Simultaneous CH–OH PLIF and stereoscopic PIV measurements of turbulent premixed flames

Abstract

Simultaneous CH and OH planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) measurement have been developed to investigate the local flame structure of turbulent premixed flames. In this system, high-speed CMOS cameras were adopted to capture clear stereoscopic particle images without contamination by the flame radiation. The effects of scattering of CH PLIF laser by tracer particles are carefully investigated to improve signal-to-noise ratio in CH fluorescence images. The developed simultaneous two radical concentrations and three-component velocity measurement on a two-dimensional plane was applied to relatively high Reynolds number turbulent premixed flames in a swirl-stabilized combustor. All measurements were conducted for methane–air premixed flames in the corrugated flamelet regime. Simultaneous CH and OH images suggest the presence of the isolated burned gas in the unburned mixture and the isolated unburned mixture in the burned side, which have been predicted by direct numerical simulations. Detailed analysis of simultaneous CH and OH images reveals that the minimum curvature radius of the flame front coincides with the Kolmogorov scale. Strong three-dimensional velocity fluctuations, which are measured by the stereoscopic PIV, imply that misunderstanding of the flame/turbulence interactions would be caused by the analysis of two-component velocity distributions in a cross-section. Furthermore, comparisons of CH–OH PLIF and three-component velocity fields show that the burned gases do not always have high-speed velocity in a relatively high Reynolds number turbulent premixed flame. The Reynolds number dependence of the flame front was clearly captured by the simultaneous CH–OH PLIF and stereoscopic PIV measurements.