Deflagration-to-detonation transition in natural gas–air mixtures

Abstract

The gas explosion test facility (GETF) previously used to study detonability of natural gas (NG)–air mixtures was modified for studies of flame acceleration and deflagration-to-detonation transition (DDT). The 73-m-long by 1.05-m-diameter tube was equipped with 15 baffles of varying blockage ratio (BR)=0.13, 0.25, or 0.50, placed near the closed end of the tube and spaced 1.52-m apart. The remaining part of the tube was smooth. Experiments used mixtures between 5.1% and 15.0% NG–air.Ignition was achieved in NG–air mixtures over the composition range 6.1–14.1%. After passing the 15 baffles, both flame and pressure wave velocity were more than 300m/s over this range. Flame velocity was increasing over the range 6.2–12.8% NG–air, and it reached the sound speed in the burned gases (800–1000m/s) over the composition range 8.0–10.8% NG–air. Pressure wave velocity was increasing over the composition range 6.1–14.1% NG–air and had reached sonic velocity over the composition range 6.2–12.6% NG–air. Shock waves with magnitude greater than 1MPa were measured in all tests over the composition range 6.5–12.4%. DDT within the baffled section of the tube and sustained detonations beyond the baffles in the smooth part of the tube were observed over the composition range 8.0–10.8% NG–air. The observed run-up length to sonic flame velocity normalized by the tube diameter, Xru/D, ranges from 16 to 23 at BR=0.13, 10 to 21 for BR=0.25, and 13 to 21 for BR=0.50. The observed run-up length to DDT normalized by the tube diameter, XDDT/D, ranges from 19 to 23 at BR=0.13, and 16 to 23 for BR=0.25 and 0.50.Coal mine safety regulations in the US require mine seals to resist an explosion pressure–time curve that rises instantaneously to 0.8MPa and remains at that level for 4s. Pressure–time curves measured in these experiments show that shock waves with near-instantaneous rise time and magnitude greater than 1MPa can develop from weak spark ignition after passing 15 turbulence-generating obstacles in test mixtures ranging from 6.5% to 12.4% NG–air.