Boiling of Dilute Emulsions.

Abstract

Although boiling in pure liquids has been studied thoroughly, boiling in other circumstances is less well understood. One area that has received little attention is boiling of dilute emulsions in which the dispersed component has a lower boiling point than the continuous component. These mixtures exhibit several surprising behaviors that were unknown until the 1970's. Generally, boiling of the dispersed component enhances heat transfer over a wide range of surface temperatures without transition to film boiling, but a high degree of superheat is required to initiate boiling. In single-phase convection the dispersed component has little effect on heat transfer. These behaviors appear to occur in part because few droplets in the emulsion contact nucleation sites on the heated surface. No detailed and physically consistent model of boiling in dilute emulsions exists at present. The unusual behavior of boiling dilute emulsions makes them potentially useful for high heat flux cooling of electronics. High-power electronic devices must be maintained at temperatures below ~85 °C to operate reliably, even while generating heat fluxes of 100 W/cm2 or more. Current research, generally focusing on single phase convection or flow boiling in small diameter channels, has not yet identified an adequate solution. An emulsion of refrigerant in water would be well-suited to this application. The emulsion retains the high specific heat and thermal conductivity of water, while boiling of the refrigerant enhances the heat transfer coefficient at temperatures below the saturation temperature of water. To better understand boiling dilute emulsions and expand the experimental database, an experimental study of boiling heat transfer from a horizontal heated wire, including visual observations, is performed. Emulsions of pentane in water and FC-72 in water are studied. These emulsions have properties suitable for practical use in high heat flux cooling applications, unlike most emulsions that have previously been studied. The range of the experimental study is extended to include enhanced boiling of the continuous component, which has not previously been observed, in addition to boiling of the dispersed component. In both regimes the heat transfer coefficient is enhanced compared to that of water. Visual observations reveal the presence of large attached bubbles on the heated wire, the formation of which coincides with the inception of boiling in the heat transfer data. At very low dispersed component fractions and low temperatures, boiling of individual dispersed droplets is not observed. The large attached bubbles represent a new boiling mode that has not been reported in previous studies and is, under some circumstances, the dominant mode of boiling heat transfer. A model of boiling dilute emulsions is developed based upon the Euler-Euler model of multiphase flows. The general balance equations as developed by Drew and Passman are applied to the present situation, thus providing a rigorous and physically consistent framework. The model contains three phases that represent the continuous component, liquid droplets of the dispersed component, and bubbles that result from boiling of individual droplets. Mass, momentum, and energy transfer between the phases are modeled based upon the behavior of and interaction between individual elements of the dispersed phases. One-dimensional simulations of a single boiling droplet in superheated liquid are also performed, and the results are used to develop the closure equations of the larger model. Droplet boiling is assumed to occur when a droplet contacts a heated surface or a vapor bubble. Collisions between droplets and bubbles and chain-boiling of closely-spaced droplets are considered. The model is limited to the dispersed component boiling regime, and thus it does not account for phase change of the continuous component. The model also does not include the large attached bubbles revealed in the visualization experiments. However, simulations of boiling match several trends observed in the experimental data. The model thus provides a physically consistent and partially validated platform for future analytical and numerical work.