Due to the increasing power of electronic devices, the use of air-cooled heat sink is becoming increasingly difficult to meet their requirements. Due to the use of air as the working fluid, it is becoming increasingly difficult to design heat sinks that can dissipate over 100 W/cm2 at the device level. Liquid cold plates have become a recognized alternative to air-cooled heat sinks due to their better performance and smaller size. In liquid cooling technology, microchannel Liquid cold plate is the most promising device level cooling technology.
Liquid Cold Plate Manufacturer — Kenfatech
Based on over 10 years of production experience, KENFA TECH adopts various advanced production processes to produce Liquid cold plates, especially optimized liquid cooled plate microchannels, which can take away 790 W/cm2 of heat.
The magic behind microchannel liquid cooled plates lies in their ability to achieve high heat transfer coefficients and create a large surface area. The heat dissipation capacity of liquid cooled heat sinks is determined by the heat conduction in solid materials and the heat convection in fluids.
Formula 1
Usually, when using highly conductive materials to manufacture heat sinks, convection is the main factor in reducing thermal resistance. For fully developed laminar flow in a square channel with constant wall temperature or wall heat flux, the Nusselt number Nu is a constant. The heat transfer coefficient h can be calculated by the following equation.
Among them, k is the thermal conductivity of the fluid, and Dh is the hydraulic diameter of the channel. The above equation indicates that the heat transfer coefficient is inversely proportional to the hydraulic diameter of the channel. By reducing the channel size, a larger heat transfer coefficient can be obtained.
The friction coefficient of laminar flow fully developed in a square channel is also a constant.
Formula 2
The formula for calculating the pressure drop δ P through the channel is as follows, where f is the friction factor, u is the flow velocity, and L is the channel length. It can be seen that the pressure drop is also inversely proportional to the hydraulic diameter of the channel. So for a constant flow rate, the smaller the channel, the greater the pressure drop.
Combining the above two formulas, there is a contradiction between improving the thermal and hydraulic performance of microchannel radiators.In order to reduce thermal resistance, the size of the channel should be reduced. However, as the channel size decreases, the pressure drop through the channel will increase. This means that more power is needed to drive the flow, which increases the demand for external pump performance.
There are many methods to improve microchannel performance. Some of them aim to increase the convective heat transfer coefficient, some focus on reducing pressure drop, and some are dedicated to these two goals. The general methods for improving performance can be summarized as follows:
Larger aspect ratio channels
Using deeper channels will increase convective heat transfer and reduce pressure drop in microchannel heaters. Compare two different channel geometries in Figure 1.
For a square groove arrangement, the hydraulic diameter is 2a, the boundary circumference is 24h, and the total cross-sectional area of the groove is 12a ². For a rectangular channel arrangement with an aspect ratio of 6, the hydraulic diameter of the channel is:
the total circumference is 56a, and the total cross-sectional area is 24 square meters. Rectangular channels have smaller hydraulic diameters and larger perimeters and cross-sectional areas.This means it can achieve smaller thermal resistance and voltage drop. Its working principle is like an air-cooled heat sink with long fins. However, there are limitations on the height of the fins in microchannel structures. This limitation is limited by manufacturing technology and internal conduction of solid materials.On the one hand, it is difficult to form large aspect ratio channels with uniform wall thickness at the microscale.
On the other hand, due to its high convective heat transfer coefficient, the efficiency of microchannel fins rapidly decreases with increasing channel height.This means that when the channel height exceeds a certain limit, the overall thermal resistance will not change, and the thermal resistance inside the solid will become significant.Therefore, there exists an optimized aspect ratio, and thermal engineers must find the optimal solution between performance and cost.
Fin enhancement
There are many ways to improve the performance of fins, such as the four fin designs shown in Figure 2.
The (a) shows a conventional straight fin layout. Figure (b) shows a layout with decomposed fins, which will improve flow mixing and reduce pressure drop. Figure (c) shows a layout with staggered wings. This layout can break through the boundary layer and achieve a larger heat transfer coefficient on a single fin. Figure 2 (d) shows a complex diamond shaped fin layout.
Flow optimization
Changing the structure of microchannel heat sinks to create better flow is another way to achieve better heat sink performance.Figure 3 illustrates an example of flow optimization. In this microchannel heat sink, there are two inlets and two outlets. The flow is separated in the middle. The thermal performance of this structure is similar to the straight fin layout shown in Figure 2 (a). As the flow length is reduced by half, the pressure drop is also reduced by half compared to Figure 2 (a).
Multi layer microchannel structure
The use of multi-layer microchannel structure can achieve good thermal and hydraulic performance. Figure 4 shows two examples of multi-layer microchannel heat sinks. The multi-layer structure reduces the hydraulic diameter of the channel, resulting in a high heat transfer coefficient and increasing the wetted area between solids and liquids. This helps to reduce the total thermal resistance. It also increases the cross-sectional area of the channel, resulting in a smaller pressure drop.
This multi-layer structure can also overcome the channel height limitation caused by manufacturing technology and make the heat sink thicker.
However, this also means that creating multi-layer microchannel structures will increase costs and manufacturing difficulties.
So, when optimizing the microchannels of liquid cooled plates, we must consider the choice of cost and processing technology, so as to make the design of liquid cooled plate with high cost-effectiveness
