Timothy John O'Hern

Gamma-densitometry tomography of gas holdup spatial distribution in industrial-scale bubble columns

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. Two-phase air/water flows were examined. 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 holdup results, which have been compared with volume-averaged gas holdups determined from differential pressure measurements and from the rise in the air/water interface during gas flow. The results agree reasonably well.

Validation of Electrical-Impedance Tomography for Measurements of Material Distribution in Two-Phase Flows

A series of studies is presented in which an electrical-impedance tomography (EXT) 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 measurements, with good results. The EIT system is then applied to solid-liquid and gas-liquid flows, and results are compared to an established gamma-densitometry tomography (GDT) system. In the solid-liquid flows, the average solid volume fractions measured by EIT are in good agreement with nominal values; in the gas-liquid flows, average gas volume fractions and radial gas volume fraction profiles from GDT and EIT are also in good agreement.

A heuristic model of turbulent mixing applied to blowout of turbulent jet diffusion flames

Abstract A phenomenological study has been conducted on jet flames near blowout for the purpose of determining the blowout mechanism. The authors show the successful blowout correlation of Broadwell et al. [ Twentieth Symposium (International) on Combustion , 1984, p. 303] can be derived from the assumptions of Vanquickenborne and van Tigglen [ Combust. Flame 10:59 (1966)], namely, that blowout is a competition between the local premixed turbulent flame speed and the local flow velocity. The authors argue that the role of coherent, large-scale, rotational structures found in jet turbulence is to enhance the turbulent flame speed near blowout. Experiments were conducted which show that nearly the entire cross-section of the jet is combusting in a premixed flame near blowout. This is distinct from a lifted flame that combusts only near the outer edge of the jet. The length and time scales used in the derivation of the blowout mechanism are compared with those observed in the experiments and found to be consistent with the data.

A Turbulence Model for Buoyant Flows Based on Vorticity Generation

A turbulence model for buoyant flows has been developed in the context of a k-{var_epsilon} turbulence modeling approach. A production term is added to the turbulent kinetic energy equation based on dimensional reasoning using an appropriate time scale for buoyancy-induced turbulence taken from the vorticity conservation equation. The resulting turbulence model is calibrated against far field helium-air spread rate data, and validated with near source, strongly buoyant helium plume data sets. This model is more numerically stable and gives better predictions over a much broader range of mesh densities than the standard k-{var_epsilon} model for these strongly buoyant flows.

Wavefront sensors for optical diagnostics in fluid mechanics: application to heated flow, turbulence and droplet evaporation

Optical measurement techniques are extremely useful in fluid mechanics because of their non- invasive nature. However, it is often difficult to separate measurement effects due to pressure, temperature and density in real flows. Using a variation of a Shack-Hartmann wavefront sensor, we have made wavefront measurements that have extremely large dynamic range coupled with excellent sensitivity at high temporal and spatial resolution. These wavefront variations can be directly related to density perturbations in the fluid. We have examined several classes of flow including volumetrically heated gas, grid turbulence and droplet evaporation.

Freshwater Algae Floc Structure in a Shear Flow.

Flocculation is a promising method to overcome the economic hurdle to separation of algae from its growth medium in large scale operations. However, understanding of the floc structure and the effects of shear on the floc structure are crucial to the large scale implementation of this technique. The floc structure is important because it determines, in large part, the density and settling behavior of the algae. Freshwater algae floc size distributions and fractal dimensions are presented as a function of applied shear rate in a Couette cell using ferric chloride as a flocculant. Comparisons are made with measurements made for a polystyrene microparticle model system taken here as well as reported literature results. The algae floc size distributions are found to be self‐preserving with respect to shear rate, consistent with literature data for polystyrene. Three fractal dimensions are calculated which quantitatively characterize the complexity of the floc structure. Low shear rates result in large, relatively dense packed flocs which elongate and fracture as the shear rate is increased. The results presented here provide crucial information for economically implementing flocculation as a large scale algae harvesting strategy. Biotechnol. Bioeng. 2013;110: 3156–3163.

Size and structure of Chlorella zofingiensis/FeCl3 flocs in a shear flow

Flocculation is a promising method to overcome the economic hurdle to separation of algae from its growth medium in large scale operations. However, understanding of the floc structure and the effects of shear on the floc structure are crucial to the large scale implementation of this technique. The floc structure is important because it determines, in large part, the density and settling behavior of the algae. Freshwater algae floc size distributions and fractal dimensions are presented as a function of applied shear rate in a Couette cell using ferric chloride as a flocculant. Comparisons are made with measurements made for a polystyrene microparticle model system taken here as well as reported literature results. The algae floc size distributions are found to be self‐preserving with respect to shear rate, consistent with literature data for polystyrene. Three fractal dimensions are calculated which quantitatively characterize the complexity of the floc structure. Low shear rates result in large, relatively dense packed flocs which elongate and fracture as the shear rate is increased. The results presented here provide crucial information for economically implementing flocculation as a large scale algae harvesting strategy. Biotechnol. Bioeng. 2013;110: 3156–3163.

Diagnostics for liquid dispersion due to a high-speed impact with accident or vulnerability assessment application

The high-speed impact and subsequent dispersion of a large liquid slug is of interest for assessing vulnerability of structures when subjected to such an event. The Weber number associated with such liquid impacts is generally between 105 and 108. Because of the experiment scale and destructive nature of these high-energy impacts, most traditional diagnostics are difficult to implement. Therefore, unique diagnostics were employed in several tests to gather information on impact force, spreading instability, slug break-up, ejection velocity, droplet deformation and spray characteristics. Measurement techniques discussed here include high-speed photometrics, particle image velocimetry (PIV), TrackEye particle analysis, speckle correlation, single-pass schlieren imaging, phase Doppler particle analyzer (PDPA) and load cell measurements as applied to large-scale, high-speed liquid impacts.

Quantitative tomographic measurements of opaque multiphase flows

An electrical-impedance tomography (EIT) system has been developed for quantitative measurements of radial phase distribution profiles in two-phase and three-phase vertical column flows. The EIT system is described along with the computer algorithm used for reconstructing phase volume fraction profiles. EIT measurements were validated by comparison with a gamma-densitometry tomography (GDT) system. The EIT system was used to accurately measure average solid volume fractions up to 0.05 in solid-liquid flows, and radial gas volume fraction profiles in gas-liquid flows with gas volume fractions up to 0.15. In both flows, average phase volume fractions and radial volume fraction profiles from GDT and EIT were in good agreement. A minor modification to the formula used to relate conductivity data to phase volume fractions was found to improve agreement between the methods. GDT and EIT were then applied together to simultaneously measure the solid, liquid, and gas radial distributions within several vertical three-phase flows. For average solid volume fractions up to 0.30, the gas distribution for each gas flow rate was approximately independent of the amount of solids in the column. Measurements made with this EIT system demonstrate that EIT may be used successfully for noninvasive, quantitative measurements of dispersed multiphase flows.

Particle-Image Velocimetry Investigation of an Oscillating Turbulent Channel Flow

Particle-Image Velocimetry is used to study the cyclic modulation of the wall shear stress and turbulence properties of an oscillating channel flow. The PIV instrument employed here utilizes a dynamically adjusted delay between the laser pulses to accommodate the wide variations in velocity encountered in the oscillating flow. Both high- and low-magnification digital PIV recordings are obtained to reveal the near-wall boundary layer structure and wall shear stress, as well as the full-field turbulence throughout the channel. We present wall-shear-stress and global turbulence data for Stokes-thickness Reynolds numbers of Reδ = 1220, 2033, and 2875. The results reveal a fully developed turbulent state, relaminarization, and an explosive transition back to turbulence. The flow is examined in detail for the case at Reδ = 1220, where instantaneous PIV realizations at low magnification reveal the structure of the flow during relaminarization and transition back to turbulence. High-magnification PIV results are used to reveal the phase modulation of the mean velocity profiles in the viscous sublayer and logarithmic layers through the half cycle and quantitative profiles of in-plane Reynolds stresses and turbulence production are presented. To our knowledge, this is the first PIV investigation of this canonical unsteady turbulent channel flow and these results represent a needed contribution to the limited turbulence data which exists for unsteady wall flows.