Mehrdad Shahnam

Radiation modelling in oxy-fuel combustion scenarios

Coal combustion was simulated in a lab-scale furnace and a full scale utility boiler at air-burn, oxy-combustion with dry- and wet-flue gas recycles. Good agreement was obtained between the numerical predictions and experimental measurements. The study confirmed that certain dry and wet recycle ratios imitate the temperature and heat transfer characteristics found in air combustion. The performances of five grey models to predict the radiative properties of gases in the simulations were examined. Emissivity correlations developed for purely CO2 media were not found to be suitable to predict the property at low H2O/CO2 ratios encountered during dry-recycle. Therefore, a new total emissivity correlation and a weighted-sum-of-grey-gases (WSGG) model were formulated to address the shortcomings and inaccuracies in existing grey gas models under oxy-firing. The new WSGG model with 4 band intervals was found to perform well in test cases that were representative of air- and oxy-firing conditions in boilers.

Validation and Application of a CFD-Based Model for Solid Oxide Fuel Cells and Stacks

The National Energy Technology Laboratory (NETL) has developed a solid oxide fuel cell (SOFC) model based on commercial computational fluid dynamics (CFD) software. This new tool is being used to support the US DOE Solid State Energy Conversion Alliance Fuel Cell Program, which will require advanced fuel cell designs in order to meet the program goal of reaching

Radiation Modeling in Oxy-Fuel Combustion Scenarios

400/kW for small (∼5kW) systems. The NETL model combines a special SOFC electrochemical model with an electrical potential field model in the finite-volume commercial CFD code from Fluent Incorporated (Lebanon NH). Mass and energy sources and sinks resulting from the electrochemical reactions and electrical current flow are coupled to the fluid flow, chemical species transport, heat transfer, porous media flow, and gas phase chemistry capabilities available in the base CFD model. The NETL SOFC model has also been recently extended to model SOFC stacks with cells connected in electrical series. The model is able to predict detailed, spatially resolved current flow through the electrolyte and through all conducting media in three-dimensional SOFC cells and cell stacks. In conjunction with the SOFC model development program, NETL has an experimental facility in place to generate data for validation of the SOFC model. The experimental program includes collaboration with the University of Utah, a supplier of test specimens and preliminary cell performance data. Well-characterized SOFC test specimens are being tested in the NETL fuel cell test stands for single cell and short-stack arrangements. Anode-supported cells with controlled electrode microstructures, electrode thickness, and electrolyte thickness are being tested. Operating data from the test stands includes cell and stack polarization curves, temperature data, and chemical composition of reactant streams. Using NETL and University of Utah data, an extensive validation program is now underway for the NETL SOFC model. The model is being tested using a simple button-cell configuration. A parametric study of varying operating conditions, cell geometries and cell properties is being performed. Good agreement between predicted and measured cell performance has been observed and is presented. The model has also been applied to planar single cell and cell stack configurations to help in the design of NETL experimental test facilities.Copyright

Coupled CFD and Process Simulation of a Fuel Cell Auxiliary Power Unit

Three gray models for the radiative properties of gases were examined for their usage in oxy-combustion simulations of a full scale boiler with flue gas recycle. Fully coupled computational fluid dynamic (CFD) simulations of a full scale boiler were carried out employing the weighted-sum-of-gray-gases model (WSGGM) at air burn, dry-recycle and wet-recycle conditions. The resulting thermal and composition fields were then frozen and the radiative properties of the gaseous media recomputed employing the Exponential Wide Band Model (EWBM) and correlations for total emissivities of gas mixtures. It is shown that when high CO2 /H2 O ratios were encountered within the boiler such as in dry-recycle scenarios, employing emissivity correlations developed for purely CO2 media within the models can result in incorrect gas properties. The errors associated with this can be significant when there are large pockets within the furnace where the gas radiation dominates the particle radiation.© 2009 ASME

Code Verification for Multiphase Flows Using the Method of Manufactured Solutions

A high-temperature auxiliary power unit (APU) based on solid oxide fuel cell (SOFC) technology is analyzed in this study using coupled computational fluid dynamics (CFD) and process simulation. The tightly integrated process flowsheet consists of a reformer, desulfurizer, SOFC stack, combustor, and various heat exchange and rotating equipment items. A detailed three-dimensional CFD model is used to represent the cross-flow, planar SOFC. Process simulations are used to calculate the overall material and energy balances. Coupled CFD and process simulations are performed over a range of fuel cell currents to generate a voltage current curve and analyze the effect of current on fuel utilization, power density, and overall system efficiency. The fuel cell APU system considered here generated 4.3 kW of power and yielded a maximum fuel-to-electricity conversion efficiency of 45.4% at a current of 18 amperes. Integrated CFD and process simulations provide a better understanding of the fluid mechanics that drive overall performance and efficiency of fuel cell systems. In addition, the analysis of the fuel cell using CFD is not done in isolation but within the context of the whole APU process.Copyright

A Study of the Electromedics Autotransfusion System, CRADA PC93-010, Final Report

Code verification is the process of ensuring, to the degree possible, that there are no algorithm deficiencies and coding mistakes (bugs) in a computational fluid dynamics (CFD) code. In order to perform code verification, the Method of Manufactured Solutions (MMS) is a rigorous technique that can be used in the absence of exact solution to the problem. This work addresses major aspects of performing code verification for multiphase flow codes using the open-source, multiphase flow code MFIX which employs a staggered-grid and a modified SIMPLE-based algorithm. Code verification is performed on 2D and 3D, uniform and stretched meshes for incompressible, steady and unsteady, single-phase and two-phase flows using the two-fluid model of MFIX. Currently, the algebraic gas-solid exchange terms are neglected as these can be tested via unit-testing. The no-slip wall, free-slip wall, and pressure outflow boundary conditions are verified for 2D and 3D flows. A newly-developed curl-based manufactured solution for 3D divergence free flows is introduced. Temporal order of accuracy during unsteady calculations is also assessed. Techniques are introduced to generate manufactured solutions that satisfy the divergence-free constraint during the verification of the incompressible governing equations. Manufactured solutions with constraints due to boundary conditions as well as due to divergence-free flow are derived in order to verify the boundary conditions. Use of staggered grid and SIMPLE-based algorithm for numerical computations in MFIX requires specific issues to be addressed while performing MMS-based code verification. Lessons learned during this code verification exercise are discussed.Copyright

CFD-DEM study of effect of bed thickness for bubbling fluidized beds

This report describes the work at PETC to evaluate flow dynamics in the Electromedics autotransfusion system. First, a literature survey was conducted for flow studies in centrifuge systems. Although no flow studies were identified for Latham-type bowls, pertinent literature for general centrifugal separation was found and reviewed. Sample measurements were taken with a Laser Doppler Velocimetry (LDV). The data indicates that LDV is a useful tool in flow analysis. Velocity, turbulence intensity, and bowl vibration are all accurately measured with LDV. For optical imaging of particle separation it is necessary to use fluorescent doped particles and color separation. This allows each type of particle to be observed in a mixture. A market survey was completed and sources for fluorescent dyed particles of three different emission wavelengths and corresponding optical bandpass filters were identified.