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Subtopic 2.3: Soot Field

Subtopic 2.3: Soot Field. POLIMI. Topic 2.0 Organizer Jose M. Garcia-Oliver Subtopic 2.3 Coordinators Michele Bolla, ETH Dan Haworth, PSU Scott Skeen, Sandia Subtopic 2.3 Contributors Experimental IFP Energy nouvelles Sandia Meiji University Modeling

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Subtopic 2.3: Soot Field

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  1. Subtopic 2.3: Soot Field POLIMI Topic 2.0 Organizer Jose M. Garcia-Oliver Subtopic 2.3 Coordinators Michele Bolla, ETH Dan Haworth, PSU Scott Skeen, Sandia Subtopic 2.3 Contributors Experimental IFP Energy nouvelles Sandia Meiji University Modeling University of Wisconsin Politecnico di Milano ETH Zurich Wisconsin Sandia

  2. Review of ECN 2 Soot Session • Dan Haworth provided discussed the physics of soot formation and CFD-based soot modeling, emphasizing the importance of radiation heat transfer (see Webex recording) • EmreCenker presented LII/LEM experiments for Spray A and a few parametric variants • Peak SVF of 2-4 ppm for Spray A (930 K, 21.8 kg/m3) • Peak SVF of 12 ppm at 1030 K • Signal trapping considered to be negligible • Two groups (ETH and U Wisconsin) submitted mean soot volume fraction data for Spray H • Models reproduced measured soot levels and trends with variations in ambient O2 and density • No definitive conclusions were drawn regarding the merits of the different modeling approaches • Recommendations from ECN 2: • Ambient temperature of ECN pre-combustion vessels should be well characterized • LII measurements exhibited significant statistical error due to jitter between the laser and camera. Future LII experiments must minimize jitter and account for it in the LII calibration • Long injection duration for measurements examining quasi-steady behavior • Begin looking at Spray A (n-dodecane) • Modelers should perform systematic parametric studies to isolate and quantify the effects of individual physical processes • Turbulence-Chemistry Interaction • Turbulence-Radiation Interaction • Nucleation, surface growth, agglomeration

  3. Subtopic 2.3: Objectives “To improve the understanding of the physical/chemical processes of soot formation and oxidation under engine-relevant conditions and to distill this improved understanding into predictive CFD-based models.” -ECN3 Guidelines • Soot Onset (Timing and Location) • How to quantify for consistency between experiments and modeling • Parametric variation (850 K, 900 K, 1000 K) (13%, 15%, 21% O2) • 2-D Soot Field • Transient progression (1.5, 2.0, 2.5, 4.5 ms ASOI) • Compare IFPEN LII with extinction imaging from Sandia at available timings • Evaluation of signal-trapping • Standardization of soot non-dimensional extinction coefficient • Soot Temperature • Comparison of 2-Color pyrometry (IFPEN) with Imaging Spectrometer (Sandia) • Soot Particle Size • What is the primary particle size at the location of peak SVF? • How does particle size change as a function of distance from the injector?

  4. Sandia Extinction Imaging Setup • Simultaneous ignition delay, quasi-steady lift-off length, and soot extinction measurements • Two incident wavelengths has proven useful for understanding optical properties of soot • Soot Measurement Resolution • 85 kHz  35 µs (2 wavelength)  23 µs (1 wavelength) • 100 µm per pixel • Lower Detection Limit (Beam-steering) • < 0.5 ppm

  5. Extinction Imaging • Soot mass is proportional to measured optical thickness (KL) • High-speed extinction imaging measurements provide time-resolved KL maps • Total mass and axial resolved soot mass do not require tomography for comparison to modeled SVF results • Mass-based soot onset timing and location provide targets for modeling efforts • Inception of soot in spray head and its progression downstream provide a difficult modeling target Spray A Masssoot= pixel area

  6. Time Sequence of LII vs. Time-Resolved Extinction *Tamb: 930 K *ρamb: 21.8 kg/m3 • Can compare progression of total soot mass as an indicator of soot onset • Appears to be a mismatch in reacting vapor penetration

  7. Soot Onset: Timing and Location • Mass-based soot onset timing and location provide targets for modeling efforts • Based on a soot mass threshold of 0.5 µg for total mass • Based on a soot mass threshold of 10 ng for axial resolved mass • Rate of total soot mass increase is very similar for IFPEN LII data and Sandia Extinction Imaging Data • 200 µs difference in soot onset potentially explained by uncertainty in IFPEN vapor penetration

  8. Soot Onset: Timing and Location 15% 850 K 15% 1000 K

  9. Soot Onset: Timing and Location 15% 850 K Full soot field was not captured, so numbers are considered low relative to reality 15% 1000 K

  10. Soot Onset: Timing and Location 13% 900 K 21% 900 K

  11. Soot Onset: Timing and Location 13% 900 K Full soot field was not captured, so numbers are considered low relative to reality 21% 900 K

  12. Soot Timing and Location Relative to Ignition • Parametric variation around Spray A in temperature and O2 concentration show a predictable trend in the time between high-temperature ignition and soot onset and the location of high-temperature ignition and soot onset.

  13. Time-Resolved Total Soot Mass • Higher ambient temperature and O2 lead to better performance of UW model • UW model scales similarly later during quasi-steady period for AR and O3 cases • Between 1 and 2 ms ASOI, POLIMI model scales similarly for all but the 21% O2 case Wisconsin

  14. Ensemble Averaged SVF (IFPEN/Sandia)

  15. Ensemble Averaged SVF sdf LII n-heptane: 15% O2, 1000 K, 1500 bar, 30 kg/m3, 100 µm orifice With sufficient statistics, ensemble average of single-shot LII yields axisymmetric images similar to time- and ensemble-averaged extinction imaging data

  16. Radial Profiles of fv • Signal trapping may cause plateau in LII data • Correction must be applied to raw LII signal before integration and calculation of fv • IFPEN used a 425 nm +/- 15 nm bandpass filter for collection of LII signal • Extinction measurements at Sandia using 406 nm incident light showed a mean KL of ~0.9 between 55 and 60 mm (KL = 0.45 for half the path length) • Signal trapping could result in 36% of the signal blocked along the centerline • Must also consider the effect of ke Sandia KL using 406 nm incident light

  17. Non-dimensional Extinction Coeff., ke • Standard ke was updated from 4.9 to 8.7 for 632.8 nm extinction measurements • ke computed from Rayleigh-Debye-Gans theory for fractal aggregates is different • Refractive index 1.75-1.03i from Williams et al. Int. J. Heat and Mass Transfer (2007) • Np primary particles per aggregate, dp primary particle diameter • Incident wavelength of 632.8 nm • Greater effect of Np for larger primary particle size • Small particles sizes in Spray A measured by TEM means uncertainty in assumption of constant Np is reduced • Greatest uncertainty remains in the refractive index of soot

  18. Non-dimensional Extinction Coeff., ke • Standard ke was updated from 4.9 to 8.7 for 632.8 nm extinction measurements • ke computed from Rayleigh-Debye-Gans theory for fractal aggregates is different • Refractive index 1.75-1.03i from Williams et al. Int. J. Heat and Mass Transfer (2007) • Np primary particles per aggregate, dp primary particle diameter • Incident wavelength of 632.8 nm • Greater effect of Np for larger primary particle size • Small particles sizes in Spray A measured by TEM means uncertainty in assumption of constant Np is reduced • Greatest uncertainty remains in the refractive index of soot O3 (21% O2)

  19. Signal Trapping • Correction based on Sandia extinction data improves plateau somewhat • Correction actually decreases mass along chosen cross section by 4% • Use uncorrected fv as ILII(x,y), make correction based on Gaussian KL from Sandia data, re-integrate new KLLII • Correction increases mass by a factor of 1.8

  20. Total Soot Mass • IFPEN calibrated with 632.8 HeNe laser extinction • ke = 8.7 was standard at the time of publication • Sandia extinction imaging with 406 nm LED • ke = 7.76 based on RDG theory with dp = 16 nm and Np = 150

  21. Total Soot Mass • IFPEN calibrated with 632.8 HeNe laser extinction • ke = 8.7 was standard at the time of publication • ke = 7.28 from RDG theory with dp = 16 nm, Np=150 as in Imaging Extinction work (20% increase in fv and soot mass)

  22. Summary • Extinction imaging measurements have provided useful targets for modeling efforts including: • Soot onset time • Soot onset location • Soot mass and/or soot volume fraction • Transient progression of the 2D soot field with high temporal resolution (35 µs) • Need to increase field of view and further reduce effects of beam steering • Comparison of LII/LEM measurements from IFPEN and Sandia’s Extinction Imaging measurements • Similar rate of soot mass increase for Spray A • Differences in reacting penetration may explain difference in soot onset time • Differences in SVF lessened by accounting for signal trapping (~400 nm) • Differences in SVF lessened further by considering ke derived from Rayleigh-Debye-Gans theory • Primary particle size as measured by IFPEN/Meiji ranges from 10-20 nm • Small primary particle sizes reduce the error associated with our assumption of constant Np throughout the soot field.

  23. Dirty Laundry-Nozzle Aging (injector 370) • Similar lift-off lengths and total soot mass, slightly short ignition delay time for later data, significantly shorter soot onset time • Mass measurements and pressure traces indicate change in discharge coefficient (more mass in later experiments) Tamb= 905 K Lift-off: 16.09 τig = 404 µs (chemi) τig = 400 µs (press) Tamb= 902.5 K Lift-off: 16.23 τig = 344 µs (chemi) faster camera τig= 370 µs (press)

  24. Outline: Soot modeling Presentationsootmodelsused (3 contributors) • UW, POLIMI and ETH Analysis C2H2 assoot „initialcondition“ • C2H2 total mass in time (UW, ETH, POLIMI and UNSW) • Spatialdistributionat 1.5 msand 4 ms (UW, ETH, POLIMI, UNSW and ANL) Analysis sootresultsforreferencecase • Total soot mass in time • Sootspatialextentat 1.5/2.0/2.5 mscomparedto KL (qualitative) • SVF comparisonat 4 ms (quantitative) • Meanparticlesizeat 4ms Analysis Sootonset • Evolution ofsoot mass andlocation Sensitivityanalysissoot model • Surfacegrowth rate Conclusions Outlook

  25. Overview ECN Soot modeling • ECN 1: No soot results presented • ECN 2: Only Spray H (n-heptane) considered • Two contributors: UW and ETH • Both used two-equation soot model • UW: G. Vishwanathan et al.,Comb. Sci. and Tech. 182 (2010) • ETH: M. Bolla et al., Comb. Sci. and Tech. 185 (2013) • Comparison of quasi-steady soot only • ECN 3: Spray A (n-dodecane) considered • Three contributors: UW, ETH and POLIMI • All used two-equation soot model • UW and ETH used the same soot model as ECN 2 • Soot modeling for Spray A at early stage (to-date no publication) • Comparison of soot temporal and spatial evolution • Focus on soot onset evolution

  26. Two-equation soot model • Solve transport equation for soot mass fraction and number density • Accounts for inception, surface growth, coagulation and surface oxidation • Calibrated reaction rates (semi-empirical) • Mono-disperse spherical soot particles assumed • Agglomeration neglected FUEL Chemical mechanism (0) ACETYLENE / PAH Inception (1) Surface Growth (2) Coagulation (5) Surface oxidation (3-4) PRODUCTS

  27. Two-equation soot model FUEL Chemical mechanism (0) (1) Particle Inception ETH and POLIMI: ACETYLENE / PAH UW: Inception (1) (2) Particle Surface Growth (3) Particle Oxidation by O2 Surface Growth (2) Coagulation (5) (4) Particle Oxidation by OH Surface oxidation (3-4) (5) Particle Coagulation PRODUCTS

  28. Modeling Approach Computational grid Related sub-models Lift-off length Onset of the averaged OH concentration Ignition delay MaxmiumdT/dt MaxmiumdOH/dt

  29. Soot Modeling Approach G. Vishwanathan et al., Combustion Science and Technology, 2010, 182(8):1050-1082.

  30. Soot modeling results Non-reacting mixing Reacting conditions

  31. Total C2H2 mass ID Different ID: UW 0.82 ms ETH 0.48 ms POLIMI 0.62 ms UNSW 0.70 ms EXPERIMENT 0.41 ms • Large differences in peak C2H2 mass (factor 4) • All simulationpredict a plateau after approx. 3 ms • Delays in startof C2H2 productioncoincideswithdifferences in ID

  32. C2H2 comparison at 1.5 and 4 ms r=0mm 1.5 ms r=0mm 4 ms LOL

  33. Total soot mass • Comparison total soot mass • Onsetofsootformation ID • UW and ETH show a comparablemagnitudeandshape • Experimental firstsootbump not capturedbythemodels • Delays in startofsootformationcoincideswithdifferences in ID

  34. Temporal evolution soot region: 1.5/2.0/2.5 ms • Experiment: KL signal • Simulation: normalized SVF • Qualitative 1.5 ms 2 ms 2.5 ms • Sootregion in qualitative agreement • Differences in sootspreadandtippenetration • Simulation hasshorterpenetrationat 2/2.5 ms

  35. Quantitative Soot volume fraction at 4 ms r=0mm LOL z=60mm [ppmv] • Sootregion in qualitative agreement • Different axial offsets LOL-soot • UW and ETH showcomparableresults • UW tighter in radius ->lesssootvolume

  36. Computed mean particle size at 4 ms [nm] • UW and ETH modelspredictlargestparticlesof 17-18 nm • Largestmeanparticlesizeatpeaksoot

  37. Soot onset: Evolution axial soot mass • UW • ETH • EXP ID=0.82 ms ID=0.48 ms ID=0.41 ms • Forsootonsetanalysis „resetprocesses“ • -> Consider time after ID • ETH shows a goodshape, soot 2 timeslower • UW is 2 timeslowerthan ETH • -> Comparable SVF but lowerspreadofthesootregion • UW overpredictslocationofsootonset • -> due to larger ID (0.82 vs. 0.41 ms)

  38. Soot onset: Evolution SVF simulation • ETH • UW ID=0.82 ms ID=0.48 ms • Evolution of SVF iscomparable • UW reaches half SVF max after ID+0.7ms and ETH takes 0.8 ms • (quasi-steady SVF maxis 6 ppmv)

  39. Soot onset: Mean particle size evolution • ETH • UW ID=0.82 ms ID=0.48 ms • UW shows a strong particlesizepeakat ID+0.1 ms • ETH shows a more smooth increaseatthebeginning (ID+0.1-0.2 ms) • Fast stabilizationofparticlesizeupstream Spray A TEM 60 mm IFPEN/Meiji

  40. Sensitivity analysis: Surface growth -33% • Soot mass ismost sensitive w.r.t. surfacegrowth (cf. e.g. Bolla et al., CST 2013) • -> most illustrative sensitivitystudy • A 33% reduction in surfacegrowthdecreases total soot mass but not theshape • Both UW and ETH reactanalogously: reductionofsoot mass by 40-50% • Radial SVF profilesarenearly down-scaled ->Sootregionremainsthe same

  41. Summary and conclusions • Detailedanalysisofsootformationperformedforreferencecase • Large differences in C2H2 andsootonset -> DIFFERENT ID • Sootonset: firstsootpeak not reproduced • Probablymixingrelated (Tipvortexdynamics) -> LES needed? • Quasi-steadysootfairly well captured (same as ECN 2) • Sensitivityanalysis on surfacegrowthassessed • Consistentresultswithandwithout TCI • Sootspatialextentremainsunchanged • -> Mostlymixturefractiondetermineswheresootis • Beforelookingat TCI andmorecomplexsootmodelsoneshould: • Assureaccuratetippenetrationandmixturefractiondistribution • Improve ID

  42. Outlook - Topic 2.3 Soot field • Experimental Soot: • Extinction Imaging in constant flow vessel (build up statistics for time-resolved tomographic reconstruction) • Gas sampling (can we measure acetylene axial profile?) • Combined laser-induced incandescence with extinction imaging • Spectrally resolved laser-induced fluorescence (progression of PAH growth) • Quantify soot in Spray A with other injectors • Multiple injections • Spray B • Sootmodeling: • Keywordforfuture: TRANSIENT • Short injection, multiple injection • Understanding thefirstsootbump • Need formoreaccuratechemicalmechanisms – ID must beimproved • Alternatively: re-visit n-heptanesprays in moredetail?

  43. LIF 355: consideration CH2O and PAH (first impression) • Simulation UW at 4 ms • Experiment IFPEN • LIF 355 at 4.7 ms • First impressionofsimulationcomparedto LIF 355 • CH2O ismoreupstreamand PAH(A4) ismoredownstreamthanexp. • LIF 355 coincidesapprox. with UW simulated C2H2

  44. Sandiaconstant-volumeSteady soot DI CMC Exp. 42 bar 85 bar • Comparable soot volumefraction • DI tight, CMC broaddistribution • Experiment is in between Source: Bolla et al., Comb. Theory Modelling (2014)

  45. Sandiaconstant-volumeQuasi-steady soot Formation Oxidation PDF C2H2 soot PDF soot CMC DI CMC DI O2 • Soot formation rate is comparable • DI predicts 500 times larger soot oxidation rate • Caused by limited mixture fraction co-existance range Source: Bolla et al., Comb. Theory Modelling (2014)

  46. Sandiaconstant-volumeTransient soot • 12% O2, 14.8 kg/m3, 1000 K • DOI=1.8 ms 3 4 2 • DI overpredicts soot oxidation after end ofinjection 1 1 2 3 4 Source Exp.: Idicheria and Pickett, IJER (2011)

  47. Pyrometry • IFPEN 2-Color Setup • Collected 425 +/- 15 nm and 676 +/- 14.5 nm • Calibrated with Santoro burner inside vessel at 1 atm • Eliminates uncertainties associated with sootemissivity • 15 images at 3.5 ms ASOI, ensemble averaged Spray A, Tsoot

  48. Pyrometry • Sandia Imaging Spectrometer Setup • System images only the central 1.4 mm along spray axis • Collects emission from entire spray event • Exposure derived from high-speed imaging • Spectra quantified using a calibrated integrating sphere

  49. Pyrometry • Two very different pyrometry approaches • IFPEN: 2-color, 2 camera pyrometry • Sandia: Imaging Spectrometer, long exposure, center 1.4 mm along spray axis

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