|
Nucleate pool boiling is enhanced by facilitating water transport to the boiling surface. Doing so in an industrial boiler could improve the energy efficiency of the Rankine cycle widely used in power plants. The fundamental problem of water transport is solved in myriad ways in biology, particularly in the vascular structure, or xylem, of plants. The structure varies, from straw like tracheids in ferns and conifers, to sponge like reticulate vessels in many angiosperms. The latter morphology is of particular interest, as fabrication processes exist for reticulate metal foams and sintered metal wicks that mimic the porous xylem of angiosperms. Couple this bioinspired bulk morphology with an anodization surface treatment, and a functional, composite material is obtained. The anodization step imparts surface micro-porosity on the already bulk-porous metal foam, further increasing surface area to heat transfer. Varying degrees of surface modification may be achieved by varying anodization voltage, treatment time, and electrolyte concentration. The combined effect is expected to increase the active nucleation site density by about one order of magnitude. In this study, a method is proposed that involves back tracking bubbles to the moment of their departure from the boiling surface. This approach enables quantification of bubbles in nucleate pool boiling even at moderate heat fluxes. An apparatus has been fabricated to measure boiling enhancement and facilitate visualization. It is expected that this study will contribute to greater theoretical and material understanding of boiling on porous anodized boiling surfaces.
|
