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Ammonia absorption refrigeration has attracted attention due to its low refrigerating temperature and the absence of crystallization as well as good performance under vacuum conditions. However, its efficiency is still lower than the mechanical compression refrigeration system at present. The quality of heat and mass transfer in absorption process is vital for improving the performance of ammonia absorption refrigeration. The objectives of this work were to experimentally investigate the enhancing influence of nanoparticles on an ammonia/water falling film absorption process under pressure reducing conditions and to further explore the mechanism of absorption enhancement by the nanofluids. Our experimental results showed that the different kinds of nanoparticles used had different enhancing influences on the ammonia falling film absorption process, and the nanoparticles had different optimal enhancement concentrations. These concentrations were 0.2%, 0.1% and 0.1% in mass concentration for the Al2O3, ZnO and ZrO2 nanofluids, respectively. The effective absorption ratios increased with increasing initial ammonia concentration, indicating that the enhancing influence of the nanoparticle addition on the absorption was more obvious at a lower ammonia absorption potential. The absorption operating pressure is an important influencing factor. The enhancing effect of the nanoparticles was not represented without a sufficient absorption pressure (driving force). The enhanced absorption could be better explained by a combination of the two-film theory, the penetration theory and the surface update theory. The transportation effect and the vortex effect caused by the nanoparticles, as well as the Marangoni convection effect induced at the phase surface, could destroy the two-film stagnation layer assumed in the two-film theory. These effects could also enhance solute permeability and expedite the update of the surface, thus enhancing the absorption performance.

The enhancing influence of nanoparticles on ammonia/water falling film absorption in binary nanofluids under pressure reducing conditions