Enhancement of Nucleate Boiling on Rough and Dimpled Surfaces with Application to Composite Spreaders for Microprocessors Immersion Cooling

Microprocessors have substantially increased total power dissipation and transistors density over the past two decades, owing to the growth in complexity, performance, and parallelism of computational systems. To continue to effectively and safely dissipate larger amounts of power, advanced methods of cooling such as immersion cooling by nucleate boiling of dielectric liquids are being considered. For electronic cooling applications, dielectric liquids are chemically inert, environmentally friendly, and have low saturation temperatures (34 — 56oC at 0.1 MPa) advantageous for keeping the chips junction temperature below that recommended by the manufacturer (85 — 115 oC), depending on the application. This research experimentally investigated the enhancement of pool nucleate boiling of PF-5060 dielectric liquid on uniformly heated, 10 x 10 x 1.6 mm rough and dimpled Cu surfaces. Fabricating these surfaces is cost effective and scalable, making them suitable for immersion nucleate boiling cooling of high powered microprocessors requiring heat spreaders of different sizes. The PF-5060 has a saturation temperature of 51.4oC at ~0.085MPa — the local pressure in Albuquerque NM, where the experiments were carried out. Because various circuit board orientations and the perpendicular mounting of add-in cards such as graphics processing units results in chip orientations that vary from 0o — 180o with respect to gravity, the effects of these surface inclinations on nucleate boiling of saturated and subcooled PF-5060, are thoroughly investigated for both the rough and dimpled Cu surfaces. In the experiments, liquid subcooling was varied up to 30 K. Experimental nucleate boiling heat transfer coefficient curves for rough Cu surfaces were used to computationally investigate the performance of composite spreaders. These spreaders removed the thermal power dissipated by a 20 x 20 mm microprocessor with and without hot spots, by saturation nucleate boiling of PF-5060. To ensure the consistency of the experimental results, all pool boiling experiments reported in this dissertation are for degassed PF-5060 liquid and uniformly heated 10 x 10 x 1.6 mm Cu surfaces. Multiple experiments performed for the same conditions, separated by at least 2 hours, and sometimes a few days, verified the reproducibility of the results. The absence of boiling hysteresis confirmed no influence by the thermal inertia of the heated Cu surface, but rather the thermophysical properties of the PF-5060 dielectric liquid and surface characteristics solely influenced the nucleate boiling results. Results on the effect of the average surface roughness, (Ra = 0.039 — 1.79μm), inclination angle (θ = 0o — 180o), and liquid subcooling (ΔTsub = 0 — 30 K) on nucleate boiling enhancement and CHF on plain Cu surfaces are presented and discussed throughout the dissertation, along with several developed correlations and comparisons with prior work. In the upward facing surface inclination (θ = 0o), increasing surface roughness, Ra, from 0.039 to 1.79 μm, increased the maximum nucleate boiling heat transfer coefficient, hMNB, by as much as ~150%, and the Critical Heat Flux (CHF) by ~39%. The hMNB, increased proportional to Ra to the power ~0.23, from ~0.67 W/cm2K for Ra = 0.039 μm, to ~1.65 W/cm2K for Ra = 1.79 μm. The corresponding values of CHF increased proportionally to Ra to the power ~0.08, from ~15.5 W/cm2 to ~21.5 W/cm2, respectively. The data of the nucleate boiling heat transfer coefficient, hNB, was correlated as: hNB = AqB. The coefficients \u27A\u27 and exponent \u27B\u27 are both functions of Ra. As Ra increases from 0.039 to 1.79 μm, the coefficient \u27A\u27 increases from 0.09 — 0.23, while the exponent \u27B\u27 decreases from 0.81 — 0.69 which is consistent with results reported by others. The effect of the inclination angle on the nucleate boiling of PF-5060 on rough Cu surfaces is independent of surface roughness. The values of both hMNB and CHF decreased as θ increased. Their lowest values in the downward facing orientation (θ = 180o) are ~40% and ~31% of their upward facing (θ = 0o) values, respectively. For the upward facing orientation (θ = 0o), the CHF increased with increased liquid subcooling, ΔTsub at a rate of 2.2%/K. This rate of increase is independent of the Ra, but depends on the surface inclination angle. It increases as θ increases, to a maximum rate of 4.0%/K in the downward facing orientation (θ = 180o). The developed correlations for hNB, hMNB, and CHF, as a functions of Ra, θ, and ΔTsub, are in good agreement with the experimental data to within + 12%, + 12%, and + 12%, respectively. The present correlation for hNB for the upward facing orientation (θ = 0o), as a function of Ra, falls within the middle of the range of predictions by other established correlations. High speed videos of saturation nucleate boiling at low applied heat flux (~0.5 W/cm2) on rough Cu surfaces were captured at 210 fps, and analyzed for the transient growth of the discrete vapor bubbles. From the transient growth measurements, the bubble departure diameter and detachment frequency were determined, and used to estimate the surface average density of nucleation sites on the smooth and rough Cu surfaces. For the smooth Cu (Ra = 0.039 μm), the determined departure bubble diameter, Dd, and detachment frequency, fd, are 655 + 53 μm and 31 + 4 Hz, respectively. On the rough Cu surfaces (Ra \u3e 0.21 μm), the measured values are 438 + 36 μm and 38 + 3 Hz, respectively, and independent of surface roughness. The present values Dd fall within a broad range of values reported by others for similar dielectric liquids. The obtained values of the fd are generally lower than those reported by others. The determined Dd and fd were used in conjunction with the experimental nucleate boiling curves, along with the estimated total wetted surface areas of the Cu surfaces, to estimate the surface average density of active nucleation sites, N, per footprint area, as a function of wall superheat. For smooth Cu, the active sites density ranges from 100 to 2000 cm-2, compared to 650 to 10,000 cm-2 for the rough Cu surfaces. For all Cu surfaces, N increases with increasing wall superheat and / or surface roughness. The performed pool boiling experiments also investigated nucleate boiling of PF-5060 on uniformly heated 10 x 10 x 1.6 mm dimpled Cu surfaces. The dimples with diameters \u03a6d = 300, 400, and 500 μm, are arranged in a triangular lattice with a fixed pitch-to-diameter ratio of 2.0. The dimples enhance nucleate boiling compared to the smooth polished Cu surface (Ra = 0.039 μm), but not as much as some of the rougher Cu surfaces with Ra \u3e 0.58 μm. In the upward facing orientation, the surfaces with \u03a6d = 300, 400, and 500 μm have hMNB of ~1.04 — 1.08, ~0.98 — 1.01, and 0.65 — 0.72 W/cm2K, respectively, and CHFs of ~19.2 — 19.5, ~18.3 — 18.7, and ~17.7 — 18 W/cm2, respectively. The effect of the inclination angle on the nucleate boiling of PF-5060 on dimple Cu surfaces is similar to that obtained for the rough Cu surfaces. The values of hMNB and CHF decreased as θ increased, to their lowest values in the downward facing orientation (θ = 180o). These values are ~40% and ~33% of their upward facing (θ = 0o) values, respectively. In the upward facing orientation, the CHF increased linearly with the liquid subcooling at a rate of 1.8%/K. This rate is 20% lower than that obtained for smooth and rough Cu surfaces for the same dielectric liquid. High speed videos of nucleate boiling on the dimple surfaces at a heat flux of ~0.5 W/cm2, revealed dimpled surfaces to have a different type of boiling than on the rough Cu surfaces. As opposed to the randomly distributed sites for bubbles nucleation on the rough Cu surfaces, the growing bubbles were almost entirely associated with the surface dimples. The larger dimples promoted more evaporation, leading to higher bubble growth rates. The bubble volumetric growth rate is highest on the dimpled surface with \u03a6d = 500 μm, and lowest on the surface with \u03a6d = 300 μm. The measured bubble departure diameter on the surfaces with \u03a6d = 300, 400, and 500 μm are 738 + 61 μm, 962 + 75 μm, and 1051 + 73 μm, respectively. The corresponding bubble detachment frequencies are 8.6 + 0.7 Hz, 10.2 + 1.0 Hz, and 13.5 + 1.8 Hz, respectively. These departure diameters depended largely on the surface tension forces holding the bubble down along the rim of the dimple, determined by the circumference of the dimple and thermophysical properties of the PF-5060 dielectric liquid. These values of the bubble departure diameter and detachment frequency are much larger and lower, respectively, than on smooth rough Cu surfaces at the same applied heat flux. The pool boiling experimental results on rough Cu with Ra = 1.79 μm were used in 3-D computational thermal analyses investigating the performance of advanced composite spreaders, for immersion cooling of high powered microprocessors. The spreaders are comprised of two thin ~0.5 mm thick Cu laments that protectively encase a layer of thermally anisotropic material such as highly ordered pyrolytic graphite (HOPG). The exposed surface of the top Cu lament is cooled by saturation nucleate boiling of PF-5060. The analyses varied the in-plane (kx = 325 — 2000 W/mK) and through-plane (kz = 5 — 20 W/mK) thermal conductivities, and thickness of the thermally anisotropic material layer (δ = 0 — 1 mm), and the spreader surface area. The impacts of each were determined on the total power removed, and maximum chip temperature, of a 20 x 20 x 0.25 mm underlying microprocessor dissipating uniform power, as well as with central and multiple hot spots with heat flux ratios up to 10. In the computations, the surface of the spreader was limited to 90% CHF, and minimum surface temperature of 1oC above that for boiling incipience in the experiments. These constraints ensure safe operation and nucleate boiling over the entire spreader surface. The total power removed by the composite spreader ranges from 6 — 400% more than that possible by an all Cu spreader of the same thickness. The enhancement in total power removed depends on the ratio of the axial to lateral absolute resistances, Rz/Rx, and the size of the spreader, w. Increasing Rz / Rx increases the total power removed by composite spreader. The Rz and Rx depend on the thickness of the anisotropic layer, δ, and thermal conductivities kx and kz. Correlations were developed to relate the impacts of kx, kz, δ, and w on the total power removed, Q, and maximum chip temperature, Tchip,max. These correlations are in good agreement with the computed data, to within + 1 — 7%. The total thermal resistance, RTOT, which the summation of the resistances from the thermal interface material, composite spreader, and that of nucleate boiling at the spreader surface, ranges from 0.16 — 0.4 oC/W, depending on the kx, kz, and δ of the thermally anisotropic layer, and w of the spreader. For industrial applications involving nucleate boiling of PF-5060 or other similar dielectric liquids such as FC-72, and specifically immersion cooling of high performance microprocessors, the presented results and developed correlations are useful design tools. Additionally, results demonstrated that composite heat spreaders with scalable surface modifications, such as surface roughening and dimples machining, are very promising for the immersion cooling of high powered microprocessors.\u2

Theoretical basis for convective invigoration due to increased aerosol concentration

The potential effects of increased aerosol loading on the development of deep convective clouds and resulting precipitation amounts are studied by employing the Weather Research and Forecasting (WRF) model as a detailed high-resolution cloud resolving model (CRM) with both detailed bulk and bin microphysics schemes. Both models include a physically-based activation scheme that incorporates a size-resolved aerosol population. We demonstrate that the aerosol-induced effect is controlled by the balance between latent heating and the increase in condensed water aloft, each having opposing effects on buoyancy. It is also shown that under polluted conditions, increases in the CCN number concentration reduce the cumulative precipitation due to the competition between the sedimentation and evaporation/sublimation timescales. The effect of an increase in the IN number concentration on the dynamics of deep convective clouds is small and the resulting decrease in domain-averaged cumulative precipitation is shown not to be statistically significant, but may act to suppress precipitation. It is also shown that even in the presence of a decrease in the domain-averaged cumulative precipitation, an increase in the precipitation variance, or in other words, andincrease in rainfall intensity, may be expected in more polluted environments, especially in moist environments. A significant difference exists between the predictions based on the bin and bulk microphysics schemes of precipitation and the influence of aerosol perturbations on updraft velocity within the convective core. The bulk microphysics scheme shows little change in the latent heating rates due to an increase in the CCN number concentration, while the bin microphysics scheme demonstrates significant increases in the latent heating aloft with increasing CCN number concentration. This suggests that even a detailed two-bulk microphysics scheme, coupled to a detailed activation scheme, may not be sufficient to predict small changes that result from perturbations in aerosol loading

Surface Engineering Solutions for Immersion Phase Change Cooling of Electronics

Micro- and nano-scale surface modifications have been a subject of great interest for enhancing the pool boiling heat transfer performance of immersion cooling systems due to their ability to augment surface area, improve wickability, and increase nucleation site density. However, many of the surface modification technologies that have been previously demonstrated show a lack of evidence concerning scalability for use at an industrial level. In this work, the pool boiling heat transfer performance of nanoporous anodic aluminum oxide (AAO) films, copper oxide (CuO) nanostructure coatings, and 1D roll-molded microfin arrays has been studied. Each of these technologies possess scalability in production, thus making them a subject of great interest to industry. To evaluate each surface modification technology, a custom pool boiling setup filled with 3MTM NovecTM HFE-7100 dielectric fluid was utilized. The pool boiling setup was autonomously operated by computer control using a custom LabVIEWTM program. Compared to natively oxidized aluminum samples, AAO samples showed improvements in surface area, but not in wickability or nucleation site density, allowing for the isolated study of the influence of increased surface area on pool boiling performance. Serving as an inverse analogue to nanoporous AAO films, protrusive CuO nanostructure coatings were shown to offer improvements in critical heat flux (CHF), wettability, and nucleation activity over their natively oxidized copper counterparts. At the micro-scale, 1D roll- molded microfin arrays were shown to have improved CHF and nucleation activity over their planar counterparts. Following the initial pool boiling evaluation of each surface finish, the practical applicability of 1D roll-molded microfin arrays was demonstrated through a comparative study of cooling solutions for a field-programmable gate array (FPGA). In this study, the junction-to-ambient thermal resistance for an immersion cooling configuration that utilized a mounted 1D roll-molded microfin array surface was found to be lower than that of both a conventional forced-air cooling system and an immersion cooling configuration with no mounted surface. This finding highlights the significance of 1D roll-molded microfin array surfaces as an industrially acceptable means of improving the capabilities of immersion cooling systems

Techniques for Enhancing and Maintaining Electrical Efficiency of Photovoltaic Systems

Demand for electricity generation from solar energy, which is a clean and renewable resource, is increasing day by day. It is desirable that the panel surface temperature is not excessively hot while generating electricity with PVT panels. High temperature causes thermal degradation and panel electric efficiency decrease. There are many studies in the literature about active thermal cooling of PVT panels used for electricity generation as well as for storing thermal energy. In this study, a review was made on methods developed to increase the thermal and electrical efficiencies of PVT panels

Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface

Evaporation of multi-component drops is crucial to various technologies and has numerous potential applications because of its ubiquity in nature. Superamphiphobic surfaces, which are both superhydrophobic and superoleophobic, can give a low wettability not only for water drops but also for oil drops. In this paper, we experimentally, numerically and theoretically investigate the evaporation process of millimetric sessile ouzo drops (a transparent mixture of water, ethanol, and trans-anethole) with low wettability on a superamphiphobic surface. The evaporation-triggered ouzo effect, i.e. the spontaneous emulsification of oil microdroplets below a specific ethanol concentration, preferentially occurs at the apex of the drop due to the evaporation flux distribution and volatility difference between water and ethanol. This observation is also reproduced by numerical simulations. The volume decrease of the ouzo drop is characterized by two distinct slopes. The initial steep slope is dominantly caused by the evaporation of ethanol, followed by the slower evaporation of water. At later stages, thanks to Marangoni forces the oil wraps around the drop and an oil shell forms. We propose an approximate diffusion model for the drying characteristics, which predicts the evaporation of the drops in agreement with experiment and numerical simulation results. This work provides an advanced understanding of the evaporation process of ouzo (multi-component) drops.Comment: 41 pages, 8 figure

ENHANCEMENT OF SATURATION BOILING OF PF-5060 DIELECTRIC LIQUID ON MICROPOROUS SURFACES

This research experimentally investigated microporous Copper (MPC) surfaces for enhancing nucleate boiling and increasing the Critical Heat Flux (CHF) of PF-5060 dielectric liquid and the potential application of the results to immersion cooling of high power computer chips. MPC surfaces of different thicknesses (80—230 \uf06dm), fabricated using conventional electrochemical deposition at high current density, have different morphology and microstructure. The PF-5060 liquid is chemically inert, environmentally friendly, and has low enough saturation temperature (~ 54oC at 0.10 MPa). This helps maintain the chip junctions temperature below that recommended by the chip manufacturer (85-120 oC, depending on the application).\u2

Assessment of Factors Contributing to Refrigerator Cycling Losses

Thermal mass effects, refrigerant dynamics, and interchanger transients are three factors affecting the transient and cycling performance of all refrigeration and air conditioning equipment. The effects of refrigerant dynamics, including refrigerant/oil solubility, off-cycle migration, and charge redistribution, were found to be the most important. These effects are quantified for a refrigerator instrumented with immersion thermocouples, pressure transducers, and microphones. The analytical methods, however, are applicable to other types of refrigeration and air conditioning systems, including those with capillary tube/suction line heat exchangers.Air Conditioning and Refrigeration Center Project 3

Mercury Wetting and Non-wetting Condensing Research

Mercury wetting and non-wetting condensatio

The materials processing research base of the Materials Processing Center

The goals and activities of the center are discussed. The center activities encompass all engineering materials including metals, ceramics, polymers, electronic materials, composites, superconductors, and thin films. Processes include crystallization, solidification, nucleation, and polymer synthesis