Influence of obstacles in coal mine roadways on smoke flow characteristics

Underground coal mine fires generate smoke that spreads along roadways, and its flow state is easily affected by internal obstacles, forming local smoke stagnation zones, increasing the accumulation risk of toxic gases, and seriously threatening the safe evacuation of personnel and rescue decision-making. Existing studies mainly focus on the effects of roadway structural variations or ventilation systems, while systematic investigations on the influences of obstacle geometric shape, height, and distance are lacking. To address this issue, this study used the Particle Image Velocimetry (PIV) experimental method to investigate the effects of three key factors—obstacle shape, obstacle height, and the distance between the obstacle and the fire source—on the evolution of recirculation vortices and velocity distribution. The results showed that the fixed geometric leading edge of a square obstacle forced strong flow separation of smoke at sharp corners, forming structurally stable and large-scale recirculation vortices, with a maximum reverse smoke velocity of −0.027 m/s and a recirculation vortex height of 184 mm, both significantly higher than those of a circular obstacle. Therefore, compared with circular obstacles, square obstacles posed a higher risk due to their stronger smoke retention capacity and were therefore considered to require priority consideration in ventilation and evacuation design. An obstacle with a height of 100 mm mainly formed small-scale, high-velocity recirculation vortices, whereas a 200 mm-high obstacle caused the recirculation vortex height to expand to 233 mm, while the maximum reverse smoke velocity decreased to −0.015 m/s, forming large-scale, low-velocity recirculation vortices. Thus, although tall obstacles weakened vortex flow velocity, the large smoke stagnation zones they formed increased the space for toxic gas accumulation and the risk of personnel entrapment. A critical distance of 200–300 mm existed between the fire source and the obstacle, at which the velocity reached its peak and the vortex structure was the most stable.