Mixture fraction, soot volume fraction, and velocity imaging in the soot-inception region of a turbulent non-premixed jet flame

Abstract

An experimental study is performed to investigate the feasibility of conducting simultaneous mixture fraction, soot volume fraction and velocity imaging in sooting jet flames. The measurements are performed in the soot-inception region of ethylene jet flames, where the yellow luminous region first appears in the flame. Three-component velocity and soot volume fraction are measured by stereoscopic particle image velocimetry and laser-induced incandescence, respectively. The mixture fraction is inferred from laser-induced fluorescence of krypton gas seeded into the fuel stream. To obtain mixture fraction from the fluorescence signal, the signal must be corrected for density and fluorescence quenching effects. This correction is accomplished by invoking an assumed state relationship that is derived from a laminar strained-flame calculation. Once properly calibrated, the krypton planar laser-induced fluorescence data give the mixture fraction, temperature and major species near the regions of soot formation. The krypton is seeded into the fuel jet at a mole fraction of approximately 4%. The fluorescence of krypton is achieved by two-photon absorption at 214.7nm and the resulting fluorescence is collected at 760.2nm. The krypton fluorescence signal is rather weak, particularly near the reaction zones where density is lowest, and so adequate signal-to-noise ratios could be achieved in a region only about 1mm in height, which effectively limited this study to a line measurement of mixture fraction. The temperature field derived from the mixture fraction field was compared to temperatures obtained from thermocouple measurements. The mean radial temperature profiles using the different techniques show excellent agreement and this serves to validate the methodology used to map from fluorescence signal to mixture fraction and temperature. The resulting data are of high enough quality as to allow the investigation of the kinematics, thermo-chemical state and even the dissipation fields near regions of soot formation.