Jack L. Parker

Effect of Surface Orientation on Nucleate Boiling of FC-72 on Porous Graphite

Effects of orientations of porous graphite and smooth copper surfaces, measuring 10 mmX 10 mm, on saturation nucleate boiling and critical heat flux (CHF) of FC-72 dielectric liquid and of liquid subcooling (0, 10, 20, and 30 K) on nucleate boiling in the upward facing orientation are investigated. Inclination angles (θ) considered are 0 deg (upward-facing), 60, 90, 120, 150, and 180 deg (downward facing). The values of nucleate boiling heat flux, nucleate boiling heat transfer coefficient (NBHTC), and CHF are compared with those measured on the smooth copper surface of the same dimensions and CHF values on both copper and porous graphite are compared with those reported by other investigators on the smooth surfaces and microporous coatings. Results demonstrated higher NBHTC and CHF on porous graphite, particularly in the downward-facing orientation (θ=180 deg). In the upward-facing orientation, NBHTCs on both surfaces decrease with increased subcooling, but increase with increased surface superheat reaching maxima then decrease with further increase in surface superheat. In saturation boiling on copper and both saturation and subcooled boiling on porous graphite these maxima occur at or near the end of the discrete bubble region, and near CHF in subcooled boiling on copper. Maximum saturation NBHTC on porous graphite increases with decreased surface superheat and inclination angle, while that on copper increases with increased surface superheat and decreased surface inclination. At low surface superheats, saturation nucleate boiling heat flux increases with increased inclination, but decreases with increased inclination at high surface superheats, consistent with previously reported data for dielectric and nondielectric liquids. The fractional decreases in saturation CHF with increased 0 on smooth copper and microporous coatings are almost identical, but markedly larger than on porous graphite, particularly in the downward-facing orientation. In this orientation, saturation CHF on porous graphite of 16 W/cm 2 is much higher than on copper (4.9 W/cm 2 ) and as much as 53% of that in the upward-facing orientation, compared to only ∼18% on copper.

Pool boiling in saturated and subcooled HFE-7100 dielectric fluid from a porous graphite surface

Experiments investigated the effects of subcooling on pool boiling of HFE-7100 dielectric liquid from porous graphite measuring 10/spl times/10 mm and 3.0 mm thick. Results indicate no temperature excursion at incipient boiling and that subcooled nucleate boiling occurs at surface temperatures well below saturation. Conversely, for a smooth copper surface the temperature overshoot at incipient boiling is as much as 37.4 K in saturation and 7.5-16 K in subcooled nucleate boiling. Nucleate boiling ensues on the porous graphite at 0.8 K in saturation, and -2.0 to -11.7 K in subcooled boiling, compared to 5.0 to 10.9 K on the smooth copper surface. The nucleate boiling heat fluxes are more than 500% higher and the surface superheats are significantly lower than those for the copper surface in same experimental setup. The critical heat flux (CHF) for the porous graphite surface is 31.8, 45.1, 55.9, and 66.4 W/cm/sup 2/ for saturation, 10 K, 20 K, and 30 K subcooled boiling, and the values for the smooth copper are 21.5, 28.1, 33.7, and 37.3 W/cm/sup 2/, respectively. In addition, the rate of increase in CHF for the porous graphite with increased subcooling /spl sim/50% higher than for the smooth copper surface.

Thermal Analyses of Composite Copper/ Porous Graphite Spreaders for Immersion Cooling Applications

The performance of composite spreaders cooled by nucleate boiling of FC-72 liquid and made of top porous graphite layer (≥ 0.4 mm or ΔPG ≥ 0.2), for enhancing nucleate boiling, and copper substrate (≤ 1.6 mm), for efficient spreading of the thermal power dissipated by underlying chips is investigated. The spreaders are of the same thickness (2.0 mm) and have square surface areas that are sized depending on the composition and cooling condition of the spreaders. The 10 × 10 mm and 15 × 15 mm chips considered in the analysis are assumed to uniformly dissipate the thermal power. With composite spreaders (ΔPG = 0.2) cooled by 30 K subcooled nucleate boiling the removed total dissipation thermal powers of 160.3 W and 98.4 W from the 10 × 10 mm and 15 × 15 mm chips, respectively, are attainable at total thermal resistances of 0.29 and 0.48 °C/W, respectively. When these spreaders are cooled by saturation nucleate boiling, however, the removed dissipation thermal powers (85.6 W and 53.4 W, respectively) and the total thermal resistances (0.12 and 0.20 °C/W, respectively) are much lower. For same cooling conditions, the removed dissipation thermal powers by the porous graphite spreaders (ΔPG = 1.0) are much lower and the thermal resistances are much higher than those with composite spreaders, because of the relatively low and anisotropic thermal conductivity of porous graphite. Results also showed that the surface temperatures of the chips are not uniform. The maximum chip temperatures at the highest removed dissipation powers by composite spreaders are < 71 °C and the temperature differentials across the chips are < 8 °C. Results demonstrated that composite spreaders are an attractive option for cooling high power computer chips at relatively low chip temperature.Copyright

Saturation and Subcooled Boiling of HFE-7100 on Pinned Surfaces at Different Orientations

Experiments are conducted to investigate nucleate boiling of HFE-7100 liquid on 10 x 10 mm copper surfaces with corner pins that are 3 × 3 mm in cross section and 2-, 3-, and 5-mm tall. Heat transfer coefficient and critical heat flux are compared for 0 (saturation), 10,20, and 30 K subcooling at orientations from θ = 0 to 180 deg. Thermal power removed in nucleate boiling at all orientations increases, partially due to the increase in surface area for boiling and the increased mixing by departing bubbles from the plane portion of the surface and sides of the pins. Increasing subcooling and pin height increases the thermal power removed and the temperature of the base surface. With 30 K subcooling, thermal power removed at critical heat flux from the copper surface with 5 mm pins at θ = 0 deg is in excess of 93 W, decreasing to 86 W at θ = 180 deg. The corresponding base temperature is ∼91 and 95° C, compared with the saturation temperature in the experiments of ∼54°C. Critical heat flux increases linearly with increased liquid subcooling, and the developed correlation, which accounts for the effects of liquid subcooling and surface orientation and area in contact with the boiling liquid, is within +10% of data.

Effect of Inclination on Pool Boiling of FC-72 Dielectric Liquid on Porous Graphite

Saturation pool boiling experiments of FC-72 liquid on a flat, porous graphite and smooth copper surfaces measuring 10 × 10 mm investigated the effect of surface orientation on nucleate boiling and Critical Heat Flux (CHF). The inclination angle of the surface increased from 0° (upward-facing) to 60°, 90°, 120°, 150°, and 180° (downward facing). Results demonstrated significant increases in the nucleate boiling heat transfer coefficient and CHF on porous graphite, compared to those on copper. At low surface superheats, increasing the inclination angle increases the nucleate boiling heat transfer coefficient, which decreases with increased inclination angle at high surface superheats. These results and the measured decreases of CHF with increased inclination angle are consistent with those reported earlier by other investigators for dielectric and non-dielectric liquids. On smooth surfaces and micro-porous coatings, the reported fractional decreases in CHF with increased inclination angle are almost identical, but markedly larger than those measured in this work on porous graphite. On these surfaces the reported CHF in the downward-facing position (180° inclination) is ∼10–20% of that in the upward-facing position (0° inclination), compared to ∼53.3% on porous graphite. The CHF values of FC-72 liquid on porous graphite, which also decreased with increased inclination angle, are correlated using the general form suggested by Kutatelatze (1961) to within ± 5% of the experimental data.Copyright

Pool Boiling in Saturated and Subcooled FC-72 Dielectric Fluid From a Porous Graphite Surface

Experiments investigated the effects of subcooling on pool boiling of HFE-7100 dielectric liquid from porous graphite measuring 10/spl times/10 mm and 3.0 mm thick. Results indicate no temperature excursion at incipient boiling and that subcooled nucleate boiling occurs at surface temperatures well below saturation. Conversely, for a smooth copper surface the temperature overshoot at incipient boiling is as much as 37.4 K in saturation and 7.5-16 K in subcooled nucleate boiling. Nucleate boiling ensues on the porous graphite at 0.8 K in saturation, and -2.0 to -11.7 K in subcooled boiling, compared to 5.0 to 10.9 K on the smooth copper surface. The nucleate boiling heat fluxes are more than 500% higher and the surface superheats are significantly lower than those for the copper surface in same experimental setup. The critical heat flux (CHF) for the porous graphite surface is 31.8, 45.1, 55.9, and 66.4 W/cm/sup 2/ for saturation, 10 K, 20 K, and 30 K subcooled boiling, and the values for the smooth copper are 21.5, 28.1, 33.7, and 37.3 W/cm/sup 2/, respectively. In addition, the rate of increase in CHF for the porous graphite with increased subcooling /spl sim/50% higher than for the smooth copper surface.

Enhanced Saturation Boiling of HFE-7100 Dielectric Liquid on Extended Copper Surfaces

Experiments are performed, which investigated the enhancement in saturation boiling of HFE-7100 dielectric liquid on copper surfaces having a footprint of 10 × 10 mm and four 3 × 3 mm corner pins that are 2, 3 and 5 mm tall. These pins increase the geometrical surface area, by 96%, 144%, and 240%, respectively, and the surfaces are prepared using #400 and #1500 emery papers to investigate the effect of roughness on boiling heat transfer. Still photographs and video footage are recorded of the boiling processes. Nucleate boiling starts at a few isolated sites on the inside of the pins, close to the common line with the base surface, markedly reducing or eliminating the temperature excursion prior to boiling incipience. Measurable enhancements are obtained in both natural convection and nucleate boiling heat transfer. On the plane Cu surfaces prepared with emery paper #1500, the maximum nucleate boiling heat transfer coefficient, based on the foot print area, hB * , is 1.16 W/cm2 .K and increases to 1.80, 2.03 and 2.37 W/cm2 .K on the surfaces with 2, 3, and 5 mm tall pins. Similarly, the Critical Heat Flux (CHF), based on the foot print area, increases linearly with increased pin height, at a rate of ∼ 32% per mm. Increased surface roughness increases both hB * and CHF by additional 15% and 10% and markedly enhances nucleate boiling heat transfer at high surface superheats (ΔTsat > ∼10 K), but causes only little enhancement at lower superheats.© 2007 ASME