A realistic approach to model LiBr-H₂O smooth falling film absorption on a vertical tube

A detailed model is developed for treating the 2-D water vapour absorption into an aqueous solution of LiBr, which is flowing over a vertical tube. Numerical predictions are presented for the spatial variation of solution temperature and concentration in the axial direction and within the LiBr-H ₂O falling film taking into account the flow and temperature rise of coolant inside the tube. Special care was given to the adequate description of the thermodynamic properties of superheated water vapour, which is treated as a real gas. Emphasis was also given in demonstrating the electrolytic nature of the aqueous solution, introducing thus the effect of variable heat of absorption on the temperature field. A parametric study for examining the effect of absorption pressure, solution mass flow rate and inlet coolant temperature on the absorption process was carried out implementing the model under laminar flow conditions. A good coincidence with experimental observations obtained for solution flows with Re<150 was witnessed, acknowledging thus the predictive capability of the numerical model.