Simultaneous LII and PIV measurements in the soot formation region of turbulent non-premixed jet flames

Abstract

An experimental study was performed to investigate the soot–turbulence interaction in the soot-formation region of turbulent non-premixed co-flowing ethylene/N2 jet flames. Simultaneous velocity and soot volume-fraction (fv) fields were obtained using two-component particle image velocimetry and laser-induced incandescence, respectively. Measurements were made for jet exit Reynolds numbers between 8500 and 12,300, and the measurement location was 10 jet diameters downstream, near the beginning of the yellow luminous region where soot is first formed. In agreement with previous studies, the peak mean fv in the production region is inversely related to the bulk strain rate. The simultaneous data show that soot is formed to the inside of the stoichiometric surface (inferred from stoichiometric velocity), but the formation region moves outside to regions of lower velocity and strain rate as the bulk strain rate is increased. Soot structures form in low strain rate regions, but their upstream edge is seen to become stretched out and aligned at a preferred angle (near 45 degrees) owing to alignment with the instantaneous principal extensive strain rate. Statistical analysis shows that the soot exists, on average, in fluid with axial velocity of about 3m/s and strain rate of 700s−1, regardless of the jet exit velocity. The radial profiles of the covariance between fv and radial velocity are consistent with a model where the soot is formed at a preferred radial location (near the reaction zone) and then is transported by turbulent fluctuations to regions of lower fv.