Experimental study on flame pulsation behavior of external venting facade fire ejected from opening of a compartment

Abstract

The present study investigated experimentally the flame pulsation magnitude (ratio of maximum flame height to mean flame height; ratio of mean flame height to continuous flame height) and flame pulsation frequency of the external venting facade fire (EVF) ejected from opening of a compartment. Experiments were conducted by employing three model-scale fire compartments with an opening and a vertical facade wall. Fifteen different opening geometries were considered. For the flame pulsation magnitude of EVF, results showed that the ratio of maximum flame height to the mean flame height was about 1.30, while the ratio of mean flame height to continuous flame height was about 1.60; both of them were independent of opening geometries and heat release rates. The flame pulsation frequency of EVF increased with increasing of heat release rate, meanwhile decreased with increasing of opening size (or namely opening characteristic lengths: ℓ1= (AH1/2)2/5; ℓ2= (AH2)1/4). The measured flame pulsation frequency of EVF showed to be much lower than the predicted values by previous classic models developed for free buoyant diffusion flames, which is due to physically the inherent special exit condition (with initial horizontal convection flow of heat) at the opening for EVF. A new formula was proposed for the flame pulsation frequency of EVF in terms of a non-dimensional function between Strouhal number (St) and Froude number (Fr), using Vf∝gZf as the characteristic flame uprising velocity and ℓ1= (AH1/2)2/5 as the characteristic length of the source exit convection flow condition at the opening. The flame pulsation frequency showed to be well represented by the proposed formula. These new observations and the proposed formula provide essential knowledge for the characterization of flame pulsation behavior of external venting facade fire (EVF) ejected through the opening of a compartment.