Laminar Flames and the Role of Chemistry and Transport Yiguang Ju and Sang Hee Won Princeton University, USA Zheng Chen Peking University, China 1 st Flame Chemistry Workshop
Advanced Engines require fuel flexibility and work near kinetic limit Synfuels in gas turbine engine HCCI • Low temperature, “Validated“mechanisms ? • High pressure, • Hundreds of fuels • Near ign./ext . limit, • Different structures • Multiple fuels, elements H, C, O, N, S…) Kinetic limiting • Thousands species • Extreme conditions Two validation targets Plasma assisted combustion • Homogeneous ignition/reactor • Inhomogeneous flames
Flame regimes in combustion How does chemistry affect flames? OH PLIF Turbulent fuel stream PIV du (u’, v’), Laser Propagating edge flame in a mixing layer dy Flame PLIF Turbulent flow reactor Reactive flow • Thin flame • Thickening flame • Local extinction Premixed flame front • Re-ignition in non-uniform flow field with complex transport and chemistry coupling
Flames “Flame” is a ignition/reaction front supported by thermal and species transport Fuel decomposition Branching/Termination • H abstraction by H reactions • Radical termination • Radicals • Heat release Fuel Oxidizer fragments Fuel Diffusion Heat release rate Fuel/Oxygen OH + CO =CO2 +H Radicals/Fragments, HCO+OH= CO2+H2O (e.g. H and C2H4) Non-uniform species/temperature distribution
1. Why is flame chemistry different from ignition? Then, what is the role of transport on kinetics? • Is flame chemistry different from that of homogeneous ignition? • How does transport and flame chemistry govern flame extinction? • How does transport and flame chemistry affect unsteady flame initiation and propagation? • How does low temperature chemistry change flame regimes?
Flames: Different fuels have different extinction limits 500 n-decane T f = 500 K and T o = 300 K n-alkanes n-nonane Extinction strain rate a E [1/s] n-heptane JETA POSF 4658 400 Princeton Surrogate iso-octane nPB 300 toluene aromatics 124TMB 135TMB 200 100 How to decouple chemistry from transport and fuel heating value? 0 0 0.05 0.1 0.15 0.2 Fuel mole fraction X f
Ignition vs. flames: n-alkanes and esters 450 T f = 500 K and T o = 300 K Extinction strain rate a E [1/s] nC16 nC10 nC9 Westbrook, 2010 350 increaing carbon # nC7 ? 250 150 Won et al., 2010 50 0.02 0.04 0.06 0.08 0.1 0.12 0.14 Fuel mole fraction, X f 450 T f = 500 K, T ox = 298 K Extinction strain rate a E [1/s] Methyl Formate Methyl Ethanoate 350 ?? Methyl Propanoate Methyl Butanoate Methyl Pentanoate 250 Methyl Hexanoate Methyl Octanoate Methyl Decanoate 150 50 0.05 0.09 0.13 0.17 0.21 0.25 0.29 Fuel mole fraction, X f
Kinetic coupling between alkanes and aromatics Blending toluene into n-decane: Extinction Limits 250 n-decane (present) LIF tested condition n-decane (Seshadri, 2007) Extinction strain rate a E [1/s] toluene (present) N-decane toluene (Seshadri, 2007) calculation (present) 200 80% n-decane + 20% toluene 60% n-decane + 40% toluene 40% n-decane +60% toluene 150 Toluene 100 50 0.02 0.06 0.1 0.14 Fuel mole fraction X f Won, Sun, Dooley, Dryer, and Ju, CF 2010
Kinetic coupling between n-decane and toluene in diffusion flames 1.50E-04 1800 60 % n-decane + 40 % toluene H Diffusion loss near extinction, X f = 0.1, a = 176 1/s 1.00E-04 Reaction rate [mole/cm 3 s] 1500 C6H5CH2+ H = C6H5CH3 5.00E-05 Temperature [K] 1200 0.00E+00 900 overall decomposition -5.00E-05 of n-decane 600 overall decomposition -1.00E-04 of toluene C10H22=C2H5+C8H17-1 C10H22=nC3H7+C7H15-1 300 -1.50E-04 C10H22=CH3+C9H19-1 C6H5CH3+H<=>C6H5CH2+H2 0.1 mm C6H5CH3+OH<=>C6H5CH2+H2O C6H5CH3+H<=>A1+CH3 -2.00E-04 0 0.85 0.9 0.95 1 1.05 1.1 Axial coordinate [cm] Won, Sun, Dooley, Dryer, and Ju, CF 2010
High pressure hydrogen kinetics: ignition and flames 1.20 2 10 H 2 /O 2 /Ar, φ=2.5 Skinner and Ringrose (1965) - 5 atm Schott and Kinsey (1958) - 1 atm T f ~1600K 1.00 Petersen et al. (1995) - 33 atm Mass burning rate (g/cm^2s) 1 10 Petersen et al. (1995) - 64 atm [O 2 ] ig (mol liter -1 s) Petersen et al. (1995) - 57 & 87 atm 0.80 Present model 0 10 Li et al. (2004) 0.60 Present experiments -1 Li et al. (2007) 10 0.40 Davis et al. (2005) Sun et al. (2007) -2 Konnov (2008) 10 0.20 H 2 /O 2 /Ar mixtures O'Connaire et al. (2004) Saxena & Williams (2006) -3 0.00 10 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 1000/T (K -1 ) Pressure (atm) Uncertainty of HO2 related kinetics at high pressure, • Ignition governed more by chain initiation and branching rates • Flames governed more by branching rate and heat release rate • Different radical pool concentration (H, OH, …) Bulke, Marcos, Dryer, Ju, CF, 2010 Burke, Chaos, Ju, Dryer, Klippenstein, IJCK 2011
Difference of kinetics in homogeneous reactor and flames Homogeneous reactor (800 K) Fuel consumption by radicals % consumed by radical reaction component OH HO 2 H O n-decane 86% 6.0% 3.6% 2.0% iso-octane 82% 6.5% 5.8% 2.3% Toluene 88% 2.8% 4.5% OH is r the most significant radical in fuel consumption Diffusion flames (~1600K) Fuel consumption by Radicals % by uni- % consumed by radical reaction component molecular H OH CH 3 O decomposition n-decane [1] 19.3% 67.2% 6.1% 5.8% 0.1% Methylbutanoate [2] 6.6% 70.8% 10.3% 8.1% 3.7% 1. Won et al. CNF 159 (2012) 2. Dooley et al. CNF 159 (2012) 1371-1384.
2. How does transport and flame chemistry govern diffusion flame extinction? 0.9N2+0.09n-decane+0.01toluene C6H5CH3 Fuel decomposition C5H10-1 • H abstraction by H C10H22 • Radical termination C3H6 • Heat release C3H8 C6H5OH A1 Fuel C4H81 Oxidizer aC3H4 fragments pC3H4 CH2CO Fuel C2H2 C2H6 C2H4 CH4 CH3 CO2 CO O OH H H2O dY H2 0 F Q ~ D dx O2 i 0 i N2 dx 0 -0.1 0.0 0.1 0.2 Diffusion sensitivity
Flames: Different fuels have different burning limits 500 n-decane T f = 500 K and T o = 300 K n-alkanes n-nonane Extinction strain rate a E [1/s] n-heptane JETA POSF 4658 400 Princeton Surrogate iso-octane nPB 300 toluene aromatics 124TMB 135TMB 200 100 How to decouple chemistry from transport and fuel heating value? 0 0 0.05 0.1 0.15 0.2 Fuel mole fraction X f
A generic correlation for extinction limit: Transport weighted Enthalpy & radical index Theoretical analysis of Extinction Damkohler number 3 2 Y T T 1 2 1 f O , 3 a Le P ( , Le , Le ) ( L , Le ) exp F F F O F F 2 Da e Y T T T T E F , f a f Extinction Strain Rate Y Q 1 F , F a * R e i C ( T T ) M / M p f F Fuel chemistry Transport Heat release/heat loss Radical production rate Transport weighted Enthalpy *Radical index Won et al. CNF 159 (2012)
A General Correlation of Hydrocarbon Fuel Extinction vs. Transport Weighted Enthalpy (TWE) and Radical Index 500 n-decane Extinction strain rate a E [1/s] n-nonane R² = 0.97 n-heptane 400 iso-octane n-propyl benzene toluene 300 1,2,4-trimethly benzene 1,3,5-trimethly benzene 200 Enabling rapid fuel screening! 100 T f = 500 K and T o = 300 K 0 0.5 1 1.5 2 Ri [Fuel] H c ( MW fuel / MW nitrogen ) -1/2 [cal/cm 3 ] 15
TWE and radical index for predicting extinction limits and synfuel fuel ranking and screening Radical Index Representing radical pool n-decane iso-octane Extinction strain rate a E [1/s] toluene High temperature reactivity 450 1st generation surrogate for Jet-A POSF 4658 Prediction from correlation Prediction from correlation Fuel Radical Index 350 Single fuel n-dodecane 1.0 250 Real fuel iso-octane 0.7 150 toluene 0.56 T f = 500 K and T o = 300 K 50 n-propyl benzene 0.67 0.02 0.06 0.1 0.14 0.18 Fuel mole fraction X f 1,2,4-trimethylbenzene 0.44 450 Extinction of diffusion flame in counterflow configuration T f = 500 K and T air = 300 K @ 1 atm 1,3,5-trimethylbenzene 0.36 400 Extinction strain rate [s -1 ] 350 JetA POSF 4658 0.79 300 S8 POSF 4734 0.86 250 Synfuels JP8 POSF 6169 200 SHELL SPK POSF 5729 JP8 POSF 6169 0.80 HRJ Camelina POSF 7720 150 HRJ Tallow POSF 6308 SASOL IPK POSF 7629 HRJ Camelina POSF 7720 0.82 100 n-alkane iso-octane 50 0.5 1 1.5 2 2.5 Transport-weighted enthalpy [cal/cm 3 ] [fuel] H c ( MW f / MW n ) -0.5
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