.

Conduction is the transfer of heat by direct contact of particles of matter, when adjacent atoms vibrate against one another, or electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. Metals (e.g. copper, platinum, gold, iron, etc.) are usually the best conductors 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. Conductivity of gases increases with temperature.

Convection is the transfer of heat energy between a solid surface and the nearby liquid or gas in motion. As fluid motion goes more quickly the convective heat transfer increases. There are two types of convective heat transfer.

Natural convection occurs when the fluid motion is caused by buoyancy forces that result from the density variations due to variations of temperature in the fluid. For example in the absence of an external source, when the mass of the fluid is in contact with a hot surface its molecules separate and scatter causing the mass of fluid to become less dense. When this happens, the fluid is displaced vertically or horizontally while the cooler fluid gets denser and the fluid sinks. Thus the hotter volume transfers heat towards the cooler volume of that fluid.

Forced convection is when the fluid is forced to flow over the surface by external source such as fans and pumps. It creates an artificially induced convection current.

Internal and external flow can also classify convection. Internal flow occurs when the fluid is enclosed by a solid boundary such as a flow through a pipe. An external flow occurs when the fluid extends indefinitely without encountering a solid surface. Both these convections, either natural or forced, can be internal or external as they are independent of each other.

Radiation is the transfer of heat energy through empty space. All objects with a temperature above absolute zero radiate energy. No medium is necessary for radiation to occur, for it is transferred through electromagnetic waves; radiation works even in and through a perfect vacuum. The energy from the Sun travels through the vacuum of space before warming the earth.

 






© 2023 :: MyLektsii.ru :: Мои Лекции
Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав.
Копирование текстов разрешено только с указанием индексируемой ссылки на источник.