Laser Based Study of Spray Auto-Ignition in a Generic Mixing Duct

Auto-ignition is one of the challenges still inhibiting the application of LPP combustion in aero-engine turbines. In this context, the auto-ignition phenomenon in a droplet-laden reactive flow was investigated. The test rig used for this investigation has optical access to the reaction zone and allows systematic parametric studies of auto-ignition. The investigations are focused on a detailed understanding of the local conditions under which auto-ignition of fuel spray may occur. The fuel droplets are injected into a rapid flow (> 30m/s) of heated, non-vitiated air at elevated gas pressure (0.8MPa). The auto-ignition and subsequent combustion of kerosene or heptane fuel spray were investigated over the technically interesting range of air temperatures (750...1100K). The global equivalence ratio of the fuel/air mixture was lean (Φ = 0.6). The chemical reactions were visualized by planar laser-induced fluorescence (LIF) of formaldehyde (CH₂O), which is formed as an intermediate species in the reactions leading to auto-ignition and subsequently consumed during combustion. The LIF of CH₂O was excited by a dye-laser at 339.3nm and captured by an ICCD camera. For the visualization of incompletely evaporated droplets, simultaneous imaging of Mie-scattering with a second laser-camera-arrangement at 532nm was performed. Additionally, high-speed sequences of the flame luminosity were taken. The chemiluminescence signal was captured broadband as well as spectrally filtered by different adequate dielectric filters. With this method it was possible to discriminate between the fractions of OH* and CH* radicals as well as from radiation at longer wavelengths that can be attributed to “young soot”. The analysis of images obtained from the Mie, LIF and chemiluminescence signals, shows the simultaneous occurrence of multiple phases in the droplet vaporization/ignition/combustion sequence. CH₂O-LIF intensity fields display pronounced large-scale inhomogeneities, with smooth spatial transitions between lower and higher CH₂O-levels, indicating different levels of reaction progress at different locations. Some images show spatially coherent zones of complete formaldehyde consumption with sharp gradients in the CH₂O-LIF fields, indicating flame-like combustion. This occurrence of isolated combusting regions suggests individual flame zones that develop from spatially separated, independent auto-ignition spots. Furthermore, individual small spots of high intensity were observed, originating from droplets that are burned under fuel rich conditions inside the hot products of the expanding flame zones. On the basis of the simultaneous LIF and Mie observations, however, a correlation of the location of ignition and the bigger residual droplets could not be confirmed, though this was inferred from single droplet investigations in literature. The impact of droplets’ boundary layers in a turbulent mixing duct flow seems to be of minor importance concerning the promotion of ignition kernels. Overall, the experiments show that auto-ignition of fuel sprays in a premixing duct requires a more detailed modeling than plain treatment of a single droplet with ambient gas phase.