Design Of Passive Two-Phase Thermosyphons For Server Cooling

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.

Two-Phase Mini-Thermosyphon Electronics Cooling, Part 4: Application to 2U Servers

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

This paper is the fourth part of the present study on two-phase mini-thermosyphon cooling. As mentioned in the first three parts, gravity-driven cooling systems using microchannel flow boiling can become a long-term scalable solution for cooling of 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. While Part 1 presented the first-of-a-kind low-height microchannel two-phase thermosyphon test results and Parts 2 and 3 showed the system scale steady and dynamic modeling and simulation results associated with this design using our in-house simulator, Part 4 deals here with an end-user application, i.e. the cooling of a 2U server. The dynamic code of Part 3 is used to model the behavior of a mini-thermosyphon that would fit within the height of a 2U server (8.9cm high), while respecting the other geometric constraints (positions of the processors, distance of the processors to the back of the blade, etc.). Thus, the simulated system consists of two parallel multi-microchannel evaporator cold plates on the top of two chips of about 11cm(2), a riser, a common water-cooled micro-condenser at the back of the blade, a liquid accumulator and a downcomer (including the piping branches to/from the two cold plates). First, an analysis of the steady-state operation highlights multiple solutions from which one is stable and one is unstable. Then, the influences of few parameters such as refrigerants, piping diameters, water coolant inlet temperature and flow rates, filling ratio and heat flux are evaluated. Simulations with unbalanced heat loads on the two chips being cooled in parallel then show the desirable flow distribution obtained in such gravity-driven systems. Finally, temporal heat load and water coolant flow rate disturbances are simulated and discussed. Noting all of these numerous influences on optimal mini-thermosyphon operation, the need for a accurate and detailed simulation code, benchmarked versus actual system tests, is seen to be imperative for attaining a good, reliable, robust design.

Ieee2016

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

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

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).

Ieee2016

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator: Preliminary analysis and performance comparison

John Richard Thome

The flow pattern based flow boiling heat transfer and two-phase pressure drop models for CO2, recently developed by Cheng et al. [L Cheng, G. Ribatski. J. Moreno Quiben, J.R. Thome, New prediction methods for CO2 evaporation inside tubes: Part I - A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops, Int. J. Heat Mass transfer 51 (2008) 111-124; L Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO2 evaporation inside tubes: Part 11 - An updated general flow boiling heat transfer model based on flow patterns, Int. J. Heat Mass transfer 51 (2008) 125-135], have been used to predict the thermal performance Of CO2 in a silicon multi-microchannel evaporator (67 parallel channels with a width of 0.223 mm, a height of 0.68 mm and a length of 20 mm) for cooling of a microprocessor. First, some simulation results Of CO2 flow boiling heat transfer and two-phase pressure drops in microscale channels are presented. The effects of channel diameter, mass flux, saturation temperature and heat flux on flow boiling heat transfer coefficients and two-phase pressure drops are next addressed. Then, simulations of the base temperatures of the silicon multi-microchannel evaporator using R236fa and CO2 were performed for the following conditions: base heat fluxes from 20 to 100 W/cm(2), a mass flux of 987.6 kg/m(2)s and a saturation temperature of 25 degrees C. These show that the base temperatures using CO2 are much lower than those using R236fa. Compared to R236fa, CO2 has much higher heat transfer coefficients and lower pressure drops in the multi-microchannel evaporator. However, the operation pressure of CO2 is much higher than that of R236fa. Based on the analysis and comparison, CO2 appears to be a promising coolant for microprocessors at low operating temperatures but also presents a great technological challenge like other new cooling technologies. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.

Elsevier2009

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator: Preliminary analysis and performance comparison

John Richard Thome

Elsevier2009

Two-Phase Mini-Thermosyphon Electronics Cooling, Part 4: Application to 2U Servers

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

Ieee2016