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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2020)

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.

    Cover page: Characteristics and Applications of Thin Liquid Films Flowing Down High-Curvature Surfaces
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2018)
    Direct-contact heat exchangers that involve energy exchange between gas and liquid streams have a variety of applications, including waste heat recovery, thermoelectric power plant cooling, and thermal desalination. Direct-contact heat exchangers are appealing as they may help mitigate potential corrosion, fouling, and scaling of solid surfaces and enhance heat transfer effectiveness. In this study, we experimentally investigate the thermohydraulic characteristics of an economic light-weight direct-contact heat exchanger that incorporates an array of strings of diameter of the order of 0.1–1 mm to sustain flows of thin liquid films. We constructed a 1.6 m-tall prototype heat exchanger with an array of as many as 112 vertically aligned strings. Thin films of a non-evaporating liquid are flown down the strings by gravity and exchange thermal energy with a counterflowing gas stream. We obtained axial liquid temperature profiles and frictional loss in the gas stream for different combinations of liquid and gas flow rates and two different string pitches. Numerical simulation is also performed to help interpret and indirectly validate our experimental results. The overall, gas-side, and liquid-side heat transfer coefficients extracted from the experimentally measured temperature profiles are examined to evaluate the impact of instability in liquid film flows and inter-bead spacing. The applicability of the Reynolds analogy is also assessed using the measured gas-stream pressure drops and air-side heat transfer coefficients. The present study helps improve our understanding of heat transfer and gas-stream pressure drop in string-based direct-contact heat exchangers and provides an experimental database to help systematically optimize their design.
      Cover page: Thermohydraulic characteristics of a multi-string direct-contact heat exchanger
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2019)
      The humidification-dehumidification (HDH) process is appealing for small-scale distributed desalination of high-salinity water. We report a new design of compact light-weight humidifiers with high effectiveness and low electric energy consumption for HDH systems. The new design consists of a dense array of vertically aligned strings, along which thin films of a heated liquid feed are allowed to flow under gravity. A counterflowing gas stream makes direct contact with liquid films carrying the generated water vapor to a dehumidifier. The unique geometric configuration of our design affords high interface-to-volume ratios necessary for high heat/mass exchange effectiveness and straight contiguous gas flow paths for reduced gas-phase pressure drops. We constructed a 0.4 m-tall prototype and examined the effects of the liquid flow rate, air velocity and feed liquid salinity on the heat/mass transfer performance and the gas stream axial pressure drop. Compared with previously reported pad humidifiers and spray columns, the present multi-string humidifier achieves almost 5 times the evaporation rate per humidifier volume at comparable gas stream pressure drops or an order of magnitude reduction in the gas stream pressure drop at comparable total evaporate rates. Our work demonstrates a highly-effective but compact and light-weight multi-string humidifier with a small gas stream axial pressure drop for HDH and related applications.
        Cover page: A highly effective multi-string humidifier with a low gas stream pressure drop for desalination
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2017)
        Thin-liquid films flowing down vertical strings undergo instability, creating wavy film profiles and traveling beads. Previous studies assumed that the liquid film thickness and velocity profiles within the healing length from a nozzle were specified by the Nusselt solution, independent of the nozzle geometry. As a result, the influence of the nozzle diameter on the flow characteristics, such as the liquid bead size, spacing, and traveling speed, was largely overlooked. We report an experimental and numerical simulation study on liquid-film flows in the Rayleigh-Plateau regime while systematically varying the nozzle diameter from 0.5 to 3.2 mm at different mass flow rates (0.02, 0.04, 0.06, and 0.08 g/s). We find that the nozzle diameter does have a strong influence on the flow regime and the flow characteristics. We identify the thickness of a nearly flat portion of a liquid film that precedes the onset of instability, which we term the preinstability thickness, as a critical flow parameter that governs the size, spacing, and frequency of liquid beads that develop downstream. By defining the liquid film aspect ratio α in terms of the preinstability thickness, we capture a flow transition from the Rayleigh-Plateau (RP) instability regime to the isolated droplet regime. Improved understanding of the flow regimes and characteristics assists in the systematic design and optimization of a wide variety of processes and devices, including fiber coating and direct contact heat and mass exchangers.
          Cover page: Effects of Nozzle Geometry on the Fluid Dynamics of Thin Liquid Films Flowing down Vertical Strings in the Rayleigh–Plateau Regime
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2017)
          Direct contact heat exchangers based on wetted string columns offer an intriguing alternative to packed beds and spray columns. We experimentally examine the flow and heat transfer characteristics of thin liquid films flowing down strings of a diameter approximately 1 mm against a counterflowing air stream. Numerical simulations are also performed to help interpret and validate our experimental results. The Rayleigh-Plateau instability caused by interplay among surface tension, gravity, and viscous forces leads to the formation of uniformly spaced drop-like liquid beads traveling down a string. The liquid mass flow rate and also the nozzle radius influence the radius and spatial/temporal frequency of liquid beads. Aerodynamic drag exerted by the counterflowing air stream deforms liquid beads. The relationship between flow characteristics and heat transfer effectiveness are examined experimentally for different combinations of the air velocities, liquid mass flow rates, and nozzle radii. We show that the liquid mass flow rate and the air velocity are two primary factors influencing heat transfer effectiveness whereas details of the liquid flow instability affect local bead-to-air heat transfer coefficients. We also compare the heat transfer effectiveness and the pressure drop between a wetted string column that consists of an array of vertical strings and a well-established structured packing that consists of vertical plates. The wetted string column is shown to deliver comparable heat transfer performance but at a lower air pressure drop than the structured packing. The present work helps improve our understanding of the flow and heat transfer performance of string-based direct-contact heat exchangers and helps build a foundation for their systematic design and optimization.
            Cover page: Experimental study of heat transfer between thin liquid films flowing down a vertical string in the Rayleigh-Plateau instability regime and a counterflowing gas stream
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2021)
            This paper presents a study on the dynamics of a thin liquid film flowing down a vertical cylindrical fibre under a streamwise thermal gradient. Previous works on isothermal flows have shown that the inlet flow and fibre geometry are the main factors that determine a transition from the absolute to the convective instability flow regimes. Our experiments demonstrate that an irregular wavy pattern and bead coalescence, which are commonly seen in the convective regime, can also be triggered by applying a thermal gradient along the fibre. We develop a lubrication model that accounts for gravity, temperature-dependent viscosity and surface tension to describe the thermal effects on downstream bead dynamics. Numerical simulations of the model show good agreement between the predicted droplet coalescence dynamics and the experimental data.
              Cover page: Thermally-driven coalescence in thin liquid film flowing down a fibre
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2024)
                Cover page: A positivity-preserving numerical method for a thin liquid film on a vertical cylindrical fiberCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2021)
                Wet electrostatic precipitators (WESP) have been widely studied for collecting fine and ultrafine particles, such as diesel particulate matter (DPM), which have deleterious effects on human health. Here, we report an experimental and numerical simulation study on a novel string-based two-stage WESP. Our new design incorporates grounded vertically aligned polymer strings, along which thin films of water flow down. The water beads, generated by intrinsic flow instability, travel down the strings and collect charged particles in the counterflowing gas stream. We performed experiments using two different geometric configurations of WESP: rectangular and cylindrical. We examined the effects of the WESP electrode bias voltage, air stream velocity, and water flow rate on the number-based fractional collection efficiency for particles of diameters ranging from 10 nm to 2.5 μm. The collection efficiency improves with increasing bias voltages or decreasing airflow rates. At liquid-to-gas (L/G) as low as approximately 0.0066, our design delivers a collection efficiency over 70% even for fine and ultrafine particles. The rectangular and cylindrical configurations exhibit similar collection efficiencies under nominally identical experimental conditions. We also compare the water-to-air mass flow rate ratio, air flow rate per unit collector volume, and collection efficiency of our string-based design with those of previously reported WESPs. The present work demonstrates a promising design for a highly efficient, compact, and scalable two-stage WESPs with minimal water consumption.Implications: Wet Electrostatic Precipitators (WESPs) are highly effective for collecting fine particles in exhaust air streams from various sources such as diesel engines, power plants, and oil refineries. However, their large-scale adoption has been limited by high water usage and reduced collection efficiencies for ultrafine particles. We perform experimental and numerical investigation to characterize the collection efficiency and water flow rate-dependence of a new design of WESP. The string-based counterflow WESP reported in this study offers number-based collection efficiencies >70% at air flow rates per collector volume as high as 4.36 (m3/s)/m3 for particles of diameters ranging from 10 nm - 2.5 μm, while significantly reducing water usage. Our work provides a basis for the design of more compact and water-efficient WESPs.
                  Cover page: Experimental study of a string-based counterflow wet electrostatic precipitator for collection of fine and ultrafine particles
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