Time-resolved study of transient soot formation in an aero-engine model combustor at elevated pressure

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Abstract

The mechanisms of transient formation and oxidation of soot in an aero-engine model combustor at elevated pressure are studied for the first time using a combination of high-speed simultaneous stereo-PIV and OH-PLIF and results from a recent detailed LES. A combined analysis of experiment and LES shows that the highly transient and intermittent evolution of soot in this combustor is governed by an unsteady interplay of distinct pockets of burned gas in the inner recirculation zone (IRZ) with either relatively rich or relatively lean composition. The former originate from reaction of fuel-rich unburned gas, whereas the latter result from additional admixture of secondary air further downstream. The analysis further enables distinction and localization of premixed and diffusion-type flame fronts within the flame zone. The time-resolved complementary measurements of velocity field and flame structure allow accurate tracking of both the burned gas pockets and soot filaments. It is seen that soot generally forms in the rich pockets if their residence time in the IRZ is sufficient, whereas oxidation occurs in the lean zones carrying OH. Correlating the dynamics of flow field and soot indicates that the intermittency of soot is driven by an intermittent flow of lean burned gas into the IRZ that affects the residence time of rich pockets. The results suggest that the formation of soot might be further reduced by a proper adjustment of secondary air injection aiming at a sufficient and more constant recirculation of lean burned gas. A remarkably good agreement of measured and simulated instantaneous flame structures is observed, indicating that flow field and gas-phase reactions are well predicted by the LES. The experimental insights into the transient mechanisms of soot formation and oxidation, on the other hand, may provide useful input for LES soot models where deviations from measurements are generally larger.

Introduction

The harmful effects of combustion-generated soot particles on human health and environment, and associated stringent emission regulations necessitate the improvement of aero-engine combustor emission characteristics [1]. While the fundamental understanding of soot formation has evolved considerably during the last decades [2], accurate predictive modeling of soot is still a major challenge in realistic combustors featuring complex turbulent swirl flows, where intricate unsteady fuel-air mixing, a wide range of time scales between high-speed jets and low-velocity recirculation zones, and possible hydrodynamic and acoustic instabilities affect soot formation and oxidation [3], [4].

Detailed experimental studies are essential for improving the understanding of soot evolution and providing accurate data for model validation. The availability of accurate experimental data for technically relevant combustors under well-defined conditions yet still remains limited. Recently our group has reported measurements of velocity field, temperature and soot concentration as well as visualization of OH and PAH for an aero-engine model combustor at elevated presssure [3], [5], [6]. The data is available on request and has since been used for improvement and validation of large eddy simulation (LES) calculations [7], [8], [9].

An advanced understanding of the highly transient and history-dependent soot formation processes, however, was limited by the low repetition rates of the reported measurements. In recent years the use of time-resolved measurements has considerably enhanced the understanding of soot formation in turbulent jet flames [10], [11], [12], [13], thus demonstrating the potential of this approach. The present work aims at elucidating the complex mechanisms of soot formation in a technically relevant aero-engine model combustor for the first time using high-speed laser-based techniques. Since the flame exhibits a complex structure with different thermochemical states and the number of measurable quantities is limited, the analysis is supported by results from a recent detailed LES of the same flame by Grader et al. [7].

Section snippets

Combustor and operating condition

The study employs a single-injector swirl combustor (shown in Fig. 1) that is representative of one sector of an annular aero-engine combustor. The injector comprises two concentric swirl nozzles for air with outer diameters of d=12.3 and 19.8 mm. The two air flows are first fed into separate plenums and then pass radial swirlers composed of eight channels for the central nozzle and 12 channels for the outer nozzle. In order to avoid additional complexities from liquid fuel atomization and

Average structure

Averages of velocity field, soot distribution and OH chemiluminescence (OH-CL, integrated along line-of-sight) measured in previous studies [3], [5] are shown in Fig. 1. Upstream of the secondary air injectors, the flow field exhibits the typical features found in confined swirl flames, namely a conical jet of unburned gas and an inner recirculation zone (IRZ) where hot burned gas is transported towards the flame base and there supports ignition of the unburned gas. Strong velocity gradients

Intermittency

Based on the knowledge of the flame structure obtained in the previous section, the mechanisms of soot formation and depletion are now analyzed using the measured time-series. In the PIV images, the Mie scattering of PIV tracer particles results in clouds of separate dots representing individual particles, whereas the much smaller soot particles are imaged by Rayleigh scattering. The time-series in Fig. 5, which is discussed further in Section 4.3, shows that soot appears in the form of smooth

Conclusions

The highly transient and history-dependent dynamics of soot studied in this work demonstrate the importance of time-resolved laser-based measurements for an advanced understanding of soot formation in aero-engine combustors. The results further highlight the essential role of additional information from detailed LES for a proper interpretation of OH-PLIF measurements in complex turbulent flames. The combined analysis showed that the evolution of soot is governed by an unsteady interplay of

Acknowledgments

Funding from the EU within the project SOPRANO (Grant No. 690724) and the Air Force Office of Scientific Research under Award No. FA9550-16-1-0044 is gratefully acknowledged. C. Carter is supported by the Air Force Windows on the World program.

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