Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment

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. 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 capable of causing the liquid choking that leads to the critical heat flux. The Zuber hydrodynamic theory is modified to account for the effect of the coating modulation-wavelength on the development of the stable vapor layer above the coated surface, which effectively chokes the liquid down-flow towards the surface (above the coating). The second liquid-choking limit occurs within the porous-layer coating when the viscous drag surpasses the available capillary pumping. The lower of these liquid-choking limits, for a given set of geometrical and thermophysical parameters, is theorized to predict the observed critical heat flux. The predicted wetted-surface regime and the two limits are compared with the experimental results and good agreement is found. 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. (C) 2001 Elsevier Science Ltd. All rights reserved.