1. ECN 3 1 Apr 2014 Subtopic 2.3: Soot Field 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 POLIMI Wisconsin Sandia

2. ECN 3 2 Apr 2014 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) Emre Cenker 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/m 3 ) –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 O 2 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. ECN 3 3 Apr 2014 Subtopic 2.3: Objectives 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% O 2 ) 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? “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

4. ECN 3 4 Apr 2014 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. ECN 3 5 Apr 2014 Extinction Imaging Spray A 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

6. ECN 3 6 Apr 2014 Time Sequence of LII vs. Time-Resolved Extinction *T amb : 930 K *ρ amb : 21.8 kg/m 3 Can compare progression of total soot mass as an indicator of soot onset Appears to be a mismatch in reacting vapor penetration

7. ECN 3 7 Apr 2014 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. ECN 3 8 Apr 2014 Soot Onset: Timing and Location 15% 850 K 15% 1000 K T amb [K]8509001000 Mean Soot Mass [µg] (quasi-steady) 21442

9. ECN 3 9 Apr 2014 Soot Onset: Timing and Location 15% 850 K 15% 1000 K T amb [K]8509001000 Mean Soot Mass [µg] (quasi-steady) 21442 Full soot field was not captured, so numbers are considered low relative to reality

10. ECN 3 10 Apr 2014 Soot Onset: Timing and Location 13% 900 K 21% 900 K O 2,amb [%]131521 Mean Soot Mass [µg] (quasi-steady) 101411

11. ECN 3 11 Apr 2014 Soot Onset: Timing and Location 13% 900 K 21% 900 K O 2,amb [%]131521 Mean Soot Mass [µg] (quasi-steady) 101411 Full soot field was not captured, so numbers are considered low relative to reality

12. ECN 3 12 Apr 2014 Soot Timing and Location Relative to Ignition Parametric variation around Spray A in temperature and O 2 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. ECN 3 13 Apr 2014 Time-Resolved Total Soot Mass Higher ambient temperature and O 2 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% O 2 case Wisconsin

14. ECN 3 14 Apr 2014 Ensemble Averaged SVF (IFPEN/Sandia)

15. ECN 3 15 Apr 2014 Ensemble Averaged SVF sdf LII n-heptane: 15% O 2, 1000 K, 1500 bar, 30 kg/m 3, 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. ECN 3 16 Apr 2014 Radial Profiles of f v Signal trapping may cause plateau in LII data Correction must be applied to raw LII signal before integration and calculation of f v 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 k e Sandia KL using 406 nm incident light

17. ECN 3 17 Apr 2014 Non-dimensional Extinction Coeff., k e Primary Particle Diameter, d p k e (N=5) k e (N=75) k e (N=150) [nm]unitless 107.037.087.12 167.047.217.28 207.067.337.47 307.127.778.04 407.258.378.77 507.459.089.62 607.729.9010.6 Standard k e was updated from 4.9 to 8.7 for 632.8 nm extinction measurements k e 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) N p primary particles per aggregate, d p primary particle diameter Incident wavelength of 632.8 nm Greater effect of N p for larger primary particle size Small particles sizes in Spray A measured by TEM means uncertainty in assumption of constant N p is reduced Greatest uncertainty remains in the refractive index of soot

18. ECN 3 18 Apr 2014 Non-dimensional Extinction Coeff., k e Primary Particle Diameter, d p k e (N=5) k e (N=75) k e (N=150) [nm]unitless 107.037.087.12 167.047.217.28 207.067.337.47 307.127.778.04 407.258.378.77 507.459.089.62 607.729.9010.6 Standard k e was updated from 4.9 to 8.7 for 632.8 nm extinction measurements k e 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) N p primary particles per aggregate, d p primary particle diameter Incident wavelength of 632.8 nm Greater effect of N p for larger primary particle size Small particles sizes in Spray A measured by TEM means uncertainty in assumption of constant N p is reduced Greatest uncertainty remains in the refractive index of soot O3 (21% O 2 )

19. ECN 3 19 Apr 2014 Signal Trapping Correction based on Sandia extinction data improves plateau somewhat Correction actually decreases mass along chosen cross section by 4% Use uncorrected f v as I LII (x,y), make correction based on Gaussian KL from Sandia data, re-integrate new KL LII Correction increases mass by a factor of 1.8

20. ECN 3 20 Apr 2014 Total Soot Mass IFPEN calibrated with 632.8 HeNe laser extinction –k e = 8.7 was standard at the time of publication Sandia extinction imaging with 406 nm LED –k e = 7.76 based on RDG theory with d p = 16 nm and N p = 150

21. ECN 3 21 Apr 2014 Total Soot Mass IFPEN calibrated with 632.8 HeNe laser extinction –k e = 8.7 was standard at the time of publication –k e = 7.28 from RDG theory with d p = 16 nm, N p =150 as in Imaging Extinction work (20% increase in f v and soot mass)

22. ECN 3 22 Apr 2014 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 k e 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 N p throughout the soot field.

23. ECN 3 23 Apr 2014 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) T amb = 905 K Lift-off: 16.09 τ ig = 404 µs (chemi) τ ig = 400 µs (press) T amb = 902.5 K Lift-off: 16.23 τ ig = 344 µs (chemi) faster camera τ ig = 370 µs (press)

24. ECN 3 Topic 2.3 – Soot fields 24 April 4 th 2014 Outline: Soot modeling Presentation soot models used (3 contributors) –UW, POLIMI and ETH Analysis C2H2 as soot „initial condition“ –C2H2 total mass in time (UW, ETH, POLIMI and UNSW) –Spatial distribution at 1.5 ms and 4 ms (UW, ETH, POLIMI, UNSW and ANL) Analysis soot results for reference case –Total soot mass in time –Soot spatial extent at 1.5/2.0/2.5 ms compared to KL (qualitative) –SVF comparison at 4 ms (quantitative) –Mean particle size at 4ms Analysis Soot onset –Evolution of soot mass and location Sensitivity analysis soot model –Surface growth rate Conclusions Outlook

25. ECN 3 Topic 2.3 – Soot fields 25 April 4 th 2014 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. ECN 3 Topic 2.3 – Soot fields 26 April 4 th 2014 Two-equation soot model ACETYLENE / PAH PRODUCTS Inception (1) Coagulation (5) Surface Growth (2) Surface oxidation (3-4) FUEL Chemical mechanism (0)  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

27. ECN 3 Topic 2.3 – Soot fields 27 April 4 th 2014 Two-equation soot model ACETYLENE / PAH PRODUCTS Inception (1) Coagulation (5) Surface Growth (2) Surface oxidation (3-4) FUEL Chemical mechanism (0) (1) Particle Inception (5) Particle Coagulation (2) Particle Surface Growth (3) Particle Oxidation by O 2 (4) Particle Oxidation by OH ETH and POLIMI: UW:

28. Modeling Approach Temp [K]800850900100011001200 O 2 [vol%]1513/15/17/21 Density [kg/m 3 ] 22.8 7.6/15.2/ 22.8/30.4 7.6/15.2/ 22.8/30.4 7.6/15.2/ 22.8/30.4 7.6/15.2/ 22.8/30.4 7.6/15.2/ 22.8/30.4 P inj [MPa]15050/100/150 Computational grid Related sub-models Lift-off length Onset of the averaged OH concentration Ignition delay Maxmium dT/dt Maxmium dOH/dt PhenomenonModel Spray breakupKH-RT instability EvaporationDiscrete multicomponent (DMC) TurbulenceGeneralized RNG k−ε model CombustionSpeedChem Droplet collisionROI model Near nozzle flowGas-jet model Soot formationMulti-step phenomenological

29. Physical processExpression Inception:A 4  soot C 2 H 2 surface growth Coagulation O 2 oxidation OH oxidation PAH condensation Transport equations G. Vishwanathan et al., Combustion Science and Technology, 2010, 182(8):1050-1082. Soot Modeling Approach

30. Non-reacting mixing Soot modeling results Reacting conditions

31. ECN 3 Topic 2.3 – Soot fields 31 April 4 th 2014 Total C2H2 mass Large differences in peak C2H2 mass (factor 4) All simulation predict a plateau after approx. 3 ms Delays in start of C2H2 production coincides with differences in ID Different ID: UW 0.82 ms ETH 0.48 ms POLIMI 0.62 ms UNSW 0.70 ms EXPERIMENT 0.41 ms ID

32. ECN 3 Topic 2.3 – Soot fields 32 April 4 th 2014 C2H2 comparison at 1.5 and 4 ms 1.5 ms 4 ms r=0mm LOL

33. ECN 3 Topic 2.3 – Soot fields 33 April 4 th 2014 Total soot mass Comparison total soot mass Onset of soot formation  UW and ETH show a comparable magnitude and shape  Experimental first soot bump not captured by the models  Delays in start of soot formation coincides with differences in ID ID

34. ECN 3 Topic 2.3 – Soot fields 34 April 4 th 2014 Temporal evolution soot region: 1.5/2.0/2.5 ms 1.5 ms2 ms2.5 ms Qualitative  Soot region in qualitative agreement  Differences in soot spread and tip penetration  Simulation has shorter penetration at 2/2.5 ms Experiment: KL signal Simulation: normalized SVF

35. ECN 3 Topic 2.3 – Soot fields 35 April 4 th 2014 Soot volume fraction at 4 ms r=0mm z=60mm Quantitative  Soot region in qualitative agreement  Different axial offsets LOL-soot  UW and ETH show comparable results  UW tighter in radius ->less soot volume LOL [ppmv]

36. ECN 3 Topic 2.3 – Soot fields 36 April 4 th 2014 Computed mean particle size at 4 ms [nm]  UW and ETH models predict largest particles of 17-18 nm  Largest mean particle size at peak soot

37. ECN 3 Topic 2.3 – Soot fields 37 April 4 th 2014 Soot onset: Evolution axial soot mass UWETHEXP ID=0.82 msID=0.48 msID=0.41 ms  For soot onset analysis „reset processes“ -> Consider time after ID  ETH shows a good shape, soot 2 times lower  UW is 2 times lower than ETH -> Comparable SVF but lower spread of the soot region  UW overpredicts location of soot onset -> due to larger ID (0.82 vs. 0.41 ms)

38. ECN 3 Topic 2.3 – Soot fields 38 April 4 th 2014 Soot onset: Evolution SVF simulation UW ETH ID=0.82 msID=0.48 ms  Evolution of SVF is comparable  UW reaches half SVF max after ID+0.7ms and ETH takes 0.8 ms (quasi-steady SVF max is 6 ppmv)

39. ECN 3 Topic 2.3 – Soot fields 39 April 4 th 2014 Soot onset: Mean particle size evolution UW ETH ID=0.82 msID=0.48 ms  UW shows a strong particle size peak at ID+0.1 ms  ETH shows a more smooth increase at the beginning (ID+0.1-0.2 ms)  Fast stabilization of particle size upstream Spray A TEM 60 mm IFPEN/Meiji

40. ECN 3 Topic 2.3 – Soot fields 40 April 4 th 2014 Sensitivity analysis: Surface growth -33%  Soot mass is most sensitive w.r.t. surface growth (cf. e.g. Bolla et al., CST 2013) -> most illustrative sensitivity study  A 33% reduction in surface growth decreases total soot mass but not the shape  Both UW and ETH react analogously: reduction of soot mass by 40-50%  Radial SVF profiles are nearly down-scaled ->Soot region remains the same

41. ECN 3 Topic 2.3 – Soot fields 41 April 4 th 2014 Summary and conclusions  Detailed analysis of soot formation performed for reference case  Large differences in C2H2 and soot onset -> DIFFERENT ID  Soot onset: first soot peak not reproduced  Probably mixing related (Tip vortex dynamics) -> LES needed?  Quasi-steady soot fairly well captured (same as ECN 2)  Sensitivity analysis on surface growth assessed  Consistent results with and without TCI  Soot spatial extent remains unchanged -> Mostly mixture fraction determines where soot is  Before looking at TCI and more complex soot models one should:  Assure accurate tip penetration and mixture fraction distribution  Improve ID

42. ECN 3 Topic 2.3 – Soot fields 42 April 4 th 2014 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 Soot modeling:  Keyword for future: TRANSIENT  Short injection, multiple injection  Understanding the first soot bump  Need for more accurate chemical mechanisms – ID must be improved  Alternatively: re-visit n-heptane sprays in more detail?

43. ECN 3 Topic 2.3 – Soot fields 43 April 4 th 2014

44. ECN 3 Topic 2.3 – Soot fields 44 April 4 th 2014 LIF 355: consideration CH2O and PAH (first impression)  First impression of simulation compared to LIF 355  CH2O is more upstream and PAH(A4) is more downstream than exp.  LIF 355 coincides approx. with UW simulated C2H2 Simulation UW at 4 ms Experiment IFPEN LIF 355 at 4.7 ms

45. Sandia constant-volume Steady soot  Comparable soot volume fraction  DI tight, CMC broad distribution  Experiment is in between DICMCExp. 42 bar 85 bar Source: Bolla et al., Comb. Theory Modelling (2014)

46. Sandia constant-volume Quasi-steady soot  Soot formation rate is comparable  DI predicts 500 times larger soot oxidation rate  Caused by limited mixture fraction co-existance range FormationOxidation PDF soot O2O2 C2H2C2H2 PDF DI CMC Source: Bolla et al., Comb. Theory Modelling (2014)

47. Sandia constant-volume Transient soot  DI overpredicts soot oxidation after end of injection 1 2 3 4 1 2 34  12% O 2, 14.8 kg/m 3, 1000 K  DOI=1.8 ms Source Exp.: Idicheria and Pickett, IJER (2011)

48. ECN 3 48 Apr 2014 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 soot emissivity –15 images at 3.5 ms ASOI, ensemble averaged Spray A, T soot

49. ECN 3 49 Apr 2014 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

50. ECN 3 50 Apr 2014 Pyrometry Two very different pyrometry approaches –IFPEN: 2-color, 2 camera pyrometry –Sandia: Imaging Spectrometer, long exposure, center 1.4 mm along spray axis

51. ECN 3 51 Apr 2014 Soot Subtopic 2.3 Contributors Experimental –Sandia extinction imaging: Time-resolved KL maps, soot mass, and f v maps during quasi-steady period Soot pyrometry (Imaging Spectrometer): Spatially resolved soot particle temperature and KL along central axis of spray flame + total radiation from broadband soot emission –IFPEN Laser-induced Incandescence & Laser Extinction: Time sequence of f v along central plane of spray flame, ensemble averaged f v during quasi-steady period Two-camera, Two-color pyrometry: 2-D map of soot particle temperature –IFPEN/Meiji Soot sampling/TEM analysis: Soot particle sizing

52. ECN 3 52 Apr 2014 Subtopic 2.3: Overall Objectives What is the soot distribution for Spray A? –How is it modified with different parametric variables? –How do different measurement techniques compare? –How accurate do different modeling approaches predict the soot field? “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 High-speed Extinction Imaging, Spray A, n-dodecane

53. ECN 3 53 Apr 2014 Soot Onset: Timing and Location 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 model results 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 T1 (800 K) Extinction due to beam steering helps define threshold. Soot extinction not detected for 800 K case. Soot mass attributed to beam steering equivalent to approx. 0.25 µg