Concentration Effects on Flame Acceleration by Obstacles in Large-Scale Methane-Air and Propane-Air Vented Explosions

Abstract

An experimental study of flame and pressure development have been performed in a large scale 50 m³ obstructed tube of diameter 2.5 m and length 10 m and with one end closed and one open to the atmosphere. Homogeneous clouds of variable concentrations of cither methane/air or propane/air mixtures have been accelerated in the tube by five regularly spaced obstacles which blocked off 30% of the tube cross-sectional area. Two types of ignition sources were used to initiate the explosions. The planar ignition mode had 19 weak match heads that covered the whole of the tube cross-section at the closed end of the tube. The point source ignition mode used two weak match heads located at the centre-line, also at the closed end of the tube. In addition to the conventional pressure and flame-arrival probes, some of the tests also included hot-wire probes for flow and turbulence measurements.

Both the pressure and flame speed data showed a strong dependence with fuel/air concentrations. Maximum pressures and flame speeds were found at slightly fuel rich concentrations. The concentration limits to explosion propagation are found to be somewhat more narrow than the conventional ones. The planar ignited explosions generated twice as high peak pressures as the point ignited ones. The hot-wire measurements revealed that turbulence is much higher along the shear layers of the obstacles than along the lube centre-line.