Local entropy generation analysis of water vapour absorption in a LiBr-H₂O solution film, over a horizontal cooled tube

This study is based on a numerical analysis of water vapour absorption in a laminar, gravity driven, viscous, incompressible liquid film of LiBr-H₂O solution, flowing over a horizontal cooled tube. The hydrodynamic description is based on Nusselt boundary layer assumptions. A local entropy generation calculation can be performed referring to velocity, temperature and concentration fields. From a general form of volumetric entropy generation, a suitable expression for the absorption process has been obtained and different irreversibility sources have been highlighted. The impact of each term (fluid friction, heat transfer, mass transfer and their coupling effects) has been locally examined. Results have been explored for different tube radii, wall temperatures and operative conditions (representing both chiller and heat transformer configurations), in order to characterise the process from a second law point of view and establish a criterion for the optimisation of the absorber.