Flame acceleration and overpressure development in a semiopen tube with repeated obstacles

Abstract

An investigation of the dependence of turbulent flame acceleration and overpressure in a semiopen tube on the configuration of obstacles has been performed in a flame propagation tube with one end closed and the other open, having an 80 mm inner diameter, a 5 m length, and repeated obstacles. Three kinds of obstacle shapes have been used for various blockage ratios and spacing of obstacles. There is evidence that the influence of the obstruction characteristics on the flame speed and overpressure is different in the various flame regimes. In the low-speed combustion regime, the highest terminal flame speed is obtained with the blockage ratio of BR=0.3–0.4. In the choking regime, the maximum flame speed is insensitive to the blockage ratio, and a blockage ratio of about 0.5 generates the highest peak overpressure. In the detonation regime, the maximum flame speed and overpressure decrease with increasing blockage ratio due to the severe momentum losses induced by the blockage effect of the obstacles, and at the same time, the detonative range is observed to become narrower. The steady flame speed is independent of the shape of obstacles when the blockage ratio is the same, and the spacing of obstacles plays a role only in determining the flame acceleration rate rather than the steady flame speed: the highest mean flame speed and peak overpressure are obtained when the spacing of obstacles is about equal to the inner diameter of the flame tube. In addition, the unsteady compressible flow model with correction for the influence of turbulence Mach number on viscosity dissipation and pressure dilatation was formulated, and numerical studies using the eddy-break-up combustion model were made to predict the flame acceleration and the development of overpressure in the tube. The comparison of calculated overpressure and flame speeds with experimental data shows good agreement.