Measurements of Turbulent Swirl Flame Dynamics in an Ethylene-fuelled Gas Turbine Model Combustor at Elevated Pressure

Abstract

N anticipation of increased regulation of soot emissions from aviation engines there has been considerable effort in recent years to better understand, model and predict soot formation in gas turbine combustors. Experimental studies in this effort generally fall into one of two categories: 1) detailed studies of chemical kinetics and mechanisms of specific sub-processes of soot formation, agglomeration and oxidation in simple, well-characterized test flames (e.g., laminar Bunsen or diffusion flames) and 2) studies focusing on system-level parameters such as global soot emissions vs fuel grade and combustor pressure at the expense of detailed understanding of specific subprocesses. Although both categories have specific strengths, the understanding yielded by each is of limited use for the purpose of predictive modelling of gas turbine combustors. The German Aerospace Center (Deutsches Zentrum fur Luftund Raumfahrt, DLR) has led an effort in recent years to bridge the gap between fundamental scientific studies of soot formation and system-level characterization of gas turbine combustors. This effort has focused on acquiring detailed measurements of a series of soot-generating flames in a generic, swirl-stabilized combustor at elevated pressure. These flames are designed to capture much of the complexity of a modern, swirl-stabilized gas turbine combustor, while maintaining excellent optical access for pointand planar laser measurement techniques. As part of this effort, Lammel et al. (2007) applied laser-induced incandescence (LII) and coherent anti-Stokes Raman scattering (CARS) spectroscopy to quantify the effects of pressure, equivalence ratio and secondary oxidation air on mean soot distribution in swirl-stabilized, sooting ethylene-air flames at pressures up to 9 bars and thermal loads up to 45 kW. Their results showed the highest soot concentrations were to be found in the lower part of the inner recirculation zone (IRZ) of the combustor, where residence times and local fluid temperatures are high. The combustor used by Lammel et al. (2007) was based on the well-characterized DLR Dual-swirl Gas Turbine Model Combustor or “DS-burner” [2,3] applied LII, CARS and particle image velocimetry (PIV) in a similar combustor at atmospheric pressure and found that injection of secondary air downstream of the flame zone results in drastic changes to soot distribution in the combustor. Geigle et al. (2014) extended these measurements (in a slightly modified burner geometry) to include flames at pressures up to 5 bars and observed that, in addition to pressure and global equivalence ratio, soot concentration and distribution were sensitive to the ratio of air flow supplied to the innerand outer swirl nozzles. Geigle et al. (2015) applied simultaneous LII and planar laser-induced fluorescence of OH (OH-PLIF) to study the spatial correlation of soot and high temperature combustion products in these flames. Their results showed the addition of air downstream of the main flame zone resulted in secondary combustion zones that consume the soot (previously observed in the IRZ) or even prevent soot formation in the first place. These studies produced a rich database of experimental measurements on sooting, swirl-stabilized flames in a gas turbine model combustor. This database, however, is limited to single-shot measurements which yield only mean and fluctuating quantities of interest. Soot formation, agglomeration and oxidation are highly dynamic processes, dependent upon multiple tightly coupled parameters. It is therefore of considerable interest to acquire time-resolved measurements of quantities such as velocity, soot distribution and reaction zone location. Furthermore, these studies have focused primarily upon globally fuel-rich flame conditions. Although this is certainly of key interest in understanding soot-dynamics, it has been observed that even (globally) lean flames can