Heat transfer

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Heat transfer is a thermal engineering discipline concerning the manufacture, use, conversion, and exchange of heat (heat) energy between physical systems. Heat transfer is classified into various mechanisms, such as heat conduction, thermal convection, thermal radiation, and energy transfer with phase change. Engineers also consider transfer of mass of different chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have different characteristics, they often occur simultaneously in the same system.

Heat conduction, also called diffusion, is a direct microscopic exchange of kinetic energy of particles through the boundary between two systems. When an object is at a temperature different from that of another body or its surroundings, heat flows so that the body and its surroundings reach the same temperature, at which point they are in thermal equilibrium. Such spontaneous heat transfer always occurs from high temperature regions to other temperatures with lower temperatures, as described in the second law of thermodynamics.

Convection of heat occurs when a large flow of liquid (gas or liquid) carries heat along with the flow of matter in the liquid. The fluid flow can be forced by external processes, or sometimes (in the gravitational field) by the buoyant force caused when the thermal energy expands the fluid (eg in a flame), thus affecting its own transfer. This latter process is often called "natural convection". All convective processes also transfer heat in part 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 device.

Thermal radiation occurs through a vacuum or transparent medium (solid or liquid). This is the energy transfer by using photons in electromagnetic waves that are governed by the same law.

Video Heat transfer

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Heat is defined in physics as the transfer of heat energy across clearly defined boundaries around the thermodynamic system. Thermodynamic free energy is the amount of work that the thermodynamic system can do. Enthalpy is a thermodynamic potential, designated by the letter "H", ie the amount of internal energy of the system (U) plus the pressure product (P) and volume (V). Joule is a unit to measure energy, work, or amount of heat.

Heat transfer is a process function (or path function), as opposed to a state function; Therefore, the amount of heat transferred in the thermodynamic process that changes the state of the system depends on how the process occurs, not only the net difference between the initial and final state of the process.

Thermodynamic and mechanical heat transfer is calculated by heat transfer coefficient, proportionality between heat flux and thermodynamic drive force for heat flow. Hot flux is a quantitative and quantitative representation of the heat flow through the surface.

In the technical context, the term heat is considered identical with heat energy. This use has its origins in the historical interpretation of heat as a fluid (Calorik ) that can be transferred by various causes, and it is also common in layman's language and everyday life.

The transport equations for heat energy (Fourier's law), mechanical momentum (Newton's law for fluids), and mass transfer (Fick diffusion law) are similar, and analogies between these three transport processes have been developed to facilitate conversion predictions from one to the other.

Thermal engineering involves generation, use, conversion, and heat transfer exchanges. Thus, heat transfer is involved in almost every sector of the economy. Heat transfer is classified into various mechanisms, such as heat conduction, thermal convection, thermal radiation, and energy transfer with phase change.

Maps Heat transfer

Mechanism

The basic modes of heat transfer are:

Advection

Advection is the mechanism of transporting liquids from one location to another, and depending on the movement and momentum of the liquid.

Conduction or diffusion

The transfer of energy between objects that are in physical contact. Thermal conductivity is the property of the material to conduct heat and is evaluated primarily in terms of Fourier's Law for heat conduction.

Convection

Transfer of energy between the object and its environment, due to fluid movement. The average temperature is a reference to evaluate properties associated with convective heat transfer.

Radiation

The transfer of energy by the emission of electromagnetic radiation.

Advection

By transferring material, energy - including heat energy - is driven by physical transfer of heat or cold objects from one place to another. It can be as simple as placing hot water in bottles and heating the bed, or the movement of the iceberg in the changing ocean currents. A practical example is thermal hydraulics. This can be explained by the formula:

${\ displaystyle Q = v \ rho c_ {p} \ Delta T}$

dimana Q adalah fluks panas (W/m²),? adalah densitas (kg/m³), ${\ displaystyle c_ {p}}  ÂÂ$ adalah kapasitas panas pada tekanan konstan (J/kg ° K),? T adalah perubahan suhu (K), ${\ displaystyle v}  ÂÂ$ adalah kecepatan (m/s).

Konduksi

On a microscopic scale, heat conduction occurs when hot, vibrating or vibrating atoms and molecules interact with adjacent atoms and molecules, transferring some of their (hot) energy to these adjacent 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 in solid or between solids in thermal contact. The liquid - especially gas - is less conductive. Thermal contact conductance is the study of the heat conduction between the contacted solids. The process of heat transfer from one place to another without the movement of particles is called conduction. Example: Heat transfer via metal rod. Steady state conduction (see Fourier's law) is a form of conduction that occurs when the temperature difference that induces conduction is constant, so that after the equilibration time, the spatial distribution of temperature in the conductor object does not change further. In steady state conduction, the amount of heat entering a part is equal to the amount of heat coming out.

Transient conduction (see heat equation) occurs when the temperature in an object changes as a function of time. The analysis of transient systems is more complex and often calls for the application of approximate theories or numerical analysis by computers.

Convection

The fluid flow can be forced by external processes, or sometimes (in the gravitational field) by the buoyant force caused when the thermal energy expands the fluid (eg in a flame), thus affecting its own transfer. This latter process is often called "natural convection". All convective processes also transfer heat in part 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 device.

Convective heat transfer, or convection, is the transfer of heat from one place to another by the movement of fluids, a process that is essentially a heat transfer through mass transfer. Bulk fluid movements increase heat transfer in many physical situations, such as (for example) between solid surfaces and liquids. Convection is usually the dominant form of heat transfer in liquids and gases. Although sometimes discussed as the third method of heat transfer, convection is typically used to describe the combined effect of heat conduction in fluid (diffusion) and heat transference by mass flow fluid flow. The process of transporting through a fluid stream is known as advection, but pure advection is a term commonly associated only with mass transport in liquids, such as river gravel advection. In the case of heat transfer in liquids, where transport with advection in a fluid is always also accompanied by transport by heat diffusion (also known as heat conduction) the heat convection process is understood to refer to the amount of heat transport by advection and diffusion/conduction.

Free, or natural, convection occurs when mass fluid movements (flow and current) are caused by buoyant forces resulting from variations in density due to temperature variations in liquids. Forced convection is the term used when the flow and current in the fluid induced by external means - such as fans, stirrers, and pumps - creates an artificially induced current convection.

Convection cooling

Convective cooling is sometimes described as Newton's cooling law:

The rate of body heat loss is proportional to the temperature difference between body and its environment .

However, by definition, the validity of Newton's Refrigeration Law requires that the rate of heat loss from convection be a linear function ("proportional to") the temperature difference that drives heat transfer, and in this convective cooling sometimes does not occur. In general, convection is not linear depending on the temperature gradient, and in some cases very nonlinear. In this case, Newton's law does not apply.

Convection vs. conduction

In the body of a liquid heated from under its container, conduction and convection can be considered to be competing for dominance. If the heat conduction is too large, the liquid that moves downward by convection is heated by conduction so rapidly that its downward motion will be stopped by its fire, while the liquid that moves up is convectionally cooled by conduction so quickly that its driving force is reduced. On the other hand, if the heat conduction is very low, a large temperature gradient can be formed and the convection may be very strong.

Angka Rayleigh ( ${\ displaystyle Ra}  ÂÂ$ ) adalah produk dari nomor Grashof dan Prandtl. Ini adalah ukuran yang menentukan kekuatan relatif konduksi dan konveksi.

${\ displaystyle Ra = GrPr = {\ frac {g \ Delta \ rho L ^ {3}} {\ mu \ alpha}} = {\ frac {g \ beta \ Delta TL ^ {3}} {\ nu \ alpha}}}  ÂÂ$

where

• g is acceleration due to gravity,
• ? is density with ${\ displaystyle \ Delta \ rho}$ becomes the density difference between the lower end and the top,
• ? is dynamic viscosity,
• ? is the Thermal Difference,
• ? is the volume of thermal expansivity (sometimes denoted ? elsewhere),
• T is the temperature,
• ? is a kinematic viscosity, and
• L is the characteristic length.

Rayleigh number can be understood as the ratio between the rate of heat transfer with convection to the heat transfer rate by conduction; or, equivalently, the ratio between the corresponding time ranges (ie the time scale of the conduction divided by the convection time scale), up to a numerical factor. This can be seen as follows, where all calculations depend on numerical factors depending on the geometry of the system.

Daya apung yang mendorong konveksi kira-kira ${\ displaystyle g \ Delta \ rho L ^ {3}}  ÂÂ$ , sehingga tekanan yang terkait kira-kira ${\ displaystyle g \ Delta \ rho L}  ÂÂ$ . Dalam keadaan stabil, ini dibatalkan oleh tegangan geser karena viskositas, dan oleh karena itu kurang lebih sama dengan ${\ displaystyle \ mu V/L = \ mu/T_ {conv}}  ÂÂ$ , di mana V adalah kecepatan fluida umum karena konveksi dan ${\ displaystyle T_ {conv}}  ÂÂ$ urutan skala waktunya. Skala waktu konduksi, di sisi lain, adalah urutan ${\ displaystyle T_ {cond} = L ^ {2}/\ alpha}  ÂÂ$ .

Convection occurs when the Rayleigh number is above 1,000-2,000.

Radiation

Thermal radiation occurs through a vacuum or transparent medium (solid or liquid). This is the energy transfer by using photons in electromagnetic waves that are governed by the same law.

Thermal radiation is the energy emitted by matter as electromagnetic waves, because the pool of heat energy in all matter with temperatures above absolute zero. Thermal radiation spreads without the presence of matter through a vacuum.

Thermal radiation is a direct result of random motion of atoms and molecules in matter. Because these atoms and molecules consist of charged particles (protons and electrons), their motion produces emission of electromagnetic radiation, which brings energy away from the surface.

Persamaan Stefan-Boltzmann, yang menggambarkan laju transfer energi radiasi, adalah sebagai berikut untuk objek dalam ruang hampa:

${\ displaystyle Q = \ epsilon \ sigma T ^ {4}.}  ÂÂ$

Untuk transfer radiasi antara dua objek, persamaannya adalah sebagai berikut:

${\ displaystyle Q = \ epsilon \ sigma (T_ {a} ^ {4} -T_ {b} ^ {4}),}  ÂÂ$

where Q is the heat flux ,? is emissivity (unity for black body) ,? is the Stefan-Boltzmann constant, and T is the absolute temperature (in Kelvin or Rankine degree). Radiation is usually only important for very hot objects, or for objects with large temperature differences.

Radiation from the sun, or solar radiation, can be harvested for heat and energy. Unlike the conductive and convective forms of heat transfer, thermal radiation can be concentrated in small places by using reflective mirrors, which are utilized in concentrating solar power plants. For example, reflected sunlight from a mirror heats up the PS10 solar tower and during the day can heat water up to 285 ° C (545 ° F).

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Transition phase

Phase transitions or phase changes, occur in thermodynamic systems from one phase or state of matter to another with heat transfer. Examples of phase changes are melting ice or boiling water. The Masonic equation describes the growth of water droplets based on the effects of heat transport on evaporation and condensation.

The transition phase involves four basic conditions of matter:

• Solid - Deposition, solidification and solid transformation into solid.
• Gas - Boil/evaporate, recombination/deionization, and sublimation.
Liquid - Condensation and melting/smelting.
• Plasma - Ionization.

Boiling

The boiling point of a substance is the temperature at which the vapor pressure of the liquid is equal to the pressure around the liquid and the liquid evaporates resulting in a sudden vapor volume change.

The saturation temperature means the boiling point. The saturation temperature is the temperature for the corresponding saturation pressure in which the liquid boils to its vapor phase. The liquid can be said to be saturated with heat energy. Each addition of heat energy produces a phase transition.

At standard atmospheric pressure and low temperature , there is no boiling and the rate of heat transfer is controlled by the usual single-phase mechanism. As the surface temperature increases, local boiling occurs and the bubble bubbles burgeon, growing into a cooling fluid around it, and collapsing. It is a sub-cooled boiling nuclei , and is a very efficient heat transfer mechanism. At the high bubble generation rate, bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is a departure from nucleation nucleation, or DNB).

At the same standard atmospheric pressure and high temperature , a hydrodynamically boiling regime is achieved. Heat flux in the steam layer is stable low, but rises slowly with temperature. Any contact between the liquid and the visible surface may lead to very rapid nucleation of the fresh vapor layer ("spontaneous nucleation"). At higher temperatures, the maximum in the heat flux is achieved (critical heat flux, or CHF).

The Leidenfrost effect shows how boiling nucleation slows heat transfer because of the gas bubbles on the heating surface. As mentioned, the gas-phase thermal conductivity is much lower than the liquid phase thermal conductivity, so the result is a kind of "gas thermal barrier".

Condensation

Condensation occurs when the vapor is cooled and converts its physique to a liquid. During condensation, the latent heat of evaporation must be released. The amount of heat is equal to that absorbed during evaporation at the same fluid pressure.

There are several types of condensation:

• Homogeneous condensation, such as the formation of fog.
• Condensation in direct contact with sub-coolant fluid.
• Condensation on direct contact with wall heat sink heat exchanger: This is the most common mode used in industry:
  • Filmwise condensation is when a liquid film is formed on a sub-cooling surface, and usually occurs when the liquid moistens the surface.
  • Dropwise condensation is when liquid drops form on the sub-cooling surface, and usually occurs when the liquid does not moisten the surface.

Condensation of droplets is difficult to maintain reliably; therefore, industrial equipment is usually designed to operate in film condensation mode.

Melting

Melting is a thermal process that produces a phase transition of solids to liquids. The internal energy of a substance increases, usually by heat or pressure, which results in a rise in temperature to the melting point, where the ordering of ionic entities or molecules in solids breaks down into irregular and solid melting conditions. Liquids generally have reduced viscosity with high temperatures; the exception to this maxim is the sulfur element, whose viscosity increases to the point due to polymerization and then decreases with higher temperatures in its liquid state.

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Modeling approach

The heat transfer can be modeled in the following ways.

Climatic model

Climate models study the transfer of radiant heat by using quantitative methods to simulate atmospheric interactions, oceans, ground surfaces, and ice.

Heat equation

The heat equation is an important partial differential equation that describes the heat distribution (or temperature variation) in a given region over time. In some cases, the exact solution of the equation is available; in other cases equations must be solved numerically using computational methods.

System analysis paralyzed

The shortest system analysis often reduces the complexity of the equations to one of the first-order linear differential equations, where heating and cooling are explained by simple exponential solutions, often referred to as Newton's cooling law.

Analysis of a system with a lumped capacitance model is a common approach in transient conduction that can be used whenever the heat conduction in an object is much faster than the heat conduction across the boundary of an object. This is an approach method that reduces one aspect of the transient conduction system - which exists within the object - into an equivalent steady state system. That is, the method assumes that the temperature inside the object is completely uniform, although its value may change over time.

In this method, the ratio of the conductive heat resistance in the object to the convective heat transfer resistance across the boundary of the object, known as Biot number , is calculated. For small Biot numbers, the estimated uniform spatial temperature in objects can be used: it can be assumed that the heat transferred to the object has the time to distribute them uniformly, because of the lower resistance to doing so, compared with the resistance to heat enters the object.

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Engineering

Heat transfer has wide applications for various device and system functions. The principles of heat transfer can be used to maintain, increase, or lower the temperature in various circumstances. Heat transfer methods are used in a variety of disciplines, such as automotive engineering, thermal management of electronic devices and systems, climate control, insulation, material processing, and power generation techniques.

Isolation, emission and resistance

Thermal insulators are materials specially designed to reduce heat flow by limiting conduction, convection, or both. Thermal resistance is a heat property and measurement in which an object or material rejects heat flow (heat per unit time or thermal resistance) to temperature difference.

Radiance or spectral light is a measure of the quantity of radiation that passes or is emitted. Radiation barriers are materials that reflect radiation, and therefore reduce the heat flow from a radiation source. A good insulator is not always a good radiation barrier, and vice versa. Metals, for example, are excellent reflectors and poor insulators.

The effectiveness of the radiation barrier is shown by its reflectivity , which is a fraction of the reflected radiation. Materials with high reflectivity (at certain wavelengths) have low emissivity (at the same wavelength), and vice versa. At certain wavelengths, reflectivity = 1 - emissivity. An ideal glowing barrier will have 1 reflectivity, and will therefore reflect 100 percent of the incoming radiation. The vacuum flask, or Dewar, is silvered to approximate this ideal. In a vacuum, satellites use multi-layer insulation, which consists of many of Mylar's aluminized (glossy) layers to greatly reduce radiant heat transfer and temperature control of satellites.

Device

A heat engine is a system that converts the flow of heat energy (heat) into mechanical energy to perform mechanical work.

Thermocouples are temperature gauges and temperature sensor types that are widely used for measurement and control, and can also be used to convert heat into electric power.

The thermoelectric cooler is a solid state electronic device that pumps (transfer) heat from one side of the device to the other when an electric current passes through it. It is based on Peltier effect.

Thermal diodes or thermal rectifiers are devices that cause heat to flow privately in one direction.

Heat exchanger

A heat exchanger is used for more efficient heat transfer or to remove heat. Heat exchangers are widely used in refrigeration, air conditioning, space heating, power generation, and chemical processing. One common example of a heat exchanger is a car radiator, in which the hot coolant is cooled by airflow over the radiator surface.

Common types of heat exchanger flow include parallel flow, counter flow, and cross flow. In parallel flow, both liquids move in the same direction while transferring heat; in the reverse flow, the liquid moves in the opposite direction; and in cross currents, the liquid moves at right angles to each other. Common construction for heat exchangers including shell and tube, double pipe, extrusion finned pipe, spiral fin pipe, tube u, and stacked plate.

Heat sinks are components that transfer heat generated in a solid material to a liquid medium, such as air or liquid. An example of a heat sink is a heat exchanger used in a cooling system and an air conditioner or radiator in a car. A heat pipe is another heat transfer device that combines thermal conductivity and phase transitions to efficiently transfer heat between two solid interfaces.

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Apps

Architecture

The efficient use of energy is the goal to reduce the amount of energy needed in heating or cooling. In architecture, condensation and air currents can cause cosmetic or structural damage. An energy audit can help assess the implementation of suggested improvement procedures. For example, insulation improvements, sealing structural leakage air or adding energy-efficient windows and doors.

• Smart meter is a tool that records the consumption of electrical energy in the interval.
• Thermal transmission is the heat transfer rate through the structure divided by the temperature difference across the structure. Expressed in watts per square meter per kelvin, or W/(mÃ,²K). The well-insulated part of a building has a low thermal transmittance, while the less insulated part of a building has a high thermal transmittance.
• Thermostats are devices for monitoring and controlling temperature.

Climate engineering

Climate engineering consists of carbon dioxide removal and solar radiation management. Since the amount of carbon dioxide determines the balance of Earth's atmospheric radiation, carbon dioxide removal techniques can be applied to reduce radiation imposition. The management of solar radiation is an attempt to absorb less solar radiation to offset the effects of greenhouse gases.

Greenhouse effect

The greenhouse effect is a process in which the thermal radiation from the planet's surface is absorbed by greenhouse gases in the atmosphere, and is re-emitted in all directions. Since this part of the re-radiation returns to the surface and the lower atmosphere, this results in an increase in average surface temperature above what would happen in the absence of gas.

Transfer of heat in the human body

The principles of heat transfer in engineering systems can be applied to the human body to determine how the body transfers heat. Heat is produced in the body by a continuous nutrient metabolism that provides energy for the body system. The human body must maintain a consistent internal temperature to maintain healthy body function. Therefore, excess heat must be removed from the body so as not to overheat. When a person is involved in increased physical activity, the body needs additional fuel that increases metabolic rate and heat production levels. The body should then use additional methods to remove the resulting extra heat to keep the internal temperature at a healthy level.

The heat transfer by convection is driven by fluid movement above the body surface. This convective fluid can be either liquid or gas. For heat transfer from the outer surface of the body, the convection mechanism depends on the body surface area, air velocity, and temperature gradient between the skin surface and ambient air. Normal body temperature is about 37Ã, Â ° C. The heat transfer occurs faster when the ambient temperature is significantly lower than normal body temperature. This concept explains why a person feels "cold" when not covering enough to wear when exposed to a cold environment. Clothes can be considered as insulators that provide heat resistance to heat flow above closed body parts. This heat resistance causes the temperature on the surface of the clothing to be less than the temperature on the surface of the skin. Smaller temperature gradients between surface temperature and ambient temperature will cause lower heat transfer rates than if the skin is not covered.

To ensure that one part of the body is not hotter than the other, heat must be distributed evenly through body tissues. The blood flowing through the blood vessels acts as a convective fluid and helps prevent the buildup of excess heat in the tissues of the body. This blood flow through the vessels can be modeled as pipeline flow in the engineering system. The heat carried by the blood is determined by the temperature of the surrounding tissue, the diameter of the blood vessels, the thickness of the fluid, the flow rate, and the heat transfer coefficient. Speed, diameter of blood vessels, and fluid thickness can all be related to Reynolds Numbers, the dimensionless numbers used in fluid mechanics to characterize fluid flow.

The latent heat loss, also known as heat loss evaporates, accounts for most of the heat loss from the body. As the body's core temperature increases, the body triggers the sweat glands in the skin to bring additional moisture to the skin's surface. The liquid is then converted into a vapor that removes heat from the body surface. The rate of evaporation heat loss is directly related to the vapor pressure on the skin surface and the amount of moisture present in the skin. Therefore, the maximum heat transfer will occur when the skin is completely wet. The body continues to lose water due to evaporation but the most significant amount of heat loss occurs during periods of increased physical activity.

Cooling Technique

Evaporative cooling

Evaporative cooling occurs when moisture is added to the surrounding air. The energy required to evaporate water is taken from the air in the form of a reasonable heat and converted into latent heat, while the air remains at the constant enthalpy. Latent heat explains the amount of heat needed to vaporize fluids; This heat comes from the fluid itself and the gas and the surrounding surface. The larger the difference between the two temperatures, the greater the effect of evaporative cooling. When the temperature is the same, there is no evaporation of clean water in the air; thus, there is no cooling effect.

Laser cooling

In quantum physics, laser cooling is used to reach temperatures near absolute zero (-273.15 ° C, -459.67 ° F) from atomic and molecular samples to observe unique quantum effects that can only occur at this heat level.

• Doppler cooling is the most common method of laser cooling.
• Sympathetic refrigeration is a process in which particles of one kind of cool particles of another type. Typically, directly coolable ionic ions are used to cool the nearest ion or atom. This technique allows the cooling of ions and atoms that can not be cooled laser directly.

Magnetic cooling

Magnetic evaporative cooling is a process for lowering the temperature of a group of atoms, having been pre-cooled by methods such as laser cooling. Magnetic cooling cools below 0.3 K, by utilizing the magnetocaloric effect.

Radiation cooling

Radiation cooling is the process by which the body loses heat by radiation. The energy that comes out is an important effect in Earth's energy budget. In the case of Earth's atmospheric systems, this refers to the process by which long-wave radiation (infrared) is emitted to balance the absorption of shortwave energy (visible) from the Sun. Convective transportation of heat and evaporative transport of latent heat both remove heat from the surface and redistribute it in the atmosphere.

Thermal energy storage

Thermal energy storage includes technologies for collecting and storing energy for later use. This can be used to balance energy demand between day and night. Thermal reservoirs can be maintained at temperatures above or below the ambient environment. Applications include indoor heating, domestic hot water systems or processes, or generating electricity.

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See also

• Forced combination and natural convection
• Heat Capacity
• Physical heat transfer
• Stefan-Boltzmann's law
• Thermal contact conductance
• Thermal physics
• Thermal resistance in electronics
• Thermal science
• Increased heat transfer

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References

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External links

• Heat Transfer Textbook - (free download).
• Thermal-FluidsPedia - An online fluid online encyclopedia.
• Hyperfysics Articles on Heat Transfer - Overview
• Interseasonal Heat Transfer - a practical example of how heat transfer is used to heat a building without burning fossil fuels.
• Heat Transfer Aspect, Cambridge University
• Thermal Fluid Center
• Energy2D: Interactive Heat Transfer Simulation for Everyone

Source of the article : Wikipedia