The effect of structural modulation of porous-layer coating on the qCHF for horizontal surfaces is examined experientally. THe surface consists of a copper substrate coated with spherical copper particles which are diffusion sintered into modulated structures inside graphite molds. Results are reported for pentane boiling on four different surfaces; a plain uncoated surface, a surface with a porous-layer coating of uniform thickness, and two surfaces with different modulated porous-layer coatings. A two-fold increase in the critical heat flux (qCHF) is found for one of the modulated porous-layer coatings. A brief explanation of the hydrodynamic theory for the qCHF is then given in which it is postulated that the qCHF occurs when columnar, vapor-escape passages throught he liquid become unstable and collapse. It is then hypothesized that the enhancement of the modulated coating results from surface-induced changes in the characteristic interfacial wavelengths that stabilize the liquid-vapor interfaces, delaying the collapse of these vapor escape paths, and ultimately increasing qCHF.
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