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An experimental study has been performed on the dynamics of a large turbulent buoyanthelium plume. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) while helium mass fraction was determined by planar laser-induced fluorescence (PLIF). PIV and PLIF were performed simultaneously in order to obtain velocity and mass fraction data over a plane that encompassed the plume core, the near-field mixing zones and the surrounding air. The Rayleigh–Taylor instability at the base of the plume leads to the vortex that grows to dominate the flow. This process repeats in a cyclical manner. The temporally and spatially resolved data show a strong negative correlation between density and vertical velocity, as well as a strong 90° phase lag between peaks in the vertical and horizontal velocities throughout the flow field owing to large coherent structures associated with puffing of the turbulent plume. The joint velocity an mass fraction data are used to calculate Favre-averaged statistics in addition to Reynolds-(time) averaged statistics. Unexpectedly, the difference between both the Favre-averaged and Reynolds-averaged velocities and second-order turbulent statistics is less than the uncertainty in the data throughout the flow field. A simple analysis was performed to determine the expected differences between Favre and Reynolds statistics for flows with periodic fluctuations in which the density and velocity fields are perfectly correlated, but have the phase relations as suggested by the data. The analytical results agreewith the data, showing that the Favre and Reynolds statistics will be the same to lead order. The combination of observation and simple analysis suggests that for buoyancy-dominated flows in which it can be expected that density and velocity are strongly correlated,phase relations will result in only second-order differences between Favre- and Reynolds-averaged data in spite of strong fluctuations in both density and velocity.

---A series of studies is presented in which an electrical-impedance tomography (EX") system is validated for two-phase flow measurements. The EIT system, developed at Sandia National Laboratories, is described along with the computer algorithm used for reconstructing phase volume fraction profiles. The algorithm is first tested using numerical data and experimental phantom 1

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 resolved scales of fluid motion that is currently not explicitly treated in the SGS turbulence models used for the LES.

Gamma-densitometry tomography (GDT) experiments have been performed to measure gas holdup spatial variations in two bubble columns: a 0.19 m inside diameter Lucite column and a 0.48 m inside diameter stainless steel vessel. Air and water were used for the measurements. Horizontal scans at one vertical position in each column were made for several air flow rates. An axisymmetric tomographic reconstruction algorithm based on the Abel transform has been used to calculate the time averaged gas holdup radial variation. Integration of these profiles over the column cross section has yielded area-averaged gas holdup results, which have been compared with volume-averaged gas holdups determined from differential pressure measurements and from the rise in the aidwater interface during gas flow. The results agree reasonably well. INTRODUCTIONBubble-column reactors are used extensively by chemical manufacturers to perform a wide variety of gas/liquid or gas/liquid/solid reactions such as oxidation, hydrogenation, chlorination, aerobic fermentation and coal liquefaction (Shah and Deckwer, 1983). Bubble-column reactors are generally tall, cylindrical vessels filled with liquid, sometimes laden with a solid catalyst, c through which a gas is injected using a sparger at or near the bottom. The gas reacts with the liquid or catalyst to form a desired product, either a gas or a liquid, that is continuously removed from the vessel. Pressures and temperatures are controlled during the reaction to optimize product distribution. One of the main benefits of slurry-phase bubble-column reactors used in catalytic reactions is the ability of the liquid phase to provide an efficient heat sink for highly exothermic reactions. Under industrial conditions, the pressure, temperature, inlet gas velocity, and column diameter may be increased, to maximize total product production rates. The effects of increasing these parameters on the multiphase flow phenomenology must be considered when attempting to scale laboratory reactors to industrial sizes and operating conditions. Development and application of noninvasive tomographic diagnostics capable of measuring gas holdup (ratio of local gas volume to total volume) spatial distributions in full-scale reactors will greatly facilitate current efforts to predict reactor performance.Gamma densitometry has been applied for measurement of local density in multiphase flows for some time (e.g., Petrick and Swanson, 1958;Swift et al., 1978;Chan and Banerjee, 1981).Standard gamma densitometry measures gamma attenuation integrated along a path through the medium, and thus lacks spatial resolution. However, spatially resolved measurements can be made by applying tomographic reconstruction algorithms to the results of measurements along many different paths. Although gamma-densitometry tomography (GDT) can measure spatially resolved gas holdup in a gas/liquid flow, neither instantaneous gas holdup nor bubble size distributions can be measured due to the time required for data acquisition. Several groups...

Holographic and Coulter Counter detection techniques were jointly used to measure the concentration density distribution of cavitation nuclei in the ocean. Comparison of the two techniques indicates that Coulter Counter analysis measures particulate contents up to an order of magnitude smaller than indicated by the holographic method and may also produce a distorted concentration density distribution. Several possible explanations of the observed discrepancies are proposed and discussed, including fundamental differences between the in situ holographic samples and the collected samples examined with the Coulter Counter, differences between the unknown electrical conductivity of the measured particles in the sea water samples and the non-conductive polystyrene spheres used to calibrate the Coulter Counter, the rupture of aggregate particles in the flow through the Coulter Counter orifice, the effect of electronic noise on the Coulter Counter signal, and the influence of statistical sampling error

Solids flow dynamics in gas-solid risers is inherently complex. Model refinement through experimental validation requires the acquisition of detailed nonintrusive measurements. In this study, noninvasive computer-automated radioactive particle tracking (CARPT) is employed to visualize and quantify in a three-dimensional domain the solids dynamics and mixing in gassolid risers. This technique has the added advantage that, along with the derived Eulerian solids flow field (time-average velocity map and various turbulence parameters such as the Reynolds stresses, turbulent kinetic energy), it also provides directly the Lagrangian description of the solids motion. The solids velocity field data are obtained in two different risers at low and high solids fluxes at varying superficial gas velocity to span both the fast-fluidized (FF) and dilute phase transport (DPT) regimes. The effect of operating conditions on solids flow and mixing is studied. Comparative analysis of the results is presented to provide insights into the complex solids flow patterns characteristic of gas-solid risers.

Cavitation phenomena were studied in the shear layer behind a two-dimensional sharp-edged plate. Observations were made under stroboscopic light and by flash photography. The first traces of cavitation appear as a series of narrow long axial “strings” suggesting that the major contributor to cavitation inception is the longitudinal (axial) secondary shear layer eddy structure. In a more developed state the cavitation takes the form of a spanwise large eddy structure but the axial “strings” are still evident. The local cavitation inception indices based on the local velocity and pressure scatter between 1.0 and 1.4 when the water is saturated with dissolved air and vary between 0.8 and 1.2 when the air content is reduced to about 30 percent of saturation. These results are in good agreement with a collection of other measurements behind sharp-edged disks that display a consistent increase of the inception index with the Reynolds number.