FLAME ACCELERATION AND TRANSITION TO DETONATION IN BENZENE-AIR MIXTURES

We report results on flame acceleration and transition to, detonation of benzene-air mixtures at room temperature. Flame acceleration experi ments were carried out in a 150-mm-diameter. 3.6-m-long steel tube. Th e entire length of the tube is filled with circular orifice plates (bl ockage or obstructed area ratio of 0.43) spaced one diameter apart. Th r fuel concentration was varied between 1.7% and 5% by volume of benze ne 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 t he speed of sound of the combustion products, 700 tu 900 m/s, and (3) a quasi-detonation regime with a wave speed ranging from 50% to 100% o f the Chapman-Jouguet value. Transition from turbulent deflagration to the choking regime occurs at an equivalence ratio of Phi = 0.65 (1.8% C6H6) and Phi = 1.8 (4.8% C6H6) on the lean and rich sides, respectiv ely. Transition from the choking to the quasi-detonation regime is obs erved at Phi = 0.88 (2.4% C6H6) on the lean side and Phi = 1.6 (4.3% C 6H6) on the rich side. Detonation cell widths were measured using a sm all charge (8 to 50 g) of solid explosive fur direct initiation of the detonation in both the 150-mm-diameter tube and a larger 300-mm-diame ter. 18-m-long, steel tube. Sooted foils are used for determining the cell size, which was 66 mm for a stoichiometric composition. A detaile d chemical reaction scheme was used to carry out numerical solutions o f the idealized Zel'dovich-von Neumann-Doring (ZND) model. The cell wi dths were approximately 20 times larger than the computed reaction zon e lengths. The ZND model was used to examine the effects of initial te mperature and dilution by steam and nitrogen,and the effects of adding hydrogen. (C) 1998 by The Combustion Institute.