Taylor N. Suess

Examination of near-wall hemodynamic parameters in the renal bridging stent of various stent graft configurations for repairing visceral branched aortic aneurysms

OBJECTIVE This study examined the flow behavior of four stent graft configurations for endovascular repair of complex aneurysms of the descending aorta. METHODS Computational fluid dynamics models with transient boundary conditions and rigid wall simplifying assumptions were developed and used with four distinct geometries to compare various near-wall hemodynamic parameters. RESULTS Graphic plots for time-averaged wall shear stress, oscillating shear index, and relative residence time were presented and compared among the four stent graft configurations of interest. CONCLUSIONS Abrupt 90° and 180° changes in stent geometry (particularly in the side branches) cause a high momentum change and thus increased flow separation and mixing, which has significant implications in blood flow characteristics near the wall. By comparison, longer bridging stents provide more gradual changes in momentum, thus allowing blood flow to develop before reaching the target vessel.

Experimental and Computational Model Comparisons of Thermal Profiles Within a Column Photobioreactor

In a bubble column-based photobioreactor, sparger design and placement govern the bubble size distribution and gas hold-up. These factors in turn influence flow pattern, effective interfacial area, rates of mass and heat transfer, and mixing. Previous computational studies of the hydrodynamic and heat transfer effects within a column photobioreactor for one sparger row have found that bubble Nusselt number and heat transfer coefficient with respect to superficial velocity do not follow any particular pattern. This study evaluates the temperature distribution and heat transfer within a photobioreactor in an effort to explain the earlier study results. Experimental and computational studies will focus on the bubble flow pattern and heat transfer within a rectangular column photobioreactor (33.65 cm long × 30.48 cm wide × 33.97 cm tall) with a single row sparger located either lengthwise or widthwise at the center of the base (27.94 cm long × 1.27 cm wide). Temperature distribution and heat transfer for both sparger positions will be compared. Carbon dioxide, water, light photons, algal cells, and nutrients need to come together continuously for successful algal production, hence mixing of the nutrients, algal cells, and carbon dioxide is essential. Instead of a light source, a heat source is used in the system. Constant electric energy is supplied to the heating pad, which converts the electric energy to thermal energy. Thermocouples are placed inside the PBR to record temperature at 36 different spatial positions. The experimental results are compared with previously developed CFD simulations. The sparger not only effects the aeration inside the PBR, but also creates mixing in the PBR. Proper design and placement of the sparger ensures proper mixing in the PBR. The present study shows the effects bubble movement and flow pattern have on the temperature distribution and how well the simulation predicts the temperature distribution inside a PBR. The present research is a continuum of previous work aimed at pursuing the optimum design of a column PBR which is commercially viable and effective at producing algal biofuels and bioproducts.Copyright

Predicting Hydrodynamic and Heat Transfer Effects of Sparger Geometry and Placement Within a Column Photobioreactor Using Computational Fluid Dynamics

This project investigates the effects of sparger geometry and placement on bubble and fluid flow patterns and convective heat transfer within a column photobioreactor (PBR) using Computational Fluid Dynamics (CFD). Experimental and computational studies have been completed that focused on the hydrodynamics and heat transfer within a rectangular column photobioreactor (34.29 cm long × 15.25 cm wide × 34.29 cm tall) with a single sparger located at the center of its base (33.02 cm × 1.27 cm) running lengthwise. This study extends previous work by investigating the flow patterns and heat transfer effects due to full bottom sparger or porous sparger. The gas bubbles and the water-based media within the photobioreactor are modeled using the Lagrangian-Eulerian approach. A low Reynolds k-Epsilon turbulence model is used to predict near-wall flow patterns. A flat surface photobioreactor is used to achieve sufficient light penetration into the system. The main interaction forces between the bubbles and the media, including drag forces, added mass forces, and lift forces are considered. The overarching goal of this research is to produce biofuels and bioproducts through the improved design of column PBRs used for microalgae production. An important factor in designing photobioreactors is the appropriate selection of sparger geometry and placement. The sparger governs the bubble size distribution and gas holdup. These factors in turn influence flow pattern, effective interfacial area, rates of mass transfer, heat transfer, and mixing. It is hypothesized that increasing the sparger width will improve uniformity of bubble distribution as well as mixing. Despite its importance, optimizing the sparger geometry and placement in PBRs for microalgae production is still largely not understood. In this study, the simulation’s results are presented for various spargers, which can be helpful in designing appropriate sparger geometry and proper placement for increased microalgae production.Copyright

Investigating the Structural Properties of Corn Stover Biomass

The purpose of this study is to analyze structural material properties of biomass materials, namely corn stover. The microstructure of the biomass is examined by using a nano-hardness testing machine (NANOVEA®). The goal of this analysis is to test the hardness and elasticity of individual fibers using nanoindentation and to develop testing techniques to perform this task. The results of the stated tests are statistically analyzed. The measured structural properties of the biomass have the potential to be used in computer simulations for structural analysis and bulk solid flows. The bulk fluid motion of the pulverized/chopped biomass can be simulated in storage and transportation equipment, including auguring screws and pneumatic conveyance systems, as well as devices for feeding biomass feedstocks in biorefineries. Traditional biochemical and thermochemical reactors operate as batch systems because of the difficulty of feeding the biomass feedstock in a continuous manner. Having a clearer background about the structural and rheological properties of biomass feedstock will help simulate and design the bulk-solid flows within storage bins and conveyance systems.Copyright

Shear accumulation as a means for evaluating risk of thromboembolic events in novel endovascular stent graft designs

Objective: This study proposes to establish a simulation‐based technique for evaluating shear accumulation in stent grafts and to use the technique to assess the performance of a novel branched stent graft system. Methods: Computational fluid dynamics models, with transient boundary conditions, particle injection, and rigid walls, simplifying assumptions were developed and used to evaluate the shear accumulation in various stent graft configurations with a healthy aorta as comparison. Results: Shear streamlines are presented for the various configurations. Shear accumulation was also calculated for each configuration. The number of particles with shear accumulations >3.5 Pa‐s for each configuration was compared with the shear accumulation values of commercially available mechanical aortic valves from the literature. Conclusions: The stent graft configuration with the diaphragm does have particles with shear accumulation >3.5 Pa‐s. However, the percentage of particles with shear accumulation above 3.5 Pa‐s is less than the two commercially available mechanical aortic valves, and more surprisingly, is smaller than in the healthy aorta. Clinical Relevance: This study includes some advanced computational fluid dynamic analysis. Shear accumulation, as discussed in this report, is an important predictor of thromboembolic events with mechanical valves; thus, the parameter is of interest to the United States Food and Drug Administration. To date we believe this analysis has not been performed on stent grafts and published in a peer reviewed journal. With stent graft designs becoming more complex and the anatomic variations of patients with complex aneurysms increasing, we determined it was an important method to introduce to the community of vascular surgeons.

Experimental Investigation of a Lab-Scale, Cross-Flow Grain Dryer for Testing of Drying Efficiency and Characteristic Profiles of a Packed Bed

This study investigates the drying mechanisms of corn when it is exposed to air at elevated temperature and velocity within a cross-flow packed bed dryer. A highly-instrumented laboratory-scale experimental test dryer was constructed to batch-dry samples of 0.03 m3 (1 ft3) of high moisture corn. This is achieved using a perforated wall drying chamber with forced air at temperatures ranging from 180–240°F. The high temperature, high velocity air entering the column is supplied by a variable speed fan and a variable Wattage electric heating coil through a 0.09 m2 (1 ft2) square air duct. This device is able to precisely control the drying air temperate and flow rate, while also measuring the temperature and humidity of the air exiting the dryer. In creating and instrumenting this apparatus, tests were performed to analyze both energy use and drying rate to determine the operating conditions that find a balance between energy and time requirements for moisture removal. This study used a variety of supply air temperatures and air flow rates in drying samples of corn at two initial moisture contents (19%MC and 24%MC) to 15%MC. This is done to determine if there are notable differences in energy requirements (Btu/pound water removed) between different operating conditions. This study determined that corn undergoes a significant pre-heating process before peak drying efficiency is achieved. Current grain dryer designs should focus the most energy just after that pre-heating process for highest overall efficiencies. Additionally, this study found an inverse relationship between dry time and energy efficiency, which showed that an optimum balance between those two factors should be identified.Copyright

Assessment of Pulsatile Blood Flow Models for the Descending Aorta Using CFD

The objective of this research is to develop a computational fluid dynamics (CFD) model of a healthy human aorta from the aortic arch to the femoral arteries to allow for a better understanding of blood flow characteristics in this significant vessel. The increasing number of patients suffering from vascular diseases has accelerated the research in this field. Pulsatile blood flow through the descending aorta has numerous mechanisms that influence the flow characteristics, including non-Newtonian fluid effects, transient effects of the cardiac cycle, and geometries within the aortic vessel, among others. Although CFD has been used to predict flow effects of rather complicated systems, the use of CFD in vascular flow is still largely not understood. This paper compares non-Newtonian fluid effects in the flow of a natural aorta as well as flow effects within the descending aorta, including the ostium flow diverter, which regulates blood flow from the aorta to the renal arteries and was discovered within the last five years. Utilizing Creo Parametric, a 3-dimensional representation of the aorta was created including physical portrayals of the renal, superior mesenteric, common iliac and celiac arteries. This geometry was imported, meshed, and analyzed using a commercially available CFD solver. Using fluid properties of blood previously characterized in prior research, pulsatile flow models were investigated using constant viscosity and the Carreau-Yasuda Non-Newtonian viscosity model. This research compares the Oscillating Shear Index results of the constant viscosity model versus non-Newtonian. Shear stress and velocity profiles are used to study the effects of each assumption on the flow of blood through the descending aorta. This will be done by using a scalar result of the shear stress and the calculated Oscillating Shear Index. Based on previous work, the boundary layers created at the entrance of the renal arteries should be reduced by the presence of the ostium flow diverter. The model with the ostium flow diverter is used in both simulations. Ultimately, the simulation may predict the effects of changes or interventions to the descending aorta caused by assuming constant viscosity or non-Newtonian.Copyright

Review of Flow Patterns in a Column Reactor for Photobioreactor Application

Photobioreactors (PBRs) and chemical reactors are often vertical columns with either circular or rectangular cross sections. The reactors are frequently referred to as column reactors and are treated as if they perform in the same manner. The reactors can have two different types of flow established in them regardless of cross sectional shape depending on the saprger/diffuser type and location within the reactor. The flow patterns in the reactors are induced by gas that is bubbled into the reactor volume usually near the bottom of the reactor. When the gas bubbles rise up through the reactor in a plug flow fashion, most of the mixing is in the radial direction which tends to make the reactor liquid and gas more homogeneous across the width of the reactor. The gas bubbles in the reactor may not move up through the reactor in a plug flow fashion, but may instead move vertically up through a portion of the reactor cross-section. This will establish a column of bubbles and liquid rising from the bottom of the reactor up to the surface and, in turn, induce a column(s) of liquid moving downward from the top of the reactor to the bottom. This behavior is similar to an air lift reactor which generally has walls physically dividing the upward (riser) and downward (down comer) flows. Without physical separation of the flows, the percent of cross sectional area of the reactor acting as the riser and down comer is established by the gas flow rate through the reactor, reactor cross sectional area, and the reactor volume. Velocity of flow(s) in the reactors is often based on the superficial gas velocity, which is the incoming gas flow rate divided by the gross cross sectional area of the reactor volume. This parameter may not relate to the two flows in the same manner. The two different flow patterns will be discussed in relation to superficial gas velocity, light in a PBR, chemical reactions in the reactor, and riser and down comer size.Copyright

Creating an Application to Predict Operational Characteristics and Efficiency of Continuous Cross-Flow Corn Drying

The purpose of this study is to create a computer simulation which numerically predicts the drying conditions within a continuous cross-flow grain drying system. The model is based on a system of four partial differential equations using energy and mass balances for the air, grain, and moisture within the column. This simulation includes: (1) a graphical user interface for varying the operating conditions, (2) a numerical scheme for solving the system of equations based on a backwards finite difference scheme, and (3) graphical and tabular output data. The output includes graphs of moisture content, air temperature, and grain temperature inside the column, as well as the predicted energy consumption of the system. Using this program, the grain drying model is analyzed in order to gain insight towards the optimal operating conditions for the grain dryer. The study also makes adjustments to the model in order to improve accuracy and ease of use. In particular, the Page equation for single-kernel drying is implemented. Model assumptions are also analyzed for validity, and the solutions are verified using experimental data collected in a previous study. The overall goal of this research is to improve grain dryer design and optimize operating conditions in order to reduce energy costs, improve grain quality, and increase the understanding of deep bed grain drying models.Copyright

Investigating the Structural Properties of Corn Stover at Macro and Fiber Levels

The purpose of this study is to analyze structural properties of biomass materials, namely corn stover. The structural properties of the biomass corn stover are examined at macro and fiber levels by performing a series of tests including three-point bending and tensile strength. Results of the stated tests are statistically analyzed. The goal of this analysis is to test the strength under loading from various directions to gather a full understanding of the structural properties of corn stalk fibers. Tests are performed using universal testing machines (UTMs). The results of these studies will be used to compile a database of the structural properties of biomass. These properties have the potential to be used in finite element computer simulations for structural analysis and bulk solid flows. The bulk fluid motion of the pulverized/chopped biomass can be simulated in storage and transportation equipment, including auguring screws and pneumatic conveyance systems, as well as devices for feeding biomass feedstocks in biorefineries. Traditional biochemical and thermochemical reactors operate as batch systems because of the difficulty of feeding the biomass feedstock in a continuous manner. Having a clearer background about the structural and rheological properties of biomass feedstock will help simulate and design the bulk-solid flows within storage bins and conveyance systems.Copyright