Fin efficiency of extended surfaces in two-phase flow

Abstract

Abstract Compact plane-fin heat exchangers and other extended surface heat exchanges are used in several two-phase flow applications, such as condensation, boiling, or moist-air cooling. Because of simplicity and lack of better information, designers often use fin efficiency formulas for two-phase flows that are derived based on uniform heat transfer coefficient. However, the heat transfer coefficient h on the fin surface under condensation or boiling may vary significantly compared to the single-phase forced convection attributable to the possible existence of different two-phase flow regimes on the fin surface, including the condition of partial dryout. Thus, the use of constant h fin efficiency formulas (of single-phase flow) for two-phase flow situation may result in serious errors. A critical assessment is made in this paper of the various methods available for calculating fin efficiency in practical fin configurations with condensation, moist air cooling, and boiling on the extended surfaces. Conditions under which the use of constant h can be made are identified. Based on the literature review, some specific design recommendations are made for the determination of the fin efficiency for two-phase flow heat exchanger applications.