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Characteristics and Applications of Thin Liquid Films Flowing Down High-Curvature Surfaces


Thin liquid films flowing down vertical fibers present a wealth of complex and interesting interfacial dynamics, including the formation of droplets and traveling wave patterns. Such dynamics is an important consideration in various applications, such as fiber coating and direct-contact heat and mass exchangers which take advantage of extended interfacial areas and larger residence time afforded by the bead formation along the fiber. A rigorous investigation on the fluid dynamics and interfacial heat and mass transfer mechanism of liquid films flowing along vertical strings is, thus, needed to enable physics-based optimization and analysis of multi-string designs for the mentioned applications. This dissertation presents a combination of experimental, numerical, and theoretical study of liquid films flowing down a vertical fiber. Additionally, we report a first-ever combined experimental and theoretical study of the instability in thin film flows of a high–surface energy low-viscosity liquid (i.e. water) along cotton threads. Utilizing our finding, we then adapted the multi-string configuration for novel applications, such as humidification, dehumidification, and particle capturing.

We started with a thorough experimental study of viscous liquids flowing down vertical fibers (i.e. polymer strings). Previous researchers suggested that the liquid film thickness and velocity profiles of nearly flat portion of a liquid film that precedes the onset of instability can be specified regardless of the nozzle geometry. As a result, they largely overlooked the effects of nozzle on the pattern and characteristics of the downstream flow. We performed a systematic experimental study by varying the nozzle inner diameter from 0.5 to 3.2 mm at various mass flow rates (from 0.02 to 0.08 g/s). We focused on experimental conditions within the Rayleigh−Plateau (RP) instability regime, where traveling wave solution emerges and generates uniformly-spaced drop-like liquid beads on vertical fibers. Our results emphasize the strong influence of nozzle geometry on the flow regime and the flow characteristics. We experimentally measured the thickness of the flat film portion after the nozzle, which we term the preinstability thickness, and identified it as a flow parameter which governs the size, spacing, and velocity of downstream liquid beads. We also performed a set of complementary numerical simulations that solves the full Navier-Stokes equations to predict the fluid dynamics of the downstream flow, such as the liquid velocity profile along the fiber.

To better understand the influence of nozzle diameter on the regime transition as well as the downstream bead dynamics, we performed a detailed theoretical study of viscous flow down a vertical fiber. We proposed a full lubrication model that includes slip boundary conditions, nonlinear curvature terms, and a film stabilization term, and compared the predicted film dynamics against the experimental results. Numerical simulations confirm that in addition to fiber sizes and flow rates, the downstream flow regime and characteristics are also significantly affected by the nozzle geometry. Moreover, the effect of film stabilization term on the flow pattern and bead characteristic is studied. We also compared our results with previously studied theoretical methods, such as CM model, linear curvature model, and full curvature model.

Additionally, we leveraged our successful demonstration of stable water flow along a vertical cotton string to construct a multi-string water vapor capturing system, where a massive array traveling water beads act as the condensation interface for water vapor in the counterflowing air stream. These water beads form through intrinsic flow instability and offer high curvature surfaces to enhance the vapor condensation rate. The effects of the water flow rate and air velocity on the condensation rates are experimentally characterized. The gas-stream pressure drop of the design is also measured. The condensation rates and gas-stream pressure drop from our multi-string dehumidifier is compared with the existing dehumidifier designs. A simplified theoretical model is also presented as the starting point for further optimizing the design parameters of our device.

Finally, we extended our investigation for potential applications of the cotton-based multi-string configuration and proposed a novel string-based particle collector. Wet electrostatic precipitators (WESP) are generally highly effective for collecting fine particles in air streams from various sources such as diesel engines, power plants, and oil refineries. However, some limiting factors, such as high water usage, poses restrictions. Our new compact particle collector utilizes an array of traveling water beads on vertical cotton strings to collect the pre-charged particles in the counterflowing air stream. The experimental and numerical investigation presented in this work is performed to determine the collection efficiency and the optimal water flow rate for our new design. The unique configuration of our string-based counterflow WESP in this study exhibits high number-based collection efficiency, > 80%, for a wide range of particle diameters, 10 nm – 2.5 m, while decreasing the water usage significantly, which can provide a basis for the design of more water-efficient WESPs.

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