Boiling Phenomenon and Heat Transfer of Emulsion in a Capillary

In this study, we investigate boiling of two different types of emulsions in a capillary. Influences of the capillary size and the mass fraction of the dispersed phase in boiling characteristics are studied. Capillaries of five different inner dimensions, 500 × 50, 1000 × 100, 200 × 200, 700 × 700, 900 × 900μm^2 are used. Two types of emulsions are chosen as our working fluids. The first type consists of sulfated castor oil as the dispersed phase and pure water as the continuous phase, where the continuous phase has lower boiling point. The second type consists of Novec 7200 as the dispersed phase and aqueous glycerin solution as the continuous phase, where the dispersed phase has lower boiling point.
When incipient boiling occurs in the square capillary, bubble may appear in the bulk continuous phase, on the interface of the two phases or on the heating sidewall. For emulsion of small mass fraction, several bubbles tend to nucleate simultaneously on the heating sidewall of a square capillary. On the other hand, only a single bubble emerges during nucleation in a rectangular capillary and the bubble advancement leads to different contact angles between the interface and two sidewalls. When bubble expands longitudinally, the bulk emulsion is flushed away from the heating surface, leaving a thin liquid film on the sidewalls.
From the results, we find that heat transfer rate of sulfated castor oil-in-water emulsion is always larger than that of pure water in a square capillary. When a small amount of sulfated castor oil is added to water, the superheat of incipient boiling is drastically augmented. Yet, superheat of incipient drops if the mass fraction of sulfated castor oil exceeds 0.001.
Interestingly, the emulsion of refrigerant Novec 7200 in aqueous glycerin solution shows different boiling characteristics. We find that heat transfer rate deteriorates when a small amount of Novec 7200 is added to aqueous glycerin solution. Moreover, increasing the mass fraction of Novec 7200 leads to the decrease in superheat of incipient boiling. When the mass fraction of Novec 7200 is larger than 0.05, however, both heat transfer rate and superheat of incipient boiling remain independent of mass fraction. For emulsion of Novec 7200 in aqueous glycerin solution, smaller capillary leads higher superheats of incipient boiling.