Immersion Cooling - Advantages
The most appealing aspect of this approach is its elegance and overall simplicity: Not only is the convective heat transfer for a liquid generally more effective compared to air at comparable velocities, but the phase change absorbs additional heat while keeping the temperature of the liquid adjacent to the heat sink at a nearly constant boiling temperature. And since the vapor bubbles have a vastly lower density relative to the liquid, they promote convective flow adjacent to the hot surfaces aiding in the circulation of the fluid inside the tank. An additional advantage is that most dielectric fluids (oils and fluorocarbons alike) are considered to be environmentally friendly compared to traditional refrigerants.
Immersion Cooling - Challenges
Obviously, there are limits to how much heat can be removed from surfaces using buoyancy-driven, two-phase immersion cooling. High bubble densities (= vapor volume fraction) near the surfaces will eventually turn into a continuous vapor film that will drastically hamper the heat transfer. The maximum achievable heat flux is called the ‘critical heat flux’. Also, simply immersing a server motherboard designed for air cooling into a tank of dielectric fluid may not realize the full potential of immersion cooling. Heat sink geometries should be designed specifically for immersion cooling or the full cooling potential may not be realized. More on this is planned for future discussions on this topic.
Dielectric Fluids
An important enabler of this technology is the availability of engineered coolants that are environmentally safe. This allows for immersion cooling at ambient temperatures and without the need for hermetically sealed containers or special handling of the coolant.
The key coolant properties from a heat transfer perspective are high thermal conductivity, high heat capacity and, most importantly for two-phase cooling, a boiling point sufficiently below the maximum junction temperature of the component to be cooled.
It should also be noted that both the conductivity and heat capacity of most dielectric fluids is usually lower than those of water. But water comes with its own set of challenges that make it less suitable for immersion cooling. There have been attempts to use water in conjunction with conformal coatings to protect electronic assemblies from short-circuiting, but these coatings invariably increase the thermal resistance between junction and fluid which partially negates the more favorable thermal transport properties of water.
The immersion cooling simulation discussed here uses the material properties of HFE-7100, a Hydrofluoroether which is available from 3M under the tradename Novec 7100 (see Figure 3).
Immersion Cooling - Background
Previous work by NSES, LLC focused on air cooling a HPC computing platform (see here). The customer’s preferred approach followed the traditional concept where each server rack employs a multitude of fans that drive cooled, ambient air past intricate heat sinks mounted to electrical components with high power dissipation. Aside from deafening fan noise, the heat transfer from convective air cooling tends to taper off at high flowrates thereby limiting the achievable power dissipation for each server rack. Alternatively, liquid cooling of individual components (common with high-end gaming rigs) is not always suitable for server hardware because the sheer number of high-powered components scattered across the mother board would lead to overly complicated and failure-prone coolant circuits. The most elegant and best performing solution for many server applications appears to be immersion cooling where the entire electronics assembly is placed into a bath of dielectric fluid (see Figure 1). Early applications of this cooling concept to computer systems date back to the 1960s by IBM (US patent US3406244A) and 1980s by Cray Research (US patent US4590538A). Both patents mention the advantages of two-phase cooling which necessitates dielectric fluids (typically fluorocarbons) with boiling temperatures sufficiently below the maximum junction temperature of the electronics components to be cooled.
Figure 1: Bath tank with server hardware (Image Source).
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Special Topic: CFD Simulation of Immersion Cooling
The boiling point of HFE-7100 at atmospheric pressure is at 61°C (334 K) which makes it a suitable candidate for a large range of immersion cooling applications though dielectric fluids with both lower and higher boiling temperatures are available (link to 3M website).
Figure 2: Vapor bubbles during operation (Image Source).
Figure 3: Phase diagram for HFE-7100 (3M Novec-7100) provided by L. Stang/B. Wilson from 3M.