Experimental Determination of the Burning Velocity of Mixtures of n-Heptane and Toluene in Engine-like Conditions

Although the burning velocities of fuel-air mixtures have been extensively studied at room temperature and pressure, there is relatively little experimental information available for elevated temperatures and pressures (the so-called engine like conditions). Therefore, the main aim of the present work is to generate accurate experimental burning velocities valid over a range of high unburned gas temperatures and pressures of a variety of mixtures of n-heptane and toluene, varying its proportion by 25% in volume each time. Two experimental combustion facilities have been used and their results compared. One facility consists of a constant volume cylindrical bomb in which Schlieren images can be recorded and used to calculate the flame front development. The second facility is a spherical combustion bomb with centred ignition in which burning velocities are calculated from pressure records by means of a two-zone model. In order to check that the pressure method is reliable, experiments with n-heptane at room temperature and pressure for different equivalence ratios carried out in the spherical combustion bomb were compared with the ones obtained at the same conditions in the cylindrical vessel equipped with the Schlieren technique. Once the validity has been checked, extensive experiments have been carried out for widely varying initial conditions of pressure between 0.3 and 0.7 MPa, temperature between 363 K and 453 K and equivalence ratios from 0.8 to 1.1. Over the ranges studied, by removing the influence of the ignition energy at the earliest stages of combustion and the quenching effects at the later ones, the burning velocities are fitted by a correlation of type , where Cc (r) , epsilon and beta depend on the equivalence ratio. The ranges of validity of the correlations obtained cover from 370 K to 700 K, from 0.3 MPa to 4.5 MPa, and from 0.8 to 1.1 equivalence ratio. A comparison with previous predicted values is also given.