(408f) Boiling of Dilute Emulsions: A State-of-the-Art Review

Boiling of dilute emulsions of two components has been long standing problem in two-phase heat transfer which has received modest attention over the past 25 years. Recent research however has identified some surprising and potentially useful characteristics of such systems. This paper reviews the current state of the art for modeling the boiling process and experimental data, and proposes a new modeling approach.

Emulsions are mixtures of two immiscible liquids in which one liquid is suspended in the other in the form of fine droplets. These droplets may range in size but can be as small as 1 µm. We consider a system in which the low-boiling-point component makes up less than five percent of the mixture by volume. One remarkable feature of such emulsions is that a very large degree of superheat is necessary before boiling begins. This delay occurs because most of the droplets never contact a solid surface, so bubble nucleation must occur separately in each droplet in the volume of the emulsion. Currently, the most detailed description of boiling dilute emulsions is based on a simple model of the thermal boundary layer and upon the buoyancy of the vapor bubbles. When applied to experimental data, the model shows that nucleation in the droplets occurs more quickly than spontaneous nucleation would occur in the pure liquid. It is speculated that the rapid nucleation is due to chain-boiling between droplets that results when a boiling droplet forms a shock wave that breaks up floccules of nanoparticles in neighboring droplets. This hypothesis has not been proven experimentally, and is not predictive.

In light of past kinetic and thermodynamic models and experiments, a new model of boiling dilute emulsions is proposed. First, the behavior of a single boiling droplet must be simulated accurately. These data can then be used to model the effects of boiling droplets on the surrounding liquid and nearby droplets, including agitation of the mixture and collisions between droplets and bubbles. Finally, the large scale behavior of the emulsion can be modeled with the theory of interacting continua, using the data of the detailed simulations to define parameters for the large-scale model. The model thus formed is amenable to use in numerical simulations of boiling dilute emulsions.