Numerical Analysis of Heat and Mass Transfer in a Fin-and-Tube Air Heat Exchanger under Full and Partial Dehumidification Conditions

Abstract

Heat exchangers are commonly used in industrial fields such as air conditioning, petrochemical and agriculture-food industries. The design and utilization of a heat exchanger should fulfill some conditions of performance, economy and space requirement. The most widely operated heat exchangers make use of fin-and-tube configuration in association with the application of heating, ventilating, air-conditioning and refrigeration systems (Khalfi & Benelmir, 2001). With regard to the fin temperature and dew point temperature of surrounding air, three situations on a fin surface can be distinguished (Lin & Jang, 2002, Benelmir et al., 2009). The fin surface is fully dry if the temperature of the whole fin is higher than the air dew point temperature. It is partially wet when the air dew point temperature is lower than the fin top temperature and is higher than the fin base temperature. Finally, the fully wet surface occurs if the temperature of the whole fin is lower than the dew point temperature. A reliable determination of the fin efficiency must account for the simultaneous heat and mass transfer on the cooling surface. Many experimental, and few numerical, studies have been carried out to study the heat and mass transfer characteristics of the fin-and-tube heat exchangers under dehumidifying conditions. It was stated by Liang et al. (2000) that the condensation of the moist air along the fin surface causes reduction of the fin efficiency. They found also that measured fin efficiency was less than the calculated one assuming a uniform heat transfer coefficient. The calculated results of Saboya & Sparrow (1974), Chen et al. (2005), Chen & Hsu (2007), and Chen & Chou (2007) concluded that the heat transfer coefficient was non-uniform under dry conditions. Due to the difficulty of considering a variable sensible heat transfer coefficient (Choukairi et al., 2006), this later was often assumed to be uniform by many investigators in the calculation of fin efficiency. Liang et al. (2000) used one-dimensional and two-dimensional models to determine the humid fin efficiency of a plate-fin-tube heat exchanger: The results obtained show comparable efficiencies with both 1-D and 2-D models. Chen (1991) analyses the fin performance under dehumidifying conditions and shows, through a 2-D model, that the humid fin efficiency was sensitive to the moist air relative humidity value. As mentioned earlier, the sensible heat transfer coefficient is often assumed uniform. Thus, the wet fin efficiency is usually determined under this assumption and by introducing a functional relation between the relative humidity and the fin temperature [Lin & Jang , 2002, Liang et