DYNAMICS OF GASEOUS AND SOLID FUEL FIRE WHIRLS

Abstract

Fire whirls (FWs) are swirling diffusion flames with significantly enhanced burning rates, flame heights, and flame temperatures. The swirling motion combined with a more intense flame poses a greater hazard than a normal diffusion flame. Therefore, a deeper understanding of FW dynamics for various fuels will help to identify and mitigate additional fire risks. In this work, experiments were conducted to study the dynamics of propane gas and wooden crib FWs in a square-based, fixed-frame apparatus. Simulations were performed using Fire Dynamics Simulator (FDS) for propane gas FWs to study the impact of additional parameters and analyze the predictability of FW dynamics under various conditions. Measurements of flame heights and temperature profiles were made with a low initial momentum propane burner using a variety of burner sizes and heat release rates (HRRs) under both FW and free burning (FB) conditions. The burner diameter and number of inlets on the apparatus did not significantly affect the flame stability, height, and temperature of FWs. Conversely, increasing HRR significantly increased flame height and temperature of FWs relative to FB conditions. Flame heights from FW and FB experiments were correlated using the equation proposed by Heskestad with a different fitting constant. The hood configuration, width of inlets, and height of walls on the apparatus strongly affected flame stability, height, and temperature. FDS predictions were generally in good agreement with the experimental data, but accuracy of FW simulations was found to depend strongly on grid resolution. Both FB and FW experiments were also conducted using three square-based wooden crib sizes with side lengths of 76.2, 152.6, and 228.6 mm, 6.35, 12.7, and 19.1 mm cross-section sticks, respectively, and six sticks per layer (giving heights of 38.1, 76.2, and 114.3 mm). All crib sizes were tested with n = 3–7 sticks per layer. Compared to FB conditions, FWs produced a less luminous and cleaner burning flame. As n increased, the burning rate for the 152.6 and 228.6 mm FW tests increased from 1.0 to 1.84 and 1.16 to 1.47 times that of the FB cribs for the same geometric configuration. The FW flame heights for the 152.6 and 228.6 mm cribs were 1.02 to 1.4 times greater than for FB conditions