F. Tang

Large eddy simulation of pollutant gas dispersion with buoyancy ejected from building into an urban street canyon

The dispersion of buoyancy driven smoke soot and carbon monoxide (CO) gas, which was ejected out from side building into an urban street canyon with aspect ratio of 1 was investigated by large eddy simulation (LES) under a perpendicular wind flow. Strong buoyancy effect, which has not been revealed before, on such pollution dispersion in the street canyon was studied. The buoyancy release rate was 5 MW. The wind speed concerned ranged from 1 to 7.5m/s. The characteristics of flow pattern, distribution of smoke soot and temperature, CO concentration were revealed by the LES simulation. Dimensionless Froude number (Fr) was firstly introduced here to characterize the pollutant dispersion with buoyancy effect counteracting the wind. It was found that the flow pattern can be well categorized into three regimes. A regular characteristic large vortex was shown for the CO concentration contour when the wind velocity was higher than the critical re-entrainment value. A new formula was theoretically developed to show quantitatively that the critical re-entrainment wind velocities, u(c), for buoyancy source at different floors, were proportional to -1/3 power of the characteristic height. LES simulation results agreed well with theoretical analysis. The critical Froude number was found to be constant of 0.7.

Vertical Temperature Profile of a Buoyant Plume in an Atrium

The temperatures of buoyant fire plumes were measured vertically and compared with the Zukoski model, the Heskestad model, and the McCaffrey model. Predictions by the Heskestad and McCaffrey models were closer to the measured value with average deviation of 18.4% and 18.6%, respectively, while those by the Zukoski model were much lower than the measured value with average deviation of 36.7%. The power law decay index of z was found to increase slightly with fire size, rather than being constant in the models. The coefficient for the variation of ΔT with Q2/3 z−5/3 it was found to be 20.9 experimentally, near to that of the Heskestad and McCaffrey models.

The Application of Snapshot POD Method in Characterization and Analysis of Numerically Simulated Fire-Induced Flows

Snapshot proper orthogonal decomposition (POD) was performed on the analysis of dominant structures of fire-induced flows. The data for POD analysis was obtained from large eddy simulation (LES). Identification and analysis of dominant flow patterns have been carried out for two important types of fire-induced flows, including vertical plumes induced by pool fire and fire-induced horizontal channel flows. The essential features or energetic motions of these fire-induced flows were identified by combination of desired orders of POD modes. For the fire plumes, the counter-rotating vortex tubes were identified as the most dominant flow patterns. It is revealed that the oscillation dynamics of fire plume were related to the vortices near these vortex tubes. A larger number of small-scale structures and more structure scales were found in the fire plumes with higher Reynolds number (or higher heat release rate). For fire-induced horizontal channel flows, both the energy fractions and the structure patterns associated with POD modes depend more strongly on Reynolds number than those for fire plumes. The energy fractions contained within the most energetic modes significantly decrease with the increase of Reynolds number (or extraction flow rate) for fire-induced channel flows. It is found that the locations of strong vortices areas identified by POD mode are higher than the interface heights estimated by Janssens’ method, especially at the positions where counter flow mixing is strong.Copyright