Experimental Investigation on the Surface Tension of LiBr/H2O Solutions with Additives at Low Pressure (ST-16-C002)

Absorption chillers represent a promising alternative to traditional vapor compression chillers, especially for the air conditioning systems. Indeed, they provide the same cooling supply without such a high electrical consumption, being driven by a low-temperature heat source. On the other side, they are not yet competitive with compression chillers because of their large size and high costs of investment. Their optimization becomes then a fundamental task to perform. Among the several components of these chillers, the absorber has been identified as the limiting one. The main obstacle is the low wetting on its tube bundle, which limits the absorption process. One way to overcome this problem is by the mean of additives. Indeed, small quantities of alcoholic surfactants in the working fluid lower the surface tension, promoting local turbulence at the vapor-liquid interface (Marangoni convection), which in turn leads to higher heat and mass transfer coefficients in the absorber. Nevertheless, only two kinds of additives are mostly used in these chillers and the experimental results available in literature are not in good agreement. The surface tension of water and aqueous lithium-bromide solution with different surfactants is experimentally investigated in this work. Common additives (e.g. 2-Ethylhexanol, 1-Octanol) as well as new additives are used. Their concentration in the solutions is varied in a wide range. The surface tension is measured according to the Pendant Drop Method. Several parameters are varied during the experiments, such as pressure, temperature and surrounding conditions. All the measurements are performed in a closed vacuum cell, in order to have the vacuum condition that occurs in the absorber, and to produce reproducible data. Results are discussed and compared with the available literature. The current study is carried out in the framework of the ITN Marie Curie “SHINE” research program financed by the European Union.