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Electronic components, especially the CPUs/GPUs used in data centers are concentrated heat-generating sources. Their large Thermal Design Power (TDP), small die area and confined packaging make their thermal management a unique challenge. While conventional single-phase cooling methods fail to dissipate such large amounts of heat efficiently, recently developed two-phase cooling systems also lack the holistic approach of combining efficient boiling and condensation mechanisms. It is hypothesized that subcooled boiling with submerged condensation and reduced saturation pressure will result in high-heat flux dissipation while maintaining low surface temperatures. The novel boiling chamber presented in this work is demonstrated in a compact configuration that fits in a 1U/2U server rack by combining submerged condensation with subcooled pool boiling. The boiling chamber is filled with 13% and 40% fill ratio of water and Novec-7000 and experimentally investigated on a thermal test vehicle. Results show that the boiling chamber dissipates about 400 W of heat with a surface temperature of less than 80 °C using Novec-7000 working fluid. When tested with water, the device dissipated more than 750 W of heat (heat flux ≈ 67 W/cm2) with a surface temperature of less than 90 °C. Though the surface temperature rose to 120°C, further testing shows the device to dissipate more than 1 kW from a 34.5 × 32 mm2 plain copper chip. High-speed images identify submerged condensation and small diameters of vapor bubbles. Further enhancements can be achieved by implementing enhanced boiling and condensation surfaces. Lastly, a guide to design considerations and future work is provided to unlock the greater performance potential of the novel boiling chamber.
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Pool Boiling of Nanofluids on Biphilic Surfaces: An Experimental and Numerical Study
This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool-boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions were developed and used as surface interfaces with different nanofluids (water with gold, silver, aluminum and alumina nanoparticles), in order to evaluate the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. To study the nucleation of a single bubble in pool boiling condition, a numerical model was also implemented. The results show an evident benefit of using biphilic patterns with well-established distances between the superhydrophobic regions. This can be observed in the resulting plot of the dissipated heat flux for a biphilic pattern with seven superhydrophobic spots, δ = 1/d and an imposed heat flux of 2132 w/m2. In this case, the dissipated heat flux is almost constant (except in the instant t* ≈ 0.9 when it reaches a peak of 2400 W/m2), whilst when using only a single superhydrophobic spot, where the heat flux dissipation reaches the maximum shortly after the detachment of the bubble, dropping continuously until a new necking phase starts. The biphilic patterns also allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic spacing between the superhydrophobic regions, which clearly contributes to cool down the surface. This effect is noticeable in the case of employing the Ag 1 wt% nanofluid, with an imposed heat flux of 2132 W/m2, where the coalescence of the drops promotes a surface cooling, identified by a temperature drop of 0.7 °C in the hydrophilic areas. Those areas have an average temperature of 101.8 °C, whilst the average temperature of the superhydrophobic spots at coalescence time is of 102.9 °C. For low concentrations as the ones used in this work, the effect of the nanofluids was observed to play a minor role. This can be observed on the slight discrepancy of the heat dissipation decay that occurred in the necking stage of the bubbles for nanofluids with the same kind of nanoparticles and different concentration. For the Au 0.1 wt% nanofluid, a heat dissipation decay of 350 W/m2 was reported, whilst for the Au 0.5 wt% nanofluid, the same decay was only of 280 W/m2. The results of the numerical model concerning velocity fields indicated a sudden acceleration at the bubble detachment, as can be qualitatively analyzed in the thermographic images obtained in this work. Additionally, the temperature fields of the analyzed region present the same tendency as the experimental results.
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- Award ID(s):
- 1605789
- PAR ID:
- 10232432
- Date Published:
- Journal Name:
- Nanomaterials
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2079-4991
- Page Range / eLocation ID:
- 125
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/nano11010125
