| Pool boiling heat transfer measurements were made on a variety of cylindrical test sections: plain, T-finned GEWA-T, and standard low fin GEWA-K. For the GEWA-T surfaces the gap width, S(,T), ranged from 0.15 mm to 0.55 mm. The working fluids were distilled water and R-113. The overall enhancement (defined as the ratio of wall superheat for the smooth tube to that of the enhanced tube, at constant heat flux) was higher in R-113 than in water. With R-113 the enhancement was a well-behaved function of S(,T). With R-113 the maximum enhancement of 2 was at S(,T) = 0.25 mm. With distilled water two different peaks for the heat transfer coefficient were obtained at S(,T) = 0.15 mm and S(,T) = 0.35 mm. The maximum enhancement of 1.6 was obtained at S(,T) = 0.35 mm.;It was observed that the GEWA-T surface enhancement in water was improved by filling the channels with a porous material. The boiling performance increased by 43% over an unfilled GEWA-T tube.;Visual observations of the actual test sections and models indicated a complex liquid-vapor exchange mechanism. The flow patterns were different from those suggested in previous studies. It was observed that the bubbles originated within the channel and expanded until their escape. The escape routes were not limited to the bottom or top portions of the test section. Bubble columns were observed at random spots along the periphery of the test section.;A critical analysis of an earlier "dynamic" model for Thermoexcel-E was performed. This analysis showed that many questionable assumptions were incorporated into the semi-analytical model.;Fin density did not prove to be a major factor in the improvement of thermal performance as indicated by the results of plain GEWA-K (740 and 1020 fins/m), and GEWA-T (740 and 1020 fins/m) in water.;Based on the visual studies and the analysis of the Thermoexcel-E model a new quantitative model was developed for the GEWA-T surface. The resulting semi-theoretical equation correlates the experimental data quite well. |
