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

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Kini, Girish Anant
Garimella, Srinivas
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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 conducted to identify the ideal surfactant based on the value of surface tension and critical concentration. It is determined that 500 PPM of 1-octanol is the preferred additive. A flow visualization study is performed to understand the impact of surfactants on flow parameters such as bubble radii, interfacial area, and velocities in bubble absorbers. The addition of 1-octanol results in enhancement in interfacial area by up to 37% due to the prevention of bubble coalescence leading to many small diameter bubbles. Ammonia-water bubble absorption experiments are conducted to evaluate the effect of adding surfactants. Enhancements in mass transfer coefficient and interfacial area result in a reduction in absorber size by 25%. A detailed heat and mass model is developed to predict the performance of the bubble absorber in the presence of surfactants with an AAD of 8.2%. The performance of a falling-film absorber in the presence of surfactants is evaluated in a two-pressure experimental facility. A reduction of 37% in the solution-side resistance is achieved due to Marangoni convection and improved wetting. In addition to understanding the changes in the absorber, these experiments demonstrate the reduction in low-side pressure and the benefits to the overall system performance by the addition of surfactants. Finally, a comparative assessment between the two absorption modes is conducted. Prototype falling-film and bubble absorbers are designed for a representative absorber that is part of a 10.5 kW cooling capacity absorption chiller. The bubble absorber is found to be favorable overall due to its performance, low cost, and simpler design. The results and insights gained from this work will guide the development of enhanced absorbers and compact sorption heat pumps.
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