Numerical Simulation of Near-Surface Environments and Particulate Clouds Generated by Large-Area Fires

摘要

The near surface environments and lofted particulate clouds produced by large area fires have been simulated with the axisymmetric DICE code. Studies with an inviscid version of the model showed that variations in the assumed area and burning rate of the fire have a strong effect on the maximum inflow winds, temperatures, and plume heights which develop. Using a multiphase version of the model, which predicts concentrations of soot, water vapor, liquid water, and ice, it was found that the particulate clouds generated by large area fires show considerable vertical dispersion, and that latent heat released by condensing water vapor can significantly enhance cloud heights. When the grid is changed from finely zoned to coarsely zoned, the effective mixing of the combustion heating over a greater depth generates cooler maximum temperatures, causing the plume to shift from oscillatory (overshooting) behavior to a steady state regime. When the mixing is explicity accounted for by introducing parameterized turbulence, steady state behavior results for the finely zoned cases as well. Using an extended version of DICE which includes a tangential velocity component, an initial swirl velocity of 10 m/sec at the edge of a 10 km radius fire was found to spin up to swirl velocities in excess of 200 m/sec in the inflow layer near the axis of the fire. Plume heights for this case, which allowed no interaction of swirl with turbulence, were about 3 km lower than a corresponding nonrotating case.