This work was funded by the US Federal Aviation Administration (FAA) Office of Environment and Energy as a part of ASCENT Project 26 under FAA Award Number: 13-C-AJFE-SU-006 and by the National Aeronautics and Space Administration under agreement number 13-C-AJFE-SU-006, and also by the US Air Force Office of Scientific Research under grant number FA9550-14-1-0235 . Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the FAA or other ASCENT Sponsors.

Project 26

Hybrid Approach to Chemical Kinetics Model Development and Evaluation

Project manager: Mohan Gupta, FAALead investigator: Hai Wang, Stanford University

Staff/Students: Rui Xu, Dongping Chen

September 27-28, 2016

Results – Fuel Oxidative PyrolysisMotivation and Objectives

Methods and Materials

Conclusions and Next Steps

Motivations

o Next generation of jet engines will operate at pressures far higher than those of currentengines for fuel efficiency improvement.

o A basic understanding of the combustion chemistry of jet fuels is critical to the design of high-pressure turbine engines.

o Alternative fuels are expected to enter into the market in the next decade. Their use incommercial aviation is hindered by the lack of quick and inexpensive fuel certificationmethods.

o Approaches that can quickly and accurately unravel the combustion chemistry of alternativefuels are essential to enable engine design and fuel certification.

Assumptions

o Early studies [1] suggest that for large hydrocarbonfuels pyrolysis precedes the oxidation of thedecomposed fragments in real, liquid fuelcombustion. These two reaction processes aredecoupled in time scales.

o In flames almost all large hydrocarbon fuels undergooxidation also in two steps. In the preheat zone ofthe flame the fuel decomposes into about a halfdozen of small molecular fragments in anendothermic process, regardless how complex thecomposition of the initial fuel is. The fragments thenenter into the flame zone and are oxidized tocombustion products in the second, exothermic step,which is rate limiting.

o The pyrolysis fragments are dominated by ethylene(C2H4), propene (C3H6), iso-butene (i-C4H8), 1-butene(1-C4H8), methane (CH4), benzene (C6H6), toluene(C7H8) and hydrogen (H2).

HyChem Approach

o The kinetic rate of the overall pyrolysis is fast as compared to oxidation, and thedistribution of the pyrolysis products has the dominant impact on radical buildup andheat release.

o The overall jet fuel reaction kinetics can be dealt with by combining an experimentallyconstrained, lumped pyrolysis model with a foundational chemistry model for theoxidation process of decomposition products (e.g., USC Mech II [2]). We term theapproach as the hybrid chemistry (HyChem) approach.

Methodology – HyChem Approach Pyrolysis Model – Reactions

o In HyChem, we write the lumped, pyrolysis model in the form of

(1)

(2)

where R = H, CH3, OH, O, O2 and HO2

Pyrolysis Model – Parameters

o The model has 11 stoichiometric parameters: ed, ea, bd, ba, α, β, γ, χ, λ3, λ4i and λ4n.Under the condition of complete reaction, elemental balances will eliminate fourparameters, which are ed, ea, bd and ba.

o The 7 independent parameters, α, β, γ, χ, λ3, λ4i and λ4n, along with 7 reaction rateconstants (ki, i = 1,2,…,7) can be directly determined from shock tube and flowreactor experiments.

Results – Fuel Pyrolysis

Shock tube pyrolysis ethylene/methane yields at 1.0 ms

Shock tube pyrolysis species time history profiles

Data: Hanson group

o The HyChem approach provides a direct path towards real, liquid fuel combustion chemistrymodeling. The resulting model is capable of predicting a wide range of combustion behaviorsof typical jet fuels.

o An automatic code for model derivation and refinement is currently under development.o Fuel X approach is under development.

Reference[1] Davidson, D. F., Z. Hong, G. L. Pilla, A. Farooq, R. D. Cook, and R. K. Hanson. "Multi-species time-history measurements during n-dodecaneoxidation behind reflected shock waves." Proceedings of the Combustion Institute 33, no. 1 (2011): 151-157.

[2] Hai Wang, Xiaoqing You, Ameya V. Joshi, Scott G. Davis, Alexander Laskin, Fokion Egolfopoulos, and Chung K. Law, USC Mech VersionII.High-Temperature Combustion Reaction Model of H2/CO/C1-C4 Compounds. http://ignis.usc.edu/USC_Mech_II.htm, May 2007.

i n

7 8

C H C H C H -C H 1-C H

C H C H H CH

m n d

d

e i

b

2 4 3 3 6 4 4 8 4 4 8

6 6 31 2

i n

7 8

C H R RH CH C H C H -C H 1-C H

C H C H H CH

m n a

a

e i

b

4 2 4 3 3 6 4 4 8 4 4 8

6 6 31 1

0

0.5

1

1.5

2

2.5

1050 1100 1150 1200 1250 1300 1350

A1 A2 A3 A2 Model

Yie

ld @

1.0

ms

Temperature, T (K)

0.73% fuel in Ar

p5 = 12.4 atm

C2H4

CH4

0.000

0.005

0.010

0 500 1000 1500 2000

Mo

le F

rac

tio

n

Time, t (s)

C2H4

CH4

Run 19

0.70% A2 in Ar

T5 = 1168 K, p5 = 12.71 atm

Flow reactor oxidative pyrolysis species time profiles

Data: Bowman group

o The 14 unknown parameters, α, β, γ, χ, λ3, λ4i, λ4n and ki, (i = 1,2,…,7) may bedetermined from shock and flow reactor experiments under a wide range ofconditions. The left figure shows the ethylene/methane yields at 1.0 ms during shocktube pyrolysis of selected jet fuels over the temperature range of 1050 to 1350 K,along with model predictions. The right figure shows one selected case of shock tubepyrolysis species time histories. The temperature sensitivity (±15 K) is shown withthe dashed lines. The fuel oxidative pyrolysis section presents the species timeprofiles during flow reactor oxidative pyrolysis for a Jet A fuel, along with modelpredictions.

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30

Mo

le F

racti

on

(P

PM

)

Time, t (ms)

Fuel

C2H4

H2

C3H6

A2 314 PPM, = 1

p = 1 atm, T = 1030 K

0

20

40

60

80

100

0 5 10 15 20 25 30

Mo

le F

racti

on

(P

PM

)

Time, t (ms)

C6H5CH3

CH4 C6H6

A2 314 PPM, = 1

p = 1 atm, T = 1030 K

Results – Fuel OxidationShock tube ignition delay time

101

102

103

104

0.7 0.8 0.9 1.0

A1-4%O2-Ar

= 1.1

= 0.5

= 2.1

= 1.2, p5 = 54.4 atm

Ign

itio

n D

ela

y,

s)

1000K/T

{p5 = 13.0 atm

102

103

104

0.7 0.8 0.9

A2 expt.A1 expt.A2 Model

Ign

itio

n D

ela

y,

(s

)

1000K/T

p5 = 1.3 atm, = 1.0

A2-4%O2-Ar

p5 = 1.1 atm, = 0.9

A2/A1-13.3%O2-Ar

A2/A1-O2-Ar

102

103

104

0.8 0.9 1.0

A2-Air

Ign

itio

n D

ela

y,

s)

1000K/T

p5 = 10.9 atm, = 1.1

Data: Hanson group

20

30

40

50

60

70

0.6 0.8 1 1.2 1.4 1.6

Equivalence Ratio,

A2, T0 = 403 K, p = 1 atm

Lam

inar

Fla

me S

peed

, S

u

o(c

m/s

)

50

100

150

200

250

300

350

400

0.06 0.065 0.07 0.075 0.08

Exti

nc

tio

n S

train

Rate

, K

ex

t (s

-1)

Fuel/N2 Mass Ratio

A2/N2 versus O2

Tu = 473 K for A2/N2

T = 300 K for O2

p = 1 atm

Laminar flame speed Non-premixed flame extinction

Data: Egolfopoulos group