1 Effects of Ignition Delay on Impulsively Initiated Premixed Jets

An experimental and numerical investigation is performed to explore the methods for achieving flame speeds that are much greater than the laminar burning velocities. A premixed fuel jet is impulsively issued into a low-speed annular airflow, and ignition is provided after a preset delay time. The temporally varying structure of the propagating flame in and around the jet is measured using OH planar laser-induced fluorescence (PLIF) and particle-image velocimetry (PIV). Experiments are performed by varying the equivalence ratio, ignition delay time and jet momentum. Calculations are also performed using a time-dependent two-dimensional code that incorporates seventeen-species finite-rate chemistry for methane fuel. The ignition process is simulated by temporarily increasing the temperature in a small region within the jet column. Calculations are performed for the stoichiometric fuel/air mixture with varying ignition delay times. Instantaneous structures of the simulated and measured flames are compared. Flame propagation speeds are obtained from the numerical results by tracking the flame location along the centerline. It is observed that flame speed increases with ignition delay. An order of magnitude increase in flame speed is achieved by igniting the jet after a delay of ~ 6 ms. Further increase in ignition delay creates unburned fuel pockets and decreases the flame speed