Pool -boiling enhancement and liquid choking limits within and above a modulated porous -layer coating.

Modulated (periodically non-uniform thickness) porous-layer coatings, as an example of capillary artery-evaporator systems, are experimentally shown to enhance the pool-boiling critical heat flux nearly three times over that of a plain surface, while maintaining low surface superheats. This enhancement is examined experimentally and discussed theoretically. This work marks the first such study on the effect of modulation of a porous-layer coating on pool boiling. The fabrication of the modulated porous-layer coating consisting of sintered, monosized, spherical copper particles is described. Measurements of the heat flux versus surface superheat, during the wetted-surface regime and up to the critical heat flux, are presented for plain surfaces and surfaces with uniform and modulated porous-layer coating. The modulation separates the liquid and vapor phases, thus reducing the liquid-vapor counterflow resistance adjacent to the surface. Theories are suggested for two independent mechanisms that are capable of causing the liquid-choking that leads to the critical heat flux. The liquid-choking limit predicted to occur first, with increasing surface heat flux, is considered to correspond to the critical heat flux experienced by the surface. The Zuber hydrodynamic theory for the critical heat flux is modified to account for the effect of the coating modulation-wavelength on the development of a stable vapor layer above the coated surface, effectively choking the liquid down-flow towards the surface (above the coating). The resulting hydrodynamic model relates this second liquid-choking limit to the inverse of the square root of the modulation wavelength. A finite-volume model of the transport in the porous-layer coating is used to predict the heat flux versus surface superheat. The second liquid-choking limit is predicted by this model and occurs within the porous-layer coating when the viscous drag surpasses the available capillary pumping. The predicted wetted-surface regime and the two liquid-choking limits are compared with the measurements and good agreement is found. All of the tested surfaces are predicted to have hydrodynamically determined heat fluxes. The theories are then used to discuss the optimization of the enhancement and suggest that completely separated liquid and vapor flow paths can result in substantial further enhancement.