An investigation of droplets and films falling over horizontal tubes

Abstract

Liquid films falling over horizontal-tube banks are frequently utilized in absorption heat pumps, heat-driven systems that provide an environmentally and economically attractive alternative to conventional CFC-based vapor compression cycles in certain applications. The widespread application of these systems has faced crucial hurdles due to the lack of aflow-mechanism based, experimentally validated theory for the binary-fluid absorption heat and mass transfer in falling films. This present work aims to advance the understanding of the behavior of falling films in horizontaltube banks. A review of the literature was conducted in three areas: mathematical modeling of falling-film absorption, experimental investigations on the performance of horizontal-tube falling-film absorbers, and the dynamics of droplet formation and impact. It was shown that, although mathematical models of falling-film absorption have grown increasingly sophisticated, even the most recent models make significant simplifications regarding the assumed flow patterns and are not capable of reliably predicting absorber performance. It was concluded that a pressing need for the continued progress in this area is improved understanding of the details of the flow patterns, and in particular, the behavior and role of droplets. The experimental investigations into falling-film absorption on horizontal tubes revealed that, though many parameters such as tube diameter and spacing affect absorption rates, the fundamental mechanism by which they do is not well understood. Experimental investigations also pointed to the fact that understanding the film and droplet behavior is key to capturing the effects of the various parameters on performance. A review of droplet formation and impact studies revealed that there are many relevant techniques recently developed for modeling the details of droplet behavior. However, only axisymmetric cases, such as the formation of droplets from capillaries and jets, are generally considered in the literature. Thus the three-dimensional nature of the problem considered here represents an extension to this body of literature. Flow visualization experiments were conducted on two horizontal-tube banks with water and aqueous Lithium-Bromide flowing in droplet mode using ahigh-speed, high-resolution digital imaging system. Qualitative analysis of the resulting images identified many of the common features and also illustrated differences between the current case and the axisymmetric cases considered in the literature including droplet stretching along the underside of the tubes and saddle wave formation upon droplet impact. A digital image analysis routine was developed to generate mathematical representations of the shape and location of the liquid-vapor interface throughout a sequence of video frames. This allowed the estimation of droplet volume and surface area versus time. The results showed that both the volume and surface area between tubes steadily rise to a maximum value at the moment of impact, after which steep decline...