Theodore J. Heindel

A Review of X-Ray Flow Visualization With Applications to Multiphase Flows

Flow visualization and characterization of multiphase flows have been the quest of many fluid mechanicians. The process is fairly straight forward only when there is good optical access (i.e., the vessel is not opaque or there are appropriate viewing ports) and the flow is transparent, implying a very low volume fraction of the dispersed phase; however, when optical access is not good or the fluid is opaque, alternative methods must be developed. Several different noninvasive visualization tools have been developed to provide high-quality qualitative and quantitative data of various multiphase flow characteristics, and overviews of these methods have appeared in the literature. X-ray imaging is one family of noninvasive measurement techniques used extensively for product testing and evaluation of static objects with complex structures. X-rays can also be used to visualize and characterize multiphase flows. This paper provides a review of the current status of X-ray flow visualization and details various X-ray flow visualization methods that can provide qualitative and quantitative information about the characteristics of complex multiphase flows. Disciplines Complex Fluids | Engineering | Mechanical Engineering | Thermodynamics Comments This article is from Journal of Fluids Engineering 133 (2011): 074001, doi:10.1115/1.4004367. Posted with permission. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/me_pubs/20 A Review of X-Ray Flow Visualization With Applications to Multiphase Flows

Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller Configurations

This study compares the power demand and gas‐liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide‐water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the “after large cavity” region (ALC) of a Rushton‐type turbine (D/T = 0.35). It is found that the dual Rushton‐type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton‐type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton‐type impellers because the concave impeller can handle higher gas flow rates prior to flooding.

Enhancement of natural convection heat transfer from an array of discrete heat sources

Abstract Single-phase, natural convection heat transfer data have been obtained for an array of highly-finned, discrete heat sources mounted to one wall of a cavity filled with a dielectric liquid (FC-77). Dense, parallel plate fin arrays were considered for both vertical and horizontal cavity orientations, and the finned surfaces were found to enhance heat transfer by as much as a factor of 24. Thermal resistances approaching 2 cm 2 °C W −1 were realized, while maintaining the temperature difference between the fin base and an opposing cold plate below 70°C. In a parallel numerical study, flow and heat transfer conditions were calculated by treating the fin array as a porous medium characterized by the Brinkman-Forchheimerextended Darcy model. The model provided a reasonable approximation which captured the experimental trends and predicted the data to within 30%.

Conjugate natural convection from an array of discrete heat sources: part 1 — two- and three-dimensional model validation

Two- and three-dimensional (2- and 3-D) numerical models have been developed for conjugate natural convection in a discretely heated cavity. Experimental results obtained for the same geometry, using water and FC-77 as the coolants, were in excellent agreement with the 3-D numerical predictions. In contrast, because of the inability to account for thermal spreading in the lateral direction, the 2-D model overpredicted measured average surface temperatures of the discrete heat sources. However, the 2-D model still predicted general trends and flow patterns that were experimentally obtained. The nature and extent of 3-D effects on the flow and heat transfer have been delineated.

Measuring carbon monoxide gas-liquid mass transfer in a stirred tank reactor for syngas fermentation.

Carbon monoxide‐liquid mass transfer rates in a water‐filled stirred tank reactor are determined using a myoglobin protein method to measure dissolved carbon monoxide concentrations as a function of time. Data are acquired over a range of stirrer speeds (200 ≤ N ≤ 600 rpm) and gas flow rates (1 ≤ Q ≤ 6 L/min), corresponding to a gas retention time range of 1.2–7 min. Volumetric CO‐water mass transfer coefficients range from 0.003 to 0.043 s−1 and are well‐correlated using the power density and the superficial gas velocity.

Modeling flotation separation in a semi-batch process

A model for a semi-batch flotation separation process has been developed, based on available microprocess probabilities, and compared to experimental data obtained using a WEMCO laboratory flotation cell. In general, the model predicts the correct experimental trends. In many cases, the model also predicts removal efficiency very well. Parametric studies reveal that the model predictions are sensitive to a stability parameter, the turbulent energy density in the flotation cell, the contact angle between the solid particle and fluid, and the ratio of initial-to-critical film thickness.

On the structure of collision and detachment frequencies in flotation models

New explicit analytical expressions are obtained for both the collision frequency and the bubble/particle detachment frequency which enter flotation separation models. The expression for the collision frequency takes into account both the particle settling velocity and the bubble rise velocity while that for the detachment frequency is motivated by analogous results for floc disruption in a turbulent flow field. In all the cases considered, it is shown that the inclusion of the particle settling velocity increases the collision frequency by a factor of approximately 1.5 and that the most significant factor affecting the collision frequency is the bubble radius.

Laminar Natural Convection in a Discretely Heated Cavity: II—Comparisons of Experimental and Theoretical Results

Three-dimensional numerical predictions and experimental data have been obtained for natural convection from a 3 X 3 array of discrete heat sources flush-mounted on one vertical wall ofa rectangular cavity and cooled by the opposing wall. Predictions performed in a companion paper revealed that three-dimensional edge effects are significant and that, with increasing Rayleigh number, flow and heat transfer become more uniform across each heater face. The three-dimensional predictions are in excellent agreement with the data of this study, whereas a two-dimensional model of the experimental geometry underpredicts average heat transfer by as much as 20 percent. Experimental row-averaged Nusselt numbers are well correlated with a Rayleigh number exponent of 0.25 for Ra Lz ≤ 1.2 X 10 8 .

Laminar Natural Convection in a Discretely Heated Cavity: I—Assessment of Three-Dimensional Effects

Two and three-dimensional calculations have been performed for laminar natural convection induced by a 3 X 3 array of discrete heat sources flush-mounted to one vertical wall of a rectangular cavity whose opposite wall was isothermally cooled. Edge effects predicted by the three-dimensional model yielded local and average Nusselt numbers that exceeded those obtained from the two-dimensional model, as well as average surface temperatures that were smaller than the two-dimensional predictions. For heater aspect ratios A htr ≤ 3, average Nusselt numbers increased with decreasing A htr . However, for A htr ≥ 3, the two and three-dimensional predictions were within 5 percent of each other and results were approximately independent of A htr .

Conjugate natural convection from an array of discrete heat sources: part 2 — a numerical parametric study

Coupled conduction and natural convection transport within a discretely heated cavity have been investigated numerically. One vertical wall of the cavity is composed of discrete, isoflux heat sources mounted in a substrate of finite thermal conductivity. The opposite vertical wall and the horizontal walls are assumed to be isothermal and adiabatic, respectively. The governing steady-state partial differential equations for the fluid and solid region are solved simultaneously using a control volume formulation, coupled with an additive correction multigrid procedure that increases the convergence rate of the solution. The fluid Prandtl number and heater/fluid thermal conductivity ratio are fixed at 25 and 2350, respectively, corresponding to a dielectric fluid (FC-77) and heaters manufactured from silicon. With increasing modified Rayleigh number (104 < RaLz∗ < 109), the cavity flow becomes more boundary layer-like along the vertical walls, and multiple fluid cells develop in the central region. Thermal spreading in the substrate increases with decreasing modified Rayleigh number and with increasing values of the substrate/fluid thermal conductivity ratio (10−1 <- Rs ≤ 103). For large Rs, the discrete heat sources lose their thermal identity, and the streamlines and isotherms resemble those associated with a differentially heated cavity. Thermal spreading in the substrate also has a significant effect on circulation in the cavity and on maximum surface temperatures.