ACERC
Abstracts - 1990 |
ACERC
Abstracts - 1990 |
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Thrust Area 5: Comprehensive Model Development |
Smith, P.J.; Sowa, W.A.
and Hedman, P.O.
Combustion and Flame, 79 (2), 111-121, 1990. Funded by Brigham Young
Unviersity.
A new design methodology is presented that allows for the use of comprehensive coal combustion codes in design applications and provides a priori information on the cost of the optimization. A statistical response surface methodology is used to determine appropriate sample points from the design space at which the computation for the comprehensive code is performed. Statistical regression analysis is used to provide interpolation functions for the optimization process. The optimum design point is then checked with a final comprehensive code calculation. The technique is demonstrated with simple examples for the design of two injectors for an entrained coal gasifier and a burner for a pulverized coal combustor. The three designs demonstrate the method as well as showing significantly different optima for different configurations. The importance of specifying operating conditions independently for different injectors or burners is demonstrated.
Hobbs, M.L. and Smith, P.J.
Fuel, 69, 1990. Funded by Utah Power & Light and Tennessee Valley Authority.
A simple two-zone zero-dimensional combustion model that estimates the influence of impurities in the fuel on the radiative energy transport has previously been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. These parameters have been obtained experimentally by others. The predictions from the two-zone model have been compared with predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
Stephenson, M.B.; Christiansen,
H.N. and Benzley, S.E.
In proceeding of Electronic Imaging West, 1990. Funded by Brigham Young
University Engineering Computer Graphics Laboratory.
As the cost of computational power decreases, more and more algorithms appear which model physical processes using digital computers rather than physical scale models or experiments. Many of these algorithms use discrete approximation methods to solve a continuous problem. The complexity of these algorithms has increased also, as computational power has increased, from hundreds of data points to a million or more, and from two dimensions to three.
Many engineering applications suffer from a lack of adequate presentation graphics that makes the interpretation of the analytical data, described above, difficult. Trends and patterns are more easily observed in graphical form when compared to reviewing tabulated data. In engineering applications, color and shading may be used to realistically portray an object and also to present additional information about it through the use of distorted shapes and color coding to classify such functions as temperature, pressure, species concentrations, etc.
The programs, described in the following sections, address the issue of visualization of finite element and finite difference data. They reflect the effort of numerous researchers over a period of approximately twenty years.
Meyers, R.J. and Stephenson,
M.B.
In proceedings of Visualization, 1990. Funded by ACERC.
An algorithm is presented for rendering scalar field data that reduces rendering times by as much as two orders of magnitude over traditional full resolution images. Less than full resolution sampling of the scalar field is performed using a fast ray tracing method. The sampling grid points are output as a set of screen based Gouraud shaded polygons that are rendered in hardware by a graphics workstation. A gradient-based variable resolution algorithm is presented which further improves rendering speed. Several examples are presented.
Brewster, B.S.; Smoot, L.D.;
Solomon, P.R. and Markham, J.R.
Tenth Annual Gasification and Gas Stream Cleanup Systems Contractors Review
Meeting, Morgantown, WV, 1990. (Also Presented at the Western States
Section/The Combustion Institute, San Diego, CA, 1990). Funded by Morgantown
Energy Technology Center and Advanced Fuel Research Co.
An advanced 2-D model for pulverized-coal combustion has been modified and applied to a laminar coal flame in a transparent wall reactor. Modifications were made to allow for the up-fired flow configuration, laminarization, and gas buoyancy. A laminarization extension to the k- turbulence model was incorporated. Particle dispersion is sensitive to laminarization and to the value of turbulent particle Schmidt number. Predicted particle velocity and residence time are sensitive to the inclusion of gas buoyancy, which increases the velocity in the center of the reactor and induces a radial, inward flow. Model predictions have been compared with flame measurements to evaluate the comprehensive model that incorporates an advanced devolatilization submodel. Predicted velocities of burning particles agree well with values determined from particle streaks on video recording. Good agreement was also obtained between measured and predicted particle burnout. Discrepancies between measured and predicted particle and gas temperature may be due to neglecting heterogeneous formation of CO2 and the variation of char reactivity with extent of burnout. Discrepancies between predicted bulk gas temperature and measured CO2 gas temperature in the ignition zone can also be explained by the fact that the combustion energy first heats the CO2 that subsequently heats the other gases. Soot decays more slowly than predicted from equilibrium concentrations of condensed carbon.
Ma, K.-L. and Sikorski,
K.
Transputer Research and Applications, 4:46, (D.L. Fielding, ed.), IOS
Press, 1990. Funded by National Science Foundation and ACERC.
This paper describes a distributed algorithm for solving the three-dimensional unsteady compressible Navier-Stokes (N-S) equations, and its implementation on the IMS T800 transputer array. Numerical experiments indicate that the algorithm offers the ultimate promise of supercomputer performance on relatively low-cost distributed memory parallel computers. In addition, we have developed a scientific visualization system, which converts generic scientific data into graphical forms. This visualization system allows us to observe our simulations with ease.
Sikorski, K.
Conference for Solution of Systems Linear Equations: State of the Art,
Italy, September 1990. Funded by the University of Utah and ACERC.
We consider the problem of approximating fixed points of contractive functions with using of the absolute error criterion. We describe two algorithms for solving this problem. The Fixed Point Envelope algorithm is the most efficient method for approximating fixed points in the univariate case. In the multivariate case it is impossible to essentially improve the efficiency of the simple iteration whenever the dimension of the domain of contractive functions is large. For a moderate dimension we exhibit a Fixed Point Ellipsoid algorithm which is much more efficient than the simple iteration for mildly contractive functions. This algorithm is based on the ellipsoid construction of Kchachiyan used for solving the linear programming problem. We list some numerical tests of the Ellipsoid algorithm.
Gillis, P.A. and Smith,
P J.
Twenty-third Symposium (International) on Combustion, The Combustion
Institute, France, 1990. Funded by Consortium and ACERC.
A three-dimensional gaseous combustion and computational fluid dynamics model is presented for simulating reacting flow in industrial furnaces and utility boilers. Data were obtained for non-reacting flow in both a tangential-fired and a wall-fired furnace. These two cases were simulated with a variety of grid resolutions to establish grid-independent solution requirements. A differencing scheme was used which assured that the numerical solution was exact for linear basis functions on an arbitrarily spaced mesh. This accuracy was demonstrated by comparing numerical results with analytic solutions of the fully coupled equation set. Several variations of the SIMPLE algorithm were incorporated into the flow model to study the importance of velocity/pressure coupling. These variations included SIMPLE, SIMPLER, SIMPLEC, SIMPLEST, and combinations of these algorithms. The robustness and speed of the SIMPLE-based methods were evaluated for a corner-fired furnace and a wall-fired furnace. The importance of temporal fluctuations due to fluid turbulence on the non-linear mixing in gaseous combustion was quantified for corner-fired furnace geometries and compared with similar studies on laboratory scale furnaces.
Hobbs, M.L.; Radulovic,
P.T. and Smoot, L.D.
Twenty-third Symposium (International) on Combustion, The Combustion
Institute, Pittsburgh, PA, 1990. Funded by Morgantown Energy Technology
Center through Advanced Fuel Research Co.
A one-dimensional model of countercurrent, fixed-bed gasification has been developed and predictions have been compared to experimental data obtained from a large-scale gasifier. The study-state model considers separate gas and solid temperatures, partial equilibrium in the gas phase, variable bed void fraction, coal devolatilization based on chemical functional group composition, oxidation and gasification of residual char with an ash layer, and axially variable solid and gas flow rates. Predictions are compared to experimental data from an atmospheric, dry-ash Wellman-Galusha gasifier for carbon conversion, effluent gas composition and temperatures, and axial profiles of temperature and pressure for a high volatile bituminous coal. The relative importance of the char oxidation resistances, bulk film diffusion, ash diffusion and surface reaction, are identified. For the cases examined, chemical resistance dominates in the cool regions at the bottom and top of the reactor while ash diffusion resistance competes with chemical resistance through most of the reactor. The importance of adequate treatment of devolatilization, gas phase chemistry, and variable bed void fraction is identified.
An accurate initial estimate of the effluent composition and temperature from a two-zone, partial equilibrium submodel was essential for efficient solution of this highly nonlinear fix-bed model. This initial estimate considers devolatilization, partial equilibrium of volatile gases, treatment of a large number of gas phase species, and tar production with potential for recirculation of effluent products. It has been shown that the submodel is adequate by itself for reliable predictions of effluent gas compositions. Effluent gas estimates from the submodel compared favorably to measured effluent temperatures and compositions from a high-pressure, dry-ash Lurgi gasifier in Westfield, Scotland for four American coals.
The importance of treating various chemical and physical processes in fixed-bed gasifiers with sufficient detail has been addressed with emphasis on coal devolatilization, char oxidation, gas phase chemistry, and bed void fraction. Calculations have shown that devolatilization in fixed-bed reactors is not an instantaneous process but is an intimate part of the overall fixed-bed process. Similarly, oxidation and gasification do not occur in separate zones, but simultaneously in certain regions of the reactor bed. Competition between endothermic gasification reactions and exothermic oxidation is evident in broad predicted and measured temperature peaks. Detailed gas phase chemistry was necessary to predict the features of temperature and concentration profiles. Variable bed void fraction was also necessary to accurately predict pressure drop, varying bed velocity, and temperature and concentration profiles.
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