Bridging the gap: How Spray details (Topic 1) affect Combustion (Topic 2)

1. Bridging the gap: How Spray details (Topic 1) affect Combustion (Topic 2) Yuanjiang Pei, Sibendu Som: Argonne National Laboratory Jose Garcia: CMT-Motores Termicos 4/5/2014

2. Objectives • Designed to bridge-the-gap between spray (Topic 1) and combustion (Topic 2) for Spray A • How do the differences in the initial boundary conditions and spray characteristics influence combustion characteristics? • Can simulations using the best boundary conditions available, capture these trends? • Why differences in spray characteristics do not seem to influence the combustion behavior? • What are the most sensitive variables for different targets of spray and combustion characteristics? - (global sensitivity analysis)

3. INERT VS REACTING SPRAY MOTIVATION • A comparison between an inert spray and a reacting one seems to be pertinent • Insight into the analysis of flame time evolution • Validation of modelling • Modelling results will be shown to enable the potential of such a comparison • Nominal Spray A under inert (0% O2) and reacting (15% O2) conditions • ETH CFD results (few available calculations for both inert and reacting conditions)

4. INERT VS REACTING SPRAY DEFINITIONS • Tip Penetration: Maximum distance from the nozzle outlet to where mixture fraction is 0.1% • Spray Radius: Location where z = 1% zcl • Fluxes from radial integrals • Mdot: • mdot: • Variables on the axis Radial integral On-axis cl values

5. INERT VS REACTING SPRAY LAYOUT PENETRATION RADIUS FLUX ON-AXIS

6. INERT VS REACTING SPRAY Before SOC – Similar spray behaviour

7. INERT VS REACTING SPRAY After SOC – Radial expansion of the spray

8. INERT VS REACTING SPRAY After SOC – Radial expansion of thespray Radius, Little effectontippenetration Mdotunbalanced Mdot = =M0nozzle ucl, zcl mdot (entrainment)

9. INERT VS REACTING SPRAY After SOC – Radial expansion of thespray

10. INERT VS REACTING SPRAY Acceleration of reactingtipoverinertone ucl, zcl

11. INERT VS REACTING SPRAY Acceleration of reactingtipoverinertone

12. INERT VS REACTING SPRAY Acceleration of reactingtipoverinertone

13. INERT VS REACTING SPRAY Acceleration of reactingtipoverinertone

14. INERT VS REACTING SPRAY Quasi-steadypenetration Mdot = Mdot = =M0nozzle mdot (entrainment)

15. INERT VS REACTING SPRAY Quasi-steadypenetration

16. INERT VS REACTING SPRAY Quasi-steadypenetration Radius StabilizedFlamelength?? Mdot = Mdot = =M0nozzle ucl, zcl mdot (entrainment)

17. INERT VS REACTING SPRAY Sequence of events • Initialidenticalpenetration • Heatrelease induces radial expansion • Flowrearrangesinternally and undergoesanaccelerationperiod as a quasi-steadyflow • Samemomentum • Lowerentrainment • Highervelocities

18. Recent investigations of nozzle to nozzle variations • ECN2 showed similar ignition delay and lift-off length measurementsamong different facilities despite the variations of (ECN2 proceedings: Ignition and Lift-off Length, 2012): • Injectors • Ambient compositions, e.g., CVP vs. CPF • Measurement techniques • A set of new Spray A injectors investigated at IFPEN (Malbec et al. SAE Paper 2013-24-0037): • Significant difference on liquid length • Much smaller dispersion of the results in the far field

19. Questions to answer: • Why differences in spray characteristics do not seem to influence the combustion behavior? • ---- Momentum driven! • What are the most sensitive boundary conditions and variables affecting different spray and combustion targets? Global Sensitivity Analysis

20. Key Steps For GSA • The fit of the response to the uncertainties leads to a variance associated with each variable (partial variance: Vi) • Calculate sensitivity coeffs., Si = Vi/V, Σ Si≅ 1, (V: total variance) Y. Pei, R. Shan, S. Som, T. Lu, D. Longman, M.J. Davis, SAE Paper 2014-01-1117, 2014. D.Y. Zhou, M.J. Davis, R.T. Skodje, The Journal of Physical Chemistry A, pp. 3569-3584, 2013. • Simulations varying all variables over uncertainty ranges simultaneously • Fit the response (ignition delay, liquid length, etc) to the uncertainties

21. Variables and their Uncertainty Range • Targets studied: • Liquid length • Vapor penetration length at 1.5 ms • Ignition delay • Lift-off length Maybe even bigger!! An example of liquid length results from 60 cases * Normalized by the baseline values

22. Lift-off length vs. ignition delay WM: 60 cases x 250 cpu hours RIF: 120 cases x 1000 cpu hours Expe: 900 K • Clear correlation between lift-off length and ignition delay: • Longer ignition delay -> longer lift-off length

23. Ignition delay and lift-off length vs. liquid length • No correlation was found for all the ambient conditions: • Ignition delay vs. liquid length • Lift-off length vs. liquid length

24. Uncertainty Quantification – Liquid length • Liquid length at 900 K: • Fuel temperature dominates liquid lengths • Trend predicted well compared with • Pickett et al. 2010-01-2106 • Meijer et al. AAS - 6083 • Ambient T is not picked up probably due to the large uncertainty of the fuel T. Pickett et al. 2010-01-2106

25. Uncertainty Quantification – Liquid length • Liquid length: • Fuel temperature dominates liquid lengths at 800 K and 1100 K. • Nozzle diameter becomes important for 1100 K condition, probably due to the faster vaporization rate.

26. Uncertainty Quantification – Vapor penetration length 900 K • Nozzle diameter ranks #1 for vapor penetration length at 900 K. • Similar for 800 K and 1100 K conditions. • Different nozzles showed 5% dispersion in Malbec et al SAE 2013-24-0037.

27. Uncertainty Quantification – ID 800 K • 800 K: • Ambient T dominates • Ambient O2 doesn’t show up • 1100 K: • Ambient T rank #1 • Comparable OH and ambient O2 1100 K [Pickett et al. SAE 2005-01-3843]

28. Uncertainty Quantification – ID 900 K • Ambient O2 dominates at 900 K. • Ambient T is not sensitive around 900 K, probably due to NTC behavior? [Pickett et al. SAE 2005-01-3843]

29. Uncertainty Quantification – LOL 900 K • Ambient O2 dominates at 900 K. • Comparable sensitivity of nozzle diameter: • Bigger nozzle diameter, longer lift-off length. In agreement with Siebers and Higgins, SAE Paper, 2001-01-0530.

30. Uncertainty Quantification – LOL 800 K • 800 K: • Ambient T dominates • Comparable OH and nozzle diameter • 1100 K: • Ambient T is most sensitive • Comparable ambient O2 and nozzle diameter 1100 K [Siebers et al. SAE 2002-01-0890]

31. Summary and Conclusions: • Clear correlation of ignition delay and lift-off length. • No clear relation for ignition delay and lift-off length vs. liquid length. • Fuel temperature is clearly important for liquid length. • Ignition delay and lift-off length: • Ambient composition and ambient temperature play significant roles. • Even though fuel temperature uncertainty is so big, it does not seem to significantly affect ignition delay and lift-off length. • Nozzle diameter seems to affect vapor penetration and lift-off length.

32. Questions to answer: • How do the differences in the initial boundary conditions and spray characteristics influence combustion characteristics? • Can simulations using the best boundary conditions available?

33. Temperature distribution in the vessel • Experiments: • Meijer et al., AAS, 2012 • ECN website • Temperature distribution in the vessels due to buoyancy • Small on spray axis after 4 mm • Small on horizontal plane • Significant on vertical direction • Especially in the region < 2 mm, close the injector • “vacuum cleaner” The near injector region, courtesy of Lyle Pickett.

34. Dilatation and entrainment effect • Non-reacting case: • (Vectors show the expected features of a transient jet) • Axial velocities peak on the centre line • A radially diverging flow around the jet head. • Entrainment is evident towards the nozzle. • Combination of the radially diverging flow at the head and the entrainment flow behind creates a counter-clockwise vortex. • Observed in experimental PIV measurements of the same case at IFPEN (ECN2 proceedings, 2012). • Reacting case: • (Similar flow structure) • Significant dilatation due to combustion, e.g., at 1.0 ms, strong outwardly expanding flow due to intense premixed burn. • Couples with the entraining flow to create an even stronger counter clockwise vortex. • Transport of hot products upstream of the flame base, accelerating ignition and promoting flame stabilisation further downstream. Temporal evolution of dilation effect of reacting condition compared to the nonreacting condition at Tamb= 900 K. The black solid line is the reacting boundary. The green dash-dot line is the non-reacting boundary. The red dashed line is flame existing. The blue arrows are the ambient velocity vectors. Y. Pei, PhD thesis, UNSW, 2013

35. T ratio – 900 K – initialization Injector • Injector protrudes into vessel 1.1 mm. • Smallest cell size 0.125 mm. • Good initialization compared to measurements on the injector centerline. Injector starts here

36. T profiles • T difference in the core region can be as high as 100 K, or even more! • At X = 2 mm, ambient T is lower than initial T indicates that the cold gas near the boundary layer is really pulled in. • At X = 10 mm, the hot and cold ambient gas in upper and lower vessel is entrained into mixing layer. Low T reaction

37. Movie – Uniform T vs. Actual T • Actual T delays ignition • Asymmetric flame found in simulation, but not systematically observed in experiments yet (SAE Paper, 2010-01-2106) • Retarded ignition will make the ignition delay predictions even worse in topic 2! • Better chemical mechanism!!

38. Random variation in T on top of the mean No T variation • Random variation in temperature on top of the mean (+/- 10 K for 900 K case) • Pickett et al. SAE 2010-01-2106 • Three random cases tested: T variation +/- 10 K

39. Conclusion and Suggestions: • Actual temperature distribution in the combustion vessel is very important. • Asymmetric flame • Significantly affect spray and combustion • Suggestions: • Experiments: temperature distribution in the < 2 mm region should be measured with capable instruments • Simulations: use this actual temperature distribution. • Better chemical mechanism for n-dodecane!!

40. Acknowledgement • Thanks Michal Davis for providing the code of global sensitivity analysis. • Thanks Lyle Pickett, Maarten Meijer and Julien Manin for the useful discussions.