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 'A' and exponent 'B' are both functions of Ra. As Ra increases from 0.039 to 1.79 μm, the coefficient 'A' increases from 0.09 — 0.23, while the exponent 'B' 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 > 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 Φd = 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 > 0.58 μm. In the upward facing orientation, the surfaces with Φd = 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 Φd = 500 μm, and lowest on the surface with Φd = 300 μm. The measured bubble departure diameter on the surfaces with Φd = 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.'
Enhancement of nucleate boiling, dielectric liquids, immersion cooling, thermally anistropic composite heat spreasders, electronics cooling, thermal management, electronics packaging
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