Ammonia-water absorption heat and mass transfer in microchannel absorbers with visual confirmation

An experimental investigation of an ammonia-water absorber that utilizes microchannel tube arrays was conducted. Liquid ammonia-water solution flows in the falling-film mode around an array of small diameter coolant tubes, while vapor flows upward through the tube array countercurrent to the falling film. Previous investigations of an absorber designed for use in a 10.55 kW cooling load (19.28 kW absorber load) residential heat pump demonstrated the potential of this technology for achieving high heat and mass transfer rates with no surface treatment or enhancement and with relatively low solution and coolant pressure drops. However, these previous integrated analytical and experimental studies indicated that solution distribution problems might be resulting in incomplete utilization of the provided surface area. In the present study, an absorber with optical access was constructed so that various improvements to the original absorber, including particularly the solution distribution mechanism, could be evaluated. Experiments covering a wide range of solution and coolant flow rates and vapor fractions were used to determine the overall and solution-side heat and mass transfer coefficients. It was found that although the surface area of this improved absorber was only 0.456 m ², approximately 30% of the surface area of the original prototype absorber, it was able to transfer duties as high as 15.1 kW, almost equaling the load of the original larger absorber. This significant increase in performance is attributed to the substantially improved flow distribution. Visual documentation of the flow in this absorber also confirmed the significantly improved flow distribution and higher participation of the surface in the heat and mass transfer process.