Flame propagation of premixed hydrogen-air explosions in bend pipes

To effectively examine the influence of elbow structures on hydrogen explosion characteristics, this paper conducts a numerical study of flame propagation during hydrogen explosions in bend pipes with different elbow angles. When the elbow angle is large, the flow field movement caused by the pressure difference can separate the "tongue" flame from the outer wall (concave wall), forming a cavity area. The influence of the elbow structure on flame propagation is mainly concentrated in the middle and later stages. In the middle stage, the elbow structure creates a relatively closed space in advance so that the speed peak-2 occurs earlier than it does in the straight pipe. When the elbow angle is alpha = 90 degrees, the speed peak-2 is the largest at 245.8 m/s. In the later stage, the acceleration effect produced by the outer wall (concave wall) plays a leading role, and the flame accelerates for the third time after passing through the elbow. When the elbow angle is alpha = 120 degrees, the best acceleration effect is obtained when the flame passes through the elbow. There is an interactive relationship between the development of explosion overpressure and flame propagation in bend pipes. In the middle stage, the interaction between the compression wave, reflected wave and shear wave causes the overpressure to rise in an oscillatory manner. The elbow structures shorten the reflection distance of the pressure wave, and the oscillation frequency and growth range of the explosion overpressure can be much higher than those in a straight pipe.