Experimental study on combustion characteristics of non-uniform premixed gas within a pipeline

In an actual roadway, stratified methane-air mixtures with concentration gradient are often formed in the closed space of roadway after gas leakage. At present, most of the researchers focus on the flame propagation characteristics of uniform premixed gas explosion. In order to explore the propagation characteristics of non-uniform premixed gas explosion flame, the methane concentration gradient distribution and the flame propagation characteristics of non-uniform premixed gas explosion flame in the pipeline under different methane diffusion time were compared through the self-built small-size explosion experiment platform. The methane concentration distribution, flame propagation structure and the maximum explosion pressure were collected by the concentration sensor, high-speed camera system and the pressure collection system respectively in the experiment. The flame propagation velocity and explosion pressure increase curve were obtained by further analysis. The results show that after methane leakage in the pipeline, it spreads along the top of the pipeline under the action of buoyancy, and at the same time, the concentration difference diffuses longitudinally to the bottom, forming a transverse and longitudinal concentration gradient field. The shorter the free diffusion time, the greater the concentration gradient. The concentration gradient field has a significant effect on the flame propagation structure and overpressure of non-uniform premixed methane explosion in the pipeline. The non-uniform premixed flame formed under the concentration gradient field propagates through four stages:sphere, finger shape, triple flame and stretch triple flame. When methane forms longitudinal concentration gradient along the pipeline, the triple flame appears in the pipeline, and the larger the concentration gradient is, the more clear the triple flame structure becomes. After the appearance of the triple flame form, the flame propagation velocity and explosion overpressure decrease rapidly. When the longitudinal concentration of methane in the pipeline is between 16%-4. 6%-0, the flame velocity is about 4. 8 m/s when the triple flame propagates steadily. With the continuous propagation of triple flame, the flame propagation velocity and explosion overpressure rise slightly. In the case of non-uniform premix of methane air in the pipeline, the explosion overpressure presents two peaks, the pressure of the back peak is about 2/3 that of the front peak, and the time between the pressure peaks decreases with the decrease of concentration gradient. Moreover, the closer the methane concentration is to the volume equivalent ratio, the faster the flame propagation speed is, and the higher the explosion overpressure is. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.