Development of a Reduced n-TetradecaneâPolycyclic Aromatic

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Development of a Reduced n-tetradecane-PAH Mechanism for Application on Two-stroke Marine Diesel Engine Xiuxiu Sun, Xingyu Liang, Gequn Shu, Yuesen Wang, Yajun Wang, and Hanzhengnan Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02708 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Energy & Fuels

An article submitted to Energy & Fuels

Development of a Reduced n-tetradecane-PAH Mechanism for Application on Two-stroke Marine Diesel Engine

Xiuxiu Sun1, Xingyu Liang1*, Gequn Shu1, Yuesen Wang1,Yajun Wang1, , Hanzhengnan Yu1,

1.

State key Laboratory of Engines, Tianjin University, Tianjin 300072, China

*Corresponding Author: Xingyu Liang

State Key Laboratory of Engines, Tianjin University, Tianjin, 300072, China

E-mail: [email protected] Tele: +86 22 2789 1285

Fax: +86 22 2789 1285

1

ACS Paragon Plus Environment

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Development of a Reduced n-tetradecane-PAH Mechanism for Application on Two-stroke Marine Diesel Engine

Xiuxiu Sun1, Xingyu Liang1*, Gequn Shu1, Yuesen Wang1, Yajun Wang1,Hanzhengnan Yu1

1.

State key Laboratory of Engines, Tianjin University, Tianjin 300072, China

Abstract

The rigorous regulation requires marine engine decrease emission. It is difficult to test all

technologies for marine engine using experiment method. The alternative fuel is important for the

simulated model. A semi-reduced kinetic mechanism can be constructed to investigate the

performance of the marine diesel engine, which consists of 341 elementary reactions and 74 species.

Then, the Directed Relation Graph with Error Propagation (DRGEP) and the sensitivity analysis

methods were used to reduce this mechanism. The reduced mechanism consists of 279 elementary

reactions and 62 species. The ignition delay times with the semi-detailed and reduced mechanisms

were validated with the experimental data. At the same time, the results were compared with the 2


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data of published mechanism. Good agreements of ignition delay times were obtained. It was also

found that the trends of in-cylinder temperature, pressure and main species mole fractions were

almost same with results of published mechanism in internal combustion engine model. The main

polycyclic aromatic hydrocarbon (PAH) mole fractions are almost the same with experimental data,

especially the species A4. The 62-species mechanism was validated with the experimental data in a

marine diesel engine model. It can be seen that the in-cylinder pressure was in good agreement with

the experimental data. The NOx had a 2.2 % error compared with the experimental data, which is

the minimum error.

Keywords: Marine diesel engine; Alternative fuel; n-tetradecane; reduced mechanism;

1. Introduction

The two-stroke diesel engine has been widely used for marine transport due to the high thermal

efficiency, wide range of power, good reliability and security. However, the increasing of

environmental concern leads to the more stringent regulation for the emission. The IMO

(International Maritime Organization) Tier III requires that the NOx quantity decrease to 3.4 g/kWh

in the Emission Control Area (ECA), which is reduced by 76 % compared with the 14.4 g/kWh in

TierⅡ1. Some available measures can be researched to reduce the marine emission 2, Exhaust Gas

Recirculation (EGR), Direct Water Injection (DWI), humid air motor (HAM) and so on 3-5. 3


Energy & Fuels

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These technologies are based on the regulate combustion process. As we all know, the

combustion is dominated by the fuel chemical kinetics 6. Thus, a detailed model, which can properly

describes fuel oxidation chemistry, is essential to simulated marine diesel combustion. Panagiotis et

al.

7

used the KIVA-3 code as the modeling platform to study the effects of advanced injection

strategies. It has been demonstrated that, by adding a pilot injection, appropriately timed, fuel

savings of the order of 1.7% can be achieved, without increasing NOX emissions. The tetradecane

can be used as alternative fuel. Michele et al. 8 researched the influence of injector diameter (0.2-

1.2 mm range) on diesel spray combustion using Star-CD. The n-dodecane had been used to

represent the liquid thermo-physical fuel properties of the diesel fuel used in the experiment. E.

Sigurdsson et al. 9 have been studied the scavenge flow and convective heat transfer in large two-

stroke marine diesel engines. The model predictions are in good agreement with the experimentally

determined mass of scavenged gases and scavenging efficiency. The convective contribution to the

in-cylinder heat loss is predicted to be 3.2% of the fuel energy flow to the cylinder. Pang et al. 10

investigate soot formation and oxidation processes under large two-stroke marine diesel engine-like

conditions using integrated CFD (computational fluid dynamics) -chemical kinetics. The n-heptane

was used as alternative fuel in the simulated model. The results show that the averaged nitrogen

monoxide concentration is 7.7 % lower when both convective and soot radiative heat losses are

accounted, but the net soot mass production is less sensitive to soot radiation. Raptotasios et al. 4


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11

investigate the NOx reduction potential of two-stroke marine diesel engines using EGR. The n-

tetradecane was used as fuels in their simulated model. The results show that the NOx can reduce

to the value of the Tier III requirement, when the EGR rate exceeded 35 % for the 4T50ME-X tested

engine. The commercial diesel is a complex mixture, which consists of hundreds of medium-high-

molecular-weight hydrocarbons. It is not feasible to consider the oxidation chemistry of all

hydrocarbons 12. So the different alkanes can be used as alternative fuel in the simulated marine

diesel engine model.

It is very important to choose a surrogate fuel to ensure the accuracy of a simulated model, due

to the diverse performance of surrogate fuels. Alkanes has been used as surrogate fuel frequently,

such as n-heptane 10, 13, 14, n-tetradecane 7, 8, 11, 15. Researchers can have studied the characteristics of

alkanes. Westbrook et al. 16analyze the detailed chemical kinetic reaction mechanism for combustion

of n-alkane hydrocarbons from n-octane to n-hexadecane. These mechanisms include both high

temperature and low temperature reaction pathways. The paper describes the pyrolysis and

oxidation of alkanes. His-Ping et al. 17 have studied the ignition delay time of n-heptane, n-decane,

n-dodecane, and n-tetradecane at elevated pressures in a shock tube. It indicates that the ignition

delay times for n-alkanes are influenced little by n-alkane chain length for C7 and longer n-alkanes

and for mixtures with common carbon content. The mechanisms of Curran et al. 18 and Westbrook

et al. (Lawrence Livermore National Laboratory (LLNL)) 16 are in agreement with this finding. John 5


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et al. 19 studied the combustion characterization and ignition delay modeling of low and high-cetane

alternative diesel fuels in a marine diesel engine. The modeled chemistry could capture relative

changes in the experimentally observed ignition delay, suggesting that the measured differences in

physical properties, which will affect spray development, do not contribute as significantly to

differences in ignition delay. The experiment and modeling study of the low-temperature oxidation

of large alkanes have been studied by Biet et al. 20. These simulations should help users of kinetic

mechanisms to know if it is necessary to consider heavier model molecules knowing that the number

of included species and reactions will then be considerably increased. Although, the detailed

chemical reaction mechanisms were developed by researchers, the reduced chemical reaction

mechanism is necessary, due to the complex chemical mechanism and the limited capacity of

computer.

Keyvan et al. 21 reduced n-heptane fuel using combined reduction methods. This combination

of methods successfully reduced the comprehensive Curran’s n-heptane mechanism (561 species

and 2539 reactions)

22, 23

to a reduced mechanism with only 118 species and 330 reactions. The

simulation time is decreased from 601 min to 8 min. Wang et al. 12developed chemical kinetics of

diesel surrogate fuel oxidation. The complete kinetic mechanism, which comprised 697 reactions

and 153 species, was reduced to a minor mechanism that included only 141 reactions and 75 species

using the sensitivity and reaction path analyses. The cetane number of n-heptane is close to the value 6


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of diesel fuel, however the n-tetradecane is an optimal choice for the character of diesel fuel. Su et

al.

24

developed a reduced model of n-heptane including 40 species and 62 reactions. The results

showed the well-known two-stage ignition characteristics of n-heptane. The optimized reduced

model generally agrees well with those of the detailed chemical kinetic model

tetradecane, which can be found in reference

23

. For the n-

25

, the simulations show good agreement with the

measurements including the laminar flame speed and extinction strain rate in premixed laminar

flame and counter flow flame, ignition delay and major species. Even though the n-tetradecane is

popular as a marine diesel alternative fuel, the chemical reaction mechanism of n-tetradecane is

insufficient yet.

On the basis of previous studies, this paper constructed a semi-reduced kinetic model to

investigate the performance of marine diesel engine, which comprised 341 elementary reactions and

74 species. Then, the Directed Relation Graph with Error Propagation (DRGEP) and the sensitivity

analysis were used to reduce this model. The reduce model consists of 279 elementary reactions and

62 species. The semi-reduced model and the reduced model are validated against the experimental

data. The three-dimensional-CFD of marine diesel engine was used to validate the reduced model.

This model can provide direction to choosing the marine diesel engine alternative fuel.

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2. Kinetic Mechanism Model 2.1 Model Development The skeletal mechanism was built upon a decoupling methodology 26. The oxidation of alkanes

was divided into two parts: a detailed mechanism for H2/CO/C1, which were detailed studied by

Klippenstein et al. 27 and the oxidation process of small molecule C2-C3, which can be gotten from

the GRI-3.0. The oxidation process of alkanes was analyzed by researchers 14, 16. The oxidation of

n-tetradecane can be built by the reaction path analysis 25, 28. These can be comprised the oxidation

process of n-tetradecane.

The polycyclic aromatic hydrocarbon was used as the precursor of soot. It is important to

predict the soot mass

29.

The reduced PAH mechanism to added into the base n-tetradecane

mechanism, which includes 20 species and 139 reactions 30. The n-tetradecane and PAH mechanism

can connect with the soot model to predict the marine engine soot mass.

The semi-reduced kinetic model investigates the performance of a marine diesel engine, which

comprised 341 elementary reactions and 74 species. Due to the marine engine model has million

cells, the mechanism needs to be simplified to save the calculated time. The DRGEP method can

delete the useless species for the oxidation of n-tetradecane, as studied by the other researchers 21,

31.

So detail information about this method will not be introduced. The CO2 is the important product

8


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species for the fuel combustion, the related species need to be kept during the simply process. The

produce process of CO2 is shown in Fig.1.

Fig.1 the CO2 reaction paths: the blue lines represent paths of HO2 in reaction, the red lines represent paths of OH in reaction

From the Fig.1, we can see that HCO, C2H2 and CH3O also have directly relationship with the

CO2 produce process. The species CO is the dominant species for the CO2 process. The OH and

HO2 are the important small species for the reactions. The Fig.2 shows the important species for A4

produce process. CH2CO is the important species for the A4, except C2H2. So, the HCO, CO, CH3O,

C2H2 and CH2CO will be retained in the simplified model.

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Energy & Fuels

Fig.2 the A4 reaction paths the blue lines represent paths of HO2 in reaction, the pink lines represent paths of OH in reaction species reaction

74

60

345

CO CO2 H2O Ignition delay time

50

72 330

315

68 66

300

40

Tolerance/ %

Species

70

Reactions number

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30

20

64 285 62 60 0

2

4

6

8

10

12

14

16

18

270 20

10

0 0

2

Simplify the number of times

4

6 8 10 12 14 Simplify the number of times

16

18

(b) Tolerance

(a) Species and reactions

Fig.3 the simplify process for the semi-detailed chemical mechanism: (a) the vary of species and reactions with the simplify number (b) the vary of tolerance with the simplify number

Fig.3 shows the simplify process using DRGEP method. The absolute tolerance set to1e-04,

the relative tolerance set to 10 %. The numbers of species and reactions decrease with increasing

number of simplifies, which can be shown in Fig.3 (a). The tolerances of species and ignition delay

time increases. The percentage of target tolerance is calculated using the following formula: the

absolute different value of master and skeletal / (the value of master * relative tolerance + absolute

tolerance). As the size of the skeletal mechanism decrease, the error value increases. The optimal 10


20

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skeletal mechanism has the smallest size possible with an error value less than 100 %. 0.5,13

R140 R139 R135 R133 R132 R114 R109 R102 R9 R8 R4 R1 -1.0

0.5,40atm

R140 R139 R135 R133 R132 R109 R102 R9 R8 R4 R3 R1

1100 1000 900

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1100 1000 900

-1.0

-0.8

-0.6

Temperature sensitivity coefficient

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0


Temperature densitivity coefficient


(a) Equivalence Ratio = 0.5, Pressure = 13 atm

(b) Equivalence Ratio =1,40 0.5, Pressure = 40 atm

1,14

R140 R139 R135 R133 R132 R109 R107 R102 R11 R9 R8 R4 R1 -1.0

R140 R139 R135 R133 R132 R109 R102 R10 R9 R8 R4 R3 R1

1100 1000 900

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-1.0

1100 1000 900

-0.8


-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Temperature sensitivity Temperature densitivitycoefficient coefficient


(c) Equivalence Ratio = 1, Pressure = 14 atm

(d) Equivalence Ratio = 1, Pressure = 40 atm

Fig.4 the temperature sensitivity analysis at different conditions

The sensitivity analysis represents the ensemble response to small disturbances by calculating

the ensemble property parameters (species concentration, temperature, and produce rate of species)

variation caused by small changes in the value of a reaction constant. The sensitivity analysis can

make us find some reactions that having influence on the special species or parameters. This method

also be used in some paper, so, it is not necessary to introduce 12, 32. Because the temperature is very

sensitive to ignition of the combustion engine, the important reactions can be kept using this method.

11


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Fig.4 shows the results of the temperature sensitivity analysis for four different conditions. These

reactions have a significant effect on the system temperature change, especially for the R4, R8, R9

and R135. The corresponding reactions can be found in accessory. These reactions were retained in

reduced chemical mechanism. The final reduced mechanism consists of 279 elementary reactions

and 62 species.

2.2 Marine Diesel Engine Model

The 3-D model is conducted based on the 6S35ME-B9. The structure parameters can be found

in table 1. The engine speed is 142 r/min, and the power is 3575 kW, which can be operated in 100 %

load.

Table. 1 6S35ME-B9 test engine specifications

cylinder number

6

Bore (mm)

350

Stroke (mm)

1550

Displacement (L)

149

connecting rod length (mm)

1550

Speed (r/min)

142

Power (kWh)

3575

The simulated model was performed using the commercial CFD code CONVERGE 2.3. Fig.

5 shows the 3-D geometric structure of the marine engine (6S35ME-B9). The model includes

scavenge box, cylinder and exhaust port. The k-ε RAN model is used for turbulence model. Kelvin 12


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Helmholtz-Rayleigh Taylor (KH-RT) is applied to spray breakup model. The breakup time and size

were set as 1.0 and 0.1respectively. The injected liquid temperature was set as 345 K. The NTC

collision is implemented to simulated fuel collision. The splash model can be used in the wall

interaction. The rebound weber number was set 5.0. The injection duration is 15.3628°CA and the

total injection mass is 0.0125915 Kg, which can be gotten from the experiment. The Extend

Zeldovich NOx model is applied to simulate the NOx product process.

Fig 5.The 3-D geometric structure of the tested marine engine

3. The Semi-detailed Chemical Mechanism Validation 3.1 Comparison to Experimental Data of Ignition Delay Times in Shock Tubes

The ignition delay time is an important parameter for the chemical mechanism, which shows

the accuracy of mechanism. All calculated data for ignition delay time were obtained using closed

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Energy & Fuels

homogeneous constant volume reactor model from the CHEMKIN. The ignition delay time is

defined as the time needed for the mixture to reach a temperature of 400 K from the initial

temperature. Fig.6 shows the comparisons of ignition delay times for new mechanism, experimental

data

17

, detailed n-tetradecane mechanism16 and the mechanism developed by Chang et al.

25

at

different equivalence ratios and pressure. 10000

P=40atm EXP Chang et al Detailed mod 74-species

P=14 atm EXP Chang et al Detailed mod 74-species

Ignition delay time/us


10000

1000

1000

100

100 0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

10 0.80

0.85

0.90

0.95

1000/T (1/K)

1.05

1.10

(b) Φ= 1 P= 40 atm 10000



10000

1.00

1000/T (1/K)

(a) Φ= 1 P= 14 atm


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1000


1000

100

100

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

10 0.80

0.85

1000/T (1/K)

0.90

0.95

1.00

1.05

1.10

1.15

1000/T (1/K) (d) Φ= 0.5 P= 40 atm

(c) Φ= 0.5 P= 13 atm

Fig. 6 Comparison ignition delay time of new mechanism, experimental data 17, detailed chemical mechanism 16 and published mechanism by Chang et al 25 at different equivalence ratios and pressure

The ignition delay times predicted by the new mechanism are in agreement with the

experimental data. The maximum errors are found in Figs.6 (a) and (c), at low pressure and the

temperature range from 900 K to 1100 K. However, the ignition delay times of new mechanism are

in agreement with of the data that published by Chang et al.. The new mechanism predicts good

14


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ignition delay times at high pressure, which can be got from Figs. 6 (b) and (d). The reason of

different ignition delay times is that the pre-exponential factors are different for significant reactions.

The ignition delay times of n-tetradecane are acceptable for this new mechanism based on the

comparisons.

3.2 Comparison to Results in Internal Combustion Engine

The semi-detailed reaction mechanism will be used as alternative fuel for internal combustion

engine. It is necessary to predict the accuracy of new mechanism. The in-cylinder pressure and

temperature, the main species mole fractions can be compared with the results from detailed

mechanism, and the developed mechanism by Chang et al.. All computer data have been obtained

by applying the closed internal combustion simulator model from the CHEMKIN. The internal

combustion engine specifications are listed here based on former study12: the bore 115 mm, the

stroke 115 mm, the connect rod length 210 mm, the clearance volume 74.656 cm3 and the engine

speed 1400 r/min.

Fig. 7 displays the comparison of the in-cylinder pressure at four different operating conditions,

which can be shown in Table.2. It can be seen that the trends of in-cylinder pressure obtained by the

74 species mechanism are in acceptable agreement with those of the published mechanism by Chang

et al.. However, the time of break pressure delay 2 °CA for condition 2 (Fig. 7(c)). The maximum

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Energy & Fuels

error was found in Fig.7 (b) by comparing the time of break pressure, which is about 4 °CA delay.

The ignition delay times directly lead to different times of break pressure. Fig.8 displays the

comparison of in-cylinder temperature at four different conditions. The in-cylinder pressure and

temperature have relationship with combustion, the same trends were also found for the in-cylinder

temperature. Therefore, the new mechanism can predict the pressure and temperature for the internal

combustion engine. 180

180 160

Chang et al Detailed mod 74-species

160


140

140 120

Pressure/atm

Pressure/atm

120 100 80 60

100 80 60

40

40

20

20

0 -40

-30

-20

-10

0

10

20

30

40

0 -40

-30

-20

-10

Crank angle/℃ A

0

10

20

30

40

Crank angle/℃ A

(a) Φ= 1 T= 325 K P= 1 atm

(b) Φ= 1 T= 350 K P= 1 atm

180 160

200


180

140

160

120

140

Pressure/atm

Pressure/atm

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100 80 60


120 100 80 60

40

40

20 0 -40

20

-30

-20

-10

0

10

20

30

40

0 -40

-30

Crank angle/℃ A

-20

-10

0

10

20

Crank angle/℃ A (d) Φ= 1.2 T= 350 K P= 1.2 atm

(c) Φ= 1.2 T= 325 K P= 1 atm

Fig.7 Comparison of the in-cylinder pressure at different conditions.

16


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40

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3500 3000



3000

2500

Temperature/K

Temperature/K

2500 2000

1500

2000

1500

1000

1000 500 -40

-30

-20

-10

0

10

20

30

500 -40

40

-30

-20

-10

Crank angle/℃ A

0

10

20

30

40

Crank angle/℃ A

(a) Φ= 1 T= 325 K P= 1 atm

(b) Φ= 1 T= 350 K P= 1 atm 3500

3000


3000


2500

Temperature/K

2500

Temperature/K

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2000

1500

1000

1500

1000

500 -40

2000

500 -30

-20

-10

0

10

20

30

40

-40

-30

-20

-10

0

10

Crank angle/℃ A

Crank angle/℃ A

(c) Φ= 1.2 T= 325 K P= 1 atm

(d) Φ= 1.2 T= 350 K P= 1.2 atm

20

30

Fig.8 Comparison of the in-cylinder temperature at different conditions. Table. 2 The combustion engine operating conditions

Equivalence ratio

Intake temperature (K)

Intake pressure (atm)

Condition 1

1

325

1

Condition 2

1

350

1

Condition 3

1.2

325

1

Condition 4

1.2

350

1.2

CO2 and H2O are important species for combustion. OH takes part in many reactions, which

can be found in Fig.1. At the same time, the OH is one kind of H2O2 pyrolysis products. The

comparison of these species can increase the accuracy of mechanism. Fig. 9 shows the comparison

of CO2, H2O, OH and H2O2 at two different operating conditions (conditions 2 and 3). It can be 17


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

observed that CO2 and H2O species mole fractions obtained by new mechanism are almost same

with that of others mechanisms at two different conditions (Fig.9 (a) and (c)). The maximum error

is that the crank angle of species increase delay 4 °CA compared with that of detailed mechanism

at the condition 1. The maximum H2O2 mole friction is smaller than that of published mechanism

by Chang et al., but it is higher than that of detailed mechanism, which can be obvious seen in Fig.9

(b) at two conditions. The crank angle of species increase also delay by compared with trend of

detailed mechanism at the condition 2. The OH mole fraction is slightly smaller than that of others

mechanism for Condition 2 (Fig .9(d)). The different value decrease gradually, the mole fraction of

detailed mechanism is 0.0026, and the value of new mechanism is 0.0022. The mole fraction almost

same for three mechanisms at condition 3. Every species has a lot of generation and consumption

paths. The paths and rates of oxidation reactions for n-tetradecane reaction lead to the different of

mechanisms. The new mechanism can well predict the trends of main species mole fractions.

18


Page 18 of 30

Page 19 of 30

0.012

H2O Mole Fraction

0.12


0.010

H2O2 Mole Fraction

0.14

0.10 0.08 0.06 0.04

0.008

0.006

0.004

0.000

0.00 -40


0.002

0.02

-30

-20

-10

0

10

20

30

40

-40

-30

-20

-10

Crank angle/℃ A

0

10

20

30

40

20

30

40

Crank angle/℃ A (b) H2O2

(a) H2O 0.014 0.12


0.012


0.10

OH Mole Fraction

0.010

CO2 Mole Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.08 0.06 0.04 0.02

0.006 0.004 0.002

0.00 -40

0.008

0.000 -30

-20

-10

0

10

20

30

40

-40

-30

-20

-10

0

Crank angle/℃ A

Crank angle/℃ A

(c) CO2

(d) OH

Condition 2 : Φ= 1 T= 350 K P= 1 atm

19


10

Energy & Fuels

0.016

0.16 Chang et al Detailed mod 74-species

0.014

0.12

0.012

0.10

0.010

H2O2 Mole Fraction

H2O Mole Fraction

0.14

0.08 0.06 0.04


0.008 0.006 0.004 0.002

0.02

0.000

0.00 -0.02 -40

-30

-20

-10

0

10

20

30

-0.002 -40

40

-30

-20

Crank angle/℃ A

-10

0

10

20

30

40

20

30

40

Crank angle/℃ A (b) H2O2

(a) H2O 0.10

0.08


0.010


OH Mole Fraction

0.008

CO2 Mole Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

0.06

0.04

0.02

0.004

0.002

0.00 -40

0.006

0.000 -30

-20

-10

0

10

20

30

40

Crank angle/℃ A

-40

-30

-20

-10

0

10

Crank angle/℃ A

(c) CO2

(d) OH

Condition 3 : Φ= 1.2 T= 325 K P= 1 atm

Fig.9 Comparison of main species mole fractions at different conditions

CO is not only the important intermediate species for the CO2 produce process, but also the

emission pollutant for the internal combustion engine. So, it is necessary to compare CO mole

fraction. Fig. 10 shows the CO mole fractions at conditions 2 and 3. It can be observed that the

trends of CO mole fraction are in agreement with that of others mechanism. The values are also

same when the crank angle is 40 °CA at conditions 2 and 3. The maximum error is that the crank

angle delay 4 °CA by compared with that of detailed mechanism at condition 2. The similarly

conclusion can be got from the Fig.9 at condition 2. The new mechanism can predict the main 20


Page 21 of 30

species mole fractions for internal combustion engine.

0.10

0.12 Chang et al Detailed mod 74-species

0.10

CO Mole Fraction

0.08

CO Mole Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.06

0.04


0.08

0.06

0.04

0.02

0.02 0.00 -40

0.00 -30

-20

-10

0

10

20

30

40

-40

-30

Crank angle/℃ A

-20

-10

0

10

20

30

Crank angle/℃ A

(a) Φ= 1 T= 350 K P= 1 atm

(b) Φ= 1.2 T= 325 K P= 1 atm

Fig.10 Comparison of CO species mole fraction at different conditions

3.3 Comparison to Results of PAHs in JSR/PFR

The (jet stirred reactor) JSR/PFR (plug flow reactor) model was used to validate the accuracy

of new mechanism for PAHs mole fractions. The temperature was set at 1630 K, and the equivalence

ratio was 2.2. The 74-species mechanism can be validated with the experimental data and the

published mechanism data, which can be got from the tutorials of chemical 33.

Fig .11 shows the comparison of main species mole fractions, H2, C2H2, A1, A2 and A4. It can

be observed that the trends and the quantity of C2H2, H2 and A1 species are almost same for three

different mechanisms. The large errors can be found in comparisons of A2 and A4. However, the

trends of species A2 and A4 are almost same with that of experiment. The mole fractions of new

mechanism main species are close to the experimental data than that of published mechanism for

21


40

Energy & Fuels

A2 and A4. C2H2 and A4 often used as nucleation species in the soot model. The new mechanism

can be used in predicted soot mass. H2

0.1

C2H2

0.01

Mole Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

1E-3

A1 1E-4

A2 1E-5

1E-6

A4

1E-7 0

4

8

12

16

20

Residence Time /ms

Fig.11 Comparison of PAHs species mole fractions (the solid symbols represent the experiment, the open symbols represent the new mechanism, the lines represent the published mechanism 33)

4. The Reduced Chemical Mechanism Validation

Due to the large structure of marine diesel engine, the new mechanism needs to be reduced.

The calculated time can be saved for using reaction mechanism to deal with combustion. The

reduced mechanism consists of 279 reactions and 62 species. The reduced mechanism will be

validated with the experimental data in 3-D marine diesel engine model.

Fig.12 shows the comparisons of ignition delay times for new and reduced mechanisms at

different pressure and equivalence ratios. It can be observed that the ignition delay times almost

have no difference with the data of new mechanism. Because the absolute tolerance and relative

tolerance of the ignition delay time can be control as 1e-06 and 10 %, respectively.

22


Page 23 of 30

10000

1000


P=13 atm 74-species 62-species P=40 atm 74-species 62-species

10000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

100

P=13 atm 74-species 62-species P=40 atm 74-species 62-species 1000

100 10 0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0.80

0.85

0.90

1000/T (1/K)

0.95

1.00

1.05

1.10

1.15

1000/T (1/K) (b) Φ= 1

(a) Φ= 0.5

Fig.12 Comparison ignition delay times of new and reduced mechanisms at different pressure and equivalence ratios

The mechanism is established to simulate the combustion of marine diesel engine fuel. Fig. 13

shows the comparison of in-cylinder pressure of marine diesel engine. The in-cylinder pressure of

reduced mechanism can be compared with the experimental data, result of published mechanism by

Chang et al. and the semi-detailed reaction mechanism. It can be shown that the in-cylinder pressure

of the reduced mechanism is in good agreement with experimental data. The comparisons of key

parameters are shown in Table.3, including the maximum compress pressure and combustion

pressure, the time of break pressure, power and break specific fuel consumption (BSFC). From the

perspective of pressure, the maximum compress pressure and combustion pressure, the time of break

pressure of new and reduced mechanism CFD models are close to the experimental data, which can

also be sown in Fig.13. The errors of new mechanism and reduced mechanism CFD models power

are 0.2 % and 2.4 %, which are acceptable.

23


Energy & Fuels

180 150

Pressure/ bar

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

EXP Chang et al 74-species 62-species

120 90 60 30 0 200

150

350

300

250

400

450

o

Crank angle/ CA

Fig. 13 Comparison of in-cylinder pressure for marine diesel engine

Table .3 The comparison of important parameters

Exp17

Chang et al.25

74-species

62-species

Maximum Compress pressure (bar)

162.47

163.96

163.93

163.90

Maximum Combustion pressure (bar)

174.83

173.92

172.79

172.05

The time of break pressure (°CA)

365.35

364.4

364.4

364.5

Power output (kW)

3575

3646.02

3583.75

3662.08

Break specific fuel consumption (g/kWh)

180

176.54

179.61

175.793

The emission pollutants have also been compared with experimental data, including NOx and

CO2, which can be observed in the Fig. 14. The quantities of CO2 using mechanism combustion

models are lower than experimental data, since the difference can be existed in simulated and

actually condition. However, the error is 1.5 %, which can be gotten by comparing the reduced

mechanism and the published mechanism by Chang et al CFD models. From the NOx aspect, we

can observe that the error of 62-species mechanism CFD model is 2.2 %, which is closer to the 24


Page 25 of 30

experimental data. The result of 74-pecies mechanism CFD model is higher than reduced

mechanism CFD model value, the reason is that the differences of temperature and oxygen

concentration distribution.

CO2 / g/kWh

570 555 540 525 510 495 480 EXP

Chang et al

74-species

62-species

EXP

Chang et al

74-species

62-species

14.4

NOx / g/kWh

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

14.0 13.6 13.2 12.8 12.4

Fig. 14 Comparison of emission pollutants in marine diesel engine

Due to the limited experiment condition, the soot cannot be validated with the marine diesel

engine. However, the PAH as the precursor of soot, which has directly relationship with the soot.

Fig. 15 displays the comparison of main species mole fraction. It can be observed that the results of

62-species mechanism model are in good agreement with that of the 74-species mechanism model.

In conclusion, the n-tetradecane can be used as alternative fuel of the marine diesel engine. It can

greatly predict the emission pollutants, especially for NOx.

25


Energy & Fuels

H2

0.1

C2H2

0.01 1E-3

Mole Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

A1 1E-4 1E-5

A2

1E-6

A4

1E-7 1E-8 0

4

8

12

16

20

Residence Time /ms

Fig.15 Comparison of PAHs species mole fractions for the 74-species and 62-species mechanisms

5. Conclusions

A semi-reduced kinetic model can be constructed to investigate the performance of marine

diesel engine, which comprised 341 elementary reactions and 74 species. Then, the Directed

Relation Graph with Error Propagation (DRGEP) and the sensitivity analysis were used to reduce

semi-reduced kinetic mechanism. The reduced mechanism consists of 279 reactions and 62 species.

The semi-detailed and the reduced mechanisms were validated with experimental data. The results

are listed as following:

1. In shock tube conditions, the ignition delay times with the new and reduced mechanisms

were validated with the experimental data. Good agreements of ignition delay times were

obtained.

26


Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2. The semi-detailed mechanism was tested under internal combustion engine conditions,

which compared with the detailed and published mechanisms. It was found that the trends

of temperature, pressure and main species mole fractions were in good agreement with the

detailed and published mechanisms.

3. The semi-detailed and reduced mechanisms were validated with the experimental data in

marine diesel engine CFD model. It can be seen that the pressure was in good agreement

with the experimental data. The NOx of reduced mechanism CFD model has a 2.2 % error

with experimental data, which is the minimum error compared with that of published

mechanism CFD model.

4. The PAHs were also validated with experimental data for the semi-detailed and reduced

mechanisms. The results of reduced mechanism were closer to the experimental data,

especially for A4.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51376136),

National Sci-Tech Support of China (2015BAG16B01) and Natural Science Foundation of Tianjin

(No. 14JCYBJC21300).

27


Energy & Fuels

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Development of a Reduced n-TetradecaneâPolycyclic Aromatic

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