The Behaviour Of Vertical Jet Fires Under Sonic And Subsonic Regimes

Accidental release scenarios such as flammable gas leaks or ruptures of pipes and pressurised process-equipment normally originate high velocity jets which, if ignited, give rise to jet fires. Although the direct effects of jet fires are considered to be relatively smaller than that associated to other types of fires, they are characterized by high heat fluxes and, especially if there is flame impingement, they can originate a domino effect, leading to a larger accident. A recent historical analysis has shown that in approximately 50% of the jet fire events registered in accident data bases another event with severe effects is originated. Nevertheless, jet fires are not well known, since most of the experimental work concerns small jet fires (up to 2.5 m in length), subsonic flames or flares. This communication presents the analysis of the measurements performed on relatively large-scale turbulent vertical jet diffusion flames of propane in still air (flame lengths of up to 10 m). The test conditions analyzed covered sonic and subsonic flows, the use of six different-sized interchangeable nozzles, three heat flow sensors located at different radial distances from the flame axis and the use of thermographic and video cameras, which images were used to determine the length and shape of jet fires. The results obtained show the flame length to increase with the orifice diameter and the fuel mass flow rate. The flame length under sonic and subsonic regimes can be predicted as a function of Reynolds number (Eq. (1)). The shape of the present vertical sonic and subsonic jet flames seems to correspond to a cylinder. The thermal radiation intensity (I) from jet fires decreases as the heat flow sensors’ radial distance from the flame axis increases and increases with the mass flow rate.