Physicochernical mechanics of surfactant-enhanced boiling heat transfer

Abstract

Boiling-employed in a variety of industrial and domestic processes such as in power stations, heating/cooling systems, and desalination plants-is involved with a major portion of the world's energy usage. Its substantial utility can be attributed to moving a large quantity of heat over small temperature differences. However, even these small temperature differences can have implications on energy efficiency, device lifetime, and performance. Surfactants, which are molecules that have hydrophobic and hydrophilic components, are known to enhance boiling by changing the way bubbles nucleate on the surface, grow, and depart from the surface. This thesis provides a mechanistic understanding of surfactant enhanced boiling from molecular and macroscopic perspectives with theory and experiments. First, a statistical mechanical model to predict equilibrium and dynamic surface tension from molecular parameters is introduced and experimentally verified. Then, models of bubble nucleation, growth, and departure are developed, taking into account the time-dependent nature of surfactant adsorption processes. From there, models are combined so as to predict the enhancement in boiling performance based primarily on molecular information of the surfactant. Pool boiling experiments conducted with a variety of surfactants have shown agreement with model predictions. With the framework presented in this thesis, large-head and long-tail surfactants were found to be desirable. However, suitable surfactants for specific needs can now be identified, which can aid in the further adoption of surfactants in practice. Finally, using insights gained about the importance of solid-liquid adsorption over liquid-vapor adsorption, a novel method of using electric fields to control surfactant adsorption wherein bubbles can be turned "on" and "off" is demonstrated. Furthermore, an ability to control boiling spatially in addition to temporally is shown. This active control of boiling can improve performance and flexibility in existing boiling technologies as well as enable emerging or unprecedented thermal applications.