| Saturated, nucleate pool boiling on a horizontal, cylindrical heater and the associated bubble dynamics in aqueous surfactant and polymer solutions are experimentally and computationally investigated. Boiling curves ( q''w vs. DeltaTsat data) for different additive concentrations and photographic records of the salient features of the ebullient behavior are presented, along with a characterization of the interfacial properties of the aqueous solutions. The surfactant additive significantly alters the nucleate boiling in water and enhances the heat transfer. The enhancement increases with concentration, with an optimum obtained in solutions at or near the critical micelle concentration (CMC) of the surfactant. On the other hand, boiling in aqueous polymer solutions shows contrary results. The enhancement is seen to increase with polymer concentration in the aqueous surface-active HEC solutions, while boiling in shear-thinning Carbopol 934 solutions is seen to decrease continuously with increasing concentrations.; In order to develop a theoretical model and understand the associated convective mechanisms, the dynamics of a single growing and departing bubble during nucleate boiling from a horizontal heated surface has been numerically simulated. The results highlight the role of the microlayer in nucleate boiling, as well as the effects of altered surface tension and viscosity, apparent contact angle, and wall superheat on the bubble dynamics and boiling behavior in aqueous surfactant and polymer solutions.; In multiphase heat transfer, the nature and dynamics of surface contact often plays a dominant role. In fact, almost all the transport processes in nucleate boiling, from the inception of an embryonic bubble to the subsequent phase change, are intricately connected with this interface. To better characterize the molecular dynamics of the additives at the interfaces, an extensive literature review is presented that delineates the surfactant adsorption process and its electrokinetic effects (zeta potential), and the concomitant surface wetting behavior. This fundamental understanding is supported by the measured interfacial properties of the solutions reported in this study. And as shown by the boiling data, the heat transfer performance is changed dramatically when the interfacial transport process is altered.; Finally, as documented by the pool boiling experiments with aqueous surfactant solutions, a markedly different ebullient behavior than not only that of water is observed, but between pre- and post-CMC solutions as well. (Abstract shortened by UMI.)... |
