Experimental investigation of Marangoni convection and surface tension of aqueous lithium bromide solutions with additives at low pressure.

Optimization of absorption chillers is a fundamental task to pursue in the refrigeration industry, since these chillers are still characterized by large sizes and low efficiency. An efficient way to achieve this aim is by the mean of alcoholic surfactants. Indeed, when added to the working fluid pair (waterlithium bromide) in small quantities, they lower the surface tension and induce a local Marangoni convection at the liquid/vapor interface. As a consequence of the better wetting of the tubes, the heat and mass transfer in the absorber is increased. Nevertheless, values of surface tension measured in literature tend to scatter and the influence of many factors on the enhancement mechanism is not yet fully understood. Surface tension of water and aqueous lithium-bromide solution with different surfactants is experimentally investigated. 1-octanol and 2-ethylhexanol are used as additives. Their concentration in the solution is varied in a wide range. Surface tension is measured by the pendant drop method. Among the two additives investigated, 1-octanol leads to the smallest values of surface tension. Highest reduction of surface tension is observed when the additive is delivered to the surface through vapor phase. Based on results obtained, a new experimental setup for measurements at low pressure is developed and first surface tension measurements at low pressure are presented.