Air cooling principle

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Air cooling principle [Copy link]

From the perspective of thermodynamics
, the heat absorption and heat release of an object are relative. Whenever there is a temperature difference, heat will inevitably be transferred from a high temperature to a low temperature. This is a very common phenomenon in nature and engineering technology. There are three ways of heat transfer: radiation, convection, and conduction, among which heat conduction is the fastest. The air-cooled heat dissipation we are going to discuss is actually forced convection heat dissipation.
Convection heat transfer refers to the heat transfer process that occurs when a solid surface or fluid in contact with a fluid has different temperatures. The heat source transfers the heat to the heat-conducting medium by heat conduction, and then the medium transfers the heat to the base of the heat sink. The base transfers the heat to the heat sink fins and conducts forced convection with air molecules through the fan to dissipate the heat into the air. The fan continuously blows cold air into the heat sink and flows out hot air to complete the heat dissipation process. Convection heat transfer is
governed by both the law of heat conduction and the law of fluid flow. It is a complex heat transfer process, which is reflected in the fact that there are many factors affecting convection heat transfer.
1. 1. According to the different reasons for the flow of fluid, it can be divided into natural convection and forced convection.
2. According to the flow properties, there are laminar flow and turbulent flow. The transition of fluid from laminar flow to turbulent flow is the result of the loss of flow stability. Generally, the size of the Reynolds number (Re) is used as the basis for judging laminar flow or turbulent flow.
3. The influence of the physical properties of the fluid on convective heat transfer. For example, viscosity, density, thermal conductivity, specific heat, thermal conductivity, etc., they vary with different fluids and change with temperature, thereby changing the effect of convective heat transfer.
4. The influence of the geometric conditions of the heat exchange surface on convective heat transfer. These include:
1) The length and shape of the inlet and outlet sections in the pipeline and the length of the flow channel itself;
2) The geometric shape and size of the surface of the object;
3) The roughness of the surface of the object, such as the wall of the pipeline, the surface of the plate, etc.;
4) The position of the surface of the object (flat, sideways, vertical, etc.) and the size of the flow space.
5. The influence of the change of the physical state of the fluid.
6. The boundary conditions of the heat exchange surface, such as constant heat flux, constant wall temperature, etc., will also affect the convective heat transfer.
7. The relationship between air volume and temperature
T = Ta + 1.76P / Q
Where
Ta - ambient temperature, ℃
P - machine power, W
Q - fan air volume, CFM
T - temperature inside the chassis, ℃
Take an example of the calculation of thermal resistance in circuit design:
Design requirements: Chip power consumption: 20 watts
The maximum temperature that the chip surface cannot exceed: 85℃
Ambient temperature (maximum): 55℃
Calculate the thermal resistance of the required heat sink.
The actual thermal resistance between the heat sink and the chip is very small, and 01℃/W is taken as an approximation. Then
(R + 0.1) × 20W = 85℃ - 55℃
to obtain R = 1.4℃/W
Only when the thermal resistance of the selected heat sink is less than 1.4℃/W can the chip surface temperature not exceed 85℃.
Using a fan can take away a lot of heat from the surface of the heat sink, reduce the temperature difference between the heat sink and the air, and reduce the thermal resistance between the heat sink and the air. Therefore, the thermal resistance parameters of the heat sink are usually expressed in a table. For example:
　Wind speed (feet/second) Thermal resistance (℃/W)
　　0 3.5
　　100 2.8
　　200 2.3
　　300 2.0
　　400 1.8

sunsea

Haha~~ thumbs up~~ Thank you OP~~~~~

supernova

Learned a lot, this is a good summary!!