Study on flame propagation dynamics and hazard characteristics of methanol vapor cloud explosions in unconfined space

Methanol vapor clouds pose explosion risks during storage, transportation, and industrial use. However, their explosion hazard characteristics in unconfined spaces have not been systematically studied. To investigate the explosion behavior of methanol vapor clouds, an 8 m³ experimental platform was established. Explosion experiments were conducted with equivalence ratios ranging from 0.7 to 1.2 to study the flame propagation mechanism and hazard characteristics of methanol vapor cloud explosions. The results show that flame propagation in methanol vapor cloud explosions generally exhibits three stages: laminar propagation, unstable acceleration, and flame decay. The stability of the flame is jointly influenced by thermal–diffusive instability and hydrodynamic instability. Under equivalence ratios of 0.7–1.1, the critical flame radius increases, whereas the onset time of self-acceleration occurs earlier with increasing equivalence ratio. At an equivalence ratio of 1.1, the critical radius reaches 0.347 m, and the onset time of self-acceleration is only 83.47 ms, representing a 91.3% increase in critical radius and a 31.4% reduction in occurrence time compared with the condition of 0.7. Based on flame radius analysis and fractal theory, the flame self-acceleration index is determined to range between 1.2 and 1.4, providing a more accurate description of the flame acceleration process. The intensity of explosion hazards is closely related to flame dynamics. As the equivalence ratio increases from 0.7 to 1.1, both the peak overpressure and the pressure rise rate increase continuously and reach their maximum values at 1.1, with values of 5.78 kPa and 57.28 kPa/s at a distance of 1 m, respectively. Under the same condition, the thermal radiation intensity increases significantly while the response time decreases, reaching 125.63 kW/m2 and 461.8 ms at 2 m, respectively. When the equivalence ratio increases to 1.2, the combustion efficiency decreases, leading to significant reductions in overpressure and thermal radiation intensity, while the duration becomes longer. These results reveal the dominant role of flame instability and self-acceleration in methanol vapor cloud explosions and clarify the coupling relationship among equivalence ratio, flame propagation dynamics, and explosion hazard intensity. The findings provide important references for explosion risk assessment and safety design in methanol-related industrial processes.