This document describes a novel algorithm developed to automatically estimate the dimensions of irregularly shaped soot sheets in turbulent flames. The algorithm combines and adapts existing computational methods to statistically quantify soot sheet lengths and widths without relying on oversimplified assumptions. It was tested on soot sheets visualized through laser-induced incandescence in a turbulent jet flame. The algorithm accounted for bending and irregular shapes, and estimated soot sheet dimensions with an uncertainty of around 11%. Measurements revealed a correlation between characteristic length and width of soot sheets. Further development could extend the approach to three dimensions.

1. Life Impact | The University of Adelaide
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A novel algorithm to estimate soot sheet dimensions
in Delft-Adelaide Flame
19th Australasian Fluid Mechanics
Conference
Dr Shaun Chan
Dr Paul R. Medwell
Professor G.J. (Gus) Nathan
Dr Shawn Kook

2. Life Impact | The University of AdelaideSlide 1
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Background
Soot in turbulent flame is distributed in thin
sheets & has high intermittency
Local flame dynamics can influence soot
distribution & emission behavior
• Recirculation of reactants within fuel-rich eddies
can result in regions with high soot concentration.
• Layers with large, dense soot sheet can
penetrate reaction zone.
Soot sheet dimension & concentration
information are important to understand soot
oxidation & emission

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Motivation
Automated method to statistically quantify soot sheet
dimensions is desired
• To reduce manual labor.
• To improve statistical reliability.
Challenges
• Soot sheets in turbulent flames are not straight, have irregular
shapes or orientations.
• Previous studies mostly rely on over-simplistic approach that is
prone to error.

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Aims & methodology
Aim
• To develop an automated approach that permit statistical
quantification of soot sheets with random orientations and shapes
Methodology
• Combines & adapts two computational methods from literature:
• Qamar et al., Combustion and Flame (2011)
• Holroyd, Journal of Computing in Civil Engineering (1999)

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Limitation
Characteristic dimensions extracted from planar images, do not
represent the true dimensions of the 3-dimensional soot sheets
Proposed method could potentially be extended into the third
dimension with the application of parallel light sheets

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Dimensions
• Nozzle diameter: 6mm
• Primary air annulus: 45mm.
Main jet
• Natural gas (~81% CH4, 14% N2)
Pilot
• C2H2/H2/Air
Flow conditions
• Ujet: 21.9m/s (Re = 9,700)
• Uann: 4.4m/s
• Ucoflow:0.3m/s
Adelaide-Delft flame
Pilot
Main
* Qamar et al., Combustion and Flame (2009)

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LII optical setup
Excitation
• Wavelength: 1064nm.
• Fluence: 0.9J/cm2.
Detection
• Wavelength: 430nm.
• Gate width: 40ns.
• Prompt detection.
• 1000 images at each measurement position.
Calibration
• Laser extinction measurement.
* Qamar et al., Combustion and Flame (2009)

8. Life Impact | The University of AdelaideSlide 7
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Website: http://www.adelaide.edu.au/cet/isfworkshop/
International sooting flame workshop
Laminar flames:
• Chemical kinetics
• Particle dynamics
Turbulent flames:
• Jet flames
• Bluff body flames
• Swirl flames
• Pool fires
• Influence of scale
Pressurised flames & sprays:
• Simplified IC engines
• Pressurised jet flames
• Shock tubes
ISF Workshop

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Algorithm steps
Selected instantaneous LII image at x/d = 80±5, with scale.
Soot sheet #15, with a bend shape, is chosen for analysis.

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Algorithm steps
Numbered steps in algorithm for soot sheet #15
• Soot sheet (shaded grey), line segment (red dashed lines).
• Anchor points (red dots), boundary boxes (black boxes).

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Characteristic length and width
Characteristic length is determined by fitting straight line
segments through anchor points
Characteristic width is derived from the averaged widths of
equivalent ellipses fitted onto the subdivided regions
• Anchor points (red diamond points).
• Ellipses (black dashed lines).

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Shape & orientation
Demonstration for (a) highly corrugated & (b) trifurcated soot
sheets
• Anchor points (red diamond points).
• Ellipses (black dashed lines).

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Uncertainty
“True” length (L) versus automated characteristic length value
(L) for soot sheets
• Anchor point method underestimated “true” length by 11%.
• Equivalent width method was previously assessed to overestimate
“true” width by 5%.
L*=0.89L
R2=0.94

14. Life Impact | The University of AdelaideSlide 13
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Joint pdf for soot characteristic width & length
Joint pdf for soot sheet characteristic width & length is
computed for LII images at x/d = 115±5
• Single population, linear relationship suggest correlation between
soot sheet characteristic width & length.
Best fit line

15. Life Impact | The University of AdelaideSlide 14
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Conclusions
A novel automated method that permits statistical
quantification of the soot sheet dimensions is developed
• Uncertainty of ~11%.
• Accounts for bending, irregular shapes & orientation.
The measurements reveals new findings in Adelaide-Delft flame
• Correlation between characteristic length & width.
Further work
• Chan et al., Experiments in fluids (2014)

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Question