Two-phase detailed numerical analysis of falling film absorption phenomena

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Abbasi Havestini, Robabeh
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Absorption refrigeration systems have the potential to provide cooling at relatively low cost by using waste heat in industrial sectors or geothermal or solar energy. The performance of an absorption system is highly affected by the design and operating conditions of the absorber. Falling film type of absorbers are commonly used to enhance the heat and mass transfer rate during the absorption process. Absorption process is complex due to the concurrent mass and heat transfer occurring in absorbent-absorbate two-phase flow and the equilibrium condition at the vapour-liquid interface. A deeper understanding of the coupled heat, mass, and momentum transfer in the absorption process using a numerical model would permit refinements needed to optimize refrigeration system performance. Previous theoretical studies were mainly focused on a single-phase numerical analysis of the absorption phenomena and neglected the effect of the gas flow at the interface. In this research, however, detailed two-phase two-dimensional numerical models are developed to study the absorption of pure water vapour into an aqueous LiBr solution flowing inside a vertical channel or over a horizontal tube. The falling film in a vertical channel is studied using a parabolic and an elliptic numerical model, then the elliptic model is applied to a horizontal tube. The numerical model results are carefully compared to available numerical or experimental data. A wide range of parameters are studied to investigate the development of co-current flows of liquid and gas in absorption phenomena. The velocity, pressure, and temperature distributions in both phases and the mass fraction distribution in the liquid phase are presented. The parabolic model predictions for the interfacial parameters are in a good agreement with the results of the elliptic model in the case of the vertical channel up to the axial point where insufficient vapour mass in the gas phase ceases the advancement of the parabolic model. The elliptic model is computationally more expensive, but has the capability of predicting recirculating flows. The elliptic model predicted the details of flow over a horizontal tube that were in good agreement with previous work.
Absorption, Falling film, Numerical model, Two-phase, LiBr solution, Elliptic equations, Sharp interface, Vertical channel, Horizontal tube