Flow Boiling of a Dilute Emulsion on a Microporous Surface

Abstract

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.