An investigation of droplets and films falling over horizontal tubes
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 a flow-mechanism based, experimentally validated theory for the binary-fluid absorption heat and mass transfer in falling films. From a review of the literature 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 in horizontal-tube banks. Only axisymmetric cases, such as the formation of droplets from capillaries and jets, are generally considered in the literature. Flow visualization experiments were conducted on two horizontal-tube banks with water and aqueous Lithium-Bromide flowing in droplet mode using a high-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. 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 declines in both occur as the primary droplet joins the film around the tube. After the initial steep decline, a period of slower decay of the thinning liquid bridges and satellite droplets was shown to be a period when the surface area to volume ratio increases significantly. Computational models utilizing the Volume-of-Fluid technique were developed and validated for 2D axisymmetric cases and the 3D case of horizontal tubes. The 2D models captured some of the characteristics observed in the flow visualization, but overall qualitative and quantitative agreement was only fair. The 3D model of droplet formation under a horizontal tube showed better qualitative and quantitative agreement with the experimental results. Many of the characteristic features, including the elongation of the drop during early formation and the saddle wave generated upon impact were well predicted by the model. Some differences could be observed in the predicted break-up of the liquid bridge. Suggestions for areas of improvement were given.