Nucleate pool boiling enhancement by means of surfactant additives

While certain surfactant additives are well known to significantly enhance boiling heat transfer, the enhancement mechanism is still not so clear for the present. In order that more conclusive information of fundamental boiling phenomena in surfactant solutions can be collected, surfactant effects on interfacial properties at gas/liquid interface, nucleate boiling heat transfer, boiling incipience, and single vapor bubble growth dynamics for three additives - including 99% SDS, 95% SDS, and Triton X-100 - were experimentally studied in this work. The nucleate boiling heat transfer coefficients were tried to correlate with the equilibrium and dynamic surface tension of the aqueous surfactant solutions. The results reveal that neither equilibrium nor dynamic surface tension can successfully explain the boiling phenomena. On the other hand, different appearance of boiling above the heater surface suggests that different heat transfer mechanism may prevail in surfactant solutions. A correlation between the nucleate boiling heat transfer coefficient enhancement and the vapor bubble occupied area increase was satisfactorily obtained. The micro-wedge model considering the evaporation at the interface of the bubble base and the mass flow in the wedge between the interface and the heated wall due to capillary forces may then be assumed for heat transfer processes in surfactant solutions. Surfactant effect on boiling incipience at a target cavity was also examined. The results show that surface tension only cannot determine boiling incipience in surfactant solutions in general. The variations in individual behavior of the successive vapor bubbles from even the same cavity, however, seems to overwhelm the effect of surfactant on single vapor bubble growth dynamics. No convincing conclusion can be drawn for bubbling periods and departure diameter.