Effect of large-scale perturbation on the critical condition of detonation formation in stoichiometric CH4-2O2 mixtures

Abstract

The effects of large-scale perturbation induced by orifice plate on the critical condition of detonation formation are investigated systematically in a circular tube with an inner diameter of 90mm and 6m long. The orifice plates with the blockage ratio (BR) of 0.5, 0.7, 0.93 and 0.96 are employed. Four pressure transducers (PCB102B06) are used to determine the average velocity while the smoked foils are employed to record the detonation cellular patterns. The experimental results indicate that a detonation can propagate at a steady velocity with small deficit when the initial pressure (P0) is greater than the critical value (Pc). The velocity deficit is enhanced sharply as the critical condition is approached by gradually decreasing the initial pressure. At P0 below the critical pressure, the detonation failure can be observed. In the smooth tube without obstacles, the critical pressure is 2.5kPa and the corresponding velocity deficit is 25% approximately. After the orifice plates are introduced into the tube, the critical pressure and the velocity deficit both are increased significantly. The critical pressure are 7.5, 11, 15 and 16kPa, respectively. The maximum velocity deficit is about 30%. By measuring the average velocity immediately after the combustion wave propagation through the orifice plates, it can be found that only when the average speed is greater than 85% times the products speed of sound, can the detonation be produced at the end of the tube. Finally, the critical condition of detonation propagation are analyzed in detail. In the cases of smaller BRs (0.5 and 0.7), the critical conditions are consistent with the criterions of d/λ>1 and L/λ>7. However, these two criterions both are invalid for BR=0.93 and 0.96 cases.