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Browsing by Author "Waikar, Ameya"

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    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.