Combined heat and mass transfer under different inlet subcooling modes during NH3-H2O falling film absorption process

Experiments were conducted for ammonia-water falling film absorption in a p late heat exchanger with, offset strip fins. The objectives of this paper w ere to analyze combined heat and mass transfer during the ammonia-water abs orption process under different inlet subcooling modes, and to obtain heat transfer coefficients (Nusselt number). This paper examined the effects of the inlet subcooling modes, the inlet concentration difference, liquid Reyn olds number, and vapor Reynolds number oil the heat transfer performance. I nlet liquid concentrations were set at 0, 5, 10, and 15 percent in mass of ammonia, while inlet vapor concentration ranged from 64.7 to 83.6 percent. Experiments were conducted in three ways according to the inlet subcooling conditions, i.e., Case A (T-0 > T-1), Case B (T-v similar to T-l), and Case C (T-v < T-l). In Case A, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption was confi rmed in. the experiments, which resulted in a lower absorption performance. The heat transfer coefficient increased as the inlet subcooling increased in all cases. The effect of inlet subcooling on heat transfer performance w as more significant in Case A than in Cases B and C. The inlet subcooling h ad more significant effect oil the heat transfer performance than the inlet concentration difference. Nusselt number increased as liquid and vapor Rey nolds numbers increased. The vapor velocity should be maximized to increase absorption performance in cocurrent ammonia-water absorption process. The parametric analysis provides fundamental understandings of the ammonia-wate r absorption process, and thus gives a guideline for heat exchanger compact ness in ammonia-water absorption systems.