Nonlinear Effects of Vent Area on Secondary Explosions in Large Curvature Radius Bent Pipes: A Comparative Study of U-Shaped, L-Shaped and Straight Pipelines

This study investigates the effects of vent area on secondary explosions in large curvature radius bent pipes (R/D = 5). The results show that the L-shaped pipe exhibits the highest flame speed, while the U-shaped pipe has the lowest flame speed but a higher overpressure than the straight pipe, revealing a nonlinear relationship. The U-shaped pipe flame exhibits distinct segmentation and instability due to the “breakdown–reorganization–re-breakdown” flow history. The vent area effect exhibits significant configuration dependence: the U-shaped pipe shows a “rise-fall” single-peak characteristic (peak at 65 mm), the L-shaped pipe shows a “fall-rise” valley-type characteristic (minimum at 65 mm), and the straight pipe shows a linear positive correlation. Quantitative analysis of the correlation coefficient η (secondary/primary overpressure) indicates that η reaches 0.88 for the U-shaped pipe (peak conversion efficiency at 65 mm), increases from 0.52 to 0.99 for the L-shaped pipe, and from 0.63 to 1.24 for the straight pipe. These findings provide a scientific basis for secondary explosion prevention in urban gas pipelines.