Microchannel cooling for the LHCb VELO Upgrade I

The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R & D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm(2) W-1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

Experimental Evaluation and Control of Two-Phase Multi-Microchannel Evaporator Cooling Systems for High Efficiency Data Center

Duan Wu

Thermal designers of data centers and server manufacturers are showing a greater concern regarding the cooling of the new generation data centers, which consume considerably more electricity and dissipate much more waste heat, a situation that is creating a re- thinking about the most effective cooling systems for the future beyond conventional air cooling of the chips/servers. Potentially, a significantly better solution is to make use of on- chip two-phase cooling, which, besides improving the cooling performance at the chip level while also consuming less energy to drive the cooling process, also adds the capability to reuse the waste heat in a convenient manner, since higher evaporating and condensing temperatures of the two-phase cooling system (from 60-95 °C) are possible with such a new “green” cooling technology. In the present thesis, three such two-phase cooling cycles using micro-evaporation technology were experimentally evaluated with specific attention being paid to (i) controllability of the two-phase cooling system, (ii) energy consumption and (iii) overall exergetic efficiency, with the emphasis on (i). The controllers were evaluated by tracking and disturbance rejection tests, which were shown to be efficient and effective. The average temperatures of the chips were maintained below the upper limit of 85 °C of computer CPU’s for all tests evaluated in steady state and transient conditions. In general, simple SISO and SIMO strategies were sufficient to attain the requirements of control. Regarding energy and exergy analyses, the experimental results showed that all these systems can be thermodynamically improved since only about 6% of the exergy supplied is in fact recovered in the condenser in the present setup. Additionally, a series of tests covering a wide range of operating conditions under steady state regime were done. The main idea was to generate a “map of performance” of the different cooling systems in terms of energy consumption, energy recovery at the condenser and heat exchanger performance. A total of 120 tests were done which considered all combinations of the variables involved. Finally, empirical and semi-empirical correlations for overall thermal conductance and performance of all components and piping of all these systems were developed based on the experimental results obtained, which can be used for simulations and validations of potential codes developed to design and/or evaluate performance of cooling systems. An overall energy balance analysis for each system using the correlations developed showed that 99.17% of the experimental data were bounded within ± 10%.

EPFL2012

Hybrid Two-Phase Cooling Technology For Next-Generation Servers: Thermal Performance Analysis

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

This paper advances the work presented at ITHERM 2020 where a novel passive two-phase technology was introduced for more efficient cooling of datacenters compared to air-based cooling solutions (e.g. In-Room, In-Row, etc.). The proposed hybrid two-phase cooling technology uses passive low-height thermosyphons to dissipate the heat generated by high power components inside the servers. The heat is then transferred to multiple rack-level thermosyphons, equipped with one or more overhead compact condensers, which in turn dissipate the total heat from the rack into a room-level water-based or air-based cooling loop. Air-cooling is still used for the low heat-generating components (e.g. memory units, secondary chips, etc.) to make the cooling technology cost effective. This study is focused on a thermal performance analysis of the hybrid two-phase cooling technology applied to a single server (2U HP ProLiant DL180 G6), where low-height thermosyphons are installed to cool the two CPUs, and the remaining server components are cooled via air in forced convection using server-level fans. The thermal performance of the low-height thermosyphons is extracted from previous experimental measurements carried out at Nokia Bell Labs and presented at ITHERM 2020. Additional measurements considering the effect of the refrigerant charge on the thermal resistance and associated chip case temperature are conducted. The air-cooling performance and flow distribution are investigated via CFD simulations performed in COMSOL that characterize the conjugate heat transfer between server components and cooling air. The experimental and numerical results presented here show that, at the maximum server IT load of 587 W, the hybrid two-phase technology ensures sufficient cooling for all the hardware components while significantly reducing the energy consumption of the fans. Specifically, the fan contribution is reduced from 24% (traditional air-cooling) to about 2% of the server effective IT load (hybrid two-phase cooling), demonstrating the potential of the proposed technology to scale existing hardware and build next-generation datacenters, while keeping low costs and being environmentally-friendly.

IEEE2021

Hybrid Two-Phase Cooling Technology For Next-Generation Servers: Thermal Performance Analysis

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

IEEE2021

Experimental Evaluation and Control of Two-Phase Multi-Microchannel Evaporator Cooling Systems for High Efficiency Data Center

Duan Wu

EPFL2012