SUMMARY

Absorption-based heating, ventilation, and air conditioning (HVAC) systems have received increased interest in recent years due to their ability to use low-grade waste heat streams and the low global warming potential of their working fluids. Advances in manufacturing have led to the development of heat exchangers with microscale features that demonstrate enhanced heat and mass transfer. While these developments have resulted in more efficient and compact absorption systems, the system performance depends significantly on the absorber, which absorbs the refrigerant ammonia vapor into the absorbent fluid. The absorber can often be the largest component in the system and dictates system size. Thus, enhancement of absorption will directly translate to more efficient and compact systems.
Surface active agents or surfactants have the potential to substantially enhance heat and mass transfer in ammonia-water absorption by reducing the surface tension of the working fluid. The enhancement is caused by improved interactions at the vapor-liquid interface that result from surface tension gradients. Absorbers are designed to operate in either the bubble mode or the falling-film mode. In this study, the effect of surfactants on absorber performance is evaluated for both configurations. A screening analysis is performed to identify the ideal surfactant based on the value of surface tension and critical concentration. A flow visualization study is performed to understand the impact of surfactants on flow parameters such as bubble radii, interfacial area, and void fractions in bubble absorbers. Detailed heat and mass transfer experiments are performed with a bubble absorber to quantify the enhancement caused by surfactants. Finally, the performance of a falling-film absorber in the presence of surfactants will be evaluated in a two-pressure experimental facility. Insights from these experiments and analyses will guide the development of enhanced absorbers and compact sorption heat pumps.