Heat transfer (or heat transfer) is a physical phenomenon in physics, which refers to the heat transfer phenomenon caused by temperature difference.In heat transfer, heat is used to measure the change of internal energy of objects.There are three basic forms of heat transfer:heat conduction、thermal radiationandThermal convection。As long as there is a temperature difference within or between objects, heat energy must be transferred from high temperature to low temperature in one or more of the above three ways.For solid heat sourcemediumWhen the temperature difference is not great (below about 50 ° C), the heat transferred from the heat source to the surrounding medium can beNewton's law of coolingTo calculate.[1-3]
Chinese name
heat transfer
Foreign name
heat transfer
Features
Heat is transferred without work.
Delivery mode
Heat conduction, heat convection and heat radiation
Heat conduction (also called heat conduction) means that when there is a temperature difference between different objects or within the same object, energy transfer will occur through the microscopic vibration, displacement and collision of molecules, atoms and electrons inside the object.The mechanism of heat conduction in different phase states is different.The heat conduction in the gas is mainly due to the irregular thermal movement of its internal molecules, which is the result of mutual collision;In a non-conductive solid, it vibrates near the equilibrium position of its lattice structure to transfer energy to adjacent molecules to achieve heat conduction;The heat conduction of metal solids is accomplished by the movement of free electrons between lattice structures.
Heat conduction is the main way of solid heat transfer.In fluids such as gas or liquid, heat conduction often occurs simultaneously with convection.
Fourier’s law It is a basic law in heat transfer, which was proposed by the famous French scientist Fourier in 1822.[2]The formula indicates that the heat conduction rate is proportional to the temperature gradient where the element is located.
Thermal conductivity is the heat conduction flux under the unit temperature gradient, so it represents the thermal conductivity of the material[1]。
The thermal conductivity of an object is related to many factors such as the composition, structure, temperature, humidity, pressure and aggregation state of the material.Generally speaking, the thermal conductivity of metal is the largest, followed by non-metal, liquid is smaller, and gas is the smallest;Inverse ratio of thermal conductivity of solid metal material to temperature, solidNon metallic materialsIs proportional to the temperature;The thermal conductivity of metallic liquids is very high, while that of non-metallic liquids is low;The thermal conductivity of gas increases with temperature.The thermal conductivity of various substances is usually measured by experimental methods[1]。
thermal radiation
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Thermal radiationelectromagnetic waveThis phenomenon is called thermal radiation.All objects whose temperature is higher than absolute zero can produce thermal radiation. The higher the temperature, the greater the total energy radiated.The spectrum of thermal radiation is continuous spectrum, and the wavelength coverage can theoretically range from 0 to ∞. Generally, thermal radiation is mainly propagated by visible light and infrared light with longer wavelength.
When the temperature is low, the radiation is mainly invisible infrared light. When the temperature is 300 ℃, the strongest wavelength of thermal radiation is in the infrared region.When the temperature of the object is above 500 ℃ to 800 ℃, the strongest wavelength component in the thermal radiation isvisible lightZone
radiation sourceThe energy emitted (or absorbed) by a surface in unit time and unit area is related to the nature and temperature of the surface. The darker the surface, the rougher it is, the stronger the ability to emit (absorb) energy.Any object radiates energy to the surrounding environment in the form of electromagnetic waves.When the radiated electromagnetic wave encounters an object on its propagation path, it will excite theMicroparticleOfThermal movementTo heat the object.
An object outwardRadiant energyAt the same time, it also absorbs the energy radiated from other objects.If the energy radiated by the object is exactly equal toat the same timeThe energy absorbed in the, then the radiation process reaches balance, calledEquilibrium radiationAt this time, the object has a fixed temperature.
Heat radiation can transform heat energy intolight speedgo throughvacuum, passing from one object to another.Any object whose temperature is higher thanAbsolute zeroIt can radiate electromagnetic waves and be absorbed by objects to become heat energy, which is called thermal rays.The propagation of electromagnetic waves does not need any medium, and thermal radiation is the only way of heat transfer in vacuum.The heat energy transmitted from the sun to the earth comes through space in the form of thermal radiation.
There are four important rules of thermal radiation:KirchhoffRadiation law, Planck radiation distribution law, Stefan Boltzmann lawWien's displacement law。These four laws are collectively referred to as the law of thermal radiation.
Thermal convection
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Thermal convection refers to the heat transfer process in which the particles in the fluid move relatively.Because the parts of the fluid are in contact with each other, in addition to the heat convection caused by the overall movement of the fluid, there is also heat conduction caused by the movement of microscopic particles of the fluid.
Thermal convection in industry can be divided into the following four types:
When the fluid has no phase change, there arenatural convection andForced convectionThe forced convection heat transfer can be divided into laminar heat transfer and turbulent heat transfer according to the different flow conditions.
When the fluid has phase change, it includes steam condensation convection and liquid boiling convection.
Convective heat transfer coefficientRepresents the convective heat transfer capability.The main factors affecting the convective heat transfer coefficient are: the causes of flow, flow conditions, fluid properties, heat transfer surface properties, etc.The convective heat transfer coefficient can be obtained by theoretical derivation, dimensional analysis, experiment, etc[1]。
