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

Abstract

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 (CH 2 O) 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.

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

“…The maximum intensity is observed between 11 and 15 mm. Further downstream, formaldehyde is consumed upon approaching the high-temperature diffusion flame or mixes and interacts with high-temperature products in the central region of the spray [21]. This presumption is consistent with the OH* chemiluminescence distribution.…”

“…Still further downstream (around 35 mm), the same 7/20/2015 observation holds. Given that injection-induced momentum diminishes upon end-of-injection, the presence of formaldehyde closer to the nozzle (up to 6 mm) seems reasonable, particularly when qualitatively compared to single-injection quasi-steady Spray A results [21,22,25]. The maximum intensity is observed between 11 and 15 mm.…”

“…Compared to the Ixt-plot of a single injection with longer injection duration (e.g. [21]), a region with two bright spots can be identified between 0.5 and 0.7 ms after SOI. Note that the relatively intense, but narrow column located exactly at 0.7 ms corresponds to the first PLIF timing, which was studied simultaneously for the displayed results.…”

“…Following this approach, the temporal evolution of OH* intensity along the spray axis can be constructed, which is commonly known as an Ixt-plot [21]. The top panel of Figure 3 depicts such an Ixt-plot for the injection schedule under study.…”

“…Although considered a well-established method to visualize CH2O, the technique is known to suffer from interference by fluorescing Poly-Aromatic Hydrocarbons (PAH) and LaserInduced Incandescence (LII) from soot. Based on spectral imaging, spatiotemporal analysis, and trends obtained by variations in boundary conditions, formaldehyde has been distinguished from PAH fluorescence, provided that they do not overlap [17,21,22].…”