A Critical Review of Condensation Heat Transfer Predicting Models-Effects of Surface-tension Force

Abstract

Condensation is defined as the transformation of vapor to its liquid state. Gravity and vapor shear are the two forces that drive the flow of the condensate film inside plain tubes. Traditional condensation predicting models generally include the effects of these two forces. For finned tubes, the surface-tension force will also be important. This paper provides a critical review of existed predicting models to correlate condensation heat transfer coefficients. Gravity-force dominated models, vapor-shear dominated models and the effects of surface-tension force are all discussed. Three basic methods have been used to model the vapor-shear dominated annular liquid film. The first model assumed that the vapor core could be replaced with an equivalent liquid flow that would yield the same value for the vapor shear. The second model assumed that the major resistance to heat transfer was offered only by the laminar liquid sublayer. The third model assumed that the velocity in liquid film could be predicted using von Karman universal velocity profile. Yang and Webb (1997) study shows that at low-mass-velocity and high-vapor-quality conditions, the effect of surface tension is comparable to that of vapor shear for condensation heat transfer inside a micro-fin tube with 13 μm fin tip radius. Owing to the manufacturing technology development of integrated circuits, smaller tubes and fin geometries can be made and used for increasing the condensation heat transfer coefficient. Because the surface-tension drainage force is proportional directly to the inverse of fin-tip radius, the importance of the surface-tension effect will increase with the development of micro-fabrication technology. The Yang and Webb (1997) model is the first model that accounts for the effects of vapor and surface-tension forces simultaneously. However, their database did not cover wide range of fluid properties and fin geometries. More work is needed on correlation and theoretically based models.