Computational analysis of binary-fluid heat and mass transfer in falling films and droplets

Abstract

Vapor absorption systems offer advantages over vapor compression systems for air-conditioning systems in some applications. They use heat as their primary energy input and hence provide opportunities to use solar energy or waste heat to drive these systems. The absorber is the most crucial component of the vapor absorption system and has the largest impact on its performance. Absorber design requires a keen understanding of the underlying heat and mass transfer processes in the absorber. The horizontal tube geometry is by far the most popular absorber geometry, due to the high absorption efficiencies achievable without incurring commensurate pressure drops.

Several models have been proposed in the literature to model the heat and mass transfer during absorption over horizontal tube banks. However all of them make very simplistic assumptions about the flow profiles in the absorber. High speed flow visualization studies in the literature have shown that the flow occurs in the form of droplets and the formation and the detachment of these droplets and their impact on the tube has significant effects on the heat and mass transfer. Most absorption models in the literature neglect these flow modes and assume the solution to flow as a uniform film.

The present study attempts to numerically model the heat and mass transfer in the absorber taking the realistic drop-wise and wavy film flow patterns into consideration. The impact of the fall of these droplets on the tube causes the lithium bromide solution present on the film on the tube to mix and present newer regions of the solution for vapor absorption. The impact of the droplets also causes waves that propagate axially over the liquid film on the tube. The mixing effect and the waves caused due to droplet impact play a very important role in the heat and mass transfer. Results obtained from this study will aid in better understanding of the vapor absorption process, and in the design of more efficient absorbers.