Laser optical investigation of turbulent transport of temperature ahead of the preheat zone in a premixed flame

Abstract

This experimental study investigates temperature profiles upstream of localized thin reaction zones in a turbulent premixed flame stabilized on a low-swirl-burner. A simultaneous dual-sheet Rayleigh/OH-LIPF measurement technique was applied using two parallel laser light sheets to measure the temperature in the preheat zone for three lean (ϕ=0.7) turbulent methane/air flames. The ratio of the turbulence intensity to the laminar burning velocity v′/sL was varied from 3.5 to 9.2 and to 18.7. The first of these flames lies on the borderline between the “corrugated flamelets” regime and the “thin reaction zones” regime, while the other two are in the “thin reaction zones” regime. The results confirm the occurrence of strong temperature fluctuations ahead of the preheat zone for flames in the “thin reaction zones” regime. Single temperature images show a significant temperature increase ahead of the preheat zones of up to 700 K for flames at the highest turbulence intensity. For statistical analysis conditional mean temperature profiles and probability density functions conditioned on the distance from the flame contour were calculated from the experimental data. Only those portions of the flame front were included which were found to be approximately normal to the two laser sheets. The resulting probability density functions show that the effect of temperature rise ahead of the preheat zone becomes significant only for flames in the “thin reaction zones” regime. The mean temperature profiles show a much smaller temperature rise which, however, increases with increasing velocity ratios v′/sL.