Laser ignition energy for turbulent premixed hydrogen air jets

Abstract

New measurements of laser ignition energy in flowing turbulent hydrogen air mixture jets are reported. Successful ignition occurs if the input by the ignition laser within the volume of laser absorption and the chemical energy released by the incipient reactions in this volume exceed the losses by conduction and radiation. The volume of laser absorption and incipient reactions is defined as the ignition kernel. The ignition kernel is labeled as the flame kernel if the ignition is successful. A cylindrical jet burner injecting a lean premixed mixture upward into ambient air with a laser focused on the axis at a fixed height above the exit is utilized. The effects of bulk velocity on the flame kernel development during 0−200μs following the laser pulse are studied. The ignition and the flame kernel are imaged using schlieren, midinfrared emission and Rayleigh scattering. The Rayleigh scattering and the midinfrared images are processed to measure the transient temperature distribution. The minimum reliable ignition energy (MRIE) increases with an increase in the bulk velocity. A phenomenological model equating the MRIE to the energy losses calculated using the flame kernel volume, the kernel temperature, the mixture properties, and the flame speed is proposed. The calculated and the measured MRIE are in good agreement.