Comprehensive Comparisons Between Evaporation and Pool Boiling on Thin Micro Porous Coated Surfaces

Abstract

The evaporation and pool boiling on micro porous coated surfaces have been shown to provide among the highest heat transfer rates achievable from any type of surfaces. The heat transfer modes in these surfaces, present a number of interesting similarities and also, some fundamental differences, which are the result of the liquid supply methods to the heated surface. For the evaporation from porous coated surfaces, the liquid return to the heated surface is assisted by the capillary pressure at the liquid-vapor interface; while for pool boiling, gravity is the principal driving force that rewets the surface. In order to better understand the physical phenomena that governs the flow behavior of both the liquid and vapor phases, and the heat transfer process inside the porous media, comprehensive comparisons between these return mechanisms and their respective characteristics, and the performance and the critical heat flux (CHF) for each have been made, based on similar physical situations. These systematic comparisons illustrate that at a lower heat flux, the evaporation and pool boiling curves are almost identical due to the similar heat transfer modes, i.e., convection and nucleate boiling. While with further increases in heat flux, the heat transfer performance of the evaporation on micro porous media is generally superior to pool boiling on an identical surface. This shift is believed to be due to the fact that for evaporation on micro porous media, the heat transfer mode is dominated by the film evaporation, while in pool boiling, it is principally the result of fully developed nucleate boiling. It was also observed that the impact of the effective thermal conductivity of the porous coating on pool boiling performance is larger than for evaporation heat transfer on the identical micro porous coated surfaces. In general, the experimental data indicated that the CHF for evaporation heat transfer is much higher than for pool boiling on the same surfaces. The mechanism of CHF for evaporation on porous coated surfaces is believed to be the capillary limit; while for pool boiling the limit is the result of the hydrodynamic instabilities. This difference in mechanisms is clearly demonstrated by the experimental observations, where initially, the dry out process of the porous coated surfaces during evaporation is gradual, while for pool boiling; the entire surface reaches dry out in a very short time. In addition, the sensitivity of the CHF to the thickness of the porous coatings at a constant volumetric porosity and pore size, as well as the various optimal volumetric porosity of the CHF at a given thickness, are clearly the results of the differences induced by the various CHF mechanisms.