Abstract
Primary energy sources for aviation gas turbines as well as direct-injection gasoline and diesel engines come in the form of liquid hydrocarbon fuels. These liquid fuels are atomized and mixed with air, prior to highly turbulent combustion and heat release processes inside engine hardware. Designing more efficient and cleaner gas turbine engines is hence dependent on the in-depth understanding of spray formation, mixing, heat release, combustion dynamics, and pollutant formation pathways in liquid-fuel spray flames. As compared to gaseous fuels, the additional steps of atomization, dispersion, and evaporation prior to turbulent mixing need to be considered for a variety of liquid fuels to enable fuel-flexible operation of these combustion hardware. Such studies can be largely facilitated by advanced laser diagnostics applied to simplified piloted liquid-spray flame configurations that can also be numerically modeled using well-defined boundary conditions. In this work, a modified configuration of a fuel-flexible piloted liquid-spray flame apparatus is used for detailed laser diagnostics studies using hydroxyl (OH) planar imaging. The configuration consists of a modified McKenna flat-flame burner fitted with a direct-injection high-efficiency nebulizer. OH radical is a primary marker of the reaction zone and a key indicator of the heat release process in flames. OH is abundant in the high-temperature combustion regions providing high signal-to-noise ratio single-laser-shot images revealing flame dynamics and instabilities. Therefore, OH planar laser-induced fluorescence (PLIF) is employed to characterize the dynamic structures of a range of piloted liquid-spray flames operated with methanol (CH3OH), n-Heptane (C7H16), iso-Octane (C8H18), dodecane (C12H26), gasoline (C4–C12), diesel (C12–C20), and kerosene (C6–C16). Single-shot and averaged OH-PLIF images show the presence of strong turbulence in the core region above the surface of the McKenna burner. The reaction zone mainly occurs around the periphery of this region, then it spreads more uniformly due to evaporation of liquid droplets downstream of the spray flame. Two-color OH PLIF thermometry in liquid spray flames operated with gasoline, diesel and kerosene, has been shown that the combustion temperature is in the range of 1200–2000 K. Overall, OH PLIF has been demonstrated to be an efficient approach for dynamic structures and temperature measurements in piloted liquid-spray flames operated with realistic fuels.
