Auto-ignition of Transient Turbulent Gaseous Fuel Jets at High Pressure 2006-01-3432

An experimental investigation of the autoignition of transient gaseous fuel jets in heated and compressed air is conducted in a shock tube facility. Experiments are performed at an initial pressure of 30 bar with initial oxidizer temperatures ranging from 1150 K to 1400 K, injection pressures ranging from 60 bar to 150 bar, and with injector tip orifice diameters of 0.275 mm and 1.1 mm. Under the operating conditions studied, increasing temperature results in a significant decrease in autoignition delay time, t_d. The smaller orifice results in an increase in ignition delay time and variability, as compared with the larger orifice. For initial temperatures below about 1250K, ignition is rarely achieved with the smaller orifice, whereas ignition is always achieved with the larger orifice down to 1150 K. Under the conditions studied, increasing the injection pressure decreases ignition delay, a result dynamically consistent with larger orifice size decreasing ignition delay time. The downstream location of the ignition kernel, Z_k, relative to the jet penetration distance, Z, is found to be in the range, 0.4 < Z_k/Z < 0.8, with both orifices, while its location relative to the equivalent orifice diameter, d∗, is in the range, 15 < Z_k/d∗ < 35, using the large orifice, and 25 < Z_k/d∗ < 65 using the small orifice. A scaling model is developed for the ignition delay time and region of initial kernel formation. The model assumes an initial period characterized by relatively high scalar dissipation and inhibited chemical reaction followed by a reaction period that proceeds at a rate determined by a local chemical timescale. Model predictions for ignition delay and region of kernel formation are in good quantitative agreement with experimental measurements from the present work and are found to be in qualitative agreement with findings from previous studies.