Turbulent flame propagation and transition to detonation in large fuel-air clouds

Abstract

The results of a series of flame acceleration tests performed in a top-vented channel, 1.8m× 1.8 m in cross-section and 15.5 m long, with repeated obstacles are reported. The tests were performed in order to assess the potential for flame acceleration and transition to detonation following an accidental spill of gaseous fuel in simulated industrial environments with repeated obstacles. Tests were performed with acetylene, propane and hydrogen sulphide fuels mixed with air. For near stoichiometric acetylene-air, the flame accelerates continuously as it propagates down the channel reaching speeds between 180 and 400 m/s prior to the occurrence of localized explosions which trigger the onset of detonation. The behavior of flames in lean acetylene (5.2% C2H2)-, propane- and hydrogen sulphide-air mixtures is much less dramatic. The observed flame speeds range from about 25 m/s up to 200 m/s, with associated peak overpressures typically less than 50 mbar. The continuous flame acceleration observed in stoichiometric acetylene-air is not observed in these mixtures. Based on these results it is concluded that the potential for flame acceleration and transition to detonation in the move open areas of a chemical plant is much smaller than in the heavily confined areas. The fact that flames in stoichiometric acetylene-air accelerate and produce detonations in these obstacle environments shows that the potential for damaging explosions does exist. However, higher levels of confinement, stronger ignition sources or denser obstacle configurations than in the present tests are required to produce such explosions in less sensitive mixtures. The results of numerical simulations of the turbulent flame propagation are also reported and compared with the observed results. These simulations successfully describe many of the observed phenomena. In particular, the flame acceleration in acetylene-air mixtures and the absence of similar flame acceleration in propane- and hydrogen sulphide-air mixtures are predicted.