The process of energy transfer from one object to another, or from one part of an object to another, is called heat transfer under the condition of no work but only temperature difference. In nature, as long as there is a temperature difference between objects or between different parts of the same object, there will be heat transfer until the temperature between objects is the same, so heat transfer is a common phenomenon.
Basic forms of heat transfer
There are three basic forms of heat transfer: heat conduction, thermal radiation and thermal convection. As long as there is a temperature difference inside or between objects, heat energy will inevitably be transferred from high temperature to low temperature in one or more of three ways: heat conduction, heat radiation and heat convection.
First, heat conduction.
Heat conduction is a way to transfer heat only through the thermal motion of particles inside an object, rather than through the relative motion caused by the temperature difference inside the object or the direct contact between two different objects.
The mechanism of internal heat conduction in different phases is different. The heat conduction in gas is mainly the result of irregular thermal motion and collision of its internal molecules; In a non-conductive solid, it vibrates near the equilibrium position of its lattice structure, transferring energy to adjacent molecules to realize heat conduction; The heat conduction of metal solid 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, the heat conduction process usually occurs at the same time as convection.
Second, thermal radiation.
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.
All objects whose temperature is higher than absolute zero will 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 mainly propagates through visible light and infrared light with longer wavelength.
When an object radiates energy outward, it also absorbs the energy radiated by other objects. If the energy radiated by an object is exactly equal to the energy absorbed at the same time, then the radiation process is balanced, which is called balanced radiation. At this time, the object has a fixed temperature.
Thermal radiation can transfer thermal energy from one object to another through vacuum at the speed of light. As long as the temperature of any object is higher than absolute zero, it can radiate electromagnetic waves and be absorbed by the object into 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.
Third, thermal convection.
Thermal convection is the two properties of fluid in an object that generate heat by thermal expansion and contraction and displacement, and transfer heat between different parts at different temperatures.
Because all parts of the fluid are in contact with each other, in addition to the thermal convection caused by the overall movement of the fluid, it is also accompanied by the thermal conduction caused by the movement of micro-particles of the fluid.
Convective heat transfer coefficient represents convective heat transfer capacity. The main factors affecting the convective heat transfer coefficient are: flow reasons, flow conditions, fluid properties, heat transfer surface properties and so on. The convective heat transfer coefficient can be obtained by theoretical derivation, dimensional analysis and experiment.