Turbulent diffusion combustion under conditions of limited ventilation: Flame projection through an opening

Development of turbulent diffuse flame in a compartment with a vertical opening is studied experimentally and numerically. Flame projection through the opening observed under conditions of limited natural-convective ventilation is considered. The measurements are performed in a laboratory box designed for compartment-fire simulation. The critical (minimum) flow rate of the fuel sufficient for flame projection is determined, as well as the delay between fuel ignition and flame projection with subsequent establishment of external combustion. Dimensionless variables for processing experimental data are proposed. A generic empirical dependence of the dimensionless time of flame projection on the dimensionless flow rate of the fuel is obtained for various opening sizes, burner positions, and box sizes. The dimensionless critical flow rate of the fuel obtained is in agreement with the previously published measurements performed for gaseous and condensed fuels. Unsteady stages of flame evolution before the projection and scenarios of flame projection through the opening are identified and analyzed. A three-dimensional numerical model is developed for calculating turbulent diffusion combustion in a compartment with an opening. The model takes into account the conjugate radiative-convective heat transfer on solid surfaces and the thermal conductivity of the wall and floor materials. The experimentally observed stages of flame development, flame projection through the opening, and stabilization of external combustion are reproduced in numerical calculations. The numerical values of flame-projection time are in good agreement with the measurement results and proposed empirical relation.