SUMMARY
Absorption heat pumps are receiving renewed attention as environmentally sound and energy-efficient alternatives to CFC-based, ozone-depleting space-conditioning systems amidst national and international interest in the global climate change problem. The absorber, where refrigerant vapor is absorbed in the absorbent, is the key component that determines the cycle viability and overall system efficiency. The ammonia-water fluid pair (unlike the LiBr/H2O) has a volatile absorbent (water), thus presenting both heat and mass transfer resistances across the respective temperature and concentration gradients in both the liquid and vapor phases. The coupled heat and mass transfer processes in both phases present challenges for analysis, modeling, experimental validation, and design. The research on NH3-H2O absorption system has resulted in seemingly conflicting conclusions with regard to the dominant heat and mass transfer resistances in the different phases. The proposed study aims to extend the understanding of coupled transport processes occurring in the absorber. A test facility is developed to conduct absorption experiments. A horizontal falling-film absorber is fabricated using 9.5 mm (3/8”) nominal OD stainless steel tubes. The absorber is installed in an outer stainless steel shell that is equipped with two sight-glasses enabling optical access for flow visualization and recording high-speed videos during the absorption process. Absorption tests are conducted over a wide range of concentrations of (5-40%), evaporator pressures (150–500 kPa), and solution flow rates. Theory based models with experimental validation will be developed to quantify the heat and mass transfer. Local temperature measurements across the tube array will be used to obtain the variation of transfer rates during absorption. The effects of surface wettability and flow mechanisms such as droplet formation, and fall and impact of falling film on absorption characteristics will be investigated.