Jonathan Olivier

Flow Boiling of R134a in a Multi-Microchannel Heat Sink With Hotspot Heaters for Energy-Efficient Microelectronic CPU Cooling Applications

This paper focuses on two-phase flow boiling of refrigerant R134a inside a copper multi-microchannel heat sink for microelectronic central processing unit cooling applications. The heat sink is composed of 100 parallel microchannels, 100 μm wide, 680 μm high, and 15 mm long, with 72-μm-thick fins separating the channels. The base heat flux was varied from 2.57 to 189 W/cm2 and the mass flux from 205 to 1000 kg/m2s, at a nominal saturation temperature of 63°C. Over 40 000 local heat transfer coefficients were measured at 35 locations using local heaters and temperature sensors, for which different heat transfer trends were identified. The main ones were that the heat transfer coefficient increased with heat flux and was independent of mass flow rate. Heat transfer coefficients as high as 270 000 W/m K (relative to the base area) were reached, keeping the chip under 85°C with a maximum of 94 kPa of pressure drop, for no inlet subcooling and a coolant flow rate of 1000 kg/m2s.

Advances in Electronics Cooling

This article highlights the advantages of on-chip microchannel cooling technology, based on first- and second-law analysis and experimental tests on two types of cooling cycles, the first driven by an oil-free liquid pump and the second by an oil-free vapor compressor. The analysis showed that the drivers of the fluid were the main culprits for major losses. It was further found that when energy recovery is of importance, making use of a vapor compression cycle increases the quality of the recovered energy, hence increasing its value. This was demonstrated by analyzing the synergy that can exist between the waste heat of a data center and heat reuse by a coal-fired power plant. It was found that power-plant efficiencies can be increased by up to 6.5% by making use of a vapor compression cycle, which results not only in significant monetary savings, but also in the reduced overall carbon footprints of both the data center and the power plant.

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

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 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 vapour 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 9 to 15˚C lower than for single-phase cooling, while the required pumping power for the CPU 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 boilings 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 multi-microchannel 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.

On-Chip Two-Phase Cooling With Refrigerant 85

Hot-spots are present in micro-electronics and are challenging to cool effectively. This paper presents highly nonuniform heat flux measurements obtained for a pseudo-CPU with 35 local heaters and temperature sensors cooled by a silicon multi-microchannel evaporator with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. A low pressure dielectric refrigerant, R245fa, was used as evaporating test fluid. The base heat flux was varied from 6 to 160 W/cm2 and the junction temperature always remained below 65°C, while the fluid inlet saturation temperature was 30.5°C. On-chip two-phase cooling was found to very effectively cool the hot-spots without inducing flow instabilities. Building on analogous uniform heat flux tests made on the same test section, the effects of position, orientation size, and strength of the hot-spots were analyzed.

\mu{\rm m}

Cooling of data centers is estimated to have an annual electricity cost of 1.4 billion dollars in the USA and 3.6 billion dollars worldwide. Currently, refrigerated air is the most widely used means of cooling data center’s servers. According to recent articles published at the ASHRAE Winter Annual Meeting at Dallas, typically 40% or more of the refrigerated airflow bypasses the server racks in data centers. The cost of energy to operate a server for 4 years is now on the same order as the initial cost to purchase the server itself, meaning that the choice of future servers should be evaluated on their total 4-year cost, not just their initial cost. Based on the above issues, thermal designers of data centers and server manufacturers now seem to agree that there is an immediate need to improve the server cooling process, especially considering that modern data centers require the dissipation of 5–15 MW of heat, and the fact that 40–45% of the total energy consumed in a data center is for the cooling of servers. Thus, the manner in which servers are cooled and the potential of recovery of the dissipated heat are all more important, if one wishes to reduce the overall CO2 footprint of the data center. Recent publications show the development of primarily four competing technologies for cooling chips: microchannel single-phase (water) flow, porous media flow, jet impingement cooling and microchannel two-phase flow. The first three technologies are characterized negatively for the relatively high pumping power to keep the temperature gradient in the fluid from inlet to outlet within acceptable limits, i.e., to minimize the axial temperature gradient along the chip and the associated differential expansion of the thermal interface material with the silicon created by it. Two-phase flow in microchannels, i.e., evaporation of dielectric refrigerants, is a promising solution, despite the higher complexity involved. The present chapter presents the thermo-hydrodynamic fundamentals of such a new green technology. Two potential cooling cycles making use of microchannel evaporators are also demonstrated. A case study was developed showing the main advantages of each cycle, and a comparison between single-phase (water and brine) and two-phase (HFC134a and HFO1234ze) cooling is given. Finally, an additional case study demonstrating a potential application for the waste heat of data centers is developed. The main aspects considered were reduction of CO2 footprint, increase of efficiency (data centers and secondary application of waste heat), and economic gains.