In most commercial absorption type refrigeration systems, a falling film heat exchanger with $LiBr-H_2O$ solution is used as the absorber and it is well known that surfactants enhance heat and mass transfer performance in the absorber considerably. Although there are many previous studies concerned with this phenomenon, the current information on the absorption performance and the understanding of the mechanism of enhancement are not sufficient to optimize the absorber design.
In this study, heat and mass transfer characteristics of $LiBr-H_2O$ solution flowing over a single horizontal tube with surfactant were examined experimentally. Parameters considered were the surfactant concentration, temperature of the solution supply at the top of the tube, and the absorber pressure. Also, the previous studies on the mechanism of the heat and mass transfer enhancement with the surfactants were reviewed.
With the surfactant, fluctuations at the liquid film surface were observed and heat and mass transfer performances were enhanced. Even with an amount of the surfactant below the solubility limit, heat and mass transfer rates were promoted tremendously. The increases of the Nusselt and the Sherwood numbers were about 70% and 340%, respectively when 10ppm of 2-ethyl-1-hexanol was added. However, an excess amount of the surfactant in the solution did not bring a further enhancement.
The absorption performance decreased with the extraction of the non-condensable gases by a vacuum pump since the vaporized surfactant was also extracted during the extraction process. Thus, it is desirable to add a sufficient amount of the surfactant (more than 10ppm) to maintain the high absorption rate.
