Date of Award
Master of Science
An integrated experimental and analytical study of ammonia-water absorption in novel microchannel geometries is presented. A prototype absorber utilizing this microchannel geometry is designed to transfer heat duties representative of those found in the residential absorption market in an extremely compact 0.178 m x 0.178 m x 0.508 m tall component. The absorber capacity, overall heat transfer coefficient, and solution-side heat transfer coefficient are studied for concentrated solution flow rates ranging from 0.010 to 0.040 kg/s with vapor production rates of between 5 and 50%. Over the range of test conditions, the overall solution mass flow rate has the largest impact on the performance of the absorber. As the solution mass flow rate is increased, the heat duty transferred by this absorber increases from 4.86 to 16.23 kW. Similarly, the overall heat transfer coefficient increases from 133.0 to 403.0 W/m2-K, and the solution-side heat transfer coefficient increases from 144.6 to 510.1 W/m2-K. The overall experimental results indicate the potential of this absorption technology to achieve large heat and mass transfer rates in a compact geometry. Some differences between the performance of the prototype and earlier analytical models are attributed to the differences in ammonia-water inlet conditions and inadequate wetting of the tubes due to solution distribution problems. To further understand the transfer processes at the local level, a segmental analysis of the absorber is conducted. An area effectiveness ratio, which accounts for potential solution distribution problems, is found to vary from 0.21 and 0.31 over the range of conditions tested. The validated model then yields good agreement between measured and predicted values throughout the absorber. These results also show that although the prototype absorber meets design duty specifications even at low area effectiveness ratios, the absorber is solution-side dominant, and any improvement in the solution-side flow distribution and surface wetting would result in increased overall absorber performance. The validated design model presented here will not only assist in the understanding of the heat and mass transfer phenomena that occur during the absorption process, but it will also help direct future design improvements to other absorption system components.
John Marcus Meacham
Meacham, John Marcus, "An integrated experimental and analytical study of ammonia-water absorption in microchannel geometries" (2002). Retrospective Theses and Dissertations. 20165.