Efficient Electronics Cooling: Using Boiling Process for CPU Cooling By Aranya Chauhan In today’s world we feel connected to people around the world because we have a reliable internet connectivity. Science and engineering have made significant developments and made 24x7 internet access a reality. But what is the source of internet? What is a cloud, where all our data is stored? All this infinite information on Google, where is it coming from? The simple answer to all these questions is Data Center. A data center is a building where thousands for servers are stored, these servers are responsible for never-ending flow of information across the internet. All the big technical companies, Google, Microsoft, Amazon, Facebook, have their data centers continuously transferring and storing information. A data center contains thousands of Central Processing Units (CPUs), each CPU acts like a small brain. One or more than one CPUs are installed in a server along with other electronic equipment like memory and storage devices. These servers are stacked in a pile to form a rack and hundreds of racks are installed in a room. Now such combination of rooms forms a data center. We have all experienced how our laptops and desktop computers become hot after using for some time, sometimes even mobile phones. And at some point, we have all heard fan noise coming from our computers. That time, the computer fan is trying to tell us that CPU is generating lot of heat and in some cases, we can feel the heat. That is the reason advanced gaming computers are built with effective cooling systems, like pumped liquid cooling because air cooling is not enough to provide efficient cooling. Now imagine thousands of such CPUs are generating heat in a data center, and these CPUs require lot of cooling. In 2018, Microsoft conducted experiment by sinking a data center unit in the ocean since heat generation in servers is expected to increase significantly in next few years. Figure 1 shows the data center unit which was installed at the bottom of the Scottish sea.
Figure 1: Microsoft's underwater data center experiment, (a) Retrieving the data center unit from the sea, (b) Cleaning the unit, (c) Layout of racks inside the data center. (Source of images - https://news.microsoft.com/innovation-stories/project-natick-underwater-datacenter/) The data center operation is a very energy expensive process. According to the report of Natural Resources Defense Council in partnership with Anthesis, in 2013 about 3 million computer rooms in data centers used enough electricity equivalent to annual output of 34 large coal-fired power plants. In 2018, worldwide data centers consumed 205 terawatt-hours of electricity, which is equivalent to 1% of total worldwide electricity consumption. A significant amount, ~40% of total power consumed by data centers is used for cooling operations, and 50 – 80% of this power is spent on CPU cooling. The conventional air or liquid based singlephase cooling techniques are not efficient to dissipate heat from high powered CPUs in data centers. The futuristic approach is to use two-phase (boiling heat transfer) cooling approach. During boiling the liquid absorbs heat and is converted into vapor phase. This liquid to vapor conversion uses latent heat of liquid, and this phenomenon can transfer significantly higher heat fluxes compared to single-phase heat transfer. The boiling heat transfer research conducted at Thermal Analysis Lab in Rochester Institute of Technology has developed an innovative tapered microgap evaporator to cool CPUs. The liquid-vapor flow mechanism during boiling heat transfer in the evaporator is shown in Fig.2. Figure 2: Vapor bubble formation, vapor removal, and liquid inflow mechanism in a tapered microgap during CPU cooling process.
28 | The ROCHESTER ENGINEER MAY 2021
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