Influence of length-diameter ratio and volume on hydrocarbon explosion overpressure characteristics in a closed square pipeline

In order to effectively predict and control the consequences of fuel-air mixtures explosions in enclosed spaces, and thereby reduce the casualties and property losses caused by accidents, this study had investigated the relationship between the explosive overpressure characteristics of fuel-air mixtures and the spatial scale of explosions. The experiment carried out closed square pipes with varying length-to-diameter ratios, volumes and lengths to examine the impact of fuel-air mixtures explosive overpressure characteristics by keeping the initial oil and gas concentration, ignition position, and ignition energy constant. The experimental results showed that the rate of overpressure rise goes through three stages, a rapid increase period, a continuous oscillation period and an attenuation termination period, which reveals the dynamic relationship between reaction rate and heat loss. Under the same volume (21.2L), with the increase of length-diameter ratio (from 7.1 to 35.7), the maximum overpressure decreased by 47.5% (from 472.9kPa to 248.5kPa), and the average overpressure rise rate decreased by 73.5% (from 1359kPa·s-1 to 360kPa·s-1), and the maximum overpressure rise rate reduced by 66.4% (from 3204kPa·s-1 to 1078kPa·s-1), and the explosion power lessened by 86.2% (from 643 (kPa)2/ms to 89 (kPa)2/ms), which is attributed to the change in the nozzle area (from 207cm2 to 71cm2) affecting the reaction rate and the change in the internal surface area of the pipeline (from 629cm2 to 1022cm2) affecting the heat loss. Further analysis of the experimental results reveals that the changes in the nozzle area affecting the flame front area and reaction rate directly, impacts on the maximum overpressure more directly and significantly. While, the changes in the inner surface area has a relatively indirect effect on the maximum overpressure by regulating energy transfer and heat loss. Additionally, pipeline length is a crucial factor affecting the time to reach maximum overpressure. The increase of the pipeline not only increases the heat loss, but also delays the superposition time point of the reflected wave and the incident wave, with the energy of the reflected wave undergoing relatively attenuation.