First of all, we should know that transfer of heat is caused by the temperature difference inside or between objects. If there is no external power input, according to the second law of thermodynamics, heat is always automatically transferred from a place with a high temperature to a place with a lower temperature.
There are three basic ways of heat transfer: heat conduction, heat convection, and heat radiation. The three heat transfer methods are introduced below.
(1) Heat conduction
When there is no relative displacement between the various parts of the object, the thermal energy transfer generated by the thermal motion of molecules, atoms and free electrons and other microscopic particles becomes heat conduction.
The basic calculation formula of heat conduction is Fourier's law: the heat transferred by heat conduction in a unit time is proportional to the cross-sectional area perpendicular to the heat flow and proportional to the temperature gradient. The negative sign indicates that the direction of heat conduction is opposite to the direction of the temperature gradient.
Thermal conductivity is an inherent physical property of a material, which represents the thermal conductivity of the material. The larger the thermal conductivity, the better the thermal conductivity of the material.
(2) Thermal convection
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 only occurs in the fluid. Because the molecules in the fluid will also perform irregular thermal motion at the same time, thermal convection is always accompanied by heat conduction.
In a common situation in engineering, a fluid flows through an object and generates heat transfer process between its surface. This phenomenon is called convective heat transfer process.
Convection heat transfer is divided into two types: natural convection and forced convection.
Natural convection is caused by the different density of the cold and hot parts of the fluid. For example, the air near the radiator is heated and flows upward.
Forced convection is due to the flow of fluid due to pressure difference. For example, the cooling water is driven by a water pump to flow instead of having a density difference.
The basic calculation formula for heat convection is the Newtonian cooling formula.
The convective heat transfer coefficient is related to many factors in the heat transfer process. For example, the physical properties of the object, the relative position of the shape and size of the heat exchange surface, and the flow rate of the fluid. In convection analysis, it is usually necessary to use theoretical analysis or experimental methods to calculate the convective heat transfer coefficient on the surface of the object.
(3) Thermal radiation
The way objects transfer energy through electromagnetic waves becomes radiation. Objects emit radiation for various reasons, among which the phenomenon of radiant energy emitted by heat is called thermal radiation.
The difference between radiation and the first two heat transfer methods is that the first two require the presence of matter, and radiation can transfer energy in vacuum, and even the most efficient transfer in vacuum.
Engineering usually considers the radiation between two or more objects, and each object in the system radiates and absorbs heat at the same time. The net heat transfer between them is calculated using the Stephen Boltzmann equation.