Chunhua Bai

Explosion and flame characteristics of methane/air mixtures in a large-scale vessel

In this study, experiments of explosions and flame characteristics in methane/air mixtures are performed in a 10‐m3 vessel. Pressure gauges and a high‐speed camera are utilized to record the pressure trajectories and the flame propagation process of ignition growth. The experimental results show that the maximum value of overpressure and the maximum rate of the explosion pressure rise are 0.596 MPa and 1.82 MPa/s for the methane (9.5% in volume)/air mixture at atmospheric conditions, respectively. Both values are higher than for other mixtures with different compositions. The results also indicate that the overpressure from the large‐scale vessel in this study is lower than that of a smaller apparatus (e.g., 5‐L closed cylindrical vessel). This difference occurs due to the cooling effect and because the reflected sonic disturbances by the vessel wall affect the explosion process and weaken the energy during the pressure attenuation stage, thus rendering the value of overpressure in the large‐scale apparatus lower than in the tiny cylindrical vessels. The maximum overpressure is observed at 0.75 m for C = 7% (“C” means the methane concentration) and 9.5% but at 1.3 m for C = 5%, 6.5%, 11.2%, and 13%. These results indicate that methane/air is an easier means to generate overpressure and that the overpressure is higher near the stoichiometric condition. Based on the analysis of the flame propagation process, the mean value of the flame speed of methane (C = 9.5%)/air is calculated to be approximately 2.43 m/s because the nonuniformity of the chemical reaction at the flame front results in a maximum fluctuation of flame speed of approximately 28.5%. The flame thickness (θ) of methane (C = 9.5%)/air fluctuates between 9.84 and 10.95 mm, with a mean value of 10.53 mm.

Influences of the Cloud Shape of Fuel-Air Mixtures on the Overpressure Field

This paper presents an experiment system in the open field, which comprises a charge structure (approximately 166.2 kg), a high-speed camera subsystem, and a pressure measurement subsystem. Through a series of experiments under the cylindrical clouds with different diameters, heights, and diameter-to-height ratios ( : ), the influences of various cloud shapes on the overpressure field were analyzed and discussed. Based on the experimental results, the overpressure field was divided into two zones: detonation wave zone and shock wave zone. It is found that the overpressure of shock waves at the same distance from the explosion center increased with the diameters, but the variations of heights had little impact on the overpressure. Therefore, the pancake-shaped cloud of fuel-air mixtures is the optimal shape for obtaining the wider overpressure field. Moreover, it is found that the overpressure field gets the maximum under the diameter-to-height ratios of 5.7 in the same distance within the studied range.