OCR Text |
Show It is further noted from various parametric case studies that the predicted combustor exit aerothermochemical profiles are relatively insensitive to initial size distribution and composition of fuel. However, as will be seen from a close examination of Figures (7) and (8), evolution of detailed, local temperature and species distributions are substantially influenced by such design parameters. Trajectories of coal particles within the combustor are displayed in Figure (10). Particle trajectories are computed until one of three conditions prevails: i) the particle is completely burnt, ii) the particle exits the combustor, or iii) the particle impacts the wall. The fate of the particle after sticking to the wall depends on the wall boundary conditions. If the wall is slagging, then unburnt particles may be retained in the flowing slag layer and undergo char gasification. The combustion process is known to be strongly dependent on the initial size distribution of the fuel. Large coal particles (> 75 microns) dominate residence time requirements for suspension burning, and are most affectively combusted via wall burning, which provides the long residence time required. The design attempts to minimize the amount of wall burning, by providing an adequate residence in suspension for the larger particles. Conditions given in Table (1) ensure a residence time of approximately 100 msec for a 75 micron particle. 3.1 DISCUSSION It will be seen from a study of the above combustor development effort and similar ones reported in the literature that mathematical modelling of combustion processes has now established itself as a viable tool that allows the designer to converge relatively rapidly on a preliminary design, in spite of the complexity of the typically encountered flow fields, stringent 12 |