Heat transfer is caused by the temperature difference inside or between objects. If there is no external work input, according to the second law of thermodynamics, heat is always automatically transferred from high temperature to low temperature.
There are three basic ways of heat energy transfer: heat conduction, heat convection and heat radiation. There are three ways of heat transfer.
(1) heat conduction
When there is no relative displacement between the parts of an object, the thermal energy generated by the thermal motion of microscopic particles such as molecules, atoms and free electrons is converted into heat conduction.
The basic calculation formula of heat conduction is Fourier law: the heat transferred by heat conduction per unit time is proportional to the cross-sectional area perpendicular to the heat flow and to the temperature gradient, and the negative sign indicates that the direction of heat conduction is opposite to the direction of the temperature gradient.
Where q represents the heat flux in w; DT/dx is the temperature gradient in c/m; A is the heat conduction area, in square meters;
λ is the thermal conductivity of the material, also called thermal conductivity, and the unit is w/(m c) or W/(mK).
Thermal conductivity is an inherent physical property of materials, which represents the thermal conductivity of materials. The greater the thermal conductivity, the better the thermal conductivity of the material.
(2) Thermal convection
Thermal convection refers to the relative displacement between different parts of the fluid caused by the macroscopic movement of the fluid, and the heat transfer process caused by the mixing of hot and cold fluids. Thermal convection only occurs in fluid, and molecules in fluid move irregularly, so thermal convection is always accompanied by heat conduction.
It is common in engineering that fluid flows through an object and produces a heat transfer process between the object and its surface. This phenomenon is called convective heat transfer process.
Convective heat transfer can be divided into natural convection and forced convection.
Natural convection is caused by the different densities of the hot and cold parts of the fluid, for example, the air near the radiator flows upward when heated.
Forced convection is the flow of fluid due to pressure difference. For example, the cooling water route is driven by water pump, not density difference.
The basic calculation formula of thermal convection is Newton's cooling formula:
Among them, q and a have the same meaning as q and a in Fourier formula, which are heat flow and area respectively.
Ts and tf represent solid surface temperature and fluid temperature, respectively;
H is the convective heat transfer coefficient, which indicates the heat flow rate per unit area under the action of unit temperature difference, and the unit is W/m2 C. The greater the convective heat transfer coefficient, the more intense the heat transfer.
The convective heat transfer coefficient is related to many factors in the heat transfer process. For example, the physical properties of the object, the shape and size of the heat exchange surface are also related to the flow rate of the fluid. In convection analysis, it is usually necessary to calculate the convective heat transfer coefficient of the object surface by theoretical analysis or experiment.
(3) Thermal radiation
The way an object transmits energy through electromagnetic waves becomes radiation. Objects will emit radiation for various reasons, and the phenomenon that heat emits radiant energy is called thermal radiation.
The difference between radiation and the first two methods of heat transfer is that both of them need substances, and radiation can transfer energy in a vacuum, even with the highest efficiency.
The radiant heat flux of an object can be calculated according to the empirical formula of Boltzmann's law:
Where a is the radiation surface area in m2;
ε is the emissivity of an object, also known as blackness, and its value is always less than 1, which is related to the type and surface state of the object;
σ is Stefan-Boltzmann constant, also called blackbody radiation constant, which is a natural constant with a value of 5.67x 10-8W/m2*k4.
φ is the heat flow radiated by the object itself, not the energy of radiation heat transfer.
Radiation between two or more objects is usually considered in engineering, and each object in the system radiates and absorbs heat at the same time. The net heat transfer between them is calculated by Stephen Boltzmann equation:
Where q is the heat flux; ε 1 is the blackness of the object; σ is Stephen Boltzmann constant;
A 1 is the area of radiation surface 1, and F 12 is the shape coefficient from radiation surface 1 to radiation surface 2;
T 1 is the absolute temperature of radiation surface 1, and T2 is the absolute temperature of radiation surface 2.
As can be seen from the above formula, thermal analysis including thermal radiation is highly nonlinear.