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

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.

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

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 4: Application to 2U Servers

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

Ieee2016

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

American Society of Mechanical Engineers2013