On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Abstract

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.

Introduction

For a long time, the effect of soot particles on the environment was considered to be negligible compared with greenhouse gases such as CO₂ and CH₄. However, recent studies have shown that soot emissions have a severe impact on global warming, to the point that they are now considered the second highest anthropogenic contributor to radiative forcing, just behind CO₂ [1]. In the field of transportation, aircraft engine emissions were estimated to be the second largest contributor in 2015 [2]. For these reasons, stringent regulatory standards concerning soot emission will come into effect in the future years. In consequence, aircraft engine manufacturers have to develop innovative combustion systems with the aim of reducing fuel consumption and soot emissions while maintaining a high combustion efficiency. Over the years, lean premixed swirl combustion has become a valuable approach to meet these regulations. However, the technology associated with this concept may produce flame instability, partial extinction or low reaction rates [3,4]. To tackle these limitations, manufacturers of aircraft engines have developed promising technologies based on partially premixing and stratifying the reaction mixture to obtain better flexibility of control of flame stability, over a large range of operating conditions. Several investigations have previously been performed to study the flame structure of academic premixed swirl and stratified flame topologies [5,6]. These studies have produced detailed information on the benefits of local mixture fraction gradients on the increase in heat release from globally fuel-lean combustion and the larger flame structure sensitivity to the specific inlet fuel-air distribution. Perhaps surprisingly, no data are available on the impact of swirled and stratified flames on the reduction of NOx and soot emissions, unlike in the case of fully premixed swirl flames in which joint numerical and experimental studies have given a detailed understanding of soot formation and oxidation [7], [8], [9].

In the context described above, a detailed investigation is required on how variations in swirl and stratification affect soot formation. The study of soot production and consumption is a complex task since the mechanisms involved are dependent on numerous parameters. Firstly, the thermal decomposition of fuel, via several chemical reactions and under suitable thermophysical conditions, forms polycyclic aromatic hydrocarbons (PAHs) [10]. PAHs are then transformed in solid particles via the nucleation process [11]. Surface growth and aggregation finally produce aggregates of 10–50 nm diameter primary particles. These growth mechanisms also compete with oxidation caused by chemical reactions between oxygenated compounds and the soot surface, which offer a way of consuming soot particles [12]. These mechanisms must be identified in order to gain an accurate understanding of soot production, and this can be achieved by performing in-situ optical measurements [13].

This work presents results on the formation of soot in swirl stratified premixed C₂H₄/air flames. This is accomplished by simultaneously imaging the velocity, the soot volume fraction, and the distributions of hydroxyl radical (OH) and PAHs, using particle image velocimetry (PIV), laser-induced incandescence (LII), and OH and PAH planar laser-induced fluorescence (PLIF), respectively. A combination of these diagnostics is shown to provide an in-depth understanding of the physical processes governing soot formation in swirl stratified flames. The links between soot, OH and precursors, highlighting the combined effects of the flame structure, the intermittency of soot formation and aerodynamics are discussed.