Experimental Characterization of a Server-Level Thermosyphon for High-Heat Flux Dissipations

This paper advances the work presented at ITHERM 2019 in which a novel thermal technology has been introduced to cool servers and datacenter racks more efficiently compared to the traditional air-based cooling solutions. As reported in the state-of-the-art and the previous papers published by the same authors, heat flux dissipation in telecom servers and high-performance computing servers is following an exponentially increasing trend in order to handle the new requirements of higher data transmission, data processing, data storage and massive device connectivity dictated by the next industrial revolution. This trend translates into the need for upgrading the capacity of existing servers and datacenter racks, as well as building new datacenters around the globe. The envisioned cooling technology, which will improve datacenter energy usage, is based on a novel combination of low-height thermosyphons operating in parallel to passively dissipate the heat generated by the servers and rack-level thermosyphons equipped with an overhead compact condenser, to dissipate the total power from the server rack to the room-level water cooling loop. The present paper is mainly focused on the experimental evaluation of the thermal performance of a 7-cm high liquid-cooled thermosyphon designed to cool a 2-U server with a maximum heat dissipation here of 200 W (but could have gone even higher) over a 4 × 4 cm
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pseudo-chip footprint. A new test setup and filling rig were designed at Nokia Bell Labs in order to accurately evaluate thermosyphon thermal performance over a wide range of heat loads, secondary side mass flow rates and inlet temperatures, using R1234ze(E) as the working fluid. A new extensive database was obtained, capturing the entire thermosyphon characteristic curve, expressed as total thermal resistance as a function of the power. Here, the experimental results are presented and discussed in detail, and they demonstrate that passive two-phase thermosyphon-based approach provides significant advantages in terms of cooling performance, energy efficiency and noise level compared to other datacenter cooling solutions available on the market or under development.