This paper describes results from an experimental study on influences of liquid fuel properties on lean blowout (LBO) limits in an aero-type combustor. In particular, this work aimed to elucidate the roles of fuel chemical and physical properties on LBO. Fuel chemical properties stem from the fuel chemical structure, thus governing chemical kinetic behaviors of oxidation characteristics (e.g., ignition or extinction time scales) and others (e.g., fuel thermal stability or sooting tendencies). Fuel physical properties affect the spray characteristics (e.g., atomization and evaporation rates). Eighteen different fuels, with a wide range of physical and chemical fuel properties, were tested. Several of these fuels were custom blends, developed to break intercorrelations between various physical and chemical properties. Fuel physical and chemical property effects were further separated by measuring blowout boundaries at three air inlet temperatures between 300 and 550 K, enabling variation in vaporization rates. The condition at 300 K corresponds to a temperature that is less than the flash point for most of the studied fuels and, therefore, forming a flammable mixture was challenging in this regime. The opposite scenario occurred at 550 K, where fuel droplets evaporate quickly, and the temperature actually exceeds the auto-ignition temperatures of some of the fuels. At 300 K, the data suggest that blowout is controlled by fuel physical properties, as a correlation is found between the blowout boundaries and the fuel vaporization temperature. At 450 and 550 K, the blowout boundaries correlated well with the derived cetane number (DCN), related to the global chemical kinetic reactivity.

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This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized, liquid fueled, swirl stabilized flame. The experiments were performed to characterize the flow field, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the strong overlap of the fuel's absorption and emission spectra with the OH fluorescence spectrum. To overcome difficulties associated with the overlap, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing. Upon data reduction, regions containing fuel, OH and a mixture of fuel and OH are identified.Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two multi-component liquid fuels, at two inlet temperatures and three pressures. These results are used to infer several important observations on coupled flow and flame physics. Specifically, the flame is "M-shaped" at higher preheat temperature and higher fuel/air ratio, as opposed to no visible reaction on the inside of the annular fuel/air jet at low temperature and fuel/air ratio conditions. While such fundamentally different flame topologies in gaseous, premixed flames are well known, these results show that there are also different families of flame shapes and heat release distributions in spray flames. In addition, the flame position with respect to the flow is different for the liquid-fueled flame than for gaseous, premixed flames-in premixed flames with this geometry, the flame lies in the low velocity shear layer separating the reactants and the recirculating products. In contrast, the flame location is controlled by the spray location in this spray flame, as opposed to the shear layer. For example, reactions are observed near the nozzle outlet in the core of the high velocity annular jet, something which would not be observed in the premixed flame configuration. Also of interest is the near invariance of the key flow features-such as jet core trajectory or shear layer locations-to the operating condition changes for this study, even as the spray penetration and distribution, and flame position change appreciably.

This paper describes an analysis of the near-lean blow off(LBO) dynamics of spray flames, including the influence of fuel composition upon these dynamics. It is motivated by the fact that, while reasonable correlations exist for predicting blowoffconditions, the fundamental reasons for why flames supported by flow recirculation actually blow offare not well understood. Prior work on gaseous systems has shown that the blowoffevent is a culmination of several intermediate processes, initiating with local extinction of reactions ("stage 1"), followed by large scale changes in flame and flow dynamics ("stage 2"), finally leading to blowoff. In this study, near-LBO dynamics were characterized for ten liquid fuels with widely varying kinetic and physical properties. Results were compared at two air inlet temperatures, 450 and 300 K, as this influences the relative importance of physical and kinetic properties in controlling LBO. Extinction, re-ignition, and recovery of the flame are evident from these data, and grow in frequency as blowoffis approached. Results show that after a near-blowoffevent, the flame can move upstream at velocities much faster than the flow velocity, corresponding to re-ignition. Nonetheless, the majority of the flame recovery events appear to be associated with convection of hot products back upstream, not re-ignition. In contrast, downstream motion of the flame faster than the flow, which would correspond to bulk flame extinction, was never observed. This indicates that "extinction events"actually correspond to convection of the flame downstream by the flow when it loses its stabilization point. The dependence of the equivalence ratio when these events appear, their frequency, and event duration were quantified as a function of fuel composition and air inlet temperature. For example, the data shows a higher percentage of recovery from near-blowoffevents through re-ignition for high DCN fuels at the 450 K air temperature condition. These extinction/re-ignition results suggest that high DCN fuels are harder to blow offthan low DCN fuels through two mechanisms: (1) by delaying the onset of LBO precursor events, and (2) because they are able to recover from these precursor events through re-ignition more often.

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