Design enhancement of an adiabatic LiBr-H2O absorber through a droplet flow optimization

Abstract

The increase of cooling demands is a widespread phenomenon that poses the problem of identifying new efficient ways to provide the necessary input energy. Waste heat is available in large quantities, and its recovery is a challenging problem. Additionally, small-scale applications are becoming more common in fields such as electronics and/or automotive. This work aims at providing a solution for the development of a small-scale waste heat-to-cool conversion system. To achieve this, a first comparison of the potential conversion systems was performed. Parameters of efficiency, cost and size were considered both separately and combined in a comprehensive performance index. From this analysis, the H2O-LiBr absorption chiller emerged as the most suitable technology for the development of a small-scale system. Further investigation of the selected technology identified the absorber as the most critical component preventing the system downsizing. A critical review of the current state of the art in the absorber design and operation strategies led to the proposition of a pin-finned, adiabatic absorber, combined with a droplet flow regime. In the next phase the proposed design was optimized. Firstly, an analytical model was developed to accurately describe the geometrical domain of a droplet forming on a solid surface, a phase which was found to be beneficial to the absorption process. The model was then validated with experiments and used to optimize the pin shape. Secondly, the absorption process was analyzed. An analytical heat and mass transfer model of the phases of a falling droplet was developed and an experimental setup, replicating the operation of the absorber, was developed to validate the proposed17 model. The validation process led to the identification of the best pin and distribution orifice sizes, completing the absorber design optimization. Finally, the performance improvement of the optimized absorber design was evaluated with respect to a standard absorption chiller configuration.