ANALYTICAL SOLUTIONS FOR THE HEAT AND MASS-TRANSFER IN A FALLING FILM ABSORBER

In this paper, fully analytical solutions for the temperature and mass fraction are obtained for a falling film absorber in an absorption heat pump. The analytical solutions described here lead to an analytical expression for the local and total mass absorbed. This situation is due to the fact that the vast majority of mass transfer takes place in a thin layer near the liquid-vapor interface and that the temperature distribution is conduction-dominated. A design procedure based on the analytical solutions is developed and results for a libr-water solution are compared with recent experiments and previous numerical solutions for the temperature and the agreement with each is found to be very good. Heat and mass transfer coefficients are outputs of the problem and it is shown that the heat transfer coefficient increases with decreasing film thickness, whereas the mass transfer coefficient decreases. This situation makes it difficult to evaluate design procedures based solely on increasing heat transfer coefficients gleaned from experiments or the open literature. It is shown that for given inlet bulk temperature, solutions may be obtained only for a limited range of mass fraction.