|Ph.D Student||Michael Koenig|
|Subject||Enhancement of Vapor Absorption by Surfactant|
Additive-Induced Marangoni Convection
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Grossman Gershon|
Surfactant additives are used in some absorption chillers due to the increase in vapor absorption rates they promote. The combined heat and mass transfer process occurring in the absorber, if left to its own devices, is characterized by laminar flow in a falling film absorber, or by a stagnant film in a pool absorber, in which mass diffusion and heat conduction must remove the heat of absorption from the interface to the cooling wall. Certain surfactant additives in systems using salt-based absorbent solutions cause Marangoni convective flows to initiate at the liquid-vapor interface, which in turn cause intense mixing of the liquid and enhanced absorption rates. Such additives have been used in absorption chillers for some time. However a unified quanititative model of the mechanisms responsible for absorption enhancement has been elusive in the literature.
This thesis involves a study of Marangoni Convection rate enhancement in a pool absorber. A numerical model was developed to quantify the effect of the dominant system parameters on the potential for enhancement and on the enhancement mechanism itself. Specific attention is given to the transport of the additive, as it adsorbs on a fresh interface at the start of the process, and thus lowers the surface tension in such a way as to promote the interfacial convection. A comparison of liquid-side and vapor-side surfactant adsorption shows the vapor-side to benefit from the increased mass flux occurring between vapor and liquid phases. A surfactant-vapor laden pool absorber experiment is described and correlated to simulations using the numerical model. Based on the findings in this study, the dominant parameters of successful additives are discussed, and specific modifications of existing absorber designs are recommended, aimed at optimizing the utilization of Marangoni convection and thus increasing absorber efficiency.