Investigation of Extinction and Turbulence Effects in Liquid-Fueled Jet Flames

Abstract

The current study investigates the effect of turbulence on flame topology of dual-fuel, combined premixed, preheated methane and n-heptane, n-dodecane, and JP-8 spray using CH and OH PLIF. The University of Michigan Hi-Pilot has been retrofitted to function with sprays, with a hypodermic needle inserted into the turbulence plate. OH-PLIF imaging is performed using the A-X (0,0) band near 311 nm to calculate flame surface statistics, with custom filters implemented to reduce interference from the sprays. CH PLIF is also performed, using the R-branch C-X (0,0) transition, also near 311 nm. Flame surface statistics of interest are flame surface density (FSD), reaction progress variable, turbulent flame area ratio, brush thickness, and flame height. The pre-mixedness/pre-vaporization of the air-fuel mixture is varied by changing the liquid fuel flow rate to pre-vaporized fuel flow rate ratio. It was observed that increasing Reynolds number increased flame height, flame surface density, and flame brush thickness. These quantities also increase when spray loading is increased. Droplet burning, fuel vapor pockets, flame fragmentation, and local extinctions are observed, but not always detected with OH PLIF, due to the nature of the lifetime of the species. Additionally, OH-PLIF imaging captures fuel vapor pockets in the product stream, which results in signal gradients similar in magnitude to a flame edge, despite no flame existing in that region of space. This likely results in an overestimation of flame statistics, in particular, flame surface density based on current edge detection methods.