Simultaneous Krypton PLIF, LII and PIV Measurements in a Sooting Jet Flame

Abstract

Experiments to measure the concentration of a conserved passive scalar (krypton), with the ultimate aim to determine the mixture fraction, by means of planar laser induced fluorescence (PLIF) were performed in the soot inception region of a turbulent non-premixed ethylene-nitrogen-krypton-air jet flame with a jet-exit Reynolds of number 8300. These measurements were conducted simultaneously to experiments to measure the distribution of soot within the flame (laser induced incandescence) and three components of velocity in a plane (stereoscopic particle image velocimetry). Krypton has an absorption peak at 214.7 nm that leads to the emission of fluorescence at 760 nm and it is this fluorescence that is subsequently detected. Whilst the technique has previously been used in non-sooting flames this study shows that it is also viable in sooting flames, and can be implemented to investigate the significance of mixture fraction in the formation and transport of soot. It is observed that concentrations of up to 8% krypton by mole fraction can be used to seed the flame without self-quenching becoming an issue at the soot inception region of the flame. Extinction of the incident 214.7 nm radiation, either through scattering by the PIV tracer particles or soot, or absorption by polycyclic aromatic hydrocarbons (PAH) present in the flame or absorption by the krypton itself plays an important role in determining the krypton mole fraction. Soot particles directly obstructing the incident 214.7 nm radiation are also noted to strongly attenuate the PLIF signal. The results confirm that soot structures in a jet flame of this Reynolds number are intermittent and “spotty” and that they tend to form in regions of low Reynolds stresses. A strong correlation is found in the region of peak mean soot volume fraction between the LII and krypton PLIF signals.