Thermal design refers to the use of appropriate cooling technologies and structural designs for heat-consuming components and the entire device or system of electronic equipment to control their temperature rise, thereby ensuring the normal and reliable operation of the electronic equipment or system. The author introduces the principles and methods of product cooling design in detail through three articles. It is hoped that this will stimulate discussion and learning among readers.
There are three basic modes of heat transfer: conduction, convection, and radiation.
(1) Thermal conduction
Due to the thermal motion of microscopic particles such as molecules, atoms, and electrons inside an object, while the matter that makes up the object does not undergo macroscopic displacement, the process of transferring heat from a high-temperature region to a low-temperature region is called thermal conduction (or heat transfer).
In gases, thermal conduction is the result of collisions between gas molecules during their irregular thermal motion. The higher the temperature of a gas, the greater the kinetic energy of its molecules. The collisions between molecules of different energy levels allow heat to be transferred from high-temperature areas to low-temperature areas.
In conductive solids, a significant number of free electrons, like gas molecules, transfer energy through collisions between crystal lattices. In non-conductive solids, heat transfer occurs through vibrations of the crystal structure, specifically the vibrations of atoms and molecules near their equilibrium positions. However, the mechanism of heat conduction in liquids is not yet universally understood: one view suggests that liquid conduction is similar to that of gas molecules through collisions, except that the distance between liquid molecules is smaller, and the intermolecular forces have a greater impact than those in gas molecules; another view proposes that liquid conduction is similar to that of non-conductive solids, primarily relying on the action of elastic waves.
(2) Convection
Convection: A mode of heat transfer caused by the relative displacement between different parts of a fluid, relying on the mixing and movement of hot and cold fluids.
Convection is the process by which a fluid (gas or liquid) transfers heat through the macroscopic flow of its various parts. Because fluids have very low thermal conductivity, very little heat is transferred through conduction; therefore, convection is the primary mode of heat transfer. Convection can be divided into natural convection and forced convection.
Natural convection: If the movement of a fluid is caused by a difference in fluid density and a temperature gradient, this heat transfer process is called free convection or natural convection.
In free convection heat transfer, the density difference between the hotter fluid at the top and the colder fluid at the bottom causes the fluid to rise.
Key factors affecting free convection thermal resistance include the temperature gradient in the fluid and the position and orientation of the surface, as shown in the table below (methods to reduce free convection thermal resistance).
Methods to reduce free convection thermal resistance
Forced convection. This heat transfer process is called forced convection if the movement of the fluid is caused by an external force (such as a fan or pump). Important factors affecting the thermal resistance of forced convection include the fluid type, its velocity, and the external characteristics of the surface (as shown in the figure).
Enhancing fluid turbulence is an effective method to increase the forced convection heat transfer coefficient.
The fluid used to remove heat is called a coolant. Air is the most important coolant. Methods to reduce thermal resistance are as follows.
Methods to reduce thermal resistance.
(3) Radiation
Radiation is the only way heat is transferred in a vacuum; it is the transfer of quanta from a hot body (radiator) to a cold body (absorber).
Opaque objects absorb some of the radiant energy falling on their surface while reflecting the rest. The amount absorbed and reflected depends on the object's surface properties, such as color and roughness. An ideal blackbody absorbs all radiant energy, while an ideal crystalline object reflects all energy. The radiation properties of a surface are characterized by a dimensionless quantity called emissivity.
Relationship between subradiative heat transfer rate and the temperature of radiator and absorber
The blackness is also between 0 and 1. The blackness of a completely black body is 1, the blackness of a completely luminous body is 0, the blackness of a real object is between 0 and 1, and the blackness of typical metallic and non-metallic materials is given in (table).
Methods to reduce radiation thermal resistance
a. Use materials with high blackness
b. The radiator must have a good viewing angle relative to the absorber.
c. Large area
Selection of cooling method
The choice of cooling method is determined based on the heat density of the heat dissipation equipment (surface heat dissipation power coefficient or volume heat dissipation power coefficient) (Figure - heat dissipation power coefficient).
The surface heat dissipation coefficient and power coefficient are suitable for selecting external heat dissipation methods for equipment; the volumetric heat dissipation coefficient is suitable for selecting internal heat dissipation methods for equipment (table-cooling method selection).
Heating power coefficient
Cooling method selection
Maximum power per unit area of commonly used cooling technologies
In the next section, the author will provide a detailed introduction to water-cooled radiators. Please continue to follow our WeChat updates.
