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An experimental study of absorption of ammonia into a constrained thin film of ammonia-water solution is presented. A large aspect ratio microchannel with one of its walls formed by a porous material is used to constrain the thickness of the liquid film. Experiments were performed at a pressure of 1, 2.5 and 4 bar absolute and a fixed weak solution inlet temperature. Weak solution flow rates were varied from 10 to 30 g/min, inlet mass concentrations from 0 to 15 percent, and gas flow rates between 1 and 3 g/min. Six geometries, including three smooth-bottom-walled channels of differing depths and three channels with structured bottom walls are considered. Results indicate that for identical rates of vapor absorption, the overall heat transfer coefficient for the 400 um smooth microchannel is significantly larger than of the 150 um and 1500 um smooth channels. For the 150 um channel, the largest overall heat and mass transfer coefficients were achieved for the highest vapor to solution flow rate ratio, where the ratio of heat generated to heat removed was unity.
A numerical model is also presented that predicts temperature and concentration profiles along the length of the absorber. In the present state, the model is not adequate for realistic sizing due to assuming instantaneous absorption of ammonia vapor. An updated model is proposed that accounts for local mass transfer phenomena that accounts for bubble size reduction and eventual absorption.
Finally, sizing estimates are introduced by scaling up the absorber using the highest flow rate ratio case, 3/10. The magnitude of the scale is dependent upon the application (residential or vehicular). Absorber dimensions for a vehicle are estimated at 4 cm x 14 cm x 28 cm, and the residential absorber dimensions, 4 cm x 29 cm x 58 cm. |
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