Jinsub Kim

Effect of Subcooling on Pool Boiling of Water from Sintered Copper Microporous Coating at Different Orientations

The subcooling effect on pool boiling heat transfer using a copper microporous coating was experimentally studied in water for subcoolings of 10 K, 20 K, and 30 K at atmospheric pressure and compared to that of a plain copper surface. A high-temperature thermally conductive microporous coating (HTCMC) was made by sintering copper powder with an average particle size of 67 μm onto a 1 cm × 1 cm plain copper surface with a coating thickness of ~300 μm. The HTCMC surface showed a two times higher critical heat flux (CHF), ~2,000 kW/m2, and up to seven times higher nucleate boiling heat transfer (NBHT) coefficient, ~350 kW/m2K, when compared with a plain copper surface at saturation. The results of the subcooling effect on pool boiling showed that the NBHT of both the HTCMC and the plain copper surface did not change much with subcooling. On the other hand, the CHF increased linearly with the degree of subcooling for both the HTCMC and the plain copper surface. The increase in the CHF was measured to be ~60 kW/m2 for every degree of subcooling for both the HTCMC and the plain surface, so that the difference of the CHF between the HTCMC and the plain copper surface was maintained at ~1,000 kW/m2 throughout the tested subcooling range. The CHFs for the HTCMC and the plain copper surface at 30 K subcooling were 3,820 kW/m2 and 2,820 kW/m2, respectively. The experimental results were compared with existing CHF correlations and appeared to match well with Zuber’s formula for the plain surface. The combined effect of subcooling and orientation of the HTCMC on pool boiling heat transfer was studied as well.

Evaporative Cooling Heat Transfer of Water From Hierarchically Porous Aluminum Coating

ABSTRACT A study of evaporative cooling of water was conducted using dual-scale hierarchically porous aluminum coating. The coating was created by brazing aluminum powders to a flat aluminum plate. The effects of particle size and thickness on evaporative heat transfer were investigated using average aluminum particle diameters of 27, 70, and 114 µm and average coating thicknesses of 560, 720, and 1200 µm. Constant ambient temperature of 24°C and relative humidity of 50% were provided throughout the study. Evaporative cooling tests on the coated surfaces were compared to the plain surface. Tested dual-scale porous coatings enhanced evaporative heat transfer significantly, compared to that of the plain surface, due to the effective wicking of water to the entire heated area. With particle size increase, both the wickability and dryout heat flux were significantly increased. The dryout heat flux with the particle size of 114 µm was 3.2 times higher than that with the particle size of 27 µm. At the fixed particle size of 70 µm the dryout heat flux increased as thickness increased, which resulted in the maximum dryout heat flux of 10.6 kW/m2 and the maximum heat transfer coefficient of 251 W/m2K at the coating thickness of 1200 µm.

Drop Impact Variation at the Extremes of Wettability

Depicted are sequences of water drop impacts on copper, taken at 16,000 fps. The copper is treated with a heated alkali solution, resulting in a highly wetting, nanoscale structured, cupric oxide layer with a static contact angle approaching 0° with water. In the top series an 11.5 µl water droplet impacts this surface from 60 mm. The interfacial forces are large compared with the inertia; the low advancing contact angle of the expanding front continues to pull the droplet outward and absorbs the droplet without any rebound. The droplet spreads to cover the entire 0.5x0.5 in2 surface in less than 500 ms. After the surface energy of the oxide layer is reduced with silane, this surface becomes highly non-wetting with a static contact angle of ~160° and a hysteresis <5°. The lower sequence shows the 11.5 µl water droplet dropped from the same height. The large advancing contact angle creates an inverted wedge at the triple line, and the advancing front quickly reaches a maximum diameter at 3 ms and begins to recede inward while the top of the droplet is still moving downward, creating a donut shape. The receding front collides at the center forcing a jet of liquid up and out. This jet pulls the remainder of the liquid upward at a decreasing velocity, relative to the head. This is apparent as the jet splits into secondary droplets at 16ms (which moves out of frame at 18 ms) and again at 22 ms, referred to as S-1 and S-2, respectively. As the S-2 splits off, surface tension force cause it to slow at 25 ms, while the parent droplet moves up to collide with, and impart momentum to S-2. They remain detached; S-2 moves out of view, the parent falls. This bouncing behavior continues until the energy is dissipated and the droplets come to rest. This can be seen as the parent drop rebounds again at 100ms, S-2 at 130 ms and S-1 in the final frame, forming a tertiary droplet. These surfaces are being studied for their effects on two phase heat transfer.