Absorption of ammonia-water mixtures in microscale geometries for miniaturized absorption systems

Abstract

Vapor absorption-based HVAC systems are attracting increased interest due to their capability to utilize low-grade waste-heat streams, and low global warming potential of the working fluids. The performance of an absorption system depends significantly on the absorber, which absorbs the refrigerant vapor into the absorbent fluid. Components with microscale features to enhance heat and mass transfer have been shown to significantly reduce the size of absorption cooling systems, making them viable for small-scale applications, such as residential and mobile use. But an incomplete understanding of the internal flow phenomena in microscale absorbers is limiting those gains. Performance limiting factors for microscale absorbers are investigated. One of the key performance limiting factors is maldistribution of vapor and liquid phases in these microscale geometries. Air-water mixtures are used to represent two-phase flow through three different microscale geometries, namely, a microchannel array, a microchannel array with mixing sections, and a serpentine pin-fin test section. The flow distribution is visually tracked along the length of the microscale geometries. Statistical distributions of void fraction and interfacial area along the microchannel array are calculated. Parameters such as the average void fraction and interfacial area intensity are used to evaluate and compare the different microscale geometries. This study also investigates the internal flow phenomena in an absorber by visualizing the process of absorption and measuring local temperatures in microscale geometries. A single unit of a microscale absorber consisting of two heat exchange plates; one with an ammonia–water mixture, and the other with a coupling fluid to absorb the heat released during absorption, is fabricated. Heat and mass transfer mechanisms in the microscale components are investigated. An open absorption system was fabricated to evaluate the absorber, as it enables control over inlet properties of the fluids. Two microscale absorber designs are evaluated in the test facility. The effects of solution flow rate, solution nominal concentration, operating pressures, and coupling fluid temperature are evaluated. A model is developed to predict heat and mass transfer in these microscale absorbers. This study provides insight into the limiting factors of current designs, and improvements that can be made in the future designs of such components.