Two-Phase Mini-Thermosyphon Electronics Cooling, Part 1: Experimental Investigation

Efficient, small, state-of-the-art passive cooling two-phase systems, i.e. advanced micro-thermosyphon cooling systems, are viable solutions for high performance datacenter servers and power electronics cooling applications. The objective of this study is to push through the "two-phase threshold" that seems to be hindering the application of this cooling technology by offering here proven experimental results (Part 1), validated steady-state and transient simulation tools (Parts 2 and 3) and a server case study (Part 4). The experimental investigation in Part 1 presents the thermal-hydraulic performance of a mini-thermosyphon loop with a small riser height, H-riser = 15.0 cm. The thermosyphon loop has a multi-microchannel copper evaporator, mounted on top of a pseudo-chip CPU emulator (heat source). Experimental results for R134a, acquired under both pumped flow and passive thermosyphon driven flow (for direct comparison) for mass flow rates up to 10 kg/hr, uniform heat fluxes, q of up to 61.4 W/cm(2) and refrigerant filling ratios up to 83% were obtained. An innovative thermal calibration method, developed as a non-intrusive mass flow measurement technique, has also been implemented to monitor the thermosyphon's operation. Summarizing in brief, the two-phase thermosyphon loop with an integrated in-line liquid accumulator offered a very sustainable cooling performance for the microchannel/pseudo-CPU package, and is a first step forward in our effort towards the integration of such two-phase passive cooling devices for data center servers and other electronic devices at heat flux of up to 80 W/cm(2) (or more).

Design Of Passive Two-Phase Thermosyphons For Server Cooling

Raffaele Luca Amalfi, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The main objective of this paper is to utilize an improved version of the simulator presented at InterPACK 2017 to design a thermosyphon system for energy-efficient heat removal from 2-U servers used in high-power datacenters. Currently, between 25% and 45% of the total energy consumption of a datacenter (this number does not include the energy required to drive the fans at the server-level) is dedicated to cooling, and with a predicted annual growth rate of about 15% (or higher) coupled with the plan of building numerous new datacenters to handle the "big data" storage and processing demands of emerging 5G networks, artificial intelligence, electrical vehicles, etc., the development of novel, high efficiency cooling technologies becomes extremely important for curbing the use of energy in datacenters. Notably, going from air cooling to two-phase cooling, not only enables the possibility to handle the ever higher heat fluxes and heat loads of new servers, but it also provides an energyefficient solution to be implemented for all servers of a datacenter to reduce the total energy consumption of the entire cooling system. In that light, a pseudo -chip with a footprint area of 4 x 4 cm2 and a maximum power dissipation of 300 W (corresponding heat flux of about 19 W/cm2), will be assumed as a target design for our novel thermosyphon-based cooling system. The simulator will be first validated against an independent database and then used to find the optimal design of the chip's thermosyphon. The results demonstrate the capability of this simulator to model all of the thermosyphon's components (evaporator, condenser, riser and downcomer) together with overall thermal performance and creation of operational maps. Additionally, the simulator is used here to design two types of passive two-phase systems, an air- and a liquid-cooled thermosyphon, which will be compared in terms of thermal -hydraulic performance. Finally, the simulator will be used to perform a sensitivity analysis on the secondary coolant side conditions (inlet temperature and mass flow rate) to evaluate their effect on the system performance.

AMER SOC MECHANICAL ENGINEERS2020

Two-Phase Flow Control of Electronics Cooling With Pseudo-CPUs in Parallel Flow Circuits: Dynamic Modeling and Experimental Evaluation

Nicolas Lamaison, Jackson Braz Marcinichen, John Richard Thome

On-chip two-phase cooling of parallel pseudo-CPUs integrated into a liquid pumped cooling cycle is modeled and experimentally verified versus a prototype test loop. The system's dynamic operation is studied since the heat dissipated by microprocessors is continuously changing during their operation and critical heat flux (CHF) conditions in the microevaporator must be avoided by flow control of the pump speed during heat load disturbances. The purpose here is to cool down multiple microprocessors in parallel and their auxiliary electronics (memories, dc/dc converters, etc.) to emulate datacenter servers with multiple CPUs. The dynamic simulation code was benchmarked using the test results obtained in an experimental facility consisting of a liquid pumped cooling cycle assembled in a test loop with two parallel microevaporators, which were evaluated under steady-state and transient conditions of balanced and unbalanced heat fluxes on the two pseudochips. The errors in the model's predictions of mean chip temperature and mixed exit vapor quality at steady state remained within +/- 10%. Transient comparisons showed that the trends and the time constants were satisfactorily respected. A case study considering four microprocessors cooled in parallel flow was then simulated for different levels of heat flux in the microprocessors (40, 30, 20, and 10 W cm(-2)), which showed the robustness of the predictive-corrective solver used. For a desired mixed vapor exit quality of 30%, at an inlet pressure and subcooling of 1600 kPa and 3 K, the resulting distribution of mass flow rate in the microevaporators was, respectively, 2.6, 2.9, 4.2, and 6.4 kg h(-1) (mass fluxes of 47, 53, 76 and 116 kg m(-2) s(-1)) and yielded approximately uniform chip temperatures (maximum variation of 2.6, 2, 1.7, and 0.7 K). The vapor quality and maximum chip temperature remained below the critical limits during both transient and steady-state regimes.

American Society of Mechanical Engineers2013

Two-Phase Mini-Thermosyphon Electronics Cooling, Part 3: Transient Modeling and Experimental Validation

Nicolas Lamaison, Jackson Braz Marcinichen, Chin Lee Ong, John Richard Thome

This paper is the third part of the present study on two-phase mini-thermosyphon cooling systems. As mentioned in the first two parts, gravity-driven cooling systems using microchannel flow boiling are a very promising long-term viable solution for electronics cooling and more specifically for datacenter servers. Indeed, the enhancement of thermal performance and the drastic reduction of power consumption together with the possibility of energy reuse and the inherent passive nature of the system offer a wide range of solutions to thermal designers. In order to design this new type of cooling system, a new novel simulation code specifically developed for this purpose is required. While Part 2 dealt with a steady-state simulation code, the present Part 3 considers the dynamic nature of the system. The dynamic simulator is a set of connected partial differential equations (temporal and spatial) solved for the four components of the thermosyphon, meaning for the micro-evaporator, riser, condenser and downcomer. Thermal inertia of the electronic package and role of a liquid accumulator are also accounted for. Predicted steady states obtained for 6 different heat fluxes (from 15.2 to 33.1 W/cm(2)) are compared to experimental results obtained with the test loop presented in Part 1 in terms of chip temperature and system pressure. Mean errors of 2.9 and 3.1% are respectively found and good performances of the heat transfer prediction methods used in the simulator are emphasized. Additionally, the dynamic response to a heat load disturbance is compared with experimental results in terms of chip temperature. Two variations with different time constants are both observed experimentally and predicted numerically. Finally, the predicted mass flow rate variations are discussed.