Heat transfer is a physical phenomenon in physics, which refers to the phenomenon of heat energy transfer caused by temperature difference. Heat is used to measure the change in the internal energy of an object. Heat transfer is a process in which the heat generated by the collision of a large number of molecules, atoms, etc., is transferred from the part with high heat inside the object to the part with low heat. In nature, as long as there is a temperature difference between objects or between different parts of the same object, there will be a phenomenon of heat transfer, which will continue until the temperature between objects is the same, so heat transfer is a universal Phenomenon.
There are three basic forms of heat transfer: heat conduction, heat radiation and heat convection. As long as there is a temperature difference inside or between objects, heat energy must be transferred from high temperature to low temperature in one or more of three ways: heat conduction, heat radiation and heat convection.
Heat conduction is a way of transferring heat by relying on the temperature difference inside the object or direct contact between two different objects without producing relative motion, but only relying on the thermal motion of particles inside the object.
Heat conduction means that when there is a temperature difference between different objects or within the same object, energy transfer occurs through the microscopic vibration, displacement and mutual collision of molecules, atoms and electrons inside the object.
The internal heat conduction mechanism of materials in different phases is different. The heat conduction inside the gas is mainly the result of the irregular thermal motion of its internal molecules which is the result of mutual collision; in non-conductive solids, the vibration near the equilibrium position of its lattice structure transfers energy to adjacent molecules to achieve heat conduction; while metal solids vibrate near the equilibrium position of their lattice structure. The thermal conductivity of ions is accomplished by the movement of free electrons between lattice structures.
Heat conduction is the primary mode of heat transfer in solids. In fluids such as gases or liquids, heat conduction and convection often occur simultaneously.
Thermal radiation is a process in which an object transmits energy through electromagnetic waves. Due to heat, the internal energy of an object is converted into the energy of electromagnetic waves. The radiation process.
All objects with a temperature higher than absolute zero can produce thermal radiation. The higher the temperature, the greater the total energy radiated. The spectrum of thermal radiation is a continuous spectrum, and the wavelength coverage can theoretically range from 0 to infinity. Generally, thermal radiation is mainly propagated by visible light and infrared rays with longer wavelengths.
When an object radiates energy, it also absorbs energy radiated from other objects. If the energy radiated by the object is exactly equal to the energy absorbed at the same time, the radiation process reaches equilibrium, which is called equilibrium radiation, and the object has a fixed temperature at this time.
Thermal radiation transfers thermal energy from one object to another through a vacuum at the speed of light. As long as the temperature of any object is higher than absolute zero, it can radiate electromagnetic waves, which are absorbed by the object and become heat energy, which is called heat rays. The propagation of electromagnetic waves does not require any medium, and thermal radiation is the only way of heat transfer in a vacuum.
Thermal convection is the transfer of heat by the fluid in the object between parts with different temperatures, using the two properties of heat caused by thermal expansion and contraction and displacement.
Since all parts of the fluid are in contact with each other, in addition to the heat convection caused by the overall motion of the fluid, it is also accompanied by heat conduction caused by the movement of the microscopic particles of the fluid.
The convective heat transfer coefficient represents the convective heat transfer capacity. The main factors affecting the convective heat transfer coefficient are: the cause of the flow, the flow condition, the properties of the fluid, and the properties of the heat transfer surface. The convective heat transfer coefficient can be obtained by theoretical derivation, dimension analysis, experiments, etc.
Heat conduction is the main method of heat transfer in solids, and in gases and liquids, heat conduction often occurs at the same time as heat convection. Thermal convection is a process in which internal energy is transferred from a high temperature part to a low temperature part by the flow of liquid or gas. Heat convection is the main way of heat transfer between liquid and gas, and the convection of gas is more obvious than that of liquid. Thermal radiation is a process in which an object directly emits energy without relying on a medium and transmits it to other objects. Thermal radiation is the main way to transmit energy over long distances.