OCR Text |
Show ters has been successfully used in the past, for example, by Enomoto et al [5] to predict heat transfer in a furnace using measured gas concentrations and temperatures. In the present particle behavior calculations, the particle is tracked through the imposed temperature and oxygen concentration fields. Reaction is calculated point by point using the relevant values of the imposed field parameters. By this procedure, the predicted behavior of single particles in the turbulent flow is followed, giving the particle path, the changes in particle temperature along the path, the changes in volatile loss, and the changes in particle diameter to burn-out. Examining the behavior of single particles of different sizes, injected into the flow and followed individually, we show that the particle trajectories vary substantially with particle size and injection location. The consequence is that particles can pyrolyze and burn out at very different rates, with different temperature histories, depending on the size and input location. Our particular focus is to analyze the effects of fluid flow fluctuations on the behavior of particles followed individually through the flame. 2 Model In the present model the flow field is computed, and temperature and oxygen fields are then imposed, using profiles measured in an experimental furnace. Consequently, the trajectory of the entering particle is dictated by the fluid flow computations, whereas local gas temperature and oxygen concentration fields, which govern the particle reaction, are given by the imposed fields. The physically based system for the model is a cold-air jet issuing from a burner of radius r with 2r / d= 0.35 into a hot furnace of radius d/2, with particles of 5, 10, 50, and 100 J.Lm diameter introduced at radial locations 2r/d = .1, .2, and .3 in the burner, for a total of twelve different trajectories through the furnace. With the fluid velocity field determined by direct solution of the incompressible time-dependent N-S equations, the trajectories of individual particles are determined by solving the particle momentum equation, considering only the drag force due to the local relative fluid velocity. Particles of different sizes intra- 3 |