The effect of surfactant vapor on marangoni convection in absorption and condensation

摘要

Mass and heat transfer enhancement by the addition of a class of surfactant additives is in common use in absorption machines based on aqueous lithium-bromide (LiBr). It is observed that the addition of on the order of 100 ppm of a surfactant such as 2-ethyl-hexanol (2EH) introduces Marangoni convection on the liquid surface and thereby enhances absorption and condensation rates. The Vapor Surfactant Theory (Kulankara and Herold, 2000) proposed that such Marangoni convection is driven by the surface tension gradient caused by surfactant that circulates through the machine and arrives at the liquid surfaces as a vapor by bulk flow along with water vapor.The objective of this work was to fully understand the vapor surfactant induced enhancement mechanism and to quantify the relationship between the enhancement and the key variables. This goal was achieved by conducting experimental and numerical analyses including the measurement of surface tension with surfactant 2EH in the vapor, determination of 2EH surface concentrations in aqueous LiBr and water, experimental study of Marangoni convection in an absorption and condensation pool with surfactant 2EH in the vapor and modeling of Marangoni convection in the presence of surfactant vapor. The surface tensions were measured with controlled 2EH concentration in the vapor by using the drop volume method. The results show that for both aqueous LiBr and water the surface tensions are reduced with increased 2EH concentration in the vapor. The 2EH concentration in the vapor is a primary variable in determining the surface tension of aqueous LiBr. Calculated surface concentrations show that the presence of LiBr results in a reduction in 2EH solubility, and that the surface concentration of 2EH is more sensitive to surfactant in the vapor than to surfactant in the liquid. Furthermore, the experimental and numerical analyses show that surfactant in the vapor alone can initiate the Marangoni convection; the strength of Marangoni convection is primarily dependent on the 2EH concentration in the vapor. The current studies show that surfactant in the vapor is a necessary condition for significant absorption and condensation enhancement.