Experimental study on the lower explosion limit and mechanism of methanol pre-mixed spray under negative pressure

Equally as important as gas and dust explosions, the lower explosion limit(LEL) of spray is an important parameter for explosion prevention and control. The impact of ambient temperature(298.15–323.15 K), pressure(0.8–1.0 bar) and injection pressure(1.3–2.1 bar) on the LEL of methanol spray under negative pressure was studied. The development of a typical spray explosion accident and the mechanism of spray explosion were analyzed. Results showed that the impact of oxygen content on the LEL of methanol spray was minor and could be ignored. Under negative pressure, the LEL increased with increasing ambient pressure with a linear relationship. The increasing ambient temperature promoted the decrease of the LEL. The reduction of LEL was facilitated by the air drive at 13 bar to 17 bar. Under the combined effect of ambient pressure and injection pressure, the LEL of methanol spray varied in line with the effect of single factor. When ambient temperature and injection pressure combined, the injection pressure had less impact on the LEL, while the impact of ambient temperature took the lead. The slope of the fitting curve was greater at ambient temperatures above 308.15 K(material temperature) when ambient pressure and temperature were combined, and the LEL of methanol spray was more significantly impacted by ambient pressure. The combined effect of ambient pressure and temperature resulted in a 43.64% reduction of the LEL, much greater than the sum of the reductions caused by the single factors. Apart from the three factors of combustion, a suitable vapour-liquid two-phase concentration was needed for the spray explosion. In the initial stage, the explosion pressure increased, the rate of pressure rise decreased and the combustion between droplets spread relatively slowly. When the explosion developed fully, the rate of pressure rise increased, the pressure reached a peak. The flame brightness was extremely high. Droplet flaming continued after the termination of the explosion. The hazards of spray explosions included debris from the explosion, fireballs, shock waves, and the scalding of flaming droplets.