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

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.

Experimental Investigation of Soot Production and Oxidation in a Lab-Scale Rich–Quench–Lean (RQL) Burner

“…The goal here is to understand better how the quenching section of the RQL burner affects the flame front and soot formation and oxidation by systematically varying the amount and location of dilution air and investigating their effects through a series of optical and laser diagnostics. This variation of jet location and flow split will extend the understanding presently available on the effect of dilution on soot levels reported in Stöhr et al (2019), Geigle et al (2011Geigle et al ( , 2013Geigle et al ( , 2015aGeigle et al ( , b, 2017. These results will also serve to validate numerical investigations such as those presented in Giusti et al (2018).…”

“…However, ex situ diagnostics are extractive processes that perturb the flow, and the method of extraction can alter the physical and chemical properties of the soot particles so that they are no longer representative of combustion environments. Additionally, probes are most widely used with laminar flames, and while they can be used to measure size distributions in turbulent flames (Boyette et al 2017), they cannot measure the high levels of spatial and temporal intermittency of soot formation in turbulent flames identified in many studies (Wang et al 2019a, b;Lammel et al 2007;Geigle et al 2019;Stöhr et al 2019;Geigle et al 2011Geigle et al , 2013Geigle et al , 2017Bartos et al 2017;Qamar et al 2005Qamar et al , 2009Mahmoud et al 2017Mahmoud et al , 2018Köhler et al 2011;Narayanaswamy and Clemens 2013). These techniques can be used as an accompaniment to in situ laser diagnostics that are currently the main diagnostic techniques used to characterize sooting behavior in turbulent flames.…”

“…Turbulent flames exhibit spatial and temporal fluctuations not present in laminar flames, as well as shorter residence times that add a level of complexity to soot studies (Wang et al 2019a, b;Lammel et al 2007;Geigle et al 2011Geigle et al , 2013Geigle et al , 2017Geigle et al , 2019Stöhr et al 2019;Bartos et al 2017;Qamar et al 2005Qamar et al , 2009Mahmoud et al 2017Mahmoud et al , 2018Köhler et al 2011;Narayanaswamy and Clemens 2013). Qamar et al (2005) investigated soot volume fraction in simple jets (SJ), precessing jets (PJ), and bluff body flames (BB).…”

“…Swirling flows provide benefits such as flame stabilization and increased mixing, and understanding soot formation in these systems is of the utmost importance. Extensive work using laser diagnostics has been done on swirl stabilized, turbulent flames, premixed and non-premixed, using mostly gaseous fuels with some more recent work looking at liquid fuels (Wang et al 2019a, b;Stöhr et al 2019;Geigle et al 2011Geigle et al , 2013Geigle et al , 2015aGeigle et al , b, 2017Roussillo et al 2018). Geigle and coworkers have looked at the effect of equivalence ratio, thermal power, pressure, and secondary air injection on soot formation in swirl stabilized, turbulent non-premixed ethylene-air flames (Stöhr et al 2019;Geigle et al 2011Geigle et al , 2013Geigle et al , 2015aGeigle et al , b, 2017.…”

“…Soot no longer propagates downstream in the central recirculation zone (CRZ) where the OH is dispersed due to the dilution air (Geigle et al 2015b). Recently, their work has focused on soot-turbulence interactions, revealing that soot forms in rich pockets when residence times are favorable, while soot is oxidized by the OH present in the lean zones (Stöhr et al 2019). While this large body of work examines the effect that dilution air has on oxidizing soot particles there is still a lack of systematic investigations into the effect of the dilution jet location and the flow split (i.e.…”