Analysis of a falling film H2O/LiBr absorber at local scale based on entropy generation

Abstract

A solution of laminar H2O/LiBr falling film flowing under the effect of gravity on a vertical plate heated by a heat transfer fluid was studied to determine the local temperature, concentration and velocity fields from a numerical resolution. In order to investigate a real absorber case, the heat transfer fluid was assumed to flow counter-current to the falling film. Different heat transfer fluid flow regimes were studied and compared with the limit cases: adiabatic and isothermal walls. A local entropy generation formulation was performed to identify the different sources of irreversibility with the aim of locating and quantifying them. The results enabled analysis of the distribution of entropy generation in the entire absorber, namely, the heat transfer fluid, the wall and the falling film. The analysis shows that an increase in the driving force generated at the film interface and of the heat transfer fluid velocity leads to an increase in the absorbed mass flow rate, but this increase is associated with higher irreversibility. Thermal entropy generation dominates near the wall for both fluids while mass entropy generation dominates at the film interface. Total entropy generation per width of absorber is mainly due to heat transfer irreversibilities through the film – more than 35% – while the irreversibilities due to mass transfer represent less than 15% under the assumptions considered.