CFD simulation of adiabatic water vapor absorption in large drops of water-LiBr solution

Vapor absorption inside free-falling drops of 4.4 mm in diameter in an adiabatic absorber is studied numerically. The full process of forming drops including sessile stage, turning to pendant shape, detachment and free-fall are investigated. In this simulation, the full Navier-Stokes equations are solved. The volume of fluid (VOF) method is used to capture the vapor/solution interface. An in-house CFD code is utilized for the simulation of the coupled heat and mass transfer through the drops. Adaptive mesh refinement (AMR) method with respect to the volume fraction gradient is used to reduce the cell numbers and assist the interface resolution as well as reducing run time. The equilibrium of vapor pressure is assumed for the calculation of LiBr concentration at liquid-vapor interface. Moreover, the effect of different absorber working pressures (subcooling effect) is investigated. A good agreement is observed within the quantitative comparison between present numerical results and experimental data. The simulation results reveal the transient drop shape changes and its effect on the velocity profile inside the falling drops. It is also observed that by increasing the working pressure from 1.32 to 2.80 kPa, the degree of subcooling increases by 13.8 K and the amount of absorbed water vapor is multiplied by 2.5.