Effect of foamability on pool boiling critical heat flux with nanofluids.

Foaming, which is of significant importance to many industrial processes, is attributed to the reduced coalescence of bubbles due to the presence of stabilizing/foaming agents such as surfactants and nanoparticles. While foams have been extensively investigated for their rheological properties, their impact on the critical heat flux (CHF) during boiling is not well understood. The technical benefits of CHF enhancement with nanofluids are lost when surfactants are added to improve their stability. The actual mechanism of this decrease is unresolved, and thermal engineers are forced to look for alternative CHF enhancement solutions. Here, we showed that nucleating bubbles formed vapor-foam and crowded the heater surface to inhibit rewetting. Less frequent rewetting forces premature dryout, which is primarily responsible for the reported CHF deterioration. In this regime, strong foaming agents such as SDS mask the effect of nanoparticles on CHF. Using these insights, we presented a master curve that captured the effect of foamability on CHF and could be used to predict the value of CHF solely based on the foamability of the solution. We further showed that the CHF mechanism switched from the foamability regime to the conventional wettability regime upon lowering the surfactant concentration and/or with weakly foaming surfactants. In such cases, an increase in the nanoparticle concentration successfully increased CHF. We believe that the important clarifications regarding the CHF mechanism with nanofluids and the master curve of CHF versus foamability presented in this study will facilitate the design of energy-efficient boiling systems.

Surfactants for Bubble Removal against Buoyancy.

The common phenomenon of buoyancy-induced vapor bubble lift-off from a heated surface is of importance to many areas of science and technology. In the absence of buoyancy in zero gravity of space, non-departing bubbles coalesce to form a big dry patch on the heated surface and heat transfer deteriorates despite the high latent heat of vaporization of water. The situation is worse on an inverted heater in earth gravity where both buoyancy and surface tension act upwards to oppose bubble removal. Here we report a robust passive technique which uses surfactants found in common soaps and detergents to avoid coalescence and remove bubbles downwards, away from an inverted heater. A force balance model is developed to demonstrate that the force of repulsion resulting from the interaction of surfactants adsorbed at the neighboring liquid-vapor interfaces of the thin liquid film contained between bubbles is strong enough to overcome buoyancy and surface tension. Bubble removal frequencies in excess of ten Hz resulted in more than twofold enhancement in heat transfer in comparison to pure water. We believe that this novel bubble removal mechanism opens up opportunities for designing boiling-based systems for space applications.