QC 20100601

The dangerous hydraulic conditions that can form downstream of a low-head dam were investigated. These dangerous hydraulic conditions have been the cause of hundreds of drowning incidents since the construction of the first low-head dams. Two primary objectives were identified for this study, each of which were primarily performed using the Computational Fluid Dynamics software, Flow-3D®, with physical models used to verify the numerical results. The first objective was the identification of a risk factor made up of easily measured parameters that could accurately predict when the dangerous hydraulic conditions are present at a low-head dam. The risk factor that was found to achieve this objective was calculated as (hu - hd)/P, where hu and hd are the upstream and downstream water depths, respectively, and P is the dam height. For the flat-topped dams tested, the dangerous condition was present within the range of risk factors from 0.343 to 0.708. For the ogee-crested dams tested, the dangerous conditions were present between risk factors of 0.093 and 0.798. The second objective was to identify possible remediation options that would be capable of eliminating the dangerous hydraulic conditions, therefore reducing risk to the public. It was also desired to keep the options easily and inexpensively implemented. Two different remediation options were found to this end, and consisted of either upstream facing ramps spaced along the width of the channel below a low-head dam, or spaced platforms protruding from the downsteam face of the dam slightly below its crest. Three different designs of each configuration were tested, with those for the ramp configuration being identified as R1, R2, and R3. The platform designs were identified as P1, P2, and P3. The options were evaluated based on how long it took for human dummies introduced into the flow to pass through the high risk region of the simulations, with the maximum allowed time being 50 seconds. Any test in which a dummy remained in the danger region for longer than 50 seconds was deemed ineffective. The option found to perform the best was the P2 design, which had an overall performance time of about 17.4 seconds.

We present a method for anisotropic mesh refinement to high-order numerical solutions. We accomplish this by assigning metrics to vertices that approximate the error in that region. To choose values for each metric, we first reconstruct an error equation from the leading order terms of the Taylor expansion. Then, we use a Fourier approximation to choose the metric associated with that vertex. After assigning a metric to each vertex, we refine the mesh anisotropically using three mesh operations. The three mesh operations we use are swapping to maximize quality, inserting at approximate circumcenters to decrease cell size, and vertex removal to eliminate small edges. Because there are no guarantees on the results of these modification tools, we use them iteratively to produce a quasi-optimal mesh. We present examples demonstrating that our anisotropic refinement algorithm improves solution accuracy for both second and third order solutions compared with uniform refinement and isotropic refinement. We also analyze the effect of using second derivatives for refining third order solutions.

Chosen as one of six Generation‒IV nuclear-reactor concepts, SuperCritical Water-cooled Reactors (SCWRs) are expected to have high thermal efficiencies within the range of 45 ‒ 50% owing to the reactor‟s high pressures and outlet temperatures. The behaviour of supercritical water however, is not well understood and most of the methods available to predict the effects of the heat transfer phenomena within the pseudocritical region are based on empirical one-directional correlations which do not capture the multi-dimensional effects and do not provide accurate results in regions such as the deteriorated heat transfer regime. Computational Fluid Dynamics (CFD) is a numerical approach to model fluids in multidimensional space using the Navier-Stokes equations and databases of fluid properties to arrive at a full simulation of a fluid dynamics and heat transfer system. In this work, the CFD code, FLUENT-12, is used with associated software such as Gambit and NIST REFPROP to predict the Heat Transfer Coefficients at the wall and corresponding wall temperature profiles inside vertical bare tubes with SuperCritical Water (SCW) as the cooling medium. The numerical results are compared with experimental data and 1-D models represented by existing empirical correlations. Analysis of the individual heat-transfer regimes is conducted using an axisymmetric 2-D model of tubes of various lengths and composed of different nodes count along the heated length. Wall temperatures and heat transfer coefficients were analyzed to select the best model for each region (below, at and above the pseudocritical region). To neutralize effects of the rest of the tube on that region, smaller meshes were used were possible. Two turbulent models were used in the process: k-ε and k-ω, with many variations in the sub-model parameters such as viscous heating, thermal effects, and low-Reynolds number correction. Results of the analysis show a fit of ±10% for the wall temperatures using the SST k-ω model in the deteriorated heat transfer regime and less than ±5% for the normal heat transfer regime. The accuracy of the model is higher than any empirical correlation tested in the mentioned regimes, and provides additional information about the multidimensional effects between the bulk-fluid and wall temperatures. Despite the improved prediction capability, the numerical solutions indicate that further work is necessary. Each region has a different numerical model and the CFD code cannot cover the entire range in one comprehensive model. Additionally, some of the trends and transitions predicted are difficult to accept as representation of the true physics of SCW flow conditions. While CFD can be used to develop preliminary design solutions for SCW type reactors, a significant effort in experimental work to measure the actual phenomena is important to make further advancements in CFD based analysis of SCW fluid behaviour. / UOIT

For natural gas combustion, there is a large amount of experience in the gas turbine industry. However, much of the design work is based on costly combustion tests due to insufficient accuracy of existing prediction tools for data such as emissions and effects due to fuel composition. In the present work, Computational Fluid Dynamics (CFD) approach is used to study partially premixed combustion in the 3rd generation DLE (Dry Low Emission) burner that is used in SGT-700 and SGT-800 gas turbines. The fuels that are studied here are natural gas and enriched hydrogen fuel. The CFD models which are used in this work are an axisymmetric and a 3D model and the softwares are ANSYS CFX and ANSYS FLUENT. One of the main objectives of this thesis is the study of flame shape and NOx emission in hydrogen enriched combustion. In the first study of the present work, effect of adding hydrogen to non-preheated gas combustion was investigated and the results were compared with the available measurement data. Calculated laminar burning velocity with CANTERA showed a good agreement with the experimental and numerical references. Also, the accuracy of generated flamelet libraries in CFD tools to calculate adiabatic flame temperature was compared with different available tools. Results showed good agreement between available tools for the ranges of interest. In addition, flame shape and NOx prediction was studied in the gas turbine burner. Adding hydrogen to the fuel increased significantly turbulent burning velocity and OH distribution in the domain. The effect of hydrogen on the central stagnation point was studied and the simulation results did not show a significant effect on the stagnation point location. Beside the flame shape, this study showed that although the CFD NOx prediction tools in ANSYS CFX and ANSYS FLUENT predict the trend of NOx and the flame propagation in the right manner, in order to use as a reliable prediction tool in the gas turbine industry they need to be improved.

The lymphatic system plays an important role in protein and solute transport as well as the immune system. Its functionality is vital to proper homeostasis and fluid balance. Lymphatic fluid (lymph) may be propelled by intrinsic (active) vessel pumping or passively. With regard to the former, nitric oxide (NO) is known to play an important role in lymphatic vessel contraction and vasodilation. Lymphatic endothelial cells (LECs) are sensitive to shear and increases in flow have been shown to cause enhanced production of NO by LECs. Additionally, high concentrations of NO have been experimentally observed in the sinus region of mesenteric lymphatic vessels. The goal of this work was to develop a computational flow and mass transfer model using physiologic geometries obtained from confocal images of a rat mesenteric lymphatic vessel to determine the characteristics of NO transport in the lymphatic flow regime. Both steady and unsteady analyses were performed. Steady models were simulated by prescribing fully developed velocity profiles ranging from 0.5 mm s^-1 to 7 mm s^-1 as the inlet boundary conditions. Unsteady simulations were generated using a velocity profile taken from experimental data from in situ experiments with rats. Production of NO was shear-dependent; basal cases using constant production were also generated. Simulations revealed areas of flow stagnation adjacent to the valve leaflets, suggesting the high concentrations observed here experimentally are due to lack of convection in this region. LEC sensitivity was found to alter the concentration of NO in the vessel, and the convective forces were found to profoundly affect the concentration of NO at a Peclet value greater than or equal to approximately 61. The quasi-steady analysis was able to resolve wall shear stress within 0.15% of the unsteady case. However, the percent error between unsteady and quasi-steady conditions was higher for NO concentration (approximately 6.7%).

Two topics from nuclear engineering field are included in this dissertation. One study is the air-ingress phenomenon during a loss of coolant accident (LOCA) scenario, and the other is a 5-by-5 bundle assembly problem under a design of PWRs. The objectives are to investigate the Kelvin-Helmholtz instability of the gravity-driven stratified flows inside a coaxial pipe and the effects caused by two types of spacers at the downstream of the rod bundle problem. Richardson extrapolation is used for the grid independent study. Simulation results give good agreements with the experiments. Wavelet analysis and Proper Orthogonal Decomposition (POD) are used to study the flow behaviors and flow patterns. For the air-ingress phenomenon, Brunt-Vaisala frequency, or buoyancy frequency, predicts a frequency of 2.34 Hz, which is confirmed by the dominant frequency of 2.4 Hz obtained from the wavelet analysis between times 1.2 s and 1.85 s. For the rod bundle study, the dominant frequency at the center of the subchannel is given as 2.4 Hz with a secondary dominant frequency of 4 Hz and a much minor frequency of 6 Hz. Generally, wavelet analysis has much better performance than POD in the air-ingress phenomenon that is a strongly transient scenario; they both appropriate for the rod bundle study. Based on this study, when the fluid pair in a real condition is used, the time which air intrudes into the reactor is predictable.

The energy requirements of forage harvesters can be quite high and can sometimes determine the size of tractor needed on a farm. Therefore, improving the energy efficiency of the forage harvester could allow a farm to reduce costs by using a smaller tractor that is less expensive and more efficient. The objective of this research was to increase the throwing distance of a forage harvester by modeling the flow of forage in the spout and the air flow in the blower and spout. These models can then be used to compare the efficiencies of prototype designs. The air flow in the blower and spout was modeled using the commercial computational fluid dynamics software FLUENT. The simulation results of air velocities and flow patterns were compared to experimental values and it was found that both were of the same order of magnitude with the model predicting slightly higher air velocities than those measured. The flow of forage in the spout was modeled analytically by taking into account the friction between the forage and the spout surface and the aerodynamic resistance after the forage leaves the spout. From this model, two improved prototype spouts that should theoretically result in longer throwing distances were designed. However, field testing of the two prototypes did not reveal any significant improvements over the current design. It was also found that the model under-predicted the throwing distance of one prototype by 2 % and over estimated the other by 12 %.

This work presents a model of the fouling mechanism and the evaluation of compressor washing on line. The results of this research were obtained from experimental and computational models. The experimental model analyzed the localization of the particle deposition on the blade surface and the change of the surface roughness condition. The design of the test rig was based on the cascade blade arrangement and blade aerodynamics. The results of the experiment demonstrated that fouling occurred on both surfaces of the blade. This mechanism mainly affected the leading edge region of the blade. The increment of the surface roughness on this region was 1.0 μm. This result was used to create the CFD model (FLUENT). According to the results of the CFD, fouling reduced the thickness of the boundary layer region and increased the drag force of the blade. The model of fouling was created based on the experiment and CFD results and was used to calculate the engine performance in the simulation code (TURBOMATCH). The engine performance results demonstrated that in five days fouling can affect the overall efficiency by 3.5%. The evaluation of the compressor washing on line was based on the experimental tests and simulation of the engine performance. This system demonstrated that it could recover 99% of the original blade surface. In addition, this system was evaluated in a study case of a Power Plant, where it proved itself to be a techno-economic way to recover the power of the engine due to fouling. The model of the fouling mechanism presented in this work was validated by experimental tests, CFD models and information from real engines. However, for further applications of the model, it would be necessary to consider the specific conditions of fouling in each new environment.

The lattice Boltzmann method (LBM) is a relatively new method for fluid flow simulations, and is recently gaining popularity due to its simple algorithm and parallel scalability. Although the method has been successfully applied to a wide range of flow physics, its capabilities in simulating turbulent flow is still under-validated. Hence, in this project, a 3D LBM program was developed to investigate the validity of the LBM for turbulent flow simulations through large eddy simulations (LES). In achieving this goal, the 3D LBM code was first applied to compute the laminar flow over two tandem cylinders. After validating against literature data, the program was used to study the aerodynamic effects of the early 3D flow structures by comparing between 2D and 3D simulations. It was found that the span-wise instabilities have a profound impact on the lift and drag forces, as well as on the vortex shedding frequency. The LBM code was then modified to allow for a massively parallel execution using graphics processing units (GPU). The GPU enabled program was used to study a benchmark test case involving the flow over a square cylinder in a square channel, to validate its accuracy, as well as measure its performance gains compared to a typical serial implementation. The flow results showed good agreement with literature, and speedups of over 150 times were observed when two GPUs were used in parallel. Turbulent flow simulations were then conducted using LES with the Smagorinsky subgrid model. The methodology was first validated by computing the fully developed turbulent channel flow, and comparing the results against direct numerical simulation results. The results were in good agreement despite the relatively coarse grid. The code was then used to simulate the turbulent flow over a square cylinder confined in a channel. In order to emulate a realistic inflow at the channel inlet, an auxiliary simulation consisting of a fully developed turbulent channel flow was run in conjunction, and its velocity profile was used to enforce the inlet boundary condition for the cylinder flow simulation. Comparison of the results with experimental and numerical results revealed that the presence of the turbulent flow structures at the inlet can significantly influence the resulting flow field around the cylinder.