Between Dec 19, 2024 and Jan 2, 2025, datasets can be submitted to DRUM but will not be processed until after the break. Staff will not be available to answer email during this period, and will not be able to provide DOIs until after Jan 2. If you are in need of a DOI during this period, consider Dryad or OpenICPSR. Submission responses to the UDC may also be delayed during this time.

Browsing by Subject "Flow Boiling"

Now showing 1 - 2 of 2
  • 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
    Flow Boiling of a Dilute Emulsion on a Microporous Surface
    (2019-01) Shadakofsky, Brandon
    In this investigation, an experimental apparatus was designed and constructed for studying flow boiling of water and dilute emulsions. Experiments were conducted in microgaps of 200, 500 and 1000 μm hydraulic diameter and mass fluxes of 150, 350 and 550 kg/m2s. The emulsions comprised droplets of FC-72 suspended in water at FC-72 volume fractions of 0.1, 0.5, 1 and 2%. Experiments were conducted for a smooth surface and three microporous surfaces of varying thickness (708, 633, 412 μm) and porosity (0.354, 0.410, 0.413). For water on the smooth surfaces, the single-phase heat transfer coefficient increases with increasing mass flux and decreasing gap size. After the onset of nucleate boiling and prior to transition to the critical heat flux, the heat transfer curves collapse to one curve. CHF increases with increasing gap size and mass flux. The effects of the liquid subcooling, applied heat flux, mass flux, and gap size on the two-phase heat transfer coefficient are correlated using the Nusselt, Jakob, Reynolds and Boiling numbers, with 98% of the experimental data within ±30% of the predicted Nusselt number. For boiling of emulsions on the smooth surface, increasing the volume fraction up to 0.1 or 0.5% enhances cooling in some cases, but increasing ε further to 1 or 2% provides no additional benefit and decreases heat transfer in some experiments. The emulsion improves heat transfer compared to water for larger gap sizes and lower mass fluxes. In almost all experiments, the heat transfer coefficient for the emulsion increases with increasing wall temperature. Based on these observations, it was posited that two heat transfer mechanisms exist. Conduction in a thin film of FC-72 impairs heat transfer due to the low conductivity of FC-72. Mixing due to boiling of FC-72 increases heat transfer. From these two mechanisms, correlations are developed for the emulsion heat transfer coefficient and the ratio of the emulsion and water heat transfer coefficients. These correlations include a new non-dimensional number, GCpd/kd, to account for conduction in the thin film and sensible heat advected from the wall. A very good fit is seen for h, with 95.7% of the experimental data falling within ±10% of the correlation. For the heat transfer coefficient ratio, 58.7% of the experimental data falls within ±30% of the predicted value, though the correlation captures the trend of the data well. For boiling of water on the porous surfaces, better heat transfer is measured, especially at higher mass flux and gap sizes. The best heat transfer for the porous surfaces is consistently displayed on Porous Surface 1. The measured pressure drop for the porous surfaces is generally higher than that for water on the smooth surface and the highest pressure drops were measured for Porous Surface 2. For boiling of emulsions on the porous surfaces, Surface 1 shows a mixture of enhanced and degraded heat transfer. For Surface 2, the emulsions enhance heat transfer for the majority of the data set, and this is especially pronounced for smaller gaps. The emulsions also decrease the measured pressure drop on Porous Surface 2 for the cases where heat transfer is increased. Porous Surface 3 shows similar behavior for the emulsions and water for most of the data set. The better emulsion heat transfer behavior for Surface 2 is likely due the open pore network seen from both the side and top of Surface 2. This allows FC-72 droplets to flow down in the porous structure and nucleate bubbles. The resulting vapor can also release more easily in this open structure. Finally, maps are given to show where the emulsions enhance or degrade heat transfer on each surface. Recommendations are given for further study in boiling of emulsions.