Characterization of Spray A flame structure for parametric variations in ECN constant-volume vessels using chemiluminescence and laser-induced fluorescence

The transient and quasi-steady flame structure of reacting fuel sprays produced by single-hole injectors has been studied using chemiluminescence imaging and Planar Laser-Induced Fluorescence (PLIF) in various constant-volume facilities at different research institutes participating in the Engine Combustion Network (ECN). The evolution of the high-temperature flame has been followed based on chemiluminescence imaging of the excited-state hydroxyl radical (OH*), and PLIF of ground-state OH. Regions associated with low-temperature chemical reactions are visualized using formaldehyde (CH2O) PLIF with 355-nm excitation. We compare the results obtained by different research institutes under nominally identical experimental conditions and fuel injectors. In spite of design differences among the various experimental facilities, the results are consistent. This lends confidence to studies of transient behavior and parameter variations performed by individual research groups. We present results of the transient flame structures at Spray A reference conditions, and include parametric variations around this baseline, involving ambient temperature, oxygen concentration and injection pressure. Key results are the observed influence of an entrainment wave on the transient flame behavior, model-substantiated explanations for the high-intensity OH* lobes at the lift-off length and differences with OH PLIF, and a general analogy of the flame structures with a spray cone along which the flame tends to locate for the applied parametric variations