Nucleate pool boiling heat transfer in aqueous surfactant solutions

Saturated, nucleate pool boiling in aqueous surfactant solutions is investigated experimentally. Also, the role of Marangoni convection, driven both by temperature and surfactant concentration gradients at the vapor-liquid interface of a nucleating bubble is computationally explored.; Experimental measurements of dynamic and equilibrium σ using the maximum bubble pressure method indicate dynamic σ to be higher than the corresponding equilibrium value, both at room and elevated temperatures. Also, nonionic surfactants (Triton X-100, Triton X-305) show larger σ depression than anionic surfactants (SDS, SLES), and a normalized representation of their dynamic adsorption isotherms is shown to be helpful in generalizing the surfactant effectiveness to reduce surface tension.; The dynamic σ has a primary role in the modification of bubble dynamics and associated heat transfer, and is dictated by the adsorption kinetics of the surfactant molecules at boiling temperatures. In general, an enhancement in heat transfer is observed, which is characterized by an early incipience and an optimum boiling performance at or around the critical micelle concentration of the surfactant. The optimum performances, typically in the fully developed boiling regime ($q\prime \prime w$ > 100 kW/m2), show a reverse trend with respect to surfactant molecular weights M, i.e., higher molecular weight additives promote lower enhancement. Normalized boiling performance using the respective solution's dynamic σ correlates heat transfer coefficient by M−0.5 for anionics and M 0 for nonionics. This has been shown to be brought about by the surfactant concentration and its interfacial activity in a concentration sublayer around the growing vapor bubble, which governs the bubble growth behavior through the mechanism of dynamic σ. The ionic nature of the surfactant influences the thickness and molecular makeup of the enveloping sublayer, thereby affecting the bubble dynamics and boiling heat transfer.; Finally, the computational modeling of Marangoni convection for boiling nuclei at short time transients shows similarity solutions for pure water, and reduced convection with a peak in circulation strength in the presence of surfactants. The peaking corresponds to the characteristic surfactant adsorption time, which has been shown to depend solely upon the surfactant bulk concentration. For the absence of surfactant surface convection, an enhancement in Marangoni convection is observed. Furthermore, for the investigated range of parameters and time scales, the surfactant adsorption at the interface is not characterized by the presence of a stagnant cap. (Abstract shortened by UMI.)...