Boiling of dilute emulsions. Mechanisms and applications

This paper reviews recent advances in pool and flow boiling of dilute emulsions, including initial theories on the boiling mechanisms. The current consensus is that pool boiling is initiated on nucleation sites on the surface and that increased heat transfer coefficients are a result of mixing produced by vapor bubbles in the thermal boundary layer. Visualization of the boiling process substantiates this conclusion. Current understanding of the influence of various quantities on the convective heat transfer coefficient is enumerated with respect to the underlying thermodynamics and energy transport. Prior research on flow boiling is limited in comparison, and much more needs to be done with respect to the boiling and enhancement mechanism. Various models for boiling of emulsions are discussed, including a RANS model that incorporates droplet contact with the heated surface, collisions between droplets and bubbles, and chain boiling due to collision during the boiling process. Key modeling elements are how interphase transport and droplet interactions affect nucleation rates and droplet behavior within the viscous and thermal boundary layers at and near a heated surface. It is clear now that boiling of a suspended phase with a lower saturation temperature than the continuous phase can produce marked increases in overall heat transfer coefficients. A re-conceptualization of two-phase heat exchange equipment is a future possibility, and various applications are identified. Boiling of dilute emulsions may be an attractive cooling strategy when restrictions on size, weight and pumping power must be addressed, such as in thermal management of avionics systems.