Soot particle size distribution measurements in a turbulent ethylene swirl flame

Abstract

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (fv) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and fv inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average fv. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, fv drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air fv reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets fv reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an fv one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures fv using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.

Comprehensive soot particle size distribution modelling of a model Rich-Quench-Lean burner

“…In terms of the total soot number density, N sim s,total , the diluted case shows a decrease of about 60% compared to the undiluted case A, whereas a total SVF leakage, f sim v,total , decrease of about 55% is observed with dilution. Notably, case B shows a total number density of approximately 5.5×10 10 cm −3 and a total SVF of 0.06 ppb, which agree reasonably well with recently obtained experimental measurements at the burner exhaust: N exp s,total = 8.6 × 10 8 ± 1.7 × 10 8 cm −3 , f exp v,total = 0.03 ± 0.01 ppb [28]. Figure 11 shows the comparison between the time-averaged simulated and the measured [28] PSDs obtained at the exhaust.…”

“…Notably, case B shows a total number density of approximately 5.5×10 10 cm −3 and a total SVF of 0.06 ppb, which agree reasonably well with recently obtained experimental measurements at the burner exhaust: N exp s,total = 8.6 × 10 8 ± 1.7 × 10 8 cm −3 , f exp v,total = 0.03 ± 0.01 ppb [28]. Figure 11 shows the comparison between the time-averaged simulated and the measured [28] PSDs obtained at the exhaust. It is found that in the highdilution case, a fairly good match is obtained with some quantitative differences, also observed from the total soot quantities reported above.…”

“…This is essentially a result of finite-rate chemistry effects and the underlying mixing field affected by the additional dilution air and also associated with a lower mean residence time. Nevertheless, on average, all locations are characterised by a uni-modal soot number density PSD, consistent with experimental observations [28] in this burner and other turbulent jet flames [72,73]. As discussed in Ref.…”

“…In both cases, the secondary air stream is deactivated, and the height of the dilution jets is h = 47 mm. Previous experimental studies in this burner have shown that these conditions drastically affect soot emission [25,26,27,28]. Similar trends have been observed via numerical investigations in similar conditions with a slightly leaner global equivalence ratio [48].…”

“…Herein, Large Eddy Simulation (LES) with the Conditional Moment Closure (CMC) combustion model is used to simulate an ethylene lab-scale burner that has been developed at the University of Cambridge and replicates the major features of the RQL concept [25,26,27], similar to the DLR burner [12,13], but with different air-fuel injection systems (axial vs angled annular) and extra degrees of freedom (variable dilution jet location, range of dilution ratios, spray capability). In particular, this work evaluates the reasons why dilution air drastically reduces the overall soot emission in the Cambridge RQL burner, as shown by measurements of soot volume fraction [26,27,28] and PSDs inflame [28] and at the exhaust [25,28]. The experiments reveal that this device is characterised by high soot production in the primary zone and strong oxidation due to dilution.…”