An experimental study of jet fires in pits

Abstract

The explosion of underground pipelines could cause a big crater in which a gas leakage occurs to induce a large jet fire. The pit wall would significantly affect the leaked gas flow and the subsequent jet fire behavior. This paper presents a systematical investigation on such jet fire in a pit (JFP). A facility consisting of a jet fire apparatus and a rectangular pit, was designed for the experimental study. Phenomenon observation shows that the JFP can be divided into the impinging jet flame (IJF), transitive jet flame (TJF) and jet flame ejected from the pit top (JFEPT), due to the variation of the mode of air entrained into the pit with the nozzle exit velocity. A dimensionless heat release rate (HRR) coupling the pit geometrical property is developed to correlate the flame area in different pits, which clarifies the critical dimensionless HRR to distinguish the IJF, TJF and JFEPT. Using the dimensionless HRR, data fitting can well develop the correlation for the flame length of TJF and JFEPT, and the correlation is well validated by the small-scale and full-scale jet fires in irregular cuboid pits. The dimensionless HRR also can be used to well correlate the flame width. The radiative fraction of IJF still follows the law of the Froude number at the nozzle exit, while that of JFEPT should be predicted using the Froude number at the top opening of pit. As indicated by the variations of air entrainment mode and radiative fraction, the flame buoyancy gradually plays a more significant role than the nozzle exit momentum in the flame behavior, as the nozzle exit velocity increases to cause the evolution of JFP from IJF to JFEPT. In addition, a line source radiation model and two solid radiation models are proposed to predict the thermal radiation of JFP.