Surface Tension Driven Film Flow due to Condensation With a Vapor Borne Surfactant

Abstract

In absorption chillers, it is well known that the presence of small amounts of alcohol can enhance the absorber heat and mass transfer significantly. Similar enhancement is also observed in the condenser. Among the theories to explain such phenomena, the recently described Vapor Surfactant theory provides the most comprehensive explanation of the available data. According to this theory, the surfactant arrives at the liquid surface primarily from the vapor. As the water vapor flows toward eventual absorption/condensation, it carries the vapor borne surfactant toward the surface. Once the surfactant arrives at the surface, it tends to concentrate there due to its low solubility and low diffusivity in the liquid as well as the natural affinity of a surfactant for the surface. Non-uniform surface concentration of the surfactant leads to surface tension forces in the liquid surface layer that drive the surface flows often observed. These Marangoni flows are what enhance the heat and mass transfer processes. Although it was often theorized that Marangoni flow plays a role in enhancement, the important role of the vapor has not been well understood. A number of previous attempts have been made to compute Marangoni flow but all such attempts have assumed that the surfactant transport in the liquid is the key. This paper presents calculations of liquid film flows driven by the surfactant as it arrives at the surface from the vapor. This completely new model involves several very interesting features. A two-dimension numerical simulation of such a flow is reported here. The geometry is a thin film (3mm × 10cm) of liquid water with a small patch of cooling centered on the bottom side. The upper surface is exposed to steam+surfactant vapor so that condensation will occur. The model computes the surface concentration of surfactant and relates that to the surface tension. Calculations show that the condensation rate is much higher when Marangoni convection is occurring, even if the surfactant concentration in the vapor is very small (120ppm). This is in accordance with observations in absorption chillers. The result helps to clarify understanding of surfactant-induced enhancement of heat and mass transfer in absorption chillers.