Modelling and experiments of falling film break-up characteristics considering mass transfer for liquid desiccant dehumidification

The falling film flow pattern and heat/mass transfer severely interact. Considering the unbalanced forces caused by liquid-gas mass transfer and Marangoni effect, this paper developed a theoretical model to predict the break-up characteristics of gas-liquid falling film convective absorption such as dehumidification. Analytical solutions were provided by calculating the uneven distribution of falling film parameters such as film thickness, interface temperature and concentration. Full consideration was also given to the influence of liquid film fluctuations and liquid-gas mass transfer coefficients that vary with operation and design conditions. Experiments of liquid desiccant dehumidification with LiCl saline aqueous were conducted for model validation. Though the model cannot reflect the volatility of film break-up positions, the trend was close and the error was less than 20% between the calculated and experimental results. Results showed that under absorbtion, except for the top of liquid film, the force caused by the surface tension difference due to liquid-gas mass transfer dominated, and its increase was most obvious as the flow distance increased. Liquid/solid friction was also significant, while gas/liquid shear force had little effect. Thus, since the upward force on film edge increased with the flow distance and mass transfer, where it was equal to gravity, the possibility of film rupture was very high (0.6-0.7 m in the conditions of this paper). All forces acting on the falling film increased, while the largest increment with liquid Re was liquid/solid friction and that with air Re was liquid/gas friction. Besides, reducing the liquid contact angle can efficiently relieve film rupture and enhance mass transfer, mainly by reducing the liquid/solid friction. This study provided a theoretical explanation for the coupling of falling film rupture and gas liquid mass transfer, which helps to improve the performance of dehumidification and other wetted-wall absorptions. (c) 2021 Elsevier Ltd. All rights reserved.