A Numerical Study Comparing Forced and Natural Convection in a High-Density Single-Phase Immersed Cooled Server
Authors: Gupta Gautam , Nair Vivek , Bansode Pratik , Suthar Rohit , Pundla Sai Abhideep , Herring Joseph , Lamotte-Dawaghreh Jacob , Sivaraju Krishna Bhavana , Agonafer Dereje , Mynampati Poornima , Sweeney Mike 
Published: November 2023

Abstract
Data centers are witnessing an unprecedented increase in processing and data storage, resulting in an exponential increase in the servers’ power density and heat generation. Data center operators are looking for green energy efficient cooling technologies with low power consumption and high thermal performance. Typical air-cooled data centers must maintain safe operating temperatures to accommodate cooling for high power consuming server components such as CPUs and GPUs. Thus, making air-cooling inefficient with regards to heat transfer and energy consumption for applications such as high-performance computing, AI, cryptocurrency, and cloud computing, thereby forcing the data centers to switch to liquid cooling. Additionally, air-cooling has a higher OPEX to account for higher server fan power. Liquid Immersion Cooling (LIC) is an affordable and sustainable cooling technology that addresses many of the challenges that come with air cooling technology. LIC is becoming a viable and reliable cooling technology for many high-power demanding applications, leading to reduced maintenance costs, lower water utilization, and lower power consumption. In terms of environmental effect, single-phase immersion cooling outperforms two-phase immersion cooling. There are two types of single-phase immersion cooling methods namely, forced and natural convection. Here, forced convection has a higher overall heat transfer coefficient which makes it advantageous for cooling high-powered electronic devices. Obviously, with natural convection, it is possible to simplify cooling components including elimination of pump. There is, however, some advantages to forced convection and especially low velocity flow where the pumping power is relatively negligible. This study provides a comparison between a baseline forced convection single phase immersion cooled server run for three different inlet temperatures and four different natural convection configurations that utilize different server powers and cold plates. Since the buoyancy effect of the hot fluid is leveraged to generate a natural flow in natural convection, cold plates are designed to remove heat from the server. For performance comparison, a natural convection model with cold plates is designed where water is the flowing fluid in the cold plate. A high-density server is modeled on the Ansys Icepak, with a total server heat load of 3.76 kW. The server is made up of two CPUs and eight GPUs with each chip having its own thermal design power (TDPs). For both heat transfer conditions, the fluid used in the investigation is EC-110, and it is operated at input temperatures of 30°C, 40°C, and 50°C. The coolant flow rate in forced convection is 5 GPM, whereas the flow rate in natural convection cold plates is varied. CFD simulations are used to reduce chip case temperatures through the utilization of both forced and natural convection. Pressure drop and pumping power of operation are also evaluated on the server for the given intake temperature range, and the best-operating parameters are established. The numerical study shows that forced convection systems can maintain much lower component temperatures in comparison to natural convection systems even when the natural convection systems are modeled with enhanced cooling characteristics.