Small Skeletal Kinetic Mechanism for Kerosene Combustion - Energy

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A small skeletal kinetic mechanism for kerosene combustion Niklas Zettervall, Christer Fureby, and Elna Johanna Kristina Nilsson Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01664 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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A small skeletal kinetic mechanism for kerosene combustion N. Zettervall1, C. Fureby1, E. J. K. Nilsson2,* 1

Defense & Security, Systems and Technology, Swedish Defense Research Agency – FOI, SE 147 25 Tumba, Stockholm, Sweden 2

Combustion Physics, Lund University, Box 118, SE 221 00, Lund, Sweden

1

ACS Paragon Plus Environment

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ABSTRACT

The development and validation of a new skeletal mechanism for kerosene combustion, suitable for reacting direct-, large-eddy and Reynolds averaged Navier Stokes Simulations, is presented. The mechanism consist of 65 irreversible reactions between 22 species, and build on a global fuel breakdown approach to produce a subset of C2 intermediates. A more detailed set of reactions for H/O/C1 chemistry largely determine the combustion characteristics. The mechanism is validated for combustion characteristics related to ignition, flame propagation and flame extinction over a wide range of pressure, temperature and equivalence ratios. Agreement with experiments and a more complex reference mechanism are excellent for laminar burning velocities and extinction strain rate, while ignition delays are over predicted at stoichiometric and rich conditions. Concentration profiles for major stable products are in agreement with reference mechanism, and also a range of intermediate species and radicals show sufficient agreement. The skeletal mechanism shows an overall good performance in combination with a numerical stability and short computation time, making it highly suitable for combustion Large Eddy Simulation (LES).

1. INTRODUCTION Kerosene-type aviation jet fuels are complex mixtures of components with carbon numbers in the range 8 to 16, including linear and branched alkanes (35-50 vol%), cyclic alkanes (30-35 vol%) and aromatics with one or two rings (20-25 vol%)

1-2

. The exact composition vary

between fuels from different sources, with average carbon number generally in the range C9-C13 3

. For the kerosene fuel Jet A the molecular formula is commonly approximated to C12H23.

2


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Gas turbines fueled with kerosene type fuels are the dominating power sources of civilian as well as military aeropropulsion, including turboshaft engines, turbofan engines, and afterburning turbojet- and turbofan engines. Aeropropulsion gas turbine engines usually have an annular combustion chamber with multiple burners sharing a common fuel supply line 4. Constraints on such gas turbines include velocity and temperature profiles delivered to the turbine (primarily affecting turbine life), pressure drop across the combustor (affecting thermal efficiency), the capability to withstand flame extinction, blow-out and pressure oscillations, the ability to relight at high altitudes, unsteady thermal loads and mechanical vibrations, as well as the ever more stringent emission regulations on CO, CO2, NOX, unburned hydrocarbons and smoke. The complexity of a normal gas turbine engine, together with the turbulent flow of air, fuel and hot combustion products, results in a complex flow. Accurate observations and quantitative measurements in real engine configurations are difficult and expensive, and are thus in short supply. An alternative to experiments is to use high fidelity Computational Fluid Dynamics (CFD) simulations to explore the fundamental physics and chemistry. Such simulations increase understanding of, among other things, combustion efficiency and pollutant formation, which enable further design improvements and combustion optimization. The rapid development in computational capability during the last decade has enabled use of Large Eddy Simulation (LES) methods that use separation of flow motions on small and large scales. LES has been successfully applied to sector models and fully annular models of gas turbines, as reviewed by Gicquel et al. 4 and recently exemplified in the work of Zettervall et al. 5

. Computational capacity is, however, still a main limiting factor for simulations of combustion.

The overall complexity and computational cost of the numerical simulation is determined by the

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geometric representation of the combustor, the simulation model employed, and the complexity and detail of the chemical reaction mechanism. A full chemical representation using a detailed chemical mechanism is not possible for LES, instead some type of reduced scheme has to be used. Zettervall et al.

5

demonstrated that a kinetics scheme consisting of 22 species and 57

irreversible reactions for kerosene combustion was suitable for use in LES of a generic turboshaft gas turbine combustor, applied to a single sector as well as fully annular multi-burner configuration. It was shown that this level of complexity of the kinetic mechanism captured important interactions between chemistry and turbulence. The performance of the mechanism was compared to more complex as well as simpler mechanisms by simulation of ignition, propagation and extinction cases. Laminar flame speeds were considered the most important targets, in good agreement with experiments and the more complex mechanism of Ranzi et al

2

over a range of equivalence ratios (0.6-1.6), temperatures (400-600 K) and pressures (1-10 atm). Weaknesses of the mechanism are that the ignition delay time is overestimated by several orders of magnitude, and for laminar flames the agreement with the reference mechanism was not satisfactory considering product formation and flame temperature at rich conditions. Detailed kinetic mechanisms for combustion of kerosene type fuels suffer from limitations as a result of both the size and complexity of the fuels; the mechanisms are very large and build on insufficient knowledge regarding many of the individual chemical reactions. The mechanism size is a result of the large number of carbons in the fuel molecules, as well as the high number of different components in a real fuel mixture. The lack of understanding of rate constants and branching in reactions is a result of difficulties of studying these low-volatility compounds in laboratory experiments. As discussed by Ranzi et al.2 these mechanisms of as much as tens of thousands of unique reversible reactions are orders of magnitude too large to be

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suitable for use in CFD simulations of real combustion applications. In recent years methods for reducing the size and complexity of the highly detailed mechanisms have been developed, as reviewed and explained by Turányi and Tomlin6. For a review of earlier, prior to 2006, kinetic mechanisms and experimental kinetic data for kerosene combustion we refer to Dagaut and Cathonnet 3. More recent overviews of kinetic models have been presented by Starik et al.7 and Narayanaswamy et al.8. For research purposes kerosene and other highly complex fuel mixtures are commonly represented by so called surrogate mixtures consisting of between two and up to 12 or more known components that together have properties that mimic the chemistry of real fuels, as described by Dooley et al. 1. These surrogate mixtures are used for experimental as well as computational studies. The choice of a surrogate mixture composition is a trade-off between the need for a simple system and the requirements to accurately predict particular parameters. Combustion properties of a fuel are closely related to the production of radicals and minor reactive species as a result of fuel oxidation and decomposition 1. The nature of the fuel molecules with respect to branching and saturation, are important for the composition of the radical pool and intermediates. A surrogate mixture needs to consist of an appropriate composition of different types of components to accurately reproduce combustion properties of the real fuel. Combustion LES using kerosene type fuels have so far used reaction mechanism of a global type, for example the two step mechanism of Franzelli et al. 9 constructed mainly for gas turbine combustion conditions. In global mechanisms the fuel is represented by a molecule with properties based on average composition of kerosene and a few reaction steps transform the fuel into the final products CO2 and H2O. The global reaction mechanisms are computationally cheap

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and commonly make accurate predictions of flame propagation, fuel consumption and final product concentrations at lean conditions. Limitations of the global mechanisms are the obvious fact that they cannot predict production of other species since the intermediate reaction steps are not taken into account and, they commonly give a too rapid production of final products, and generally perform poorly at rich conditions. Some level of chemical detailed can be obtained from quasi-global reaction mechanisms10-11 where fuel decomposition is done in one irreversible global step to give CO and H2 or H2O, while the H/O/CO chemistry is represented by several reversible reactions, and where no intermediate hydrocarbons are included. As a result of the increasing computational capacity and need for more understanding of the intermediate chemistry in combustion, it has in recent years been motivated to develop intermediate sized mechanism with explicit treatment of the most important steps in the combustion chemistry. These mechanisms can either treat the fuel as one component, like the global mechanisms, or as a mixture of selected species giving a suitable surrogate. Ranzi et al. 2 developed a skeletal mechanism for kerosene surrogate components with 231 species and 5591 reactions, with a high temperature version further reduced to 121 species and 2613 reactions. The starting point was a highly detailed mechanism with 17848 reversible reactions among 451 species, which was subject to lumping and reduction procedures.. The skeletal mechanism, in the following referred to as POLIMI_KEROSENE_231, was developed targeting operating conditions of T=600-1700K, P=1-40 atm and φ=0.5-2.0. This mean that the reduced scheme is in good agreement with the detailed scheme at these conditions, but it should be noted that the accuracy of the detailed mechanism over the full range of conditions have not been confirmed since experimental data is not available at all conditions.

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Skeletal mechanisms with a one-component fuel can be made significantly smaller than the surrogate fuel mechanisms. These mechanisms can have a detailed treatment of the H/O/C1 chemistry governing the overall reactivity of the system, but with the fuel decomposition and oxidation chemistry heavily reduced. A mechanism optimized for kerosene combustion in internal combustion engines12 consist of 48 species and 152 reactions, combining a detailed treatment of H/O/C1, a skeletal sub-mechanism for C2/C3 chemistry, and a few global steps breaking down the fuel molecule to the C2/C3 components. A similar approach was used for development of the 57 step mechanism5 that preceded the present work, but with even fewer reactions included in the fuel breakdown subset and the intermediate hydrocarbon chemistry. The present work is a continuation of our previous mechanism development effort, and present a skeletal mechanism of similar complexity as the 57 step mechanism5. The new mechanism does, however, include several additional reactions to improve predictions of several combustion characteristics, in particular at rich conditions. In addition the new mechanism presented here has been more extensively validated over a wider range of combustion relevant conditions. The aim of the work is thus to present a new skeletal mechanism for combustion of kerosene, small enough to use in 3D CFD finite rate LES. Since flame chemistry is mainly governed by reactions of small species belonging to the H/O/C1 subset of reactions, these are treated with sufficient detail to accurately predict production of important major and minor species. The fuel decomposition and oxidation steps are treated with a global approach by transferring the fuel directly to C2-species. In the following the mechanism development approach is explained and the choice of reactions and rate constants are motivated. The mechanism is then thoroughly validated by comparison to modeling using the mechanism of Ranzi et al.2 and with experimental data.

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2. MECHANISM DEVELOPMENT 2.1 MECHANISM DEVELOPMENT METHODOLOGY Mechanism development starts with selection of a relevant set of chemical reactions and reaction rate parameters. Guided by results from experiments and more detailed kinetic mechanisms the skeletal mechanism is then tuned by adjustment of rate parameters. A kinetic scheme for combustion needs to accurately predict the key flame properties such as the laminar burning velocity, Su, flame temperature, Tflame, and production of major species (CO2, CO, H2O, H2). Ideally the mechanisms should also be able to predict intermediate hydrocarbons (C2H4, C2H2, CH2O) and radicals (OH, H, CH, HO2, O, CH3) in the flame and other combustion properties like ignition delay time, τig, and the extinction strain rate, σext. Since gas turbines operate at elevated temperatures (~700 K) and pressures (~20 bar) combustion properties at these conditions need to be targeted. The skeletal mechanism POLIMI_KEROSENE_231, by Ranzi et al.2 was used as a reference, when possible in combination with experimental data. The choice of surrogate mixture in the present study is based on an evaluation by Dooley et al.1 and considerations from Slavinskaya et al.13: n-hexadecane (40%), iso-octane (30%), n-propylbenzene (23%) and trimethyl-benzene (7%). This mixture has an average composition C11.5H22.6 which gives a molecular weight of 160.9 g/mol and a H/C ratio of 1.97, which is sufficiently close to the C12H23 fuel proposed as a Jet-A average. The main development targets, i.e. the combustion characteristics that the mechanism was tuned to accurately predict, were the laminar burning velocity for a wide range of initial gas pressures and temperatures together with flame temperature and major species (CO2, CO, H2O,

8


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H2) concentrations. Achieving a well predicted laminar burning velocity is crucial for combustion LES since it represents an important link between the chemical kinetics and the fluid flows, together with the extinction strain rate. A well predicted flame temperature is equally important since the thermal decomposition and chemical reaction of a fuel and its products are highly temperature dependent. Well predicted major species concentrations are of importance in order to estimate quantities such as complete combustion and pollutant formation. Secondary development targets were species concentrations of intermediate hydrocarbons and minor species and radicals, meaning that the mechanism was tuned to improve agreement with the reference mechanism considering these parameters, but without compromising the accuracy concerning the main development targets mentioned above. Two intermediate C2-hydrocarbons found in the high concentrations in combustion of larger hydrocarbon fuels are C2H4 and C2H214. Achieving a good prediction of these species is important for overall flame characteristics close to a fuel inlet since the diffusivity of the cloud of species present there will then be well predicted. For a mechanism that is to be used for scientific research applications coupling experimental and computational work, it is valuable if the minor species and radicals commonly measured experimentally, using laser diagnostics techniques, are well reproduced. Important species for research purposes are OH, CH and CH2O and these where therefore chosen as development targets. In addition to these three species the hydrogen atom, H, was also chosen due to its importance in the overall characteristics of the mechanism meaning that a well predicted hydrogen atom concentration was necessary in order to get a well-functioning mechanism.

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Figure 1. Reaction path diagram for the reduced kerosene mechanism, for a laminar flame at 400 K, 1 atm and φ=1.1. Dashed boxes enclose the three sub-mechanisms: red – fuel breakdown, blue – intermediate hydrocarbon oxidation, green – base mechanism. The kerosene mechanism presented here is divided into three steps; fuel breakdown, intermediate hydrocarbon oxidation and base mechanism. Figure 1 shows a reaction path diagram of the C species in the skeletal mechanism, with the dashed lines enclosing the species involved in the three subsets. The fuel decomposes into C2-hydrocarbons (fuel breakdown, red box) followed by transfer of the C2-hydrocarbons (intermediate hydrocarbon oxidation, blue box) to the C1 components of the base mechanism (green box). The role of the fuel breakdown and intermediate hydrocarbon chemistry is to deliver a correct distribution of small species to the base mechanism governing the important flame chemistry. Combustion characteristics are strongly governed by small radicals like O, H, OH and CH3 and as mentioned in the introduction the composition of a real fuel with respect to branching, saturation and presence of cyclic 10


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structures largely determine the composition of the radical pool. In a heavily reduced mechanism like the one presented here it is necessary to tune the mechanism by incorporation of particular reactions and modifying rate constant, in order to control the production and loss of radicals. These radicals are then taking part in the important base subset of reaction, and this chemistry can be less compromised compared to the chemistry dealing with the fuel and the intermediate C2-hydrocarbons. To achieve a mechanism small enough to be suitable for finite rate LES, species and reactions included must be chosen with care. To begin with all hydrocarbons larger than C2 are excluded, as well as low and medium temperature chemistry, and the chemistry controlling the intermediate oxygenates. The remaining reactions, belonging to the C2 subset and the basic H/C/O chemistry are fundamental to prediction of combustion characteristics and heat release. The skeletal reaction mechanism presented here consists of 22 species taking part in 65 irreversible reactions, presented in Table 1. The mechanism is from now on referred to as Z65. In the following sections the three sub sections of the mechanism are presented. All reactions mentioned in text are numbered in the same way as in Table 1.

Table 1. The complete skeletal reaction mechanism Z65 (k=A×Tn×exp(-Ea/RT), units: s, mole, cm3, cal, K). #

Reaction

A

n

Ea

Reference

1

C12H23 → 5 C2H4+C2H3

3.00E+11

1.5

60000

a

2

C12H23+H → 6 C2H4

3.00E+10

0

5000

a

3

C12H23+OH → 6 C2H4+O

2.00E+09

1

8942

a

4

C2H4+H+M → C2H5+M

4.17E+10

0

11030

15

5

C2H5+H → CH3+CH3

3.16E+13

0

0

15

11


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6

C2H4 → C2H3+H

1.00E+12

0

50000

a

7

C2H4+H → C2H5

1.00E+12

1

11500

a

8

C2H4+O → HCO+CH3

3.31E+12

0

1130

15

9

HCO+CH3 → C2H4+O

1.58E+11

0

25000

15, c

10

C2H4+OH → C2H3+H2O

4.79E+12

0

1230

15

11

C2H3+H2O → C2H4+OH

1.20E+12

0

14000

15

12

C2H4+CH3 → C2H3+CH4

1.00E+13

0

13000

15

13

C2H3+CH4 → C2H4+CH3

3.02E+13

0

12580

15

14

C2H2+H+M → C2H3+M

1.23E+11

1

10360

15

15

C2H3+H → C2H2+H2

2.00E+13

0

2500

15

16

C2H+H+M → C2H2+M

1.10E+09

1

770

15

17

C2H2+H → C2H+H2

2.00E+14

0

19000

15

18

C2H2+OH → C2H+H2O

8.00E+12

0

5000

15, b, c

19

C2H+H2O → C2H2+OH

5.37E+12

0

16360

15

20

C2H2+O → C2H+OH

3.24E+15

0.6

12000

15, c

21

C2H+OH → C2H2+O

2.95E+14

0.6

910

15

22

C2H+O2 → HCO+CO

1.00E+13

0

6500

15, c

23

HCO+CO → C2H+O2

8.51E+12

0

138400

15

24

CH4 (+ M) → CH3 + H (+ M)d

25

16

kf

6.30E+14

0

104000

kf0

1.00E+17

0

86000

CH3 + H (+ M) → CH4 (+ M)d

16

kf

5.20E+12

0

-1310

kf0

8.25E+14

0

-19310

26

CH4 + H → CH3 + H2

2.20E+04

3

8750

16

27

CH3 + H2 → CH4 + H

9.57E+02

3

8750

16

28

CH4 + OH → CH3 + H2O

1.60E+06

2.1

2460

16

29

CH3 + H2O → CH4 + OH

3.02E+05

2.1

17422

16

30

CH3 + O → CH2O + H

6.80E+13

0

0

16

12


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31

CH3 + O2 → CH3O + O

5.00E+13

0

25652

16, b

32

CH3 + OH → CH2 + H2O

7.60E+06

2

5000

14

33

CH3O + H → CH2O + H2

2.00E+13

0

0

16

34

CH3O + M → CH2O + H + M

2.40E+13

0

28812

16

35

CH2 + O → CO + H2

3.00E+13

0

0

14

36

CH2 + OH → CH + H2O

4.0E+07

2

3000

14, b

37

CH2O + H → HCO + H2

5.00E+13

0

3991

16, b

38

CH2O + OH → HCO + H2O

1.20E+14

0

1100

16, b, c

39

CH + O → CO + H

5.70E+13

0

0

14

40

CH + OH → HCO + H

3.00E+13

0

0

14

41

CH + O2 → HCO + O

3.30E+13

0

0

14

42

CH + CO2 → HCO + CO

8.40E+13

0

200

14

43

HCO + H → CO + H2

4.00E+13

0

0

16

44

HCO + M → CO + H + M

1.60E+14

0

14700

16

45

CO + OH → CO2 + H

1.51E+07

1.3

-758

16

46

CO2 + H → CO + OH

1.57E+09

1.3

19200

16, c

47

H + O2 → OH + O

2.75E+14

0

16800

16, b

48

OH + O → H + O2

1.40E+13

0

690

16, b

49

O + H2 → OH + H

1.80E+10

1

8826

16

50

OH + H → O + H2

8.00E+09

1

6760

16

51

H2 + OH → H2O + H

1.17E+09

1.3

3626

16

52

H2O + H → H2 + OH

7.00E+09

1.3

18588

16, b

53

OH + OH → O + H2O

6.00E+08

1.3

0

16

54

O + H2O → OH + OH

5.90E+09

1.3

17029

16

55

H + O2 + M → HO2 + Me

1.20E+18

-0.8

0

16, b

56

H + HO2 → OH + OH

1.50E+14

0

1004

16

57

H + HO2 → H2 + O2

2.50E+13

0

700

16

58

OH + HO2 → H2O + O2

2.00E+13

0

1000

16

59

HO2 + HO2 → H2O2 + O2

8.00E+13

0

0

16, b

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60

H2O2 + M → OH + OH + M

1.30E+17

0

45500

16

61

OH + OH + M → H2O2 + M

9.86E+14

0

-5070

16

62

H2O2 + OH → H2O + HO2

1.00E+13

0

1800

16

63

H2O + HO2 → H2O2 + OH

2.86E+13

0

32790

16

64

OH + H + M → H2O + M

2.20E+22

-2

0

16

65

H + H + M → H2 + M

1.80E+18

-1

0

16

a: This work, see text b: Pre exponential factor has been modified c: Activation energy has been modified d: Collisional coefficients C12H23:3 CH4:6.5 CO:0.75 CO2:1.5 H2:1 H2O:6.5 N2:0.4 O2:0.4 e: Collisional coefficients CH4:6.5 CO:0.75 CO2:1.5 H2:1 H2O:6.5 N2:0.4 O2:0.4

2.2. BASE MECHANISM The base mechanism is a customized version of the 35 step reaction mechanism by Smooke & Giovangigli16, SG35 (reactions R24-31, 33, 34, 37, 38, 43-65 in Table 1), for combustion chemistry of CH4-O2, CO-O2 and H2-O2. The mechanism has been shown to perform satisfactory for CH4-air combustion, but with generally worse performance at rich conditions. To improve predictions at fuel rich conditions a CH2/CH sub mechanism from Glassman and Yetter 14

, consisting of seven reactions (R32, 35, 36, 39-42), was included, to give a base mechanism of

in total 42 reactions. To a large extent, the reaction rate expressions in the present mechanism are adopted from16 and14, but to optimize mechanism performance pre-exponential factors or activation energy were adjusted for a few reactions, see comments in Table 1. The CH2/CH subset mainly affects the chemistry at fuel rich conditions since the initiation to the CH2/CH sub-mechanism comes via a reaction with CH3, R32, a species mostly affecting the chemistry at fuel rich conditions. An important result of including the CH2/CH

14


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subset is the accurate prediction of CO and CO2 at rich conditions, mainly governed by reaction R42.

2.3. FUEL BREAKDOWN REACTIONS The common understanding is that in the high temperature oxidation of large hydrocarbons the main chain initiation step is controlled by breaking of a C-C bond14. For a large fuel like C12H23 the number of breakdown paths are obviously large, and to minimize the mechanism size a global approach to fuel decomposition via R1-R3 was implemented. Thermal decomposition, R1, initialize the combustion by creation of mainly C2H4. The initiation sequence continues by thermal decomposition of C2H4 to form C2H3 and H (R6). The H atom rapidly reacts with O2 to produce OH and O (R47) or HO2 (R55), and build up of H and OH start the oxidation reactions R2 and R3. The radicals H and OH are of fundamental importance since they are the major species governing reactivity at rich and lean conditions, respectively. The products from the fuel breakdown reactions are the intermediate C2-hydrocarbons C2H4 and C2H3 which then decompose and oxidize further, eventually connecting to the underlying CO-O2 and H2-O2 chemistry. C12H23 → 5 C2H4 + C2H3

(R1)

C12H23 + H → 6 C2H4

(R2)

C12H23 + OH → 6 C2H4 + O

(R3)

2.4. INTERMEDIATE HYDROCARBON CHEMISTRY

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This reaction subset connects the C2-species from the fuel breakdown to the base mechanism. The reactions (R4-23) and their rate parameters have mainly been adopted from the work of Sher and Refael15. Transfer of carbon to the base mechanism occurs via CH3, HCO and CO produced in the intermediate C2-hydrocarbon subset. The breakdown of C2H4 occurs via thermal decomposition (R6) and radical reactions (R4, 7, 8, 10, 12). Reactions R7 and R8 are essential since H and O are highly reactive, resulting in either a C2H5 molecule, which in turn reacts further via reaction R5 into two methyl radicals, or in one CH3 and one HCO radical via reaction R8. According to Glassman and Yetter14 the main attack on C2H4 comes from an O atom, hence R8 is an important reaction for oxidation of ethylene. The C2-subset contains more reactions for C2H4 and C2H2 than C2H5 and C2H3 and the reason for this is because both C2H4 and C2H2 are found in large concentrations and are needed to be treated in more detail since they greatly affects the overall characteristics of the mechanism. Another reason for the thorough C2H4 and C2H2 treatment is because one of the development targets was to predict those two species well and so requiring more reactions for them. C2H4 → C2H3 + H

(R6)

C2H4 + H → C2H5

(R7)

C2H4 + O → HCO + CH3

(R8)

C2H5 loss is taking place in reaction R5 producing methyl radicals, connecting fuel breakdown to the base mechanism. The oxidation of the vinyl radical, C2H3, proceeds via reactions R11, R13, R14 and R15, out of which R15 is the dominating reaction. This means that through R5 and R15, the radical H is rate limiting for the transfer of C2 species to the base mechanism.

16


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C2H5 + H → CH3

+ CH3

(R5)

C2H3 + H → C2H2 + H2.

(R15)

The acetylene produced in R15 will further react with H, OH and O radicals: C2H2 + H → C2H + H2

(R17)

C2H2 + OH → C2H + H2O

(R18)

C2H2 + O → C2H + OH

(R20)

Finally the closing of the C2H oxidation pathway uses mainly C2H + O2 → HCO + CO

(R22)

3. MODELING DETAILS Modeling of laminar burning velocities and ignition delays were performed using CHEMKIN software release 1011217. Laminar flame simulations were performed using the PREMIX code in CHEMKIN with curvature and gradient parameters set to 0.02 and 0.03, respectively, resulting in a grid of about 400-600 points. Ignition delays were calculated using the closed homogenous bath reactor module under constant volume, adiabatic conditions. Extinction strain rate simulations were performed in CHEMKIN PRO18 using a two-point temperature controlling method in the extinction flame reactor model. Thermochemical properties are from the recent database of Goos et al. 19, and transport parameters from the San Diego mechanism20, except for C12H23 for which transport parameters are for the specie C12H22 as implemented in the mechanism of Ranzi et al. 2.

4. MECHANISM VALIDATION

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Validation targets are in a first step the combustion characteristics used as mechanism development targets; laminar burning velocity, flame temperature and concentrations of selected species. In addition validation has been performed for extinction of flames, ignition of fuel/air mixtures, and chemical composition during combustion processes. The reference mechanism POLIMI_KEROSENE_231 is referred to as a skeletal mechanism by its authors, but has a relatively high level of complexity suitable for simulation of laboratory flames and oxidation processes. The current validation cover a wide range of conditions including equivalence ratios in the range 0.5 to 1.8, pressures up to 20 atm and initial gas temperatures from about 400 K to 750 K. The reference mechanism is employed to cover the full range of conditions and where available comparison with experimental data is included. Experimental studies suitable for validation of a kinetic mechanism are laminar burning velocity determinations, species composition in laminar flames, ignition delay times in shock tube, oxidation studies in flow reactors and shock tubes, and extinction strain rate determinations. Table 2 present a summary of validation targets employed in the present work.

Table 2. Validation targets. Reference simulations 2

Experiments

Type

P (atm)

T (K)

φ

P (atm)

T (K)

φ

Ref.

Laminar flames

1-20

373 – 750

0.5 – 1.8

1

470

0.75 – 1.45

21

1

400 – 470

0.7 – 1.4

22

1–3

400

0.7 – 1.4

23

1–8

373 – 473

0.67 – 1.43

24

1

318

1.7

25

1 – 20

650 – 1500

0.2 – 2.0

26

Ignition delay

1 – 20

1000 – 1800

0.2 – 1.0

18


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Extinction strain rate

1

403

1.0 – 1.3

1

403

0.7 – 1.4

27

4.1. LAMINAR BURNING VELOCITY AND FLAME TEMPERATURE The laminar burning velocity, Su, is an important variable for combustion LES whether it be a finite rate chemistry or a flamelet approach. Detailed kinetic mechanisms commonly show excellent agreement with experimental data, and many skeletal and global mechanisms can accurately predict Su for fuel lean conditions, while it is generally overpredicted at fuel rich conditions. A number of experimental studies determining Su of kerosene

21-24

and its

components have been performed in recent years, including investigations of the temperature 24

and pressure

23-24

22,

dependence. Investigations of different classes of compounds have shown

that straight chain alkanes burn faster compared to branched isomers, while the aromatic compounds are the slowest burning ones. Shorter chained alkanes burn faster than longer chained, but the difference is essentially within the uncertainty limits of the experiments 27. The laminar burning velocity of kerosene fuels is affected by the composition and is somewhere in between the faster burning alkanes that commonly are the main constituents and the slower burning aromatics 23. In Figures 2 and 4 simulated laminar burning velocities of Z65 (full drawn lines) and POLIMI_KEROSENE_231 (dashed lines) are presented together with available experimental data for a range of conditions. Figure 2 present data over the fully investigated range of equivalence ratios for different initial gas mixture temperatures, 2a, and pressures, 2b. The Z65 and POLIMI_KEROSENE_231 mechanisms are in good agreement but with the trend of Z65 predicting higher laminar burning velocities by up to 5 cm/s at rich conditions. At lean and stoichiometric conditions the Z65 and POLIMI_KEROSENE_231 mechanisms predict similar laminar burning velocities, but with a trend of increasing deviations at increasing pressures, as evident from Figure 2b. Agreement with 19


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experimental data is good at all conditions except for the highest preheat temperature, 470 K, at 1 atm, Figure 2a. It should be noted that there is also disagreement between the two experimental studies 21-22 presented in the figure and that the data at 470 K are unexpectedly high compared to those at 450 K. In general the increase in laminar burning velocity is expected to be close to linear within the temperature range (400-470 K) presented in Figure 2a, this is indeed the case for the simulated laminar burning velocities but not for the experiments.

Figure 2. Laminar burning velocities for kerosene at (a) 1 atm and 400, 450 and 470 K, and (b) 400 K and pressures 1, 2, 3 and 10 atm. Experimental laminar burning velocities (symbols) and modeling using Z65 (solid lines) and the mechanism of Ranzi et al. 2 (dashed lines). Using sensitivity analysis the reactions of importance for the laminar burning velocity can be identified. Figure 3 present the ten most sensitive reactions at lean, stoichiometric and rich conditions for a laminar flame at 400 K and 1 atm, for the reference mechanism, 3a, and Z65, 3b. From 3a it is seen that the chemistry governing the laminar burning velocity in the reference mechanism is dominated by reactions in the H/O/C1 subset, but also that C2 species are

20


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of some importance. The sensitivity spectra for Z65, Figure 3b, include the same or similar reactions and from this it can be concluded that the selection of reactions for Z65 is reasonable for a chemically sound representation of laminar burning velocity. In comparison of the sensitivity spectra it is important to remember that the reference mechanism have reversible reactions while Z65 build on irreversible reactions. As a result of this some reactions in 3a correspond to two reactions in 3b.

Figure 3. Sensitivity spectra with 10 reactions most sensitive to flow rate at 1 atm and 400 K, for the mechanism of a) Ranzi et al. 2 and b) Z65. Figure 4 shows the laminar burning velocities for kerosene as a function of temperatures at 1 atm, 4a, and pressure at 400 K and 473 K, 4b, for selected equivalence ratios where experimental data are available for comparison. The equivalence ratios chosen represents fuel lean (φ1.0) conditions as well as an equivalence ratio close to the maximum laminar flame speed (φ≈1.1). Figure 4a displays a consistent trend of Su increasing with temperature for both mechanisms, with POLIMI_KEROSENE_231 giving about 5% higher

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values at high temperatures, which is considered acceptable. In the temperature range where experimental data are available the model predictions are within the spread in experimental data and the 5% uncertainty of the data of Vukadinovic et al.24. The agreement between the mechanisms continues at elevated pressures, Figure 4b. Here simulations go as high as 20 atm, well beyond available experimental data, but within the operating conditions of some gas turbines running on larger hydrocarbons such as kerosene.

Figure 4. Laminar burning velocities for kerosene as a function of (a) temperatures at 1 atm, and (b) pressure at 400 K and 473 K. Experimental laminar burning velocities (symbols) and modeling using Z65 (solid lines) and the mechanism of Ranzi et al. 2 (dashed lines). Figure 5 presents the maximum temperature from simulations. The temperatures profiles are essentially identical, but with some difference at peak temperature and highly fuel rich conditions. For both mechanisms the peak temperature is achieved at φ=1.05, with values of 2329 K and 2353 K respectively for Z65 and POLIMI_KEROSENE_231. At highly fuel rich conditions Z65 predict a higher temperature than the reference mechanism, with at most 50 K.

22


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All in all, the laminar burning velocity and temperature predictions are highly satisfying for the Z65 mechanism, achieving excellent agreement compared to experimental data and reference simulations, over a wide range of equivalence ratios and initial gas conditions. The goal for the present mechanism is accurate predictions of flame properties at a low computational cost. As an example of this a comparison of the CPU time for modeling the laminar burning velocity at stoichiometric conditions at 400 K and 1 atm on a desktop computer is made. For the reference mechanism this computation takes 59 minutes, while for Z65 the time consumption is only 8 seconds. This means that Z65 use about 0.2% of the computational time to reach an essentially identical result.

Figure 5. Flame temperature in a laminar flame at 400 K and 1 atm. Modeling using Z65 (solid lines) and the mechanism of Ranzi et al. 2 (dashed lines).

4.2. SPECIES CONCENTRATIONS IN LAMINAR FLAMES In this section maximum molar concentrations versus equivalence ratio at 1 atm and 400 K, are presented for Z65 and POLIMI_KEROSENE_231. The reference mechanism is expected to be 23


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reliable for predictions of stable species, but it is well known that even detailed mechanisms might not accurately predict minor and unstable species concentrations. No experimental data are available for comparison. Figure 6 show the concentrations for the major species, CO2, CO, H2O and H2. The CO2 concentration, Figure 6a, prediction matches that of POLIMI_KEROSENE_231 well but with small deviations at equivalence ratios larger than 1.4. The standard CO2 creating reaction R45 is responsible for most of the CO2 being produced, and is implemented in essentially all combustion mechanisms. However, other global and skeletal often miss-predicts CO2 concentrations at rich conditions, which in the present work is solved by introduction of R42 and adjustment of R46. CO, Figure 6b, is overestimated compared to POLIMI_KEROSENE_231 for all equivalence ratios although the deviation is consistent and generally small, and both mechanisms show a small kink in CO at φ>1.5 suggesting a similar chemistry in both mechanisms.

The H2O concentration,

Figure 6c,

is

in

excellent

agreement

with

POLIMI_KEROSENE_231, except for equivalence ratios above 1.4 where Z65 overpredicts the concentration. This overprediction however is never larger than 10 %. The H2 concentrations, Figure 6d, also show a good agreement with POLIMI_KEROSENE_231, giving in total a very good agreement between Z65 and POLIMI_KEROSENE_231 for all major species. The importance of accurate CO2 and H2O predictions to combustion LES and industrial applications cannot be understated and any skeletal or global mechanism designed for use in combustion LES has to accurately predict both of these species if it is going to be useful for industrial applications.

24


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Figure 6. Comparison of different C12H23-air reaction mechanism predictions at 400 K and 1.0 atm in terms of the maximum molar fractions of (a) CO2, (b) CO, (c) H2O and (d) H2. Legend: black full drawn line - Z65 and red dashed line - Ranzi et al. 2.

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The intermediate hydrocarbons of interest are the validation targets C2H4 and C2H2, presented in Figure 7. These species play an important role in combustion of larger hydrocarbons, being stable species found in relatively large concentrations. Figure 7a shows concentration of C2H4, the main product from the fuel breakdown subset of Z65. While showing the same trend of increasing C2H4 concentration towards richer conditions, as POLIMI_KEROSENE_231, Z65 have higher concentration of the compound over the whole range. Concentration of C2H2, Figure 7b, on the other hand shows agreement with the reference mechanism up to about φ=1.4 but diverge significantly at the richer conditions. These discrepancies are a result of the necessity to restrict the number of reactions in Z65. Production and loss of C2H2 is mainly governed by reactions R20 and R21, connecting to C2H via oxygen containing radicals. At rich conditions these radicals are essentially not produced and the C2 fragments proceed via a set of reactions including C2H3 without passing C2H2 before it is turned in to C1 compounds. The C2 speciesnormally found in high quantities, have different diffusive properties than many of the smaller species present further down in the combustion chain, and subsequently later in the combustor, or combustion domain. It is therefore important to include them in a combustion LES, in Z65 the individual C2 species are not accurately predicted at all conditions, but the sum of them represent C2 species well.

26


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Figure 7. Comparison of different C12H23-air reaction mechanism predictions at 400 K and 1.0 atm in terms of the maximum molar fractions of (a) C2H4, (b) C2H2. Legend: black full drawn line - Z65 and red dashed line - Ranzi et al.2. Figure 8 presents concentrations of radicals OH, H and CH, and the intermediate CH2O, essential for the H/O/C1 combustion chemistry. High CH concentrations in a combustion LES may indicate highly fuel rich areas, providing essential information from the simulation. OH indicates where the post flame region is in the simulation and the region between the CH2O and OH concentrations are sometimes used as a marker for where the flame front is located, whereas the H concentration can give you an indication where the reactivity of your mixture may peak. These considerations motivate the importance of accurate prediction of these species. Figure 8a presents the CH concentration where Z65 is in good agreement with POLIMI_KEROSENE_231 for lean, stoichiometric and moderately fuel rich conditions, and with overpredictions at highly fuel rich conditions. The OH concentration, Figure 8b, shows good agreement between Z65 and POLIMI_KEROSENE_231 up to stoichiometric conditions,

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and with Z65 overpredicting slightly at fuel rich conditions. The prediction of CH2O, Figure 8c, show a behavior where CH2O have a similarly shaped curve as that of the reference mechanism, with occasional under- and overpredictions. The H concentration, shown in Figure 8d, again displays a similarly shaped curve as that predicted by POLIMI_KEROSENE_231 but with generally higher values. When considering the uncertainty of smaller radicals in detailed mechanisms the results from CH, CH2O, OH and H are all within acceptable limits.

28


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Figure 8. Comparison of different C12H23-air reaction mechanism predictions at 400 K and 1.0 atm in terms of the maximum molar fractions of (a) CH, (b) OH, (c) CH2O and (d) H. Legend: black full drawn line - Z65 and red dashed line - Ranzi et al. 2.

4.3. FLAME PROFILES The species profiles presented in the previous section can unfortunately not be experimentally verified. One experimental study of species profiles in a kerosene flame is available in the literature, a burner stabilized flame at rich conditions (φ=1.7), studied by Douté et al.25. The flame was studied at atmospheric pressure and initial gas mixture temperature of 473 K. Species concentrations were measured using gas chromatography. This flame has been used for characterization of a recent mechanism by Narayanaswamy et al.8, who point at that the temperature profile given by Douté et al. has an uncertainty of about ±100 K, which possibly hampers the ability of the models to get predictions in agreement with experiments. Figure 9 present profiles of major species, 9a, and intermediate hydrocarbons C2H2 and C2H4, 9b. The agreement of the present mechanism with major species is satisfactory, with overall shape of the profiles in good agreement with experiments and modeling using reference mechanism. The present mechanism overpredict CO and slightly under predict CO2, which is likely a result of the previously discussed difficulties in balancing the chemistry of these species at rich conditions. Production of the intermediates C2H4 and C2H2 in the modeling using Z65 are deviating from experiments and reference modeling in the sense that mainly C2H4 is produced and that it is produced earlier in the flame. The dominance of C2H4 over C2H2 at rich conditions is consistent with what is seen in Figure 7, where Figure 7b present a sharp decrease in C2H2 at equivalence

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ratios above 1.4. The early production is a result of the simplified chemistry of the skeletal mechanism, where C2 species are produced directly from the fuel instead of proceeding via a number of larger species.

Figure 9. Flame profiles in rich (φ=1.7) mixtures of kerosene (a) major species, (b) hydrocarbon intermediates. Experimental data from Douté et al.25 (symbols) and modeling using Z65 (full drawn lines) and the mechanism by Ranzi et al.2 (dashed lines). Color coding for modeling is same as for experiments, given in the legend.

4.4. EXTINCTION STRAIN RATES Accurate prediction of extinction strain rate is important in a combustion LES since too high or low extinction limits cause a flame to be artificially intact or extinct. Large molecules like the C8 to C16 hydrocarbons considered here have been shown to have rather similar extinction behavior, but with the smaller compounds having slightly more resistance to strain, i.e. higher extinction strain rate28. Branched, and aromatic compounds that are constituents of

30


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kerosene have less resistance to strain compared to the n-alkanes, and thus kerosene can be expected to have lower extinction strain rate than n-alkane of similar molecular weight. Figure 10a shows extinction strain rates for kerosene modeled at 403 K and 1 atm. Conditions are the same as those for the experiments on C12H26 by Ji et al.27 using measurements performed in a single flame configuration with an ambient temperature N2 flow counterflowing with a fuel/air flow at 403 K. The experimental results for C12H26 are included in figure 10a, also presenting the turning point in the calculations, i.e. the maximum strain rate as a function of equivalence ratio. In Figure 10b full strain rate curves at a few equivalence ratios are plotted as a function of temperature. Unfortunately the POLIMI_KEROSENE_231 mechanism showed convergence problems at rich and lean conditions, and simulations only performed successfully from equivalence ratio 1.0 to 1.3. The turning point values plotted in Figure 10a show slightly lower extinction strain rate for Z65 mechanism compared to the reference mechanism, in the narrow range of conditions where results from the reference mechanism were obtained. Z65 is, however, matching the experimental values well for all equivalence ratios. Since presence of branched and aromatic compounds is expected to lower the extinction limit one could expect the kerosene, and the surrogate mixture used as reference, to have lower extinction strain rates than C12H26. On the other hand the smaller components included in the surrogate mixture (and real kerosene) are less prone to extinction, and this could possibly cancel out the effect of branching and aromaticity. In this context the results for C12H26 is considered a good approximation to the extinction strain rate of kerosene. Figure 10b presents the simulated extinction curves at lean (0.7), peak (1.2) and rich

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(1.5) conditions. Here we can see a difference in the performance of the mechanisms: Z65 reach extinction at lower flame temperatures (50-100 K) compared to the reference mechanism.

Figure 10. Extinction strain rate at 403 K and 1 atm. Simulation with the Z65 mechanism (ful drawn lines) and mechanism of Ranzi et al. 2 (dashed lines). Symbols represent experiments on n-C12H26 from Ji et al. 27. In (a) the extinction strain rate is plotted vs equivalence ratio and (b) show the simulated extinction curves for equivalence ratios of 0.7, 1.2 and 1.5.

4.5. IGNITION DELAY TIME Ignition delay times of kerosene fuels with air have been experimentally determined over a relatively wide range of temperatures (700-1500 K), pressures (1-30 atm) and equivalence ratios (0.2-2.0)

26, 29-30

. Studies of Jet-A29 and Chinese RP-326 jet fuels at overlapping ranges of

conditions reveal that even though there are some differences in composition of these fuels the ignition behavior at high temperatures are essentially identical. Low temperature ignition (

Small Skeletal Kinetic Mechanism for Kerosene Combustion - Energy

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