| OCR Text |
Show Figure (3) displays a typical grid distribution (37x16x9) used for majority of the parametric cases examined, although finer grids were employed to perform limited grid dependency studies. The coal particle size distribution, adequately represented by a Rosin-Rammler expression assumes eight size ranges with a mass median particle size of approximately 30 microns. Additional relevant data regarding adopted combustor configuration and design conditions are provided in Table (1). Typical convergence histories for mass, momentum (axial velocity) and energy (enthalpy) obtained are shown in Figure (4). Using an unoptimized code, incorporating computational techniques that could be improved substantially by including recent numerical procedures, Figure (4) thus implies residual levels of approximately one percent (for mass) and corresponding typical computational times of four to five CPU hours on an Alliant FX-8 mini supercomputer. Although the coal particle size distribution (typically 80% 200 mesh grind) would be better represented by using 20 size ranges, considerable saving in computational resources is achieved by the more modest distribution adopted in this study. The problem of representing the upper 'tail' of the size distribution curve is met by placing all the larger particles in the upper size band. In practice, these particles would usually impinge on the combustor wall and burn there. The combustor configuration derived via primarily three-dimensional case studies is presently just starting to be tested and hence experimental data to be compared with predictions discussed below is for the most part lacking. However, computational results obtained to date appear Qualitatively plausible and data to be obtained during the course of the hardware development effort will be used to validate flow and coal combustion aspects of the model on an iterative basis. Furthermore, such data will aid in delineating the potential shortcomings identified in the physical and numerical models incorporated in the code; this is discussed below. 10 |
