Application of FDS to under-ventilated enclosure fires with external flaming

Numerical simulations are conducted to investigate the accuracy of the fire dynamics simulator (FDS 6.0.1) for under-ventilated enclosure fires with external flaming. The accuracy is discussed first in terms of mass balance at the steady-state stage for the enclosure volume. The required fineness of the grid is determined by analyzing the mass balance using two non-dimensional length scales. The first considers the ratio of the ventilation factor to the grid cell size, and the second considers the ratio of the hydraulic diameter of the opening to the grid cell size. When these two length scales are larger than 10, the corresponding mass balance error as obtained from post-processing the output is lower than 4%. The simulation results, including flow through the vertical opening, heat release rate inside the enclosure, gas temperature inside the enclosure, neutral plane height, and external flame height are compared with experimental data and empirical correlations. The air inflow rate through the opening is found to correlate linearly to the ventilation factor as m˙=0.41*A*H^(1/2). For the heat release rate inside the enclosure, the predictions follow the empirical correlation Qin=1131*A*H^(1/2). This is directly related to the air inflow rate and incomplete combustion with the inflowing oxygen. Time-averaged gas temperatures inside the enclosure are under-predicted by maximum 13.1% compared with experimental data at the corner near the opening. Neutral plane height values at the opening, determined from the velocity profile of the vent flow, show good agreement with empirical estimations (Zf=0.4⋅H). Two methods are employed to determine the external flame height, namely a temperature based method and a volume heat release rate based method. The trends are captured correctly.

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