Flame acceleration and transition to detonation in benzene–air mixtures

Abstract

We report results on flame acceleration and transition to detonation of benzene–air mixtures at room temperature. Flame acceleration experiments were carried out in a 150-mm-diameter, 3.6-m-long steel tube. The entire length of the tube is filled with circular orifice plates (blockage or obstructed area ratio of 0.43) spaced one diameter apart. The fuel concentration was varied between 1.7% and 5% by volume of benzene in the fuel–air mixture. Three regimes of propagation were observed: (1) a turbulent deflagration with typical flame speeds less than 100 m/s, (2) a “choking” regime with the flame speed corresponding to the speed of sound of the combustion products, 700 to 900 m/s, and (3) a quasi-detonation regime with a wave speed ranging from 50% to 100% of the Chapman-Jouguet value. Transition from turbulent deflagration to the choking regime occurs at an equivalence ratio of Φ = 0.65 (1.8% C 6H 6) and Φ = 1.8 (4.8% C 6H 6) on the lean and rich sides, respectively. Transition from the choking to the quasi-detonation regime is observed at Φ = 0.88 (2.4% C 6H 6) on the lean side and Φ = 1.6 (4.3% C 6H 6) on the rich side. Detonation cell widths were measured using a small charge (8 to 50 g) of solid explosive for direct initiation of the detonation in both the 150-mm-diameter tube and a larger 300-mm-diameter, 18-m-long, steel tube. Sooted foils are used for determining the cell size, which was 66 mm for a stoichiometric composition. A detailed chemical reaction scheme was used to carry out numerical solutions of the idealized Zel’dovich–von Neumann–Döring (ZND) model. The cell widths were approximately 20 times larger than the computed reaction zone lengths. The ZND model was used to examine the effects of initial temperature and dilution by steam and nitrogen, and the effects of adding hydrogen.