The hoodoo is introduced as a beneficial surface structure for enhancing boiling heat transfer. A full parametric study was conducted to determine which attributes of the hoodoo structure promote boiling heat transfer enhancement. Hoodoo size and spacing were observed to have the most profound effect on boiling heat transfer, nucleation site activation, and critical heat flux (CHF). The CHF enhancement factor, defined as the ratio of CHF on the structured surface to that of a smooth surface, varies from 1.05 to 1.67 for FC-72 and hexane working fluids. Droplet spreading studies confirm the hemiwicking properties of the hoodoo surface, and it is postulated to be the primary mechanism for CHF enhancement. Measured wicking front speeds varied from 12 to 40 mm/s and were observed to obey a power-law dependence on time with an exponent of approximately 0.5. Plausible thermohydraulic mechanisms for CHF enhancement on the hoodoo surfaces are discussed.
Enhanced thermal conductivity oxide fuels offer increases in both safety and efficiency of commercial light water reactors. Low-temperature oxidative sintering and Spark Plasma Sintering (SPS) techniques have been used to produce UO2-SiC composite pellets. Oxidative sintering performed for 4 hours at 1200∼1600oC and SPS was employed only for 5 mins at the same temperature. While oxidative sintering failed to achieve enhanced thermal conductivity, the SPS sintered pellet obtained promising features such as higher density, better interfacial contact, and reduced chemical reaction. Thermal conductivity measurement at 100oC, 500oC, and 900oC revealed maximum 62% higher thermal conductivity value, when compared to UO2 pellets, in SPS sintered UO2-10vol% SiC composite pellet. The result shows that the SPS technique is required to sinter UO2-SiC nuclear fuel pellets with a high value of thermal conductivity.
The pool boiling heat transfer characteristics of smooth single crystal and densely packed cylindrical cavity surfaces were investigated using two highly wetting fluids, petfuoro-nhexane (FC-72} and n-hexane. Three single crystal copper surfaces and five undoped single crystal silicon surfaces with different plane orientations were considered. In addition, silicon surfaces with densely packed cylindrical cavities with diameters ranging from 9 to 75 ßm, depth ranging from 9 to 20 ßm, and spacing ranging from 75 to 600 ßm were tested for comparison. It is observed that the copper single crystal surfaces show increasing heat transfer coefficient with decreasing atomic planar density. The single crystal silicon surfaces show increasing heat transfer coefficient with increasing atomic planar density. Plausible molecular scale mechanisms are discussed. In contrast, the silicon surfaces seeded with cylindrical cavities having diameters of 27ßm or less generally yield higher heat transfer coefficients than the single crystal silicon surfaces. A decrease in the cavity spacing results in a larger number of cavities on the surface, and the heat transfer coefficient increases as a result. Cavity depths of 6 and 20 ßm result in the same heat transfer coefficient irrespective of cavity diameter. The nucleation site density for the cylindrical cavity surfaces is measured and reported at low superheat using a novel imaging technique.
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