OCR Text |
Show range from adequate representation of turbulence-particle interaction to the excessive computational time and storage required to incorporate realistic chemical kinetics to predict pollutant concentrations simultaneously with other variables. The task of incorporating turbulent particle diffusion into the model requires evaluations of various possible approaches. A particular difficulty with the present formulation is that the fluid properties are constant within a cell, while the particles are tracked through a cell in a series of time steps smaller than the cell dimensions. Thus, the particles at several time step positions within a cell experience only one set of mean flow properties. Of the two basic approximate techniques for simulating turbulent particle diffusion, namely the random walk method and the diffusive drift method, the latter appears to be more appropriate for "fully-coupled" computations of particle combustion modelling. However, both approaches assume an isotropic turbulence field. An aspect of practical significance with this particular combustor configuration concerns particle-wall interactions and associated slag transport processes. Specifically, the fate of a particle hitting the wall depends on the wall boundary conditions, i.e., if the walls are slagging, the particle is captured by the slag, flow with the slag and in the process react via pyrolysis or a char burning mechanism. Since the present model does not resolve the specific details of wall slag flow, two limiting cases of particle-wall interaction are considered. In the first case it is assumed that any particle hitting the wall is completely lost from further consideration, while the second case assumes that a particle hitting the wall completely decomposes into gaseous fuel at constant temperature. The latter ; process is generally approached in the limit of extremely hot and sticky slag 15 |