Flame propagation behaviors and temperature characteristics in polyethylene dust explosions

Abstract

To reveal the flame propagation mechanism in polyethylene (PE) dust explosions, the flame propagation behaviors and temperature characteristics of polyethylene dust clouds were experimentally studied in an open duct. Flame propagations in polyethylene dust clouds with different concentrations and particle size distributions were recorded using high-speed photography. The flame temperatures were measured using two fine thermocouples comprising Pt–Pt/Rh13% wires of diameter 25 μm. Because of severe agglomeration, the minimum explosible concentration of polyethylene particles with diameter < 75 μm was higher than that of the particles with diameter 75–100 μm. It was observed that the flame front gradually became continuous as the particle size increased with the same polyethylene dust concentration of 300 g/m3. With higher polyethylene dust concentration, the flame front of the polyethylene particles of size <75 μm could quickly become discrete, and the floating flame appeared early, while the flame front of the 100–212 μm polyethylene particles transformed from a continuous flame into several independent diffusion flames. It was demonstrated that the flame propagation velocities were not constant, and that they fluctuated owing to the turbulence and expansion of the combustion products. The average flame propagation velocity increased initially, and then decreased with increasing dust concentration. The peak velocity of the polyethylene particles of size <75 μm was 4.79 m/s at a mass density of 300 g/m3, while the peak velocity of the 75–100 μm particles was 4.55 m/s at a mass density of 400 g/m3, and that of the 100–212 μm particles was 2.67 m/s at a mass density of 500 g/m3. In addition, it was found that the maximum flame temperatures of the different polyethylene particles were approximately the same; the temperatures were 1585.4 °C, 1511.8 °C, and 1508.4 °C for the polyethylene particles of sizes of 100–200 μm, <75 μm, and 75–100 μm, respectively. This phenomenon may be caused by the combustion behavior of the molten polyethylene particles. When the dust concentration is increased within the optimum concentration, flame temperature increased with the increase in flame propagation velocity. The flame temperature of the polyethylene particles of size <75 μm decreased as the flame propagation velocity decreased, while the flame temperature of the 75–100 μm polyethylene particles did not change significantly.