Thermal-Hydraulic Characterization of Thermosyphon Cooling System for Highly Compact Edge MicroDataCenter. Part II: Solver Validation
Authors: Minazzo Enzo M. , Rouaze Gautier , Marcinichen Jackson B. , Thome John R. , Buining Fred 
Published: November 2023

Abstract
JJ Cooling Innovation specialises in developing innovative two-phase cooling technologies for computers and servers, mobile electronics, power and aerospace electronics, telecommunications, hybrid auto electronics and batteries, EUV lithography, etc. We have our own exclusive in house engineering simulation codes for microchannel evaporator, cold plates, condensers, pulsating heat pipes and micro-two-phase thermosyphon cooling systems, we fabricate test sections, and we have in-house bench testing capabilities. Presently, this paper is focused on Edge computing, which is a rapidly expanding sector of the Internet of Things, and one of the keys for gaining high performance with the integration of 5G, AI, VR, etc. with a goal of drastically reducing energy consumption for cooling.
In the European project BRAINE, an extremely efficient and passive cooling system was developed for the new very compact edge computer architecture, which has high heat fluxes to dissipate on individual computer cards and very high heat dissipation rate per unit volume in the dense array of these cards. In this case, the thermosyphon is passive and the heat is then dissipated into the environment by fans. For more details, see Part 1. This is a perfect match for two-phase micro-channel cooling plates integrated into a passive thermosyhon (gravity-driven) cooling system, moving the heat outside to be dissipated to indoor or outdoor air or water lines under datacenter floors. Using our experience, we developed very thin two-phase cooling plates, adapted to handle the array of computer cards with their minimal spacing. Working with our partners so as to fit into the compact architecture, a new innovative micro-thermosyphon cooling system with versatile attributes was developed to provide a very low energy consumption cooling solution using a new European approved environmentally-friendly refrigerants as the working fluid.
The new micro-evaporator coldplates for the CPUs, GPUs, etc. has multiple microchannels for high heat dissipation and low pressure drop, allowing the system’s working fluid (new environmentally-friendly low GWP refrigerant) to be circulated by gravity-driven thermosyphon flow for an energy-free solution. The heat is dissipated to the environment by a compact finned air coil with a very low fan energy consumption compared to 2U servers.
This study presents a basic description of the system (will be patent pending by time by October 2023), a literature description of recent micro-thermosyphon cooling technology, a description and a validation of the solver (already described in previous publications) used to design the two-phase closed loop thermosyphon cooling system for this Edge MicroDataCenter (EMDC) for which experimental results have been presented in companion Part I. The solver validation was performed by comparing pressure drop of the different components as well as the maximum temperature of heaters mimicking CPUs for 247 simulated datapoints and proved extremely good accuracy without any scaling or empirical factors: 87% datapoints for the evaporator pressure drop, 67% datapoints for the riser pressure drop, 57% datapoints for the condenser pressure drop and 72% datapoints for the total pressure drop were within ±30% of the experimental results, which is accuracy comparable to the best two-phase pressure drop correlations in the literature, so a good validation result. For the temperature validation, 98% of the datapoints were within ±5 °C of the experimental measurements, highlighting the robustness of the solver.