Three ways of heat transfer: heat conduction, heat convection, heat radiation

    There are three basic ways of heat energy transfer: heat conduction, heat convection and heat radiation.

    Heat conduction, also known as heat conduction, refers to the transfer of heat energy generated by the thermal movement of microscopic particles such as molecules, atoms, and free electrons. For example, the internal heat of the solid is transferred from the higher temperature part to the lower temperature part, and the higher temperature solid transfers the heat to another solid with lower temperature in contact with it. Thermal conductivity is a parameter that characterizes the thermal conductivity of a material. The thermal conductivity of different materials is different, even if it is the same material, the thermal conductivity is also related to factors such as temperature. Generally, metal materials have the highest thermal conductivity, followed by liquids, and gases are the smallest.

    Thermal convection refers to the relative displacement between various parts of the fluid caused by the macroscopic movement of the fluid, and the heat transfer process caused by the mixing of cold and hot fluids. Thermal convection can only occur in the fluid, and because the molecules in the fluid are undergoing irregular thermal motion at the same time, the thermal convection must be accompanied by the phenomenon of heat conduction. There are two causes of flow: natural convection and forced convection. Natural convection is caused by the different densities of the hot and cold parts of the fluid. If the flow of fluid is caused by a water pump, fan or other pressure difference, it is called forced convection. In addition, engineering often encounters convective heat transfer problems in which liquids boil on hot surfaces and vapors condense on cold surfaces. They are convective heat transfer accompanied by phase changes. The convective heat transfer of water is stronger than that of air, the phase change is better than the one without phase change, and the forced convection is higher than the natural convection.

    Thermal radiation refers to the radiant energy emitted by an object due to heat. Various objects in nature continuously emit thermal radiation into space, and at the same time continuously absorb thermal radiation emitted by other objects. The combined result of the radiation and absorption process is to cause the heat transfer of the object in the form of radiation. When the object is in thermal equilibrium with the surrounding environment, the radiation transfer amount is equal to zero, but the radiation and absorption process continues.

    The two heat transfer methods of heat conduction and convection can only be realized under the condition of the existence of matter, and heat radiation can be transferred in vacuum, and in fact, the transfer of radiant energy in vacuum is the most effective. At this time, thermal radiation is different from the basic characteristics of heat conduction and convective heat transfer. Another feature of radiant heat transfer is that it not only produces energy transfer, but is also accompanied by a conversion of energy form, that is, it converts from thermal energy to radiant energy when it is emitted, and converts from radiant energy to thermal energy when it is absorbed.

    (huiputech)