Simulation of thermal convection in the. Colors span from red and green to blue with decreasing temperatures. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and cold material from the top moves downwards.Heat transfer is a discipline of that concerns the generation, use, conversion, and exchange of between physical systems. Heat transfer is classified into various mechanisms, such as, and transfer of energy. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a different from another body or its surroundings, flows so that the body and the surroundings reach the same temperature, at which point they are in.
Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described in the.Heat convection occurs when bulk flow of a fluid (gas or liquid) carries heat along with the flow of matter in the fluid. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called 'natural convection'. All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.Thermal radiation occurs through a or any ( or or ).
It is the transfer of energy by means of in governed by the same laws. Main article:On a microscopic scale, heat conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring particles. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is the most significant means of heat transfer within a solid or between solid objects in. Fluids—especially gases—are less conductive. Is the study of heat conduction between solid bodies in contact. The process of heat transfer from one place to another place without the movement of particles is called conduction, such as when placing a hand on a cold glass of water - heat is conducted from the warm skin to the cold glass, but if the hand is held a few inches from the glass, little conduction would occur since air is a poor conductor of heat.
Steady state conduction is an idealized model of conduction that happens when the temperature difference driving the conduction is constant, so that after a time, the spatial distribution of temperatures in the conducting object does not change any further (see ). In steady state conduction, the amount of heat entering a section is equal to amount of heat coming out, since the change in temperature (a measure of heat energy) is zero. An example of steady state conduction is the heat flow through walls of a warm house on a cold night - inside the house is maintained at a high temperature, and outside the temperature stays low, so the transfer of heat per unit time stays near a constant rate determined by the insulation in the wall, and the spatial distribution of temperature in the walls will be approximately constant over time.Transient conduction (see ) occurs when the temperature within an object changes as a function of time. Analysis of transient systems is more complex, and analytic solutions of the heat equation are only valid for idealized model systems. Practical applications are generally investigated using numerical methods, approximation techniques, or empirical study.
Convection. Main article:The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called 'natural convection'.
All convective processes also move heat partly by diffusion, as well. Another form of convection is forced convection. In this case the fluid is forced to flow by using a pump, fan or other mechanical means.it is the process in which heat transfer through the fluid as air, or convection, is the transfer of heat from one place to another by the movement of, a process that is essentially the transfer of heat via.
Bulk motion of fluid enhances heat transfer in many physical situations, such as (for example) between a solid surface and the fluid. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as a third method of heat transfer, convection is usually used to describe the combined effects of heat conduction within the fluid (diffusion) and heat transference by bulk fluid flow streaming. The process of transport by fluid streaming is known as advection, but pure advection is a term that is generally associated only with mass transport in fluids, such as advection of pebbles in a river.
In the case of heat transfer in fluids, where transport by advection in a fluid is always also accompanied by transport via heat diffusion (also known as heat conduction) the process of heat convection is understood to refer to the sum of heat transport by advection and diffusion/conduction.Free, or natural, convection occurs when bulk fluid motions (streams and currents) are caused by buoyancy forces that result from density variations due to variations of temperature in the fluid. Forced convection is a term used when the streams and currents in the fluid are induced by external means—such as fans, stirrers, and pumps—creating an artificially induced convection current. Convection-cooling. Is a highly visible form of transfer and is an example of plasma present at Earth's surface. Typically, lightning discharges 30,000 amperes at up to 100 million volts, and emits light, radio waves, X-rays and even gamma rays. Plasma temperatures in lightning can approach 28,000 kelvins (27,726.85 °C) (49,940.33 °F) and electron densities may exceed 10 24 m −3.or phase change, takes place in a from one phase or to another one by heat transfer.
Phase change examples are the melting of ice or the boiling of water.The explains the growth of a water droplet based on the effects of heat transport on and condensation.Phase transitions involve the:. – Deposition, freezing and solid to solid transformation. – Boiling / evaporation, /, and. – Condensation and. –.Boiling. Nucleate boiling of water.The of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid resulting in an abrupt change in vapor volume.Saturation temperature means boiling point.
The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase. The liquid can be said to be saturated with thermal energy. Any addition of thermal energy results in a phase transition.At standard atmospheric pressure and low 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 vapor 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, or DNB).At similar standard atmospheric pressure and high temperatures, the hydrodynamically-quieter regime of is reached. Heat fluxes across the stable vapor layers are low, but rise slowly with temperature. Any contact between fluid and the surface that may be seen probably leads to the extremely rapid nucleation of a fresh vapor layer ('spontaneous '). At higher temperatures still, a maximum in the heat flux is reached (the, or CHF).The demonstrates how nucleate boiling slows heat transfer due to gas bubbles on the heater's surface. As mentioned, gas-phase thermal conductivity is much lower than liquid-phase thermal conductivity, so the outcome is a kind of 'gas thermal barrier'.Condensation occurs when a vapor is cooled and changes its phase to a liquid.
During condensation, the must be released. Ice meltingis a thermal process that results in the phase transition of a substance from a to a. The of a substance is increased, typically with heat or pressure, resulting in a rise of its temperature to the, at which the ordering of ionic or molecular entities in the solid breaks down to a less ordered state and the solid liquefies. Molten substances generally have reduced viscosity with elevated temperature; an exception to this maxim is the element, whose viscosity increases to a point due to and then decreases with higher temperatures in its molten state.
Modeling approaches Heat transfer can be modeled in various ways.Heat equation The is an important that describes the distribution of heat (or variation in temperature) in a given region over time. In some cases, exact solutions of the equation are available; in other cases the equation must be solved numerically using.Lumped system analysis Lumped system analysis often reduces the complexity of the equations to one first-order linear differential equation, in which case heating and cooling are described by a simple exponential solution, often referred to as.System analysis by the is a common approximation in transient conduction that may be used whenever heat conduction within an object is much faster than heat conduction across the boundary of the object. This is a method of approximation that reduces one aspect of the transient conduction system—that within the object—to an equivalent steady state system.
That is, the method assumes that the temperature within the object is completely uniform, although its value may be changing in time.In this method, the ratio of the conductive heat resistance within the object to the convective heat transfer resistance across the object's boundary, known as the, is calculated. For small Biot numbers, the approximation of spatially uniform temperature within the object can be used: it can be presumed that heat transferred into the object has time to uniformly distribute itself, due to the lower resistance to doing so, as compared with the resistance to heat entering the object. Climate models study the by using quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice.Engineering. Heat exposure as part of a fire test for firestop productsHeat transfer has broad application to the functioning of numerous devices and systems. Heat-transfer principles may be used to preserve, increase, or decrease temperature in a wide variety of circumstances. Heat transfer methods are used in numerous disciplines, such as, and engineering.Insulation, radiance and resistance are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both.
Is a heat property and the measurement by which an object or material resists to heat flow (heat per time unit or thermal resistance) to temperature difference.or spectral radiance are measures of the quantity of radiation that passes through or is emitted. Are materials that 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 a poor insulator.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 percent of incoming radiation., or Dewars, are to approach this ideal.
In the vacuum of space, satellites use, which consists of many layers of aluminized (shiny) to greatly reduce radiation heat transfer and control satellite temperature. Devices. Schematic flow of energy in a heat engine.A is a system that performs the conversion of a flow of (heat) to to perform.A is a temperature-measuring device and widely used type of temperature sensor for measurement and control, and can also be used to convert heat into electric power.A is a solid state electronic device that pumps (transfers) heat from one side of the device to the other when electric current is passed through it. It is based on the.A or is a device that causes heat to flow preferentially in one direction.Heat exchangers A is used for more efficient heat transfer or to dissipate heat. Heat exchangers are widely used in, and chemical processing. One common example of a heat exchanger is a car's radiator, in which the hot is cooled by the flow of air over the radiator's 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 to each other. Common types of heat exchangers include, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate. Each type has certain advantages and disadvantages over other types.
A is a component that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems or the radiator in a car. A is another heat-transfer device that combines thermal conductivity and phase transition to efficiently transfer heat between two solid interfaces.Applications Architecture is the goal to reduce the amount of energy required in heating or cooling. In architecture, condensation and can cause cosmetic or structural damage. An can help to assess the implementation of recommended corrective procedures. For instance, insulation improvements, air sealing of structural leaks or the addition of energy-efficient windows and doors. is a device that records electric energy consumption in intervals.
is the rate of transfer of heat through a structure divided by the difference in temperature across the structure. It is expressed in watts per square meter per kelvin, or W/(m²K). Well-insulated parts of a building have a low thermal transmittance, whereas poorly-insulated parts of a building have a high thermal transmittance. is a device to monitor and control temperature.Climate engineering. An example application in climate engineering includes the creation of through the process.
Thus, storing greenhouse gases in carbon reduces the radiative forcing capacity in the atmosphere, causing more long-wave radiation out to Space.consists of. Since the amount of determines the of Earth atmosphere, carbon dioxide removal techniques can be applied to reduce the.
Solar radiation management is the attempt to absorb less solar radiation to offset the effects of.Greenhouse effect. A representation of the exchanges of energy between the source (the ), the Earth's surface, the, and the ultimate sink.
The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect.The is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases.Heat transfer in the human body. See also:The principles of heat transfer in engineering systems can be applied to the human body in order to determine how the body transfers heat. Heat is produced in the body by the continuous metabolism of nutrients which provides energy for the systems of the body. The human body must maintain a consistent internal temperature in order to maintain healthy bodily functions.
Therefore, excess heat must be dissipated from the body to keep it from overheating. When a person engages in elevated levels of physical activity, the body requires additional fuel which increases the metabolic rate and the rate of heat production.
The body must then use additional methods to remove the additional heat produced in order to keep the internal temperature at a healthy level.is driven by the movement of fluids over the surface of the body. This convective fluid can be either a liquid or a gas. For heat transfer from the outer surface of the body, the convection mechanism is dependent on the surface area of the body, the velocity of the air, and the temperature gradient between the surface of the skin and the ambient air. The normal temperature of the body is approximately 37 °C. Heat transfer occurs more readily when the temperature of the surroundings is significantly less than the normal body temperature.
This concept explains why a person feels cold when not enough covering is worn when exposed to a cold environment. Clothing can be considered an insulator which provides thermal resistance to heat flow over the covered portion of the body. This thermal resistance causes the temperature on the surface of the clothing to be less than the temperature on the surface of the skin.
This smaller temperature gradient between the surface temperature and the ambient temperature will cause a lower rate of heat transfer than if the skin were not covered.In order to ensure that one portion of the body is not significantly hotter than another portion, heat must be distributed evenly through the bodily tissues. Blood flowing through blood vessels acts as a convective fluid and helps to prevent any buildup of excess heat inside the tissues of the body.
This flow of blood through the vessels can be modeled as pipe flow in an engineering system. The heat carried by the blood is determined by the temperature of the surrounding tissue, the diameter of the blood vessel, the, velocity of the flow, and the heat transfer coefficient of the blood. The velocity, blood vessel diameter, and the fluid thickness can all be related with the, a dimensionless number used in fluid mechanics to characterize the flow of fluids.loss, also known as evaporative heat loss, accounts for a large fraction of heat loss from the body.
When the core temperature of the body increases, the body triggers sweat glands in the skin to bring additional moisture to the surface of the skin. The liquid is then transformed into vapor which removes heat from the surface of the body. The rate of evaporation heat loss is directly related to the vapor pressure at the skin surface and the amount of moisture present on the skin. Therefore, the maximum of heat transfer will occur when the skin is completely wet. The body continuously loses water by evaporation but the most significant amount of heat loss occurs during periods of increased physical activity.Cooling techniques Evaporative cooling.
A traditional air cooler in,happens when water vapor is added to the surrounding air. The energy needed to evaporate the water is taken from the air in the form of sensible heat and converted into latent heat, while the air remains at a constant. Latent heat describes the amount of heat that is needed to evaporate the liquid; this heat comes from the liquid itself and the surrounding gas and surfaces. The greater the difference between the two temperatures, the greater the evaporative cooling effect. When the temperatures are the same, no net evaporation of water in air occurs; thus, there is no cooling effect.Laser cooling In, is used to achieve temperatures of near (−273.15 °C, −459.67 °F) of atomic and molecular samples to observe unique that can only occur at this heat level. is the most common method of laser cooling.
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is a process in which particles of one type cool particles of another type. Typically, atomic ions that can be directly laser-cooled are used to cool nearby ions or atoms. This technique allows cooling of ions and atoms that cannot be laser cooled directly. Magnetic cooling. Main articles: andis a process for lowering the temperature of a group of atoms, after pre-cooled by methods such as laser cooling. Magnetic refrigeration cools below 0.3K, by making use of the.Radiative cooling is the process by which a body loses heat by radiation. Energy is an important effect in the.
In the case of the Earth-atmosphere system, it refers to the process by which long-wave (infrared) radiation is emitted to balance the absorption of short-wave (visible) energy from the Sun. Convective transport of heat and evaporative transport of latent heat both remove heat from the surface and redistribute it in the atmosphere.Thermal energy storage includes technologies for collecting and for later use. It may be employed to balance energy demand between day and nighttime. The thermal reservoir may be maintained at a temperature above or below that of the ambient environment.
Applications include space heating, domestic or process hot water systems, or generating electricity.See also.References.
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Energy Transformations. 3-5: 4E/E2b. When warmer things are put with cooler ones, heat is transferred from the warmer ones to the cooler ones. 6-8: 4E/M2.
Energy can be transferred from one system to another (or from a system to its environment) in different ways: 1) thermally, when a warmer object is in contact with a cooler one; 2) mechanically, when two objects push or pull on each other over a distance; 3) electrically, when an electrical source such as a battery or generator is connected in a complete circuit to an electrical device; or 4) by electromagnetic waves. 6-8: 4E/M3. Thermal energy is transferred through a material by the collisions of atoms within the material.
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Over time, the thermal energy tends to spread out through a material and from one material to another if they are in contact. Thermal energy can also be transferred by means of currents in air, water, or other fluids. In addition, some thermal energy in all materials is transformed into light energy and radiated into the environment by electromagnetic waves; that light energy can be transformed back into thermal energy when the electromagnetic waves strike another material. As a result, a material tends to cool down unless some other form of energy is converted to thermal energy in the material.