This paper describes a series of studies to understand the mechanism of boiling in structured surfaces having sub-surface tunnels, and surface pores. Innovative visualization that allowed observation within the tunnels conclusively shows that in saturated boiling, the tunnel is vapor filled, except for thin liquid films on the tunnel walls and menisci in the corners. Evaporation on liquid menisci in the tunnel corners is the principal boiling mechanism for hte structured surfaces. Experiments were performed to defien the effect of pore diameter, pore pitch, and tunnel size on performance in structured boiling surfaces. The Dry-Our Heat Flux increases with the increase of total open area. At a certain reduced heat flux, part of the tunnel will become flooded and the performance will be reduced. Smaller pore size will inhibit flooding at reduced heat flux. Visualization experiments were performed to determine the bubble departure diameter, bubble frequency, waiting and growth periods, and nucleation site density. These data provided the basid for models to predict these parameters as a function of surface geometry and fluid properties. a mechanistically based model was developed to predict the boiling heat flux as a function of (T_w -T_s), pore and tunnel dimensions, and fluid propertiel. The model predicted the heat transfer data for R-11, R-123, R-134a, and R-22 within +-33
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