On the mechanisms of flame propagation in methane-air mixtures with concentration gradient

In this study, the tulip flame in methane-air mixtures with concentration gradient are investigated experimentally in a square tube with 3-m long and 30 x 30 cm cross section. The concentration gradient is produced by installing 54 (27 x 2) evenly spaced intake gas device on the top wall. Six pressure transducers are used to record the pressure evolution and the high-speed camera is adopted to obtain the flame image. The results indicate that three flame propagation modes are observed, i.e., spherical, curved and tulip. Various concentration gradients can be achieved by changing the ignition delay time (T-d) and the methane concentration (C-CH4) in the tube. At T-d = 10min, asymmetric double tulip flame is seen firstly. As the flame moves forward, the flame upper skirt squeezes downward until the edges overlap and the tulip flame disappears. At T-d = 20min, the flame velocity and shape are similar to that be obtained in the homogeneous concentration case, indicating the methane-air mixtures are nearly homogeneity. Moreover, for the case of C-CH4 = 7%, the asymmetric double tulip flame is also found. However, the disappearance of tulip flame is caused by the acceleration of tulip tip, filling up the hollow area. The formation of asymmetric double tulip flame may be attributed to the R-T instabilities. (C) 2020 Elsevier Ltd. All rights reserved.