Jagannath R. Nanduri

Assessment of RANS-Based Turbulent Combustion Models for Prediction of Emissions from Lean Premixed Combustion of Methane

Reynolds-Averaged Navier-Stokes (RANS) simulations of Lean Premixed Combustion (LPC) of methane–air in a bluff-body stabilized combustor were performed with several widely used turbulent combustion methodologies in order to assess their prediction capabilities. The methods employed are the Eddy Dissipation Concept (EDC), the Composition Probability Density Function (CPDF) and the Joint Velocity–Frequency-Composition PDF (VFCPDF) models. Where needed, two different models were employed for turbulent transport closure, namely the Renormalization Group (RNG) k-ϵ and Reynolds Stress Transport (RSM) models. The combustion chemistry was represented by two separate augmented reduced mechanisms (ARM9 and ARM19) in order to assess the influence of chemical mechanisms on calculations. Mean temperature and major species predictions of all of the employed methodologies compared well with the experimental data. Intermediate and emission species predictions were sensitive to the resolution of turbulence viscosity, which changes the effective diffusivity of the species. NO emissions predictions were in error by an average ±5 ppm with the EDC models and the CPDF model, with the VFCPDF model showing a somewhat better prediction of NOx. Calculations for some intermediate species (especially H2) deviated qualitatively from the experimental data, which highlights some of the limitations of these methodologies commonly used in detailed prediction of emissions for various fuel blends.

A ONE DIMENSIONAL MATHEMATICAL MODEL FOR URODYNAMICS

Millions of people in the world suffer from urinary incontinence and overactive bladder with the major causes for the symptoms being stress, urge, overflow and functional incontinence. For a more effective treatment of these ailments, a detailed understanding of the urinary flow dynamics is required. This challenging task is not easy to achieve due to the complexity of the problem and the lack of tools to study the underlying mechanisms of the urination process. Theoretical models can help find a better solution for the various disorders of the lower urinary tract, including urinary incontinence, through simulating the interaction between various components involved in the continence mechanism. Using a lumped parameter analysis, a one-dimensional, transient mathematical model was built to simulate a complete cycle of filling and voiding of the bladder. Both the voluntary and involuntary contraction of the bladder walls is modeled along with the transient response of both the internal and external sphincters which dynamically control the urination process. The model also includes the effects signals from the bladder outlet (urethral sphincter, pelvic floor muscles and fascia), the muscles involved in evacuation of the urinary bladder (detrusor muscle) as well as the abdominal wall musculature. The necessary geometrical parameters of the urodynamics model were obtained from the 3D visualization data based on the visible human project. Preliminary results show good agreement with the experimental results found in the literature. The current model could be used as a diagnostic tool for detecting incontinence and simulating possible scenarios for the circumstances leading to incontinence.Copyright

Dispersion Study in a Giant Intracranial Aneurysm Using Computational Fluid Dynamics Techniques

The formation and growth of intracranial aneurysms is partly attributed to various hemodynamic factors such as shear stress and pressure. This issue is further investigated in the present paper using some new insight gained from passive scalar dispersion in the blood flow. Intracranial aneurysms are best visualized using selective catheter angiogram technique where a contrast agent added to the blood flow is visualized and filmed dynamically. In the current study, the 2D threshold images produced by “3D rotational X-ray angiography technique” are used in an ‘in-house’ program to construct a 3D volumetric grid which is then used to calculate the blood flow through the aneurysm. Dispersion of a fluid material with properties similar to that of the contrast agent is also modeled in the blood flow. Unlike previous studies, we investigated the dispersion of a pulsed scalar to better understand the flow dynamics. Results from a large aneurysm are compared to those for a relative small one to show the effect of geometry. The dispersion and flow patterns show the localization of high stress in stagnation areas, which may be the potential regions for the origin and growth of aneurysms. We also investigated the dependence of flow properties on the initial and boundary conditions to calculate the pressure and the wall shear stress values for a given geometry.© 2007 ASME

Application of Journal of Fluids Engineering CFD Verification Procedure to Flow Past Bluff Body Using Two Different Turbulence Models

The ASME Journal of Fluids Engineering (JFE) recently adopted the grid convergence index (GCI) method as one of the methods among others to estimate the discretization error in CFD applications. The GCI method (based on Richardson extrapolation (RE)) is applied to CFD simulations of confined flow past a bluff-body and is used to evaluate the performance of two different turbulence models in predicting the flow. Four different grid densities (coarse, fine, finer and finest) are used in a 2D axisymmetric geometry along with the RNG k-e model and the Reynolds stress transport (RST) turbulence models to predict the flow. A spline extrapolation (SE) method for the approximate error is also used to estimate the extrapolated value of the flow variables. The two models show monotonic and oscillatory convergence in various regions of the flow field. Results from the two turbulence models show different convergence behaviors. The results from the RE method as well as the SE method are assessed by comparing to the solution obtained on the finest grid as well as experimental data. The relative advantage of each of the methods is discussed. This study is a contribution towards testing the procedure proposed by JFE.Copyright

Application of Reactive Molecular Dynamics to Simulate Diffusion and Reaction in a Solid Oxide Fuel Cell Pore

In a typical solid oxide fuel cell (SOFC), the kinetics of the gas phase reactions in the porous anode and electrochemical reactions at the triple phase boundary are generally unknown. Due to the unavailability of non-destructive experimental methods, factors affecting the performance of SOFC systems, especially the loss in performance due to contaminants, are usually deduced from many days of experiments. In this paper a Reactive Molecular Dynamics (ReMoDy) model based on collision theory is introduced and applied to simulate the behavior of species inside a SOFC pore. Using novel simulation methods, algorithms and visualization techniques ReMoDy has the ability to simulate chemical reactions involving tens of millions of molecules and determining the thermo-physical properties of the fluids from intermolecular energies and forces. In the current work two cases of molecular dynamics simulation inside a micro pore were analyzed. In the first case diffusion of hydrogen molecules was studied inside a 0.03125 μm3 cube. The diffusion coefficients obtained from this simulation are compared to the ones obtained using Chapman-Enskog correlations. In the second case gas phase and surface reactions were modeled for Syngas oxidation in a 1 μm3 cube representing a SOFC electrode pore. For this case detailed gas phase and surface reaction mechanisms involving 13 species and 63 reactions is included. Future studies will include the calculation of diffusion coefficients, rates of formation of different species, and comparison with published data. The results can be used for the verification of continuum models.Copyright

Direct Simulation of Mass Transport in a SOFC Cathode Using Microchannels

Fuel cells are clean and efficient power generation devices which are being widely investigated under the efforts to reduce the impact of greenhouse gases on the environment. Solid oxide fuel cells (SOFCs), especially, are suitable for stationary power generation using a wide range of alternative fuels. Performance of a SOFC strongly depends on the mass transport inside the porous electrodes which are essentially composed of a network of microchannels. In this study the mass transport inside a SOFC cathode is studied using direct simulation of mass transport in microchannels along with statistical analysis. A virtual cathode is built using microchannels that are representative of continuous flow paths between the cathode/air stream interface and cathode/electrolyte interface of a SOFC. Different representative microchannel flow paths are built with varying tortuosity and channel diameters. The numbers of channels of each kind are chosen according to a normal distribution and they are randomly arranged in an appropriately sized cuboid to construct a unit block of the virtual cathode. The normal distribution is modulated with average and standard deviation values for real world electrodes found in literature. Microchannels are tightly packed to achieve the desired porosity. Mass transport in each of the channels is studied separately using commercial CFD software FLUENT. Three dimensional simulations of momentum and specie transport equations (for oxygen and nitrogen) are performed. The results from individual channel simulations are used to assess the global mass transfer characteristics of the virtual cathode. Results obtained using this approach will be compared with those from a continuum Fick’s law type diffusion model used to simulate mass transport in porous media. The primary objective is to test the assumptions employed within the context of continuum mass transport model.Copyright

Quantification of Discretization Error in Wall Shear Stress Calculations for Aneurismal Flows Using CFD

Many recent studies suggest that hemodynamic factors such as wall shear stress (WSS) and pressure contribute to the genesis and growth of intracranial aneurysms. Recently there have been a number of computational hemodynamics studies that calculate the values of wall shear stress in arterial and aneurismal flows. However there is a lack of comprehensive error analysis in many of the computational hemodynamics studies. This is perhaps the reason for speculative and ambiguous conclusions drawn by various studies as to the nature of wall shear stress responsible for aneurysm growth. In the current study, geometry involving an actual aneurysm is built from angiogram images. Another geometry consisting of the primary artery where the aneurysm formed is also built by removing the aneurysm volume. The two geometries are meshed using three different grid densities. Second order schemes are used to simulate the pulsatile hemodynamics through each of the geometries. Various representative planes along the geometries are considered and the major flow variables and WSS are plotted as a function of grid densities. The procedure for estimation of discretization error, suggested by ASME Journal of Fluids Engineering, is applied at various representative locations along the aneurysm and arterial geometry. The results suggest high dependence of calculated WSS on local grid density. The contours of WSS in the arterial geometry suggest that high WSS does not necessarily occur at the location where the aneurysm originated. Possible remedies are suggested so that this uncertainty could be eliminated from future studies.Copyright