EFFECT OF AROMATIC COMPONENTS IN SURROGATE FUELS ON SOOT IN CO-FLOW ...

226 Pages·2012·5.15 MB·English

The Pennsylvania State University

The Graduate School

College of Engineering

EFFECT OF AROMATIC COMPONENTS IN SURROGATE FUELS
ON SOOT IN CO-FLOW FLAMES AND A MODEL GAS TURBINE COMBUSTOR

A Dissertation in

Mechanical Engineering

by

Venkatesh R. Iyer
© Venkatesh R. Iyer

Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy

December 2012
The dissertation of Venkatesh R. Iyer was reviewed and approved* by the following:

Thomas A. Litzinger
Professor of Mechanical Engineering
Dissertation Advisor
Chair of Committee

Robert J. Santoro
George L. Guillet Professor of Mechanical Engineering

Daniel C. Haworth
Professor of Mechanical Engineering

André L. Boehman
Professor of Fuel Science and Materials Science and Engineering,
and Professor of Mechanical Engineering

Karen A. Thole
Professor of Mechanical Engineering
Department Head of Mechanical and Nuclear Engineering

*Signatures are on file in the Graduate School
iii

ABSTRACT

This study is part of a larger effort to formulate surrogate fuels for the military jet fuel,
JP-8. In order for the surrogate fuel to match the combustion properties of JP-8, the composition
of the surrogate fuel was selected so that its H/C ratio, molecular weight, derived cetane number,
and the threshold soot index (TSI) match the corresponding values for the JP-8. The first part of
this study focuses on establishing the utility of TSI as the parameter to match the sooting
tendency of the surrogate fuels and JP-8. Measurements were conducted to compare the soot
volume fractions for JP-8 and its TSI-matched surrogates in laboratory flames and in a model gas
turbine combustor. Because the sooting tendency of the aromatic component has a critical
influence on the TSI of the surrogate fuel, it is necessary to understand the relationship of TSI and
soot formation for aromatic compounds in order to pick the most suitable aromatic component for
the surrogate fuel. The second part of the thesis focuses on understanding the relationship
between TSI and soot characteristics for aromatic-fueled laboratory flames.
A laminar co-flow wick burner was developed for use with liquid hydrocarbon fuels. The
burner was calibrated to correlate the sooting tendencies of all the fuels studied using TSI. Peak
soot volume fractions for binary mixtures of iso-octane and toluene were correlated with the
smoke heights using Kent‘s model which showed that the wick burner was consistent with the
burners of Kent and Olson et al. In order to establish the extent to which the soot fields of
surrogate fuels match that of JP-8, laser extinction measurements were used to obtain soot fields
for JP-8 and surrogate fuels on the wick burner. In addition, net soot production for JP-8 and
solvent-based surrogate fuels were measured on a model gas turbine combustor for JP-8 and its
TSI-matched surrogate fuels. The soot fields in the flames and the soot concentrations in the
combustor showed excellent agreement within ± 15% which confirmed that TSI works well as an
indicator of sooting tendency of surrogate fuels matched to the TSI of JP-8.
iv

The study of the relationship of TSI and the soot characteristics of aromatic compounds
involved four fuels. 1,3,5-trimethylbenzene and n-propylbenzene were studied because they were
potential aromatic components for the surrogate fuels in the MURI program. iso-propylbenzene
and m-xylene were selected because they had similar TSI to 1,3,5-trimethylbenzene and n-
propylbenzene respectively, but substantially different soot volume fractions (20-30%). Axial
soot measurements and radial soot profiles for the aromatic fuels were used to characterize the
formation and oxidation dominated regions in the flames. 1,3,5-trimethylbenzene and m-xylene
showed about 20% higher soot volume fractions than the propylbenzenes. Differences were also
observed in the overall soot formation and oxidation rates among the fuels. Estimates of
formation and oxidation based on temperature measurements from the literature showed that
differences in temperatures cannot fully account for differences in sooting. Consequently, studies
were done to determine whether morphology of soot could account for the differences.
Morphology was investigated using multi-angle scattering (MAS) and thermophoretic
sampling/TEM analysis. MAS showed that primary particle sizes were the same within the
experimental uncertainty of ± 15% for all the fuels. 1,3,5-trimethylbenzene and m-xylene had 2.5
times and 2 times higher particle number densities, respectively, than n-propylbenzene. TEM data
for n-propylbenzene and m-xylene also showed that the fuels had similar particle sizes.
Thermogravimetric measurements of oxidation rates of soot from all four aromatics showed that
the mass specific rates of oxidation were identical for all four fuels. Analysis of these results
provided additional confirmation that primary particle sizes were the same. These results pointed
towards different particle number densities for the fuels as the reason that they form different
amounts of soot. Since number densities are related to nucleation, modeling analysis were
performed to examine nucleation rates for m-xylene and n-propylbenzene using state-of-the-art
chemical kinetic mechanisms. Higher concentrations of PAH and acetylene from the simulations
v

were consistent with the hypothesis that m-xylene had a greater number of primary particles than
n-propylbenzene.
vi

TABLE OF CONTENTS
LIST OF FIGURES……………………………………………….………………………..viii
LIST OF TABLES………………………………………………………………………….xiii
ACKNOWLEDGEMENTS………………………………………………………………...xiv
Chapter 1 INTRODUCTION ................................................................................................... 1
1.1 Background and Motivation ....................................................................................... 1
1.2 Need for surrogate fuels .............................................................................................. 1
1.3 MURI Surrogate Development ................................................................................... 3
1.3.1Need for matching sooting propensity ............................................................. 4
1.3.2Aromatic components for MURI surrogates .................................................... 5
1.4 Research Objectives .................................................................................................... 7
1.5 Research Questions ..................................................................................................... 8
1.5.1Possible factors driving the differences in soot volume fraction of aromatic
compounds ........................................................................................................ 9
1.6 Overall Research Method ........................................................................................... 11
Chapter 2 BACKGROUND RELATED LITERATURE ........................................................ 12
2.1 Soot formation ............................................................................................................ 12
2.1.1Formation of soot precursors ............................................................................ 13
2.1.2Inception of soot particles ................................................................................ 15
2.1.3Growth of soot particles ................................................................................... 16
2.1.4Soot oxidation .................................................................................................. 17
2.2 Soot formation in laminar diffusion flames ................................................................ 17
2.2.1Smoke point and the tendency to soot .............................................................. 21
2.2.2Threshold Soot Index (TSI) .............................................................................. 23
2.2.3Yield Sooting Index (YSI) ............................................................................... 27
2.2.4Micropyrolysis Index (MPI) ............................................................................. 32
Chapter 3 EXPERIMENTAL FACILITIES ............................................................................ 33
3.1 Wick burner system .................................................................................................... 33
3.1.1Burner design ................................................................................................... 33
3.1.2Wick tube and burner translation ..................................................................... 35
3.1.3Air delivery system .......................................................................................... 37
3.1.4Wick installation .............................................................................................. 38
3.1.5Chimney ........................................................................................................... 40
3.1.6Camera set-up ................................................................................................... 40
3.2 Model Gas Turbine Combustor .................................................................................. 41
3.3 High pressure co-flow burner ..................................................................................... 45
Chapter 4 EXPERIMENTAL METHODS .............................................................................. 47
vii

4.1 Smoke Point ................................................................................................................ 47
4.2 Diagnostic Methods .................................................................................................... 49
4.2.1Laser Extinction ............................................................................................... 49
4.2.2Laser Induced Incandescence ........................................................................... 56
4.2.3Multi-angle Scattering ...................................................................................... 61
4.2.4Thermophoretic Sampling (TS)........................................................................ 72
4.2.5Soot Oxidative Reactivity Study ...................................................................... 77
Chapter 5 RESULTS AND DISCUSSIONS ........................................................................... 79
5.1 Calibration of the Wick Burner for TSI measurements .............................................. 79
5.2 Correlating peak line-of-sight soot volume fraction to TSI through axial
extinction scans on binary fuel mixtures .................................................................. 84
5.3 Comparing soot profiles of JP-8 and its TSI-matched surrogates in the wick
burner ....................................................................................................................... 89
5.3.1Surrogates for JP-8 4658 .................................................................................. 90
5.3.2Surrogates for JP-8 5699 .................................................................................. 97
5.4 Characterizing the soot formation and oxidation regions in the wick burner
flames with aromatic compounds ............................................................................. 116
5.4.1Overall rates of soot formation and oxidation .................................................. 126
5.5 Determination of soot morphology using multi-angle scattering and extinction ........ 130
5.6 Determination of soot primary particle sizes using thermophoretic sampling and
TEM analysis ........................................................................................................... 145
5.7 Relative oxidative reactivity of soot samples using thermogravimetric analysis ....... 154
5.8 Comparison of concentrations of soot precursors through simulations with
chemical mechanisms ............................................................................................... 160
5.8.1Chemical Mechanisms ..................................................................................... 161
5.8.2Modeling conditions ......................................................................................... 162
5.8.3Analysis ............................................................................................................ 163
5.9 Comparison of sooting behavior in nitrogen-diluted ethylene flames ........................ 170
Chapter 6 CONCLUSIONS AND FUTURE WORK .............................................................. 175
6.1 Conclusions ................................................................................................................. 175
6.1.1MURI Surrogate Development ........................................................................ 175
6.1.2Fundamental Research ..................................................................................... 175
6.2 Future Work ................................................................................................................ 177
APPENDIX A Initial Development of Wick Burner ....................................................... 179
APPENDIX B Burner Drawings ...................................................................................... 182
APPENDIX C Scattering cross-sections for nitrogen and ethylene ................................. 185
APPENDIX D Correcting for incident beam attenuation and scattering self-
absorption ................................................................................................................. 186
APPENDIX E Uncertainty Analysis ................................................................................ 190
APPENDIX F Gas chromatograms .................................................................................. 194
APPENDIX G Predicting TSI using Structural Group Additivity ................................... 197
REFERENCES ................................................................................................................. 200
viii

LIST OF FIGURES

Figure 1-1 Peak line-of-sight soot concentration versus Threshold Soot Index. Source:
Olson et al. [23]. ............................................................................................................... 6
Figure 1-2 Measured maximum soot volume fractions against predicted maximum soot
volume fractions. Aromatic compounds indicated by diamonds. Source: Kent [24]. ...... 7
Figure 1-3 Two parts of the thesis: MURI surrogate development – compare sooting of
JP-8 and its surrogates (red), and fundamental research – understand sooting of
aromatic compounds (blue). ............................................................................................. 8
Figure 1-4 Possible factors driving differences in soot volume fractions among the
aromatic compounds. ....................................................................................................... 10
Figure 2-1 Rough picture of soot formation in premixed flames. Source: Richter and
Howard [35]. .................................................................................................................... 13
Figure 2-2 Vertically polarized scattering, depolarized scattering, and fluorescence
signals for an ethylene diffusion flame, z = 10 mm. Source: Santoro et al. [38]. ............ 18
Figure 2-3 Soot processes in an ethylene diffusion flame. Source: Puri et al. [51]. ................ 19
Figure 2-4 Integrated soot volume fraction with height above the burner for an ethylene
diffusion flame at different flow rates. Source: Kent and Wagner [53]. .......................... 20
Figure 2-5 Shape of the flame above the smoke point – A, at the smoke point – B, and
below the smoke point – C. Source: ASTM D1322-97 [62]. ........................................... 22
Figure 2-6 Photograph of an iso-octane/toluene diffusion flame at the smoke point on the
ASTM smoke point lamp. Source: Mensch [63]. ............................................................. 23
Figure 2-7 Fuel mass flow rate plotted against flame height. Arrows indicate the local
minimum coincident with the smoke point. Source: Olson et al. [23] ............................. 25
Figure 2-8 Peak line-of-sight soot concentration versus Threshold Soot Index. Source:
Olson et al. [23] ................................................................................................................ 26
Figure 2-9 YSI values of alkylbenzenes plotted versus the number of Carbon atoms.
Source: McEnally and Pfefferle [71]. .............................................................................. 29
Figure 2-10 Possible naphthalene formation pathways for methylbenzene (left), n-
propylbenzene (center), and iso-propylbenzene (right). Reproduced from McEnally
and Pfefferle [71]. ............................................................................................................ 31
Figure 3-1 Schematic of burner assembly (left) and a photograph of the burner set-up
(right). .............................................................................................................................. 36
ix

Figure 3-2 Modified fuel delivery set-up. ................................................................................ 37
Figure 3-3 Schematic of the air delivery system. ..................................................................... 38
Figure 3-4 Parts used for wick installation. ............................................................................. 39
Figure 3-5 Schematic of the model gas turbine combustor. Source: Mordaunt, Kalaskar
[76,79]. ............................................................................................................................. 43
Figure 3-6 Schematic of the liquid fuel injection system. Source: Mordaunt [76]. ................. 44
Figure 3-7 (a) High pressure chamber (b) Co-flow burner. Source: Menon [80]. ................... 46
Figure 4-1 Typical flame appearances near the smoke point [62]. .......................................... 48
Figure 4-2 Optical set-up for laser extinction on the wick burner. .......................................... 54
Figure 4-3 Optical set-up for laser extinction on the model gas turbine combustor. ............... 56
Figure 4-4 Optical set-up for LII in the high pressure burner. Source: Menon [80]. ............... 61
Figure 4-5 Optical set-up for multi-angle laser scattering measurements on the wick
burner. .............................................................................................................................. 68
Figure 4-6 Schematic to describe beam attenuation and scattering self-absorption. ............... 71
Figure 4-7 Scattering profiles before and after correcting for beam attenuation and self-
absorption of scattered light. ............................................................................................ 71
Figure 4-8 Schematic of Thermophoretic Sampling system. ................................................... 75
Figure 4-9 Shroud plug, TEM grid, and grid holder. ............................................................... 76
Figure 4-10 Probe exposure time measurement ...................................................................... 76
Figure 4-11 Quartz rod for collecting soot samples for TGA .................................................. 78
Figure 5-1 Image of a decalin flame at the smoke point. ......................................................... 81
Figure 5-2 TSI calibration curve for the wick burner. TSI values are taken from Mensch
[69]. Line represents uncertainty-weighted least square fit to wick burner smoke
point data yielding TSI = 3.62*(MW/SP) – 4.3. .............................................................. 82
Figure 5-3 TSI values for binary mixtures of n-dodecane and 1,3,5-trimethylbenzene. ......... 83
Figure 5-4 TSI values for binary mixtures of iso-octane and toluene. ..................................... 84
Figure 5-5 Maximum line-of-sight soot volume fractions plotted against mixture
composition for iso-octane/toluene mixtures. .................................................................. 85
x

Figure 5-6 Maximum line-of-sight soot volume fractions plotted against MW/SP for iso-
octane/toluene mixtures. .................................................................................................. 86
Figure 5-7 Left hand side of equation 3.3 plotted versus H*SP/k for iso-octane/toluene
mixtures. Line represents least squares fit to the data. ..................................................... 88
Figure 5-8 Maximum soot volume fraction. Symbols represent experimentally measured
values, curve represents values obtained using correlation constants from Figure 5-7 ... 89
Figure 5-9 Photographs of flames at their smoke points on the wick burner. .......................... 92
Figure 5-10 I/I profiles for JP-8 4658 at four heights. ............................................................ 93
0
Figure 5-11 Radial soot volume fraction profiles for JP-8 4658 at four heights. .................... 93
Figure 5-12 Radial soot profiles for JP-8 (left) and 2nd generation surrogate (right). ............ 96
Figure 5-13 Radial soot volume fraction profiles for JP-8 5699 at three heights. ................... 100
Figure 5-14 Comparison of radial soot volume fractions for JP-8 5699 (left) and JP-8
4658 (right) ...................................................................................................................... 101
Figure 5-15 Radial soot profiles for JP-8 (left) and the solvent-based surrogates (right) at
non-matching z locations ................................................................................................. 103
Figure 5-16 Radial soot profiles for JP-8 (left) and the solvent-based surrogates (right) at
matching z locations ......................................................................................................... 107
Figure 5-17 Line of sight soot volume fractions for JP-8 5699 and the solvent based
surrogates. ........................................................................................................................ 108
Figure 5-18 Sample signals from the photodiodes: Fuel – JP-8, φ = 1.5. ................................ 110
Figure 5-19 Sample plot of I/I versus equivalence ratio φ for a JP-8 test. ............................. 111
0
Figure 5-20 Line-of-sight soot volume fraction data for JP-8 5699 as a function of
equivalence ratio. Two runs presented together shown with different symbols. ............. 111
Figure 5-21 Polynomial fit for JP-8 data ................................................................................. 112
Figure 5-22 Line-of-sight soot volume fraction data for JP-8 and the solvent-based
surrogates as a function of equivalence ratio on the model gas turbine combustor. ........ 115
Figure 5-23 Photograph of the 1,3,5-trimethylbenzene flame at the smoke point: Left –
resized image (scale in mm), Right – actual size (to scale). ............................................ 117
Figure 5-24 Line-of-sight soot volume fractions with axial locations above the burner. ........ 120
Figure 5-25 Maximum soot volume fraction. Diamonds represent experimentally
measured values for aromatic compounds, squares represent experimentally