Nucleate Pool Boiling Heat Transfer in Aqueous Surfactant Solutions

Nucleate Pool Boiling Heat Transfer in Aqueous Surfactant Solutions
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Release: 2001
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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. Boiling of surfactant solutions is important in many key industrial applications such as hygiene and personal care, pharmaceutical, and food processing, among others. Moreover, surfactants in trace amounts are present in boilers of conventional power plants and have also been shown as promising agents to augment the nucleate boiling performance of water. Their efficient use, however, requires precise knowledge of their solutions' boiling behavior. This need is addressed here through systematic experimentation and quantification of boiling performance with respect to surfactant's ionic nature and molecular weight. In addition, the computational modeling attempts to quantify the extent of Marangoni convection during initial bubble growth transients. An exhaustive literature review demonstrates significant property (primarily surface tension and to some degree steady-state shear viscosity) variations and altered boiling behavior of water in the presence of surfactants. A generalized quantification of boiling behavior is not available however, though the available results and analyses suggest the possibility of dynamic surface tension being a primary correlating parameter for the boiling heat transfer. Experimental measurements of dynamic and equilibrium surface tension using the maximum bubble pressure method indicate dynamic surface tension to be higher than the corresponding equilibrium value, both at room and elevated temperatures.

Computational Modeling of Bubble Growth Dynamics in Nucleate Pool Boiling for Pure Water and Aqueous Surfactant Solutions

Computational Modeling of Bubble Growth Dynamics in Nucleate Pool Boiling for Pure Water and Aqueous Surfactant Solutions
Author: Bradley J. Romanchuk
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Total Pages: 93
Release: 2014
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A computational model has been developed to simulate growth of an isolated vapor bubble during nucleate pool boiling of pure water and aqueous surfactant solutions at saturated conditions on a surface with a constant temperature. The governing equations of mass, momentum, and energy conservation are solved in the liquid and the vapor phases using a finite volume method. The volume-of-fluid (VOF) method is employed to capture the deforming liquid-vapor interface. The computational domain includes a microlayer near the liquid-solid-vapor contact line and macro region that contains the vapor bubble and the surrounding liquid. Solution of the governing equations in the microlayer provides source terms for the heat transfer and phase change for the macro region. The computational model was validated by comparing with results available in the literature for pure water. Simulations of bubble growth from incipience to departure are conducted for pure water and surfactant solutions of sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and octylphenol ethoxylate (Triton X-100) at twice the critical micelle concentration (2xCMC). Wall superheats of 4K and 8K are considered and their respective growth cycle characteristics are discussed in detail. The results show that the predicted bubble departing volume and growth rate increases leading to a faster departure time as the wall superheat is increased. The time-dependent surface tension relaxation at the liquid-vapor interface along with increased surface wettability at the liquid-solid interface result in smaller bubbles departing with higher frequency in surfactant solutions compared to boiling in pure water. The dynamic surface tension and surface wettability play an important role in governing bubble growth dynamics in nucleate pool boiling of surfactant solutions.