Convection is a heat transfer process in which a fluid contacts a warmer surface and removes heat from it. When external devices such as pumps or fans are used to increase flow rate, this is called "forced convection" and is widely used for cooling electronic systems. Let us introduce heat sink design for forced convection as following.
Increase the thermal conductivity
1. This is a very straightforward way to increase the air rate. You can do this with a fan with high wind speed.
2. Cross cutting of plate fin heat sink, as is shown in below pictures, cuts the plate fin into several short parts, which reduces heat dissipation, but increases thermal conductivity and pressure. This heat sink design is more appropriate when the wind direction is variable. Such as motorcycle heat sink.
3. Pin fin heat sink design is lighter and smaller in size, at the same time, it also has high volume efficiency, and more importantly, it has equal directivity, so it is suitable for forced convection heat sink, as is shown in below pictures. The shape of the fin can be divided into rectangle, circle and oval. The rectangular fin is made of aluminum extruded crosscut, we called crosscutting fin. The round fin can be forged or cast into shape. The thermal conductivity of the oval or droplet fin is higher, but the shape is more difficult.
4. Impingement cooling uses the airflow from the top of the fin to the bottom. This cooling method can increase the thermal conductivity, but attention must be paid to the direction of the wind to suit the overall design.
5. For the common fan blow down design placed above the heat sink, more accurate design is needed due to the need to match the fan characteristics. Axial fans have a rotating effect and the location of the shaft is not easily reached by the wind, so many radiating finsare designed, as is shown in below picture.
Some fins are also designed to different length or curved tops to guide the wind. The other way is by side blowing. Generally speaking, the sideblow heat sink can be blown through the fin due to the airflow, and the flow resistance is less, so for high and dense fin, with the top cover design to prevent bypass, sideblow than downblow can have a better effect.
6. In order to maximize heat transfer in convection, it is beneficial to increase the free surface available for heat transfer. Therefore, the heat sink is usually designed with a structure called "heat sink fin". Air speed is high due to the forced convection and high wind speeds, and so it makes sense to use a very small gap between fins. Different technologies can be applied to realize heat sinks with reduced fin spacing. With CAB brazing technology, it’s possible to obtain cost-effective heat sinks where the gap and position of the fins are engineered to optimize the cooling system’s thermal performance.
The following show forced convection heat sinks realized with CAB technology.
Design method of forced convection heat sink
1. Add corrugated fin on the surface of the forced convectionheat sink. The depth of corrugated fin should be less than 0.5mm.
2. Increase the number of forced convectionheat sink fins, at present, the advanced extrusion equipment and technology in the world have been able to achieve 23 aspect ratio, while the maximum aspect ratio in China can only reach 8. It is recommended to use low temperature vacuum brazing cold plate with fin spacing up to a minimum of 2mm to provide sufficient concentrated air cooling. Besides, the advance skiving fin technolpgy can process the fin spacing up to a minimum of 0.5 mm.
3. The design method of pin fin is adopted to increase the disturbance of fluid and improve the convective heat transfer coefficient between heat sink fin.
4. When the wind speed is greater than 1m/s(200CFM), the effect of buoyancy on surface heat transfer can be completely ignored.