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Research Papers

Green Cooling of High Performance Microprocessors: Parametric Study Between Flow Boiling and Water Cooling

[+] Author and Article Information
Jonathan A. Olivier, Jackson B. Marcinichen, Arnaud Bruch, John Thome

e-mail: john.thome@epfl.ch Laboratoire de Transfert de Chaleur et de Mass, École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland

J. Thermal Sci. Eng. Appl 3(4), 041003 (Oct 24, 2011) (12 pages) doi:10.1115/1.4004435 History: Received September 21, 2010; Accepted May 16, 2011; Published October 24, 2011; Online October 24, 2011

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up to 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapor can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water–ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature from 9 to 15 °C lower than for single-phase cooling, while the required pumping power for the central processing unit cooling element for single-phase cooling was on the order of 20–130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling’s dependency on the local heat flux, with junction temperatures being 20 to 30 °C lower when compared to water and the 50% water–ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser, and a multimicrochannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%), and the exergy destroyed, i.e., irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Cross-sectional view of a cooling element used in Aquasar [19]

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Figure 9

Critical heat flux of HFC134a for various heat and mass fluxes

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Figure 10

Critical heat flux of all the refrigerants as a function of the heat flux for a mass flux of 300 kg/(m2 s) and 1000 kg/(m2 s)

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Figure 11

Heat transfer coefficients as a function of the base heat flux

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Figure 12

Junction temperature as a function of the base heat flux for a mass flux of 900 kg/(m2 s)

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Figure 13

Junction temperature uniformity as a function of the base heat flux for a mass flux of 900 kg/(m2 s)

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Figure 14

Pressure drop as a function of the base heat flux for a mass flux of 900 kg/(m2 s)

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Figure 2

Flow diagram of simulation program

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Figure 3

Schematic of a multichannel micro-evaporator

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Figure 4

Local heat transfer coefficients for water for various mass fluxes

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Figure 5

Local heat transfer coefficients for 50% water–ethylene glycol mixture for various mass fluxes

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Figure 6

Length-averaged heat transfer coefficients for water as a function of the base heat flux for various mass fluxes

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Figure 16

Performance of the fluids at a mass flux of 900 kg/(m2 s)

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Figure 17

Liquid pumping cooling cycle

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Figure 18

Exergetic efficiency versus secondary fluid temperature

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Figure 19

Simulated hot-spot map

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Figure 20

Local heat transfer coefficient with hot-spots

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Figure 21

Local junction temperatures with hot-spots

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Figure 7

Nusselt number for water for all the heat fluxes and mass fluxes simulated as a function of the dimensionless distance

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Figure 8

Local heat transfer coefficients for HCFC123 as a function of the local vapor quality for various mass fluxes

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Figure 15

Required pumping power as a function of the base heat flux for a mass flux of 900 kg/(m2 s)

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