CFD Analysis of Falling Film Hydrodynamics for a Lithium Bromide (LiBr) Solution over a Horizontal Tube

Abstract

Falling film evaporators are used in applications where high heat transfer coefficients are required for low liquid load and temperature difference. One such application is the lithium bromide (LiBr)-based absorber and generator. The concentration of the aqueous LiBr solution changes within the absorber and generator because of evaporation and vapor absorption. This causes the thermophysical properties to differ and affects the film distribution, heat, and mass transfer mechanisms. For thermal performance improvement of LiBr-based falling film evaporators, in-depth analysis at the micro level is required for film distribution and hydrodynamics. In this work, a 2D numerical model was constructed using the commercial CFD software Ansys Fluent v18.0. The influence of the liquid load corresponding to droplet and jet mode, and the concentration, on film hydrodynamics was examined. It was found that the jet mode was more stable at a higher concentration of 0.65 with ±0.5% variation compared to lower concentrations. The recirculation was stronger at a low concentration of 0.45 and existed until the angular position (θ) = 10°, whereas at 0.65 concentration it diminished after θ = 5°. The improved heat transfer is expected at lower concentrations due to lower film thickness and thermal resistance, more recirculation, and a higher velocity field.

Heat Transfer Study from a Salty Droplet Film on a Horizontal Tube in Mechanical Vapor Recompression Crystallization System

“…Moreover, there are significant impacts of physical properties on evaporative heat transfer in horizontally drop films. Tahir et al [19] found that with a lower thermal resistance and film thickness, higher velocity field, and increased recirculation, lithium bromide solutions enhanced heat transfer at low concentrations. Shen et al [5] found that h θ of seawater decreased slightly with the increasing temperature of evaporation, increased with increasing tube spacing, and decreased obviously with increasing salinity.…”

Falling Film Flow and Heat Transfer of Cryogenic Liquid Oxygen on Different Structural Surfaces

“…Mao et al [15,16] simulated the heat recycles from the oily wastewater film by falling on a horizontal tube, and the effects of oil content, tube diameter, spray density on heat transfer were analyzed. Tahir et al [17] set up a numerical model to calculate the film flow of LiBr-H 2 O solution on the horizontal tube, and the effects of the liquid load and the concentration on the film hydrodynamics were examined. In addition, the falling film absorber using horizontal tubes was theoretically modelled with an aqueous alkaline nitrate solution as working fluid [18].…”

“…The working fluids studied in the existing literature are mainly concentrated in pure water [5][6][7][8]10,11,13,19,23,25], seawater [3], oil sewage [15,16], LiBr solution [17], freon refrigerant [9,20,22], ethylene glycol aqueous solution [14], potassium formate aqueous solution [24], ionic fluid [21,22]. For the cryogenic liquid, Pavlenko et al [26] measured the film flow of cryogenic liquid nitrogen on the surfaces with complex geometry, and found the microstructure has a significant influence on the film spreading progress.…”