ABSTRAKT

This work identifies preferred integral-fin geometries for steam condensation on low conductivity, integral-fin tubes (admiralty, copper-nickel, and titanium). Although much work has been done to measure and predict condensation coefficients for refrigerants on high thermal conductivity copper tubes, very little has been done for the problem of present interest. Because of the low tube thermal conductivity, and condensate retention, it is necessary to solve a conjugate problem with tube side coolant flow. An adaptation of a model previously published by Adamek and Webb is used for the steam side, and the heat transfer to the coolant, accounting for circumferential wall heat conduction is included in the model. The model was validated by predicting 53 data points for steam, R-11 and R-113. Ninety four percent of the data were predicted within ± 15%. A parametric study was performed to determine the effect of fin height, fin spacing, and fin shape on the condensing coefficient for steam condensing at 35°C on the three tube materials. The results show that the enhancement level decreases as the tube thermal conductivity decreases. The predicted enhancement level for admiralty, copper-nickel, and titanium (or stainless steel) increases as the fin height is reduced from 1.0 mm to 0.5 mm. The preferred fin geometry for titanium, copper-nickel, and admiralty tubes is a 0.5 mm fin height, 0.2 mm tip thickness, and 0.9 mm base thickness. A maximum enhancement level is achieved at 512 fins/m (13 fins/m) for admiralty, copper-nickel, and titanium, for 0.5 mm fin height. The economic optimum fins/in is expected to be less than 512 fins/m. This work has resulted in the identification of preferred fin geometries for low thermal conductivity materials, which are different from those commercially available.