Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

Abstract

The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH2O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme. OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (Σ2D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region as the flames approached blow-off. No significant variation in Σ2D, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH2O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, heptane and kerosene flames showed more pockets of CH2O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties.