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 serves to mix the refrigerant vapor with 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.
In the present work, 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. Parameters such as the average void fraction and interfacial area intensity are used to evaluate and compare the different microscale geometries.
The proposed study also aims to investigate 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. The study aims to investigate the heat and mass transfer mechanisms in the microscale components. An open absorption system was constructed to evaluate the absorber, as it enables control over inlet properties of the fluids. Two microscale absorber designs will be evaluated in the test facility. The effects of solution flow rate, solution nominal concentration, operating pressures and coupling fluid temperature will be evaluated in the system. A detailed model will be developed to predict heat and mass transfer in these microscale absorbers. The study will provide insight into the limiting factors of current designs, and improvements that can be made in the next generation of components.