Investigation on rapid consumption of fine scale unburned mixture islands in turbulent flame via 10 kHz simultaneous CH–OH PLIF and SPIV

Abstract

A system based on high repetition rate simultaneous CH–OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) measurement has been developed. The high-speed simultaneous measurement system has an acquisition rate of 10kHz and its measurement duration time exceeds 1.0s. The developed system is applied to a methane–air turbulent jet premixed flame in order to investigate the flame and flow dynamics of the turbulent combustion field. The obtained simultaneous CH and OH PLIF images well represent the turbulent flame dynamics in shear flow. Under highly turbulent conditions, the number of flame wrinkles increases and the flame front shows a very complex geometry. Since large scale vortical structures develop due to Kelvin–Helmholtz instability, large scale unburned mixture islands are often created and ejected into the downstream. Around the flame tip under the condition of high Reynolds number (U0=20m/s, Reλ=257), the formation of fine scale unburned mixture islands is observed and its contribution to enhance the turbulent burning velocity is investigated by statistically analyzing the local consumption rate. The consumption rate is estimated from the OH and CH PLIF images on the basis of the spherical and pillar assumptions. The most probable consumption rates obtained from all adopted methods are much higher than the laminar burning velocity (0.39m/s). The importance of a high repetition rate CH acquisition as a flame marker is also discussed.