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

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)

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) INERT VS REACTING SPRAY

On-axis cl values Radial integral

ECN 3 – Bridge the gap 5 April 5 th 2014 PENETRATION RADIUS FLUXON-AXIS INERT VS REACTING SPRAY LAYOUT

ECN 3 – Bridge the gap 6 April 5 th 2014 INERT VS REACTING SPRAY Before SOC – Similar spray behaviour

ECN 3 – Bridge the gap 7 April 5 th 2014 INERT VS REACTING SPRAY After SOC – Radial expansion of the spray

ECN 3 – Bridge the gap 8 April 5 th 2014  Radius, Little effect on tip penetration  mdot (entrainment) Mdot unbalanced Mdot = =M0nozzle  ucl, zcl INERT VS REACTING SPRAY After SOC – Radial expansion of the spray

ECN 3 – Bridge the gap 9 April 5 th 2014 INERT VS REACTING SPRAY After SOC – Radial expansion of the spray

ECN 3 – Bridge the gap 10 April 5 th 2014 INERT VS REACTING SPRAY Acceleration of reacting tip over inert one  ucl, zcl

ECN 3 – Bridge the gap 11 April 5 th 2014 INERT VS REACTING SPRAY Acceleration of reacting tip over inert one

ECN 3 – Bridge the gap 12 April 5 th 2014 INERT VS REACTING SPRAY Acceleration of reacting tip over inert one

ECN 3 – Bridge the gap 13 April 5 th 2014 INERT VS REACTING SPRAY Acceleration of reacting tip over inert one

ECN 3 – Bridge the gap 14 April 5 th 2014 Mdot = Mdot = =M0nozzle INERT VS REACTING SPRAY Quasi-steady penetration  mdot (entrainment)

ECN 3 – Bridge the gap 15 April 5 th 2014 INERT VS REACTING SPRAY Quasi-steady penetration

ECN 3 – Bridge the gap 16 April 5 th 2014 Stabilized Flame length?? INERT VS REACTING SPRAY Quasi-steady penetration  ucl, zcl  mdot (entrainment) Mdot = Mdot = =M0nozzle  Radius

ECN 3 – Bridge the gap 17 April 5 th 2014 INERT VS REACTING SPRAY Sequence of events Initial identical penetration Heat release induces radial expansion Flow rearranges internally and undergoes an acceleration period as a quasi-steady flow –Same momentum –Lower entrainment –Higher velocities

Recent investigations of nozzle to nozzle variations  ECN2 showed similar ignition delay and lift-off length measurements among different facilities despite the variations of ( ECN2 proceedings: Ignition and Lift-off Length, 2012 ):  Injectors  Ambient compositions, e.g., CVP vs. CPF  Measurement techniques Dispersion Liquid length17% Vapor penetration length5% at 1 ms Ignition delayLess than 5% Lift-off length7%  A set of new Spray A injectors investigated at IFPEN ( Malbec et al. SAE Paper ):  Significant difference on liquid length  Much smaller dispersion of the results in the far field

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

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

Variables and their Uncertainty Range ParametersMinMax BoundariesVessel wall temperature (K) Initial gas velocity fluctuation (m/s)0.011 Ambient temperature (K) Ambient pressure (MPa) Ambient O OH (ppb)016 CO206.4 H2O011.6 Duration of injection (ms) Fuel temperature (K) Discharge coefficient Nozzle diameter (micron) Fuel propertiesCritical temp (K) Density* Heat of vopoization* Vapor pressure* Viscosity* * Normalized by the baseline values Maybe even bigger!!  Targets studied:  Liquid length  Vapor penetration length at 1.5 ms  Ignition delay  Lift-off length An example of liquid length results from 60 cases

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

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

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

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.

Uncertainty Quantification – Vapor penetration length  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 K

Uncertainty Quantification – ID 800 K  800 K:  Ambient T dominates  Ambient O 2 doesn’t show up  1100 K:  Ambient T rank #1  Comparable OH and ambient O K [Pickett et al. SAE ]

Uncertainty Quantification – ID 900 K  Ambient O 2 dominates at 900 K.  Ambient T is not sensitive around 900 K, probably due to NTC behavior? [Pickett et al. SAE ]

Uncertainty Quantification – LOL 900 K  Ambient O 2 dominates at 900 K.  Comparable sensitivity of nozzle diameter:  Bigger nozzle diameter, longer lift- off length. In agreement with Siebers and Higgins, SAE Paper,

Uncertainty Quantification – LOL 800 K 1100 K  800 K:  Ambient T dominates  Comparable OH and nozzle diameter  1100 K:  Ambient T is most sensitive  Comparable ambient O2 and nozzle diameter [Siebers et al. SAE ]

 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. Summary and Conclusions:

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

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 The near injector region, courtesy of Lyle Pickett.  Especially in the region < 2 mm, close the injector  “vacuum cleaner”

Dilatation and entrainment effect Temporal evolution of dilation effect of reacting condition compared to the nonreacting condition at T amb = 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.  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. Y. Pei, PhD thesis, UNSW, 2013

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

 T difference in the core region can be as high as 100 K, or even more! T profiles  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

Movie – Uniform T vs. Actual T % 900 K Liquid length17.23 Ignition delay16.0 Lift-off length K  Actual T delays ignition  Asymmetric flame found in simulation, but not systematically observed in experiments yet (SAE Paper, )  Retarded ignition will make the ignition delay predictions even worse in topic 2! Better chemical mechanism!!

Random variation in T on top of the mean No T variation T variation +/- 10 K  Random variation in temperature on top of the mean (+/- 10 K for 900 K case)  Pickett et al. SAE  Three random cases tested: % 900 K Liquid length 0.8% Ignition delay2.3 Lift-off length3.9

 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!! Conclusion and Suggestions:

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