Timothy J. O’Hern

Large eddy simulation and experimental measurements of the near-field of a large turbulent helium plume

Large eddy simulations (LES) are conducted of a large, 1 m in diameter, turbulent helium plume. The plume instability modes and flow dynamics are explored as a function of grid resolution with and without the use of subgrid scale (SGS) models. LES results reproduce well-established varicose puffing mode instabilities as well as secondary “finger-like” azimuthal instabilities leading to the breakdown of periodically shed toroidal vortices. Simulation results of time-averaged velocity and concentration fields show excellent agreement with experimental data collected from Sandia’s FLAME facility using particle image velocimetry and planar laser induced fluorescence measurement techniques. For locations very near the base of the plume, i.e., X/Dp<0.5, the LES overpredicts the measured root-mean squared streamwise velocity and concentration and, in addition, is found to be highly sensitive to grid resolution. The cause of these discrepancies is attributed to unresolved buoyancy-induced vorticity generation on ...

Bubble oscillations and motion under vibration

Bubbles under vibration can behave in unusual ways, e.g., moving downward against the force of buoyancy. While the bubble downward motion due to the Bjerknes force is well known at acoustic frequencies close to the bubble resonant frequency, these experiments demonstrate that these effects can be observed at relatively low frequencies as well. Experiments were performed in a thin, quasi-two-dimensional rectangular acrylic box partially filled with 20-cSt PDMS silicone oil with overlying ambient air. The apparatus was subjected to sinusoidal axial vibration that produced breakup of the gas-liquid free surface, producing liquid jets into the air, droplets pinching off from these jets, gas cavities in the liquid from impacts of these droplets, and bubble transport below the interface. Vibration conditions for the attached videos are 280 Hz frequency, 15 g acceleration, and 94 micron peak-to-peak displacement. Behaviors shown in the videos include the following. 1. Free surface breakup into jets and droplets, and formation of bubbles under the free surface. 2. Bubbles thus generated moving downward in the cell. 3. Bubbles attracted to the first bubble deep in the cell and eventually merging to form a large bubble at the base of the cell. 4. Bubble cluster at the base of the cell merging to form a larger bubble, which stabilizes at a levitated location below the free surface and acts to damp out the surface breakup. 5. The levitated bubble interface and its breakup are similar to the free surface breakup into jets and droplets, but the jets in the bubble are facing downward. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energys National Nuclear Security Administration under contract DE-AC04-94AL85000.

An Experimental Investigation of the Effect of Walls on Gas-Liquid Flows Through Fixed Particle Beds

The effect of particle diameter on downward co-current gas-liquid flow through a fixed bed of particles confined within a cylindrical column is investigated. Several hydrodynamic regimes that depend strongly on the properties of the gas stream, the liquid stream, and the packed particle bed are known to exist within these systems. This experimental study focuses on characterizing the effect of wall confinement on these hydrodynamic regimes as the diameter d of the spherical particles becomes comparable to the column diameter D (or D/d becomes order-unity). The packed bed consists of polished, solid, spherical, monodisperse particles (beads) with mean diameter in the range of 0.64–2.54 cm. These diameters yield D/d values between 15 and 3.75, so this range overlaps and extends the previously investigated range for two-phase flow. Measurements of the pressure drop across the bed and across the pulses are obtained for varying gas and liquid flow rates.Copyright

Multiphase particle-in-cell simulations of flow in a gas-solid riser.

The commercial computational fluid dynamics (CFD) code Arena-flow is used to simulate the transient, three-dimensional flow in a gas-solid riser at Sandia National Laboratories. Arena-flow uses a multiphase particle-in-cell (MP-PIC) numerical method. The gas flow is treated in an Eulerian manner, and the particle flow is represented in a Lagrangian manner by large numbers of discrete particle clouds with distributions of particle properties. Simulations are performed using the experimental values of the gas superficial velocity and the solids mass flux in the riser. Fluid catalytic cracking (FCC) particles are investigated. The experimental and computed pressure and solid-volume-fraction distributions are compared and found to be in reasonable agreement although the experimental results exhibit more variation along the height of the riser than the computational results do. An extensive study is performed to assess the sensitivity of the computational results to a wide range of physical and numerical parameters. The computational results are seen to be robust. Thus, the uncertainties in these parameters cannot account for the differences between the experimental and computational results.Copyright

Incorporating Research Into Capstone Design Projects

Mechanical Engineering Capstone projects traditionally involve design and fabrication of a piece of hardware needed to meet the specifications of an industrial sponsor. Such projects provide an opportunity for the students to apply their classroom knowledge to a practical project and to interact and collaborate with a motivated sponsor. Howard University and Sandia National Laboratories have collaborated for the past seven years on developing research-focused projects from a national laboratory that are appropriate for Capstone design student teams. The students are exposed to a research environment and learn how to use their designs to perform experiments that can acquire high-quality data. As part of these problem-based learning projects, the students employ various Computer Aided Engineering (CAE) tools available as part of their design work, build apparatus, acquire data, and perform data analysis. The projects are focused on design, but lead to an experimental apparatus that is tested, leading to students’ experience with equipment such as data acquisition systems, high-speed cameras, image analysis, evaluation of experimental uncertainty, and comparing data with models. Two example projects will be presented in this paper, one focused on development of apparatus for testing flocculation of small particles, and another on developing a vibration test platform for experiments on bubbles under vibration.Copyright

Behavior of Levitated Bubbles Under Vibration

Gas bubbles in liquid generally rise due to buoyancy but can be forced to move downward or to be stably levitated by subjecting the liquid to vibration (Jameson, 1966; Hashimoto and Sudo, 1980; Leighton et al., 1990; Brennen, 1995; Ellenberger and Krishna, 2007). We have performed experiments in a quasi-two-dimensional test cell in which the motion of bubbles can be observed and measured. This paper presents observations and data regarding the generation and motion of levitated bubbles when the vibration conditions are varied.While bubble downward motion due to the Bjerknes force is well known at acoustic frequencies close to the bubble resonant frequency, these experiments, like those of Ellenberger and Krishna (2007), demonstrate that such effects can be observed at relatively low frequencies as well. Experiments were performed primarily in one of several thin, quasi-two-dimensional rectangular acrylic boxes partially filled with polydimethylsiloxane (PDMS) silicone oil or deionized water with ambient air above. The apparatus was subjected to sinusoidal axial vibrations for frequencies of 100–300 Hz, displacements up to 200 microns and accelerations up to 35 times standard gravity. Bubbles generated either by direct injection deep in the cell or by free-surface breakup into jets and droplets can, under appropriate vibration conditions, move downward in the test cell, where they are attracted together and merge to form a large coalesced bubble at the base of the cell. That large bubble can then rise until it reaches a location of stable levitation. The bubble damps free surface breakup above it. Under some vibration conditions, the levitated-bubble interface breakup is similar to the free surface breakup into jets and droplets.© 2013 ASME

Experimental Investigation of Selective Withdrawal and Light Layer Entrainment of Stratified Immiscible Liquids

The withdrawal of fluid from only one layer of a vertically stratified immiscible fluid system is often referred to as selective withdrawal. For a given withdrawal location, the ability to predict the maximum flow rate at which fluid from one layer can be withdrawn before an adjacent layer of fluid is also entrained is critical for many applications. The Strategic Petroleum Reserve (SPR) offers one such example where oil is stored above an underlying brine layer in large underground caverns. When oil is added to a cavern, a corresponding volume of brine must be withdrawn through a hanging string that extends through the oil into the brine layer. If the depth of the string below the oil-brine interface is insufficient for a given flow rate, oil may be inadvertently withdrawn along with the expected brine — potentially introducing oil into the brine handling system and leading to costly cleanup to prevent environmental contamination. Laboratory experiments with two immiscible liquids (silicone oil and water or brine) have been conducted to investigate this behavior for typical SPR cavern conditions. In these experiments the higher density fluid is withdrawn through a tube below the liquid-liquid interface. As the withdrawal point is raised closer to the interface for a given flow rate, or the flow rate is increased for a given submergence, the overlying lower density layer begins to entrain along with the higher density liquid. The critical withdrawal depth at which transition to light layer entrainment occurs is measured for a given flow rate of the lower liquid, and the process is repeated for different flow rates. Most prior literature concerning the transition from selective withdrawal has examined removal of the lower density fluid and the transition to entraining the higher density fluid, whereas this work focuses on the inverse. Experiments were performed for a range of different light layer silicone oils and heavy layer water or brine, covering a range of density and viscosity ratios. Three separate withdrawal tubes of differing diameter were placed in two orientations to establish the depth at which selective withdrawal began as a function of fluid properties. The data show that, at the highest flow rates, the transition to light layer entrainment can occur when the withdrawal point is up to two diameters below the liquid-liquid interface, depending on the lower fluid density. Particle Image Velocimetry (PIV) was performed to map the instantaneous velocity field. The strength of the velocity vectors increased dramatically near the withdrawal tube opening showing the region in which inertial forces dominated the flow pattern.Copyright