Browsing by Subject "Emulsion"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Boiling of Dilute Emulsions.
    (2010-06) Roesle, Matthew Lind
    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.
  • Thumbnail Image
    Item
    Flow Boiling of A Dilute Emulsion In the Transition Regime
    (2020-05) Waikar, Ameya
    This investigation investigates heat transfer of water and flow boiling of dilute emulsion in transition and turbulent regime. The gap heights for microgap of 500 and 1000 μm and nominal Reynolds number of 1600 and 2800. The emulsion in this study is an oil-in-water emulsions, where FC-72 is the oil whose droplets are suspended in water. The volume fractions for the emulsions are 1% and 2%. The heated test section is smooth. For single phase experiments, the heat transfer coefficient of water with increasing Reynolds number and decreasing the hydraulic diameter. The Nusselt number in the single-phase region is correlated to the Reynolds number, Prandtl number and aspect ratio of the channel. The Nusselt number varies linearly with ????????????ℎ.????????.????ℎ???? . In emulsion heat transfer on the smooth surfaces, the value of the heat transfer coefficient increases only for a volume fraction of 2% of the disperse component under certain conditions. Reducing the concentration to 1% provides no additional benefit and decreases heat transfer coefficient for all gap sizes and Reynolds number. The 2% emulsion has a larger overall heat transfer coefficient than that in water for lower hydraulic diameter and higher Reynolds number. The heat transfer coefficient increases with increasing wall temperature and plateaus at higher wall temperatures. The interaction between turbulence and boiling is also an area of interest in this investigation. When the emulsion boils, there is enhanced mixing in the flow, also leading to further agitation of the flow causing more turbulence. There is significant increase in pressure drop for the 2% emulsion with increasing wall temperature. Based on these observations and the previously suggested heat transfer mechanism, the following mechanisms are posited: conduction in thin film of FC-72 which reduces the heat transfer due to lower conductivity of FC-72; enhanced mixing due to boiling of FC-72 which increases heat transfer; and the boiling further increases the turbulence, enhancing the convection of the flow. These effects are quantified by correlations developed by using seven different non-dimensional parameters, and an empirical correlation is derived for calculating the heat transfer coefficient for the emulsion. The correlation is a good fit with 93.8% of data lying within ±30% of the predicted values. Further conclusions about the mechanisms involved in the flow boiling of emulsions have been made, and the data set for the flow boiling of emulsions has been further expanded into transitional and turbulent regimes.
  • Thumbnail Image
    Item
    Nanoemulsion-like Polymersomes for Nanoreactors
    (2015-07) So, Soonyong
    Self-assembly of block copolymers in various selective solvents provides a means to control nanostructures. Among selective solvents, ionic liquids (ILs) are of great interest as reaction media, with the possibility of replacing organic solvents. However, the implementation ILs is limited by their high viscosity and cost. Phase transfer of IL-filled polymer vesicles (polymersomes) from the IL phase to water produces a very stable kind of "nanoemulsion"�. Nanoemulsion-like polymersomes have great potential as they confine a catalyst within the interiors, thus mitigating the mass transfer limitations of ILs while simultaneously providing a facile route to quantitative catalyst recovery The issues in the nanoreactor system and the mechanism of the phase transfer in the biphasic system are discussed. First, a new reversible reaction process with the thermo-responsive shuttling of the IL-filled polymersomes between the phases was designed. In nanoreactor applications, a narrowly distributed, small vesicle size is required. The size of polymersomes having rubbery and glassy membranes was controlled through mechanical and kinetic approaches. In the mechanical approach, the extrusion method was employed. For the kinetic approach, the amount of co-solvent and the hydrophilic fraction of amphiphilic block copolymer were varied and its effects on the size and dispersity were studied. Transport phenomena across the glassy and rubbery bilayer membranes was elucidated by NMR techniques to quantify the mobility inside and outside the polymersomes, plus the rate of exchange through the membrane. The dependence of the membrane thickness, glass transition temperature of the membranes and the partition coefficient of tracer molecules in the IL/water were also examined. We demonstrated a general boundary for the phase transfer of polymersomes in terms of a reduced tethering density for poly(ethylene oxide) (PEO), and analyzed the phenomena thermodynamically. The tethering density can be increased by increasing the block length of PEO and the size of the polymersomes, and the increased tethering density induces the phase transfer. Interfacial tension-related phase transfer led to develop a novel separation method in the biphasic system of the IL and water. By controlling the interfacial tension between the hydrophobic membrane and water, worm-like micelles and polymersomes were successfully separated.
  • Thumbnail Image
    Item
    Surfactant Effects On Pool Boiling Of Dilute Emulsions On A Horizontal Surface
    (2020-05) Proper, John
    Previous research has demonstrated that heat transfer of water may be enhanced by either the addition of surfactants or through emulsifying the water with a small volume of volatile disperse phase. However, the combination of surfactant and emulsion has not yet been thoroughly investigated. To accomplish this, experiments of boiling heat transfer were conducted with dilute FC-72 in water emulsions and Tween-20 surfactant. Boiling occurred over a flat upward-facing aluminum nitride heater in a vessel nearly 1 L in volume. FC72-in-water emulsions with volume fractions of 0.1, 0.5, and 1.0 % were tested with concentrations of 0, 10, 60, and 100 ppm Tween-20 surfactant. Turbulent mixing induced by pumping through a check valve caused the fluid to emulsify. Size distributions of oil droplets in the emulsions were measured via laser diffraction. The resulting boiling curves for emulsion without surfactant confirmed previously observed trends, with an increase in heat transfer occurring near 80 °C. Boiling with Tween-20 surfactant did not show any increase in heat transfer, in both aqueous surfactant systems and emulsions with surfactant. Laser diffraction imaging showed that emulsions with different volume fractions can have different particle size distributions even though the emulsification process was the same. Addition of surfactant to the emulsion tended to cluster the droplet diameter distribution around a mean of 2 µm. The similarity of boiling behavior between mixtures of water and Tween-20 and emulsions with Tween-20 suggests that the boiling behavior is characterized mostly by interface chemistry unique to Tween-20, rather than a secondary effect of droplet size distribution. However, there is considerable variety in surfactants, and there is still value in testing the effect of other surfactants on the boiling behavior of dilute oil-in-water solutions.