Falling films are employed in a variety of commercial and industrial systems, specifically in advanced ammonia-water heat pumps. This paper describes an analytical and numerical tool developed to enable prediction of heat and mass transfer during the absorption process of ammonia-water vapor into a liquid layer falling inside a vertical tube. This numerical model can help to quantify the absorption process along the cooled vertical tube and provide insight for design considerations of such equipment. The physical model is based on conservation of mass, momentum, energy, and species. The conservation equations were written along with the boundary conditions expressing thermodynamic equilibrium and continuity of temperature, heat, and mass fluxes at the vapor-liquid interface. The numerical method for solution is based on the so-called finite volume method. The calculation domain of the heat and mass transfer problem that needs to be solved is nonuniform, which required a coordinate transformation that is based on stream function normalization, making the calculation domain uniform. The results of the model describe the behavior of the absorption process and its dependence on the various problem parameters. Temperature and concentration profiles have been calculated and the heat and mass fluxes have been evaluated, which yield heat and mass transfer coefficients. Special consideration is given to the interdiffusion to find its effect on the absorption rate as opposed to previous models that have not taken it into account.

Units: Dual