A critical review of models of coupled heat and mass transfer in falling-film absorption

In absorption space-conditioning systems, the performance of the absorber is critical to the overall system performance, size, and first-cost. The objective of this paper is to provide a comprehensive review of the significant efforts that researchers have made to mathematically model the coupled heat and mass transfer phenomena that occur during falling-film absorption. A detailed review of the governing equations, boundary conditions, assumptions, solution methods, results, and validation of these investigations is presented. This review excludes experimental work in this area, the effect of additives, and the effect of non-absorbable gases. It is shown that most work found in the literature has focused on the particularly simplified case of absorption in laminar vertical films of water-lithium bromide. Fewer researchers have considered the important situations of wavy films, turbulent films, and films on horizontal tubes. Investigations of the ammonia-water fluid pair have been generally more empirical in nature and/or restricted to vertical laminar films. This review is used to highlight key areas which need attention such as film and vapor hydrodynamics, especially the non-periodicity, instability, and recirculatory motion of waves in the vertical wall case and droplets and waves in the horizontal tube case. Also the potential interaction of the heat and mass transfer process on the film hydrodynamics, surface wetting, heat transfer in the vapor phase, and common simplifications to the governing equations should all be considered carefully. Finally, emphasis must be placed on experimental validation of the local conditions and transfer processes within the absorber, not just overall transport values.