Hello everyone, after half a year of learning and using thermal simulation software, I finally found that the best software to use and get started with is Flotherm XT. It is very friendly to novices. However, since the software was only released in recent years, there are very few online tutorials. I will explain it later. Here we will continue to analyze the experience of using the software and learn with everyone.
The first point to share today is about hot appetizers, basic knowledge points:
Three Theorems of Heat:
There are three ways of heat transfer: heat conduction, convection heat transfer and radiation heat transfer. During the heat dissipation process of terminal equipment, all three methods occur. The heat transferred by the three heat transfer methods is calculated by the following formula:
Fourier heat conduction formula: Q=λA(Th -Tc)/δ
Newton's convection heat transfer formula: Q=αA(Tw -Tair )
Fourth power law of radiation: Q=5.67e-8*εA(Th^4-Tc^4)
Among them, λ, α and ε are the thermal conductivity, convection heat transfer coefficient and surface emissivity respectively, A is the heat transfer area, and δ is the material thickness.
Thermal conductivity:
When there is no relative displacement between various parts of an object, the heat generated by the thermal motion of microscopic particles such as molecules, atoms, and free electrons is called thermal conduction. For example, heat transfer within a solid and heat transfer between different solids through contact surfaces are both thermal conduction phenomena. The main way the chip transfers heat to the outside of the casing is through thermal conduction.
The heat transferred during the heat conduction process is calculated according to Fourier's law of heat conduction:
Q=λA(Th-Tc)/d
in:
- A is the area perpendicular to the direction of heat transfer, the unit is m^2;
- Th and Tc are the temperatures of the high and low temperature surfaces respectively;
- δ is the distance between the two surfaces, the unit is m;
- λ is the thermal conductivity of the material, the unit is W/(m*℃), which indicates the thermal conductivity of the material.
Generally speaking, the thermal conductivity of solids is greater than that of liquids, and that of liquids is greater than that of gases. For example, the thermal conductivity of pure copper at room temperature is as high as 400W/(m*℃), that of pure aluminum is 236W/(m*℃), that of water is 0.6W/(m*℃), and that of air is only 0.025W. /(m*℃) or so. Aluminum has high thermal conductivity and low density, so radiators are basically made of aluminum alloy. However, in order to improve the heat dissipation performance of some high-power chips, aluminum radiators are often embedded with copper blocks or copper radiators.
Convection heat transfer
Convection heat transfer refers to the heat exchange process that occurs between a moving fluid and a solid surface when it flows through a solid surface with a different temperature. This is the most widely used heat exchange method in communication equipment heat dissipation. According to different causes of flow, convection heat transfer can be divided into two categories: forced convection heat transfer and natural convection heat transfer. The former is caused by pumps, fans or other external power sources, while the latter is usually caused by the uneven density field caused by the inhomogeneity of the fluid's own temperature field, and the resulting buoyancy force becomes the driving force for movement. The fan cooling commonly used in cabinets is the most typical forced convection heat exchange. In the end product, natural convection heat transfer is mainly used. Natural convection heat dissipation is divided into large space natural convection (such as heat exchange between the terminal shell and the outside air) and limited space natural convection (such as the single board in the terminal and the air in the terminal). It is worth noting that when the distance between the terminal shell and the veneer is less than a certain value, natural convection cannot be formed. For example, there is only heat conduction using air as the medium between the veneer and the casing of a mobile phone.
The heat transferred by convection heat transfer is calculated according to Newton's cooling law:
Q=hA(Tw -Tf )
in:
- A is the area perpendicular to the direction of heat transfer, in m2;
- Tw and Tf are the temperatures of the solid wall and fluid respectively,
- h is the convection heat transfer coefficient. In natural convection, the heat transfer coefficient is in the order of 1~10W/(℃*m2). In practical applications, it generally does not exceed 3~5W/(℃*m2). In forced convection, the heat transfer coefficient is in the range of 1~10W/(℃*m2). 10~100W/(℃*m2) level, generally not exceeding 30W/(℃*m2) in actual applications.
The convection heat transfer coefficient and surface heat flux density values that can be achieved by commonly used cooling methods in electronic equipment are shown in the table on the right:
Heat radiation
- T refers to the absolute temperature value of the object = Celsius temperature value + 273.15 ;
- ε is the blackness or emissivity . This value depends on the type of material, surface temperature and surface condition, and has nothing to do with external conditions or color . The blackness of the polished aluminum surface is 0.04, the blackness of the oxidized aluminum surface is 0.3 , the blackness of the painted surface reaches 0.8 , and the blackness of the snow is 0.8 .
Black body ≠ black
Let’s end it first and continue later. . .