The Woodruff School

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, system performance depends significantly on the absorber, which absorbs the refrigerant ammonia vapor into the absorbent. The absorber can often be the largest component in the system and dictates system size. Thus, enhancement of absorption will directly lead 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. In this study, the effect of surfactants on the performance of bubble and falling-film absorbers is evaluated. A screening analysis is performed, and 500 PPM of 1-octanol is identified as the optimal additive. A flow visualization study is performed, and up to 37% enhancement of interfacial area is observed due to a reduction in bubble coalescence. Detailed heat and mass transfer experiments are performed with a bubble absorber to quantify the enhancement caused by surfactants. Enhancements in mass transfer coefficient and interfacial area result in a reduction in absorber size by 25%. A two-pressure experimental facility is developed to investigate the effect of surfactants on a falling-film absorber and the performance of the overall system. A comparative study between the two absorption modes is presented and it is found that the bubble absorber is favorable due to its performance, low cost, and simpler design. Insights from these experiments and analyses will guide the development of enhanced absorbers and compact sorption heat pumps.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Girish Anant Kini

TIME:	Wednesday, December 1, 2021, 2:00 p.m.

PLACE:	https://bluejeans.com/373827729/0273, Remote

TITLE:	Ammonia-water absorption in the presence of surface active agents

COMMITTEE:	Dr. Srinivas Garimella, Chair (ME) Dr. S. Mostafa Ghiaasiaan (ME) Dr. G. P. "Bud" Peterson (ME) Dr. Ryan Lively (CHBE) Dr. Fani Boukouvala (CHBE)

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, system performance depends significantly on the absorber, which absorbs the refrigerant ammonia vapor into the absorbent. The absorber can often be the largest component in the system and dictates system size. Thus, enhancement of absorption will directly lead 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. In this study, the effect of surfactants on the performance of bubble and falling-film absorbers is evaluated. A screening analysis is performed, and 500 PPM of 1-octanol is identified as the optimal additive. A flow visualization study is performed, and up to 37% enhancement of interfacial area is observed due to a reduction in bubble coalescence. Detailed heat and mass transfer experiments are performed with a bubble absorber to quantify the enhancement caused by surfactants. Enhancements in mass transfer coefficient and interfacial area result in a reduction in absorber size by 25%. A two-pressure experimental facility is developed to investigate the effect of surfactants on a falling-film absorber and the performance of the overall system. A comparative study between the two absorption modes is presented and it is found that the bubble absorber is favorable due to its performance, low cost, and simpler design. Insights from these experiments and analyses will guide the development of enhanced absorbers and compact sorption heat pumps.