FIN-EFFICIENCY OF EXTENDED SURFACES IN 2-PHASE FLOW

Compact plate-fin heat exchangers and other extended surface heat exch anges are used in several two-phase flow applications, such as condens ation, boiling, or moist-air cooling. Because of simplicity and lack o f better information, designers often use fin efficiency formulas for two-phase flows that are derived based on uniform heat transfer coeffi cient. However, the heat transfer coefficient h on the fin surface und er condensation or boiling may vary significantly compared to the sing le-phase forced convection attributable to the possible existence of d ifferent two-phase flow regimes on the fin surface, including the cond ition of partial dryout. Thus, the use of constant h fin efficiency fo rmulas (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 o n the extended surfaces. Conditions under which the use of constant h can be made are identified. Based on the literature review, some speci fic design recommendations are made for the determination of the fin e fficiency for two-phase flow heat exchanger applications. (C) 1997 by Elsevier Science Inc.