Visualization and model development of Marangoni convection in NH3–H2O systemVisualization et développement d'un modèle de convection de Marangoni dans un système à NH3–H2O

The objectives of this paper are to obtain experimental data of surface tension and interfacial tension, and to develop a new model of Marangoni convection for the best selection of heat transfer additive in ammonia–water absorption systems. The basic mechanism of Marangoni convection in absorption systems was reviewed from the viewpoints of the surface tension and the interfacial tension gradients. Marangoni convection was successfully visualized using a shadow graphic method. The solubility limits of the additives in ammonia–water solution ranged from 500 to 3000 ppm depending on the heat transfer additives. These values are much higher than those in LiBr–H₂O solution in which the solubility ranged from 70 to 400 ppm. The temperature gradient of the surface tension should not be a criterion for Marangoni convection inducement in NH₃–H₂O system. The concentration and temperature gradients of the interfacial tension should not be a criterion for Marangoni convection inducement in NH₃–H₂O system. The magnitude of the interfacial tension did not affect the occurrence of Marangoni convection either. It was found that addition of the heat transfer additive beyond the solubility limit assisted Marangoni convection occurrence, but should not be a criterion for Marangoni convection inducement. It was proposed that the radical-out model should be a criterion for Marangoni convection inducement within the solubility limit in NH₃–H₂O system.