Thermal conduction ismediumThere is no heat transfer phenomenon when macro motion occurs inside, which can occur in solids, liquids and gases. But strictly speaking, only in solids is pure heat conduction, and even if the fluid is in a static state, it will also be caused bytemperature gradientResulting from density differencenatural convection , therefore, in the fluidThermal convectionSimultaneously with heat conduction.
Chinese name
heat conduction
Foreign name
thermal conduction
Introduction
heat energyPartial transfer from high temperature to low temperature
The temperature difference within an object or system is a necessary condition for heat conduction.In other words, as long as there is a temperature difference in or between the media, heat transfer will occur.The heat conduction rate depends on the distribution of the temperature field in the object.
heat conduction
The phenomenon that heat is transferred from one part of the system to another or from one system to another is calledconduct heat。Heat conduction is one of the three heat transfer modes (heat conduction, convection and radiation).It is the main way of heat transfer in solids. It is transferred layer by layer in the non flowing liquid or gas layer, and is often connected withThermal convectionsimultaneous.
Heat conduction is essentially composed ofMolecular thermal motionThe process by which energy is transferred from the high temperature part of an object to the low temperature part, or from the high temperature object to the low temperature object, by mutual impact.[1]In the solid, the microscopic process of heat conduction is that in the part with high temperature, the vibration kinetic energy of particles on the nodes in the crystal is larger.In the low temperature part, the vibration kinetic energy of particles is small.Due to the vibration interaction of particles, the heat energy in the crystal is transferred from the part with large kinetic energy to the part with small kinetic energy.The conduction of heat in a solid is the transfer of energy.
In the conductor, due to the existence of a large number offree electron, making irregular thermal movement continuously.commonlylatticeThe vibration energy is small, and free electronsMetallic crystalIt plays a major role in heat conduction.So generalElectrical conductorIt is also a good conductor of heat.The heat conduction in liquid is shown as: the heat movement of liquid molecules is relatively strong in the area with high temperature. Due to the interaction between liquid molecules, the energy of heat movement will gradually transfer to the surrounding layers, causing the heat conduction phenomenon.Due to heat conductioncoefficientSmall, slow conduction, similar to solid;Different from liquid, the distance between gas molecules is relatively large. Gas relies on the irregular thermal movement of molecules and the collision between molecules to generate energy transfer in the gas, thus forming a macroscopicHeat transfer。
The way in which heat is transferred from the higher temperature part of an object to the lower temperature part along the object is called heat conduction.
Definitions in the dictionary
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A mode of heat transfer.Heat conduction is a process in which a large number of molecules, atoms or electrons collide with each other to transfer energy from the higher temperature part of an object to the lower temperature part.It is the main mode of heat transfer in solids.In gas or liquid, heat conduction often occurs simultaneously with convection.Conduction and convection are also one of the ways for human body to dissipate heat.The blood circulation brings the heat in the body to the body surface, and the skin transfers the heat to the air layer close to the skin (conduction and heat dissipation). The temperature of the heated air layer increases, the density decreases, and thus the flow increases. The surrounding cold air flows to the skin surface to fill the air flowing away (convection and heat dissipation).Therefore, human body conduction heat dissipation and convection heat dissipation are always connected.[2]
Also called heat conduction, it is one of the three basic ways of heat transfer.The main way of heat transfer in solids is to transfer heat from a higher temperature part of an object to a lower temperature part or to another object with a lower temperature in contact with it by means of the collision of molecules, atoms or electrons in the object.In liquid or gas, it is often carried out simultaneously with convective heat transfer.All objects have heat conduction as long as there is a temperature difference, regardless of whether there is relative motion between particles inside or not.In industry, there are many heat transfer processes mainly based on heat conduction, such asRubber productsHeat vulcanization, heat treatment of steel forgings, etc.Heat conduction laws are required in the design and calculation of kilns, heat transfer equipment and thermal insulation, and the design of high-temperature and high-pressure equipment (such as waste heat boiler in ammonia converter).[3]
Mathematical expression
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Fourier’s law
When the temperature distribution in an object only depends on a spatial coordinate, and the temperature distribution does not change with time, heat is only transferred along the direction of temperature reduction, which is called one-dimensionalStationary stateHeat conduction.The heat conduction at this time can be described by the following formula:
Where, q ″xbe for the purpose ofHeat flux, that is, on the unit area perpendicular to the transmission directionHeat transfer rate；TIs the temperature;xbyheat transferCoordinate of direction;kbyThermal conductivity。This equation indicates thatqProportional totemperature gradientdT/dxBut the direction of heat flow is opposite to that of temperature gradient.This law was developed by French physicistsFourier It was first proposed in 1822, so it is calledFourier's law。
Thermal diffusion equation
In the most general heat conduction, the temperature changes with time and three spatial coordinates, and is accompanied by heat generation or consumption (for example, reaction heat).The heat conduction at this time is called three-dimensionalUnsteady stateHeat conduction can be described by heat equation[1]：
WhereτIs time;x、y、zbyAxis；ρIs density;cpIs constant pressureSpecific heat capacity；The thermal diffusion equation shows that at any point in the medium, the net heat conduction rate per unit volume plus the heat energy generation rate per unit volume must be equal to the change rate of energy stored per unit volume[1]。
If the thermal conductivity k is a constant, the thermal diffusion equation can be described as follows:
expressUnsteady stateIn the process of heat conduction, the ability of the internal temperature of the object to become uniform, that is, the larger the thermal conductivity coefficient, the faster the temperature tends to become uniform;q.dotIs generated by heat per unit volumerate。
Example
Calculation of one-dimensional steady state heat conduction
Taking the heat transfer through the furnace wall in the continuous operation furnace as an example, the heat is transferred from the inner wall to the outer wall according toFourier’s law The heat flow is calculated as:
WhereT1 andT2 is the temperature of the inner and outer sides of the wall;AIs the furnace wall area;LIs the thickness of furnace wall;T1-T2 is the driving force of heat transfer;QIs the heat transfer rate.According to the concept that the current is equal to the ratio of potential difference to resistance,L/kAIt is flat wall heat conductionthermal resistance。Since the rate of heat conduction is proportional toThermal conductivity, soHeat exchangerMaterials with high thermal conductivity (such as copper, steel, graphite, etc.) are used as heat transfer partition materials.In thermal insulation facilities, materials with low thermal conductivity (such as asbestos and air) are used asThermal insulation material。
For multi-layer flat walls with equal wall surfaces, according to the concept of series thermal resistance, the heat flow calculation formula is:
Where ΔTIs the temperature difference between the inner side of the innermost wall and the outer side of the outermost wall;nIs the number of layers.
Unsteady state heat conduction calculation: if the operation is intermittent or periodic, such asHeat accumulator(See heat exchanger), when the heat conduction is unsteady.For objects with simple shapes (such as flat platesBox, cylinder and sphere), which can be combined with certaininitial condition Boundary conditionsAnalytic solution, but usually the solution is very complex, and usuallyInfinite seriesFormal representation.In order to facilitate the application, these results are often expressed as lines.abouttwo-dimensional, 3D and other more complex heat conduction, difficult to useanalytic methodSolution, generally availablenumerical methodSolve, or use numerical simulation software such as Ansys or Comsol for calculation.[4]
Industrial applications
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In industry, there are manyHeat transfer process, such as heating of rubber productsvulcanization, heat treatment of steel forgings, etc.The law of heat conduction plays an important role in the design and calculation of kilns, heat transfer equipment, thermal insulation and the analysis of temperature distribution of catalyst particles.In high-temperature and high-pressure equipment (such asAmmonia converterAnd largeethyleneIn deviceWaste heat boilerIn the design of), the heat conduction law shall also be used to calculate thetemperature distribution For thermal stress analysis.
Other heat transfer modes
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Thermal convection
heat conduction
Gas or liquid dependentflowcomeconduct heatIs calledThermal convection。The process of circulating flow between the hotter part and the cooler part of a liquid or gas to make the temperature uniform.
Because of its own temperature, an object has aelectromagnetic waveThe ability to emit energy in the form ofheat transferThis is called thermal radiation.Although heat radiation is also a way of heat transfer, it is different from heat conduction and convection.
It can transfer heat directly from one system to another without depending on the medium.Thermal radiation toelectromagnetic radiationThe higher the temperature, the stronger the radiation.The wavelength distribution of radiation also varies with the temperature. For example, when the temperature is low, it is mainly invisibleinfrared lightConduct radiation. When the temperature is 500 ℃ or higher, the visible light will be emitted successively toUltraviolet radiation。thermal radiationIt is the main way of long-distance heat transfer. For example, the heat of the sun is transmitted to the earth through space in the form of thermal radiation.
Thermal conductor
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All kinds of objects can conduct heat, but the thermal conductivity of different materials is different.An object that is easy to transfer heat is called a good conductor of heat. An object that is not easy to transfer heat is called a bad conductor of heat. Metals are good conductors of heat.Porcelain, wood, bamboo, leather and water are all bad conductors.Silver is the best heat transfer metal, followed by copper and aluminum.Wool, feather, fur, cotton, asbestos, cork and other soft materials are the least good at heat transfer.Asbestos is often used as an insulating material.Liquids, except mercury, are not good at heat transfer, and gases are worse at heat transfer than liquids.[5]
