methane–air flame

Fuel 136 (2014) 37–45

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Experimental and numerical study of laminar premixed dimethyl ether/methane–air flame Huibin Yu, Erjiang Hu ⇑, Yu Cheng, Xinyi Zhang, Zuohua Huang ⇑ State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

h i g h l i g h t s  We investigate DME addition effect on flame speed, flame structure, Markstein lengths and flame instability of methane.  Laminar flame speeds are measured at different temperatures, pressures, dilutions and DME blending ratios.  Markstein lengths are presented under different DME blending ratios.  Formaldehyde, methyl radical and formyl radical play an important role in the high temperature DME oxidation.

a r t i c l e

i n f o

Article history: Received 19 May 2014 Received in revised form 10 July 2014 Accepted 14 July 2014 Available online 26 July 2014 Keywords: Laminar flame speed Markstein length DME Methane Flame structure

a b s t r a c t Laminar flame speeds of dimethyl ether/methane–air mixtures were measured in a constant volume combustion vessel at different initial temperatures (303–453 K), initial pressures (0.1–0.7 MPa), dilution ratios (0–25%), equivalence ratios (0.7–1.6) and over wide range of DME blending ratios (0–100%). Markstein lengths were also obtained at different blending ratios and equivalence ratios. Zhao DME model and NUIG Aramco Mech 1.3 were used to predict laminar flame speeds under the experimental conditions for the validation of the two kinetic models. Results show that laminar flame speeds increase almost linearly with the increase of DME blending ratio and the decrease of dilution ratio. The sensitivity coefficients of pressure are decreased with the increase of the blending ratios of DME. The simulated results of Zhao model agree well with the experimental data and those of previous studies, whereas the predicted results of NUIG Aramco Mech 1.3 give over-predictions especially at fuel-rich mixtures. Large increase in formaldehyde and small increase in methyl are resulted from the increase of DME. The laminar flame speed is found to have the linear correlation with H + OH mole fraction at maximum H mole fraction position. The unburned and burned gases Markstein length exhibit the similar trend and do not change monotonically with the increase of equivalence ratio for the mixtures with 20% DME addition. The intrinsic hydrodynamic instability is increased with the increase of DME blending ratio. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction With strengthening emission regulations and demand for energy diversity, more researches have been paid on the alternative fuels. Dimethyl ether (DME) is the simplest ether and its production is not restricted to one feedstock (i.e. synthesis gas, coal), making it to be the promising alternative fuel for gas turbine and IC engines [1]. Besides, DME is easier and safer to transport on board because of its similar physical properties to those of liquid petroleum gas (LPG) [2]. DME as one of the alternative clean fuels has no carbon-carbon bond and high oxygen content, making it ⇑ Corresponding authors. Tel.: +86 29 82665075; fax: +86 29 82668789. E-mail addresses: [email protected] (E. Hu), [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.fuel.2014.07.032 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

soot-free combustion. Therefore, DME has been regarded as a promising substitute for liquid petroleum gas and natural gas. In addition, DME is a good ignition and combustion improver or additive. Liang et al. [3] studied the combustion and emissions of DME/methanol SI engine under idle condition. They found that the indicated thermal efficiency was increased and the coefficient of cyclic variations in engine speed was decreased when DME was added. Meanwhile, HC emissions were remarkably decreased with the increase of DME ratio. Chen et al. [4] performed both experimental and numerical study on the methanol/DME mixture combustion. Their results showed that methanol concentration and injection time has a strong influence on the heat release process. Lee et al. [5] studied the effect of n-butane and propane on the combustion characteristics in a DME/LPG SI engine. Their results showed that engine operation was stable with propane/

38

H. Yu et al. / Fuel 136 (2014) 37–45

Nomenclature rf / Pu Tu XDME /r

j Sb S0u

raw flame radius (cm) equivalence ratio initial pressure (MPa) initial temperature (K) DME fraction in blending mixtures dilution ratio stretch rate (s1) stretched flame propagation speed (cm s1) laminar flame speed (cm s1)

S0b

unstretched flame propagation speed (cm s1) density of unburned gas mixtures (kg m3) density of burned gas mixtures (kg m3) adiabatic temperature (K) burned Markstein length (mm) unburned Markstein length (mm) laminar flame thickness (cm) density ratio of the unburned gas to the burned gas

qu qb Tad Lb Lu d

r

Table 1 Experimental conditions. P (MPa)

T (K)

XDME (%)

/

/r

0.1 0.1 0.1 0.1, 0.3, 0.5, 0.7

303 303 303, 333, 373, 413, 453 303

0–100 0–100 0–100 0–100

0.7–1.6 1.0 1.0 1.0

0 0%, 5%, 10%, 15%, 20%, 25% 0 0

(a)

40

30

-1

Su(cm•s )

35

Pu=0.10MPa,Tu=303K

25

0

Present data Davis et al.[24] Tahtouh et al.[20] Hassan et al.[19] Chen et al.[7] Lowry et al.[8]

20 15 10 0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Equivalence ratio 60

(b)

50

0

-1

Su(cm•s )

DME blend compared to n-butane/DME blend. In addition, combustion stability, brake specific fuel consumption and engine power output fueled with the propane/DME blend were comparable to those fueled with pure LPG operation. Natural gas has been widely used in SI engine due to its good anti-knocking property and low emissions. Besides, Natural gas is also a clean gas turbine fuel for power generation. Methane is the primary component of natural gas and its combustion characteristic has been fully studied. However, limited study on the DME/ CH4 binary fuel is conducted. Ano and Dryer [6] studied the ignition process of methane with small amounts of DME addition in a flow reactor at engine related conditions. Their results showed that DME was effectively in the promotion of auto-ignition and oxidation of methane. Chen et al. [7] experimentally and numerically studied the effect of DME addition on the burning properties and high temperature auto-ignition of methane/air mixtures. Their results revealed that DME significantly decreased the high temperature ignition delay and increased the flame speed. Lowry et al. [8] investigated the laminar burning characteristics of DME/CH4 binary fuel at two DME mixing ratios (20% and 40%) and three initial pressures (1, 5, 10 atm). Their results indicated that the flame speed was increased with DME addition. Besides, Markstein lengths were highly influenced by the blending ratios, while small variation in Lewis numbers when 20% DME was added. Tang et al. [9] studied the ignition delay times of DME/CH4 mixtures over a wide range of temperatures (1134–2105 K), pressures (1, 5, 10 bar) and DME blending ratios (0–100%) in a shock tube. Their results showed that the ignition delay times decreased largely with small addition of DME, and model predictions agreed well with measured data. Laminar flame speed embodies the physicochemical properties of combustible gas (i.e. diffusivity, reactivity), and is the elementary parameter in modeling turbulent flame speed and simulating engine combustion. Besides, laminar flame speed has been the fundamental data for developing and validating the chemical kinetics mechanism [10]. Laminar flame speeds have been investigated for the DME–air flame [10–15], and methane–air flame [16–24]. As previously mentioned, only Chen et al. [7] and Lowry et al. [8] experimentally and numerically reported the laminar flame speeds under limited initial conditions and fuel blending ratios. Laminar flame speeds at high initial temperatures and diluted conditions

40

Pu=0.10MPa,Tu=303K

30

Present data Vries et al.[10] Wang et al. [11] Daly et al. [12] Zhao et al. [14]

20

10 0.6

0.8

1.0

1.2

1.4

1.6

1.8

Equivalence ratio Fig. 1. Laminar flame speeds of DME/CH4–air mixtures versus equivalence ratio (symbol: experimental data; dash line: NUIG Aramco Mech 1.3; solid line: Zhao model).

39

H. Yu et al. / Fuel 136 (2014) 37–45

50 45

(a)

Pu=0.10MPa,Tu=303K

35 30

0

-1

Su(cm•s )

40

CH4

25

20/80 DME/CH4

20

40/60 DME/CH4 60/40 DME/CH4

15

2. Experimental and computational approaches

80/20 DME/CH4

10

DME

5 0.6

0.8

1.0

1.2

1.4

1.6

Equivalence ratio φ 50 45

(b)

Pu=0.10MPa,Tu=303K

40

0

-1

Su(cm•s )

35 30 25

Present 20/80 DME/CH4

20

Present 40/60 DME/CH4

15

Lowry et al.20/80 DME/CH4[8]

The experimental apparatus was reported in the previous publications [27,28]. Here, a brief description is given. The system consists of a combustion chamber wrapped by a heating tap, an ignition and data acquisition system, a mixture preparation and high-speed schlieren photography system. The combustion chamber is the cylindrical chamber with its inner diameter of 180 mm and length of 210 mm. A thermal-couple with an uncertainty of ±3 K and pressure transmitter are used to detect temperatures and pressures. When all components are introduced into the chamber according to their partial pressures, the centrally located electrodes are used to ignite the premixed mixtures after waiting at least 5 min to ensure full mixing and gas motionless in the chamber. Flame propagation record is conducted using the highspeed camera (Phantom V611) operating at 10,000 frames/s. To

Lowry et al.40/60 DME/CH4[8]

10 5 0.6

In this study, laminar flame speeds of DME/CH4–air mixtures were measured at different DME blending ratios, initial temperatures and initial pressures. The experimental conditions are specified in Table 1. Laminar flame speeds of DME/CH4 mixtures diluted by nitrogen were also measured. The simulated laminar flame speeds using the NUIG Aramco Mech 1.3 [25] and Zhao model [26] were compared with the experimental data. The dependences of burned and unburned Markstein lengths on blending ratio and equivalence ratio were presented. The effects of DME addition on flame structures were also investigated.

Tu=303K,φ = 1.0

45

0

Su-DME=23.23*(1/P)

Chen et al.[7]

0.8

1.0

S

1.2

1.4

-1

Su(cm•s )

35

55

Pu=0.10MPa,Tu=303K

0.30

0

0.33

30

S

0 u-20/80 DME/CH4

S

0 u-CH4

=16.69*(1/P)

=15.62*(1/P)

0.34

0.35

0

(c)

25

45 40

20

0

-1

Su(cm•s )

=22.26*(1/P)

(a) 0.29

0

Su-40/60 DME/CH4=18.19*(1/P)

Equivalence ratio φ

50

0.28

Su-60/40 DME/CH4=20.34*(1/P)

40

1.6

0 u-80/20 DME/CH4

35 15 0.0

30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pressure/MPa 25 Present 60/40 DME/CH 4 20

Present 80/20 DME/CH 4

80

15

φ = 1.0

20/80 DME/CH 4

1.2

1.4

70

1.6

Equivalence ratio φ Fig. 2. Laminar flame speeds of binary fuel–air mixtures versus equivalence ratio (symbol: experimental data; sold line: Zhao-60/40 DME/CH4; dash line: Zhao-80/20 DME/CH4; dot line: NUIG-60/40 DME/CH4; dash dot line: NUIG-80/20 DME/CH4).

have not reported yet. Besides, laminar flame speeds at high DME blending ratios are rare and the latest chemical model- NUIG Aramco Mech 1.3 [25] has not been validated by laminar flame speeds of DME/methane binary fuel and Zhao model [26] was only validated by few data of laminar flame speeds of DME/methane–air mixtures. Therefore, it is necessary to extend the study on laminar flame speeds of DME/methane–air mixtures.

40/60 DME/CH 4 60/40 DME/CH 4

-1

1.0

80/20 DME/CH 4

60

DME

0

0.8

Su(cm•s )

10 0.6

(b)

Pu = 0.1MPa

CH4

50

40

30

300

320

340

360

380

400

420

440

460

Temperature / K Fig. 3. Laminar flame speeds versus blending ratios at different pressures and temperatures and equivalence ratio of 1.0.

40

H. Yu et al. / Fuel 136 (2014) 37–45

50 45

/r ¼

Pu = 0.1MPa,Tu = 303K

CH4

φ = 1.0

20/80 DME/CH4

ð6Þ

2200

3. Results and discussions

40/60 DME/CH4 60/40 DME/CH4

35

80/20 DME/CH4

-1

Su(cm•s )

2300

where Vi is the volume fraction of i species. To remove the effect of ignition energy and pressure rise, the flame radii between 6 mm and 25 mm are used to calculate the laminar flame speed [23,31]. In addition, the maximum flame radius was also restricted by the occurrence of the cellular structure at high pressure conditions. Laminar flame speeds were calculated using the Chemkin and Premix codes [32,33]. In this study, Zhao model [26] and NUIG Aramco Mech 1.3 [25] were used to calculate laminar flame speeds. The NUIG Aramco Mech 1.3 model is the latest model which contains 253 species and 1542 elementary reactions developed by Curran et al. However, this model is not validated by the laminar flame speed of DME and DME/CH4 blended fuels. Zhao DME model contains 55 species and 292 elementary reactions and has been validated by the laminar flame speeds of DME/CH4 blends. All simulations were made using five continuation options and the finial values of adaptive grid parameters (GRAD and CURV) are both set at 0.02 for each case to obtain grid-independent solutions. There are more than 800 mesh points at the final solutions. This convergence level is enough for the accuracy of the calculations. The mixture-averaged transport option was adopted in determining the species diffusion coefficients.

40

DME

0

30 25 20 15 10

0

5

10

15

20

25

Dilution ratio Fig. 4. Laminar flame speeds versus dilution ratio at different DME blending ratios.

2400

Pu=0.10MPa,Tu=303K

Tad/K

V N2  100 V fuel þ V N2 þ V air

2100

3.1. Model validation

CH4 20/80 DME/CH4

2000

Fig. 1 illustrates the measured and simulated laminar flame speeds of CH4–air mixtures and DME–air mixtures. The comparison

40/60 DME/CH4 60/40 DME/CH4

1900

80/20 DME/CH4

CH3OCH3+H=CH3OCH2+H2

DME 1800 0.6

0.8

1.0

1.2

1.4

1.6

Equivalence ratio φ

HCOOH=HCO+OH

CH4

H+O2(+M)<=>HO2(+M)

40/60 DME/CH4 DME

HO2+OH<=>H2O+O2

Pu = 0.1MPa

CH3+H(+M)<=>CH4(+M)

Fig. 5. Adiabatic temperatures versus equivalence ratio at different DME blending ratios.

Tu = 303K φ = 1.0

HCO+H=CO+H2 H+OH+M=H2O+M C2H4+H(+M)<=>C2H5(+M)

ensure the accuracy of experimental data, each test was repeated 3 times.For the outwardly spherical freely propagating flame, the stretched flame propagation speed is calculated from original flame radius (rf)-time (t) history via

Sb ¼ dr f =dt

j¼

1 dA 2 ¼ Sb A dt r f

ð3Þ

ð4Þ

Here qb and qu are the burned and unburned gas density. DME fraction (XDME) and dilution ratio (/r) are defined as,

X DME

V DME ¼  100 V DME þ V CH4

CH3+HO2<=>CH3O+OH CO+OH=CO2+H HCO+M=H+CO+M H+O2=O+OH

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

CH3+CH3<=>H+C2H5

where S0b is the unstretched flame propagation speed and Lb is the burned Markstein length. From mass conservation across the finite flame front, the unstretched laminar flame speed, S0u , is,

S0u ¼ S0b qb =qu

CH3+OH=CH2(S)+H2O

ð2Þ

In the case of weak stretch, the stretch flame propagation speed can be linearly correlated with stretch rate [23,30] through

S0b ¼ Sb  Lb j

O+H2=H+OH

ð1Þ

Flame stretch rate, j, which consists of the strain rate and the stretch rate caused by flame curvature [29], can be derived through

(a)

Zhao model

CH3+CH3(+M)=C2H6(+M)

ð5Þ

NUIG model

HO2+OH<=>H2O+O2 H2+O2<=>H+HO2

0.4

(b)

CH4

HCO+OH<=>CO+H2O

40/60 DME/CH4

H+O2(+M)<=>HO2(+M)

DME

HCO+H<=>CO+H2

Pu = 0.1MPa

H+OH+M<=>H2O+M CH3+H(+M)<=>CH4(+M)

Tu = 303K φ = 1.0

C2H4+H(+M)<=>C2H5(+M) OH+H2<=>H+H2O O+H2<=>H+OH HO2+H<=>OH+OH CH3+HO2<=>CH3O+OH CO+OH<=>CO2+H HCO+M<=>H+CO+M H+O2<=>O+OH

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Fig. 6. Sensitivity analysis for different blending ratios using NUIG Aramco Mech 1.3 and Zhao model.

41

H. Yu et al. / Fuel 136 (2014) 37–45

with the previous and simulated values is also presented. For the CH4–air mixtures, the present experimental data agree fairly well with the previous data and good agreement with the simulated values with Zhao model [26] is presented. For the DME–air mixtures, the measurements in this study agree well with those of the previous publications, with only slight higher at fuel-lean mixtures. It was found that Zhao model [26] give relatively better prediction on laminar flame speeds than NUIG Aramco Mech 1.3 [25] does. For methane–air and DME–air mixtures, NUIG Aramco Mech 1.3 [25] gives good prediction under the fuel-lean conditions but over-prediction under the stoichiometric and fuel-rich conditions. NUIG Aramco Mech 1.3 [25] agrees well with the results of Davis et al. [24] for the CH4–air mixtures and agrees well with the results of Zhao et al. [14]. The comparison with the previous data indicates the accuracy of the experiments and data processing. 3.2. Laminar flame speeds of DME/CH4–air mixtures Fig. 2 gives measured laminar flame speeds of DME/CH4–air mixtures versus equivalence ratio and the comparison with previous data of Chen et al. [7] and Lowry et al. [8] along with the simulated results using Zhao model [26] and NUIG Aramco Mech 1.3 [25]. As shown in Fig. 2a, laminar flame speeds give their peak values at the equivalence of 1.1 regardless of DME blending ratio (XDME) and they increase monotonically with the increase of XDME. Fig. 2b and c indicates that Zhao model [26] gives good prediction on the laminar flame speed of DME/CH4–air mixtures, whereas NUIG Aramco Mech 1.3 [25] shows over-prediction to

experimental data. In addition, the experimental data of present study agree well with those of previous data, but are slightly higher under fuel-lean conditions. Laminar flame speeds of the stoichiometric mixture at different pressures and temperatures are plotted in Fig. 3a and b. As expected, laminar flame speeds increase monotonically with the increase of initial temperature and decreases with the increase of initial pressure. Meanwhile, the decline with the increase of pressure at relatively low pressure for given blending ratio is more obvious. The increase in initial temperature results in an increase in the adiabatic temperature, leading to the increase of reaction rate and promoting the overall reaction progress. The adiabatic temperature is insensitive to initial pressure, and reaction rates of chain branching reactions are also insensitive to initial pressure. When pressure is increased, the chain termination reactions are enhanced, resulting in inhibiting influence on the whole reaction progress [34,35]. The curves in Fig. 3a are the empirical formulas in the form of g(1/P)b, where g and b are the constants, and the unit of P is MPa. Fig. 3a indicates that sensitivity of laminar flame speeds to pressure is increased with the decrease of DME concentration and the laminar flame speed of methane is most sensitive to pressure. Additionally, the correlations of the laminar flame speed to pressure are also presented in Fig. 3a. Fig. 4 gives the laminar flame speeds of the diluted DME/CH4/air flames at / = 1.0. As shown in Fig. 4, laminar flame speeds decrease monotonically with the increase of dilution ratio (/r). Since the adiabatic temperature, Tad, has dominant impact on the S0u through

0.012

0.012 H CH3

Species mole fraction

0.010

O OH CH2O

0.00798

0.008

C 2H 6

0.006 0.00495

0.004 0.0025

0.002

H CH3

80/20 DME/CH4

0.00189 0.00122

0.010

Species mole fraction

CH4

0.00937

O OH CH2O

0.008 0.00705

0.00722

C 2H 6

0.006 0.0038

0.004 0.00246

0.002

0.00242

0.00096

0.000

0.000

0.0

0.1

0.2

0.3

0.00

0.4

0.05

0.10

Distance/cm

O OH CH2O

0.00883

Species mole fraction

0.30

0.35

0.40

0.008

C2H6

0.00615

H CH3

DME

0.0047 0.00322 0.002

0.010

Species mole fraction

0.010

0.002

0.25

0.012

H CH3

40/60 DME/CH4

0.004

0.20

Distance/cm

0.012

0.006

0.15

0.008

0.00958

O OH CH2O

0.00821 0.00743

C2H6

0.006 0.00404

0.004 0.00264

0.00258

0.002

0.00208

0.000

0.000 0.00

0.05

0.10

0.15

0.20

Distance/cm

0.25

0.30

0.35

0.40

0.00

0.05

0.10

0.15

0.20

Distance/cm

Fig. 7. Simulated premixed stoichiometric flame structures at Tu = 300 K, Pu=0.1 MPa.

0.25

0.30

0.35

0.40

42

H. Yu et al. / Fuel 136 (2014) 37–45

R50 CH3+O=CH2O+H R250 CH3OCH2=CH2O+CH3 R73 CH3O+M=CH2O+H+M R66 CH OH+O =CH O+HO 2 2 2 2

0.006

0.020

(a)

φ =1.3

φ =1.0

0.004 0.002 0.000 -0.002

R44 CH2O+H=HCO+H2 R46 CH2O+OH=HCO+HO2 Total rate of production

-0.004

750

900

1050

1200

0.014 0.012 0.010 0.008 0.006

1350

1500

1650

1800

0.004

1950

0

20

40

Temperature / K

(b)

R250 CH3OCH2=CH2O+CH3 R56 CH4+H=CH3+H2 R57 CH4+O=CH3+OH R58 CH4+OH=CH3+H2O Total rate of production

0.006 0.004

60

80

100

XDME

0.008

Fig. 9. Mole fraction of (H + OH) at maximum H mole fraction position versus DME blending ratio at pressure of 0.1 MPa and temperature of 303 K.

45

Pu=0.10MPa,Tu=303K

0.002

40

0.000 35

φ =0.7 φ =1.0 φ =1.3

-1

S u(cm• s )

-0.002 R100 CH +OH=CH (S)+H O 3 2 2 R50 CH +O=CH O+H 3 2 R54 CH3+CH (+M)=C H (+M)

-0.004

3

2

30

0

3

H+OH

H+OH

H+OH 0.016

-0.006

Rate of production/ mole/ (cm • s)

φ =0.7

0.018

Species mole fraction

3

Rate of production/ mole/ (cm • s)

0.008

6

25

-0.006 600

800

1000

1200

1400

1600

1800

2000 20

Temperature / K Total rate of production R54 CH3+CH3(+M)=C2H6(+M) R105 C2H6+H=C2H5+H2

3

Rate of production/ mole/ (cm • s)

0.0016 0.0012

(c)

15 0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

H+OH mole fraction Fig. 10. Laminar flame speed versus (H + OH) mole fraction at maximum H mole fraction position.

0.0008 0.0004

pressure of 0.1 MPa and temperature of 303 K using both NUIG Aramco Mech 1.3 [25] and Zhao model [26]. The reactions which have positive sensitivity coefficient promote the combustion process and those have negative sensitivity coefficient inhibit the combustion process. As shown in Fig. 6b, for NUIG Aramco Mech 1.3 [25], the most sensitive chain-branching reaction is R1 and most chain-termination reaction is R2.

0.0000 -0.0004

R106 C H +O=C H +OH 2 6 2 5 R107 C H +OH=C H +H O 2 6 2 5 2

-0.0008 600

800

1000

1200

1400

1600

1800

2000

Temperature / K

H þ O2 () O þ OH

ðR1Þ

CH3 þ H ðþMÞ () CH4 ðþMÞ

ðR2Þ

Fig. 8. ROP analysis for 40/60 DME/CH4 mixture (/=1.0, Tu=303 K, Pu=0.1 MPa).

the Arrhenius kinetics [35]. The diluted gas, nitrogen, is inactive gas. The decrease in Tad with the increase of /r contributes significantly to the decrease of the S0u . Fig. 5 shows the relationship of the adiabatic temperature to equivalence ratio. As expected, equivalence ratio has a strong influence on the adiabatic temperature. Furthermore, the adiabatic temperature increases monotonically with the increase of DME blending ratio. The monotonous increase of Tad contributes to the monotonous increase of S0u by means of Arrhenius kinetics. Fig. 6 gives the normalized sensitivity coefficients of laminar flame speed for different DME blending ratios (0%, 40%, 100%) at

For Zhao model [26], however, the most sensitivity chain-termination reaction is H + OH + M = H2O + M for the DME–air mixture as shown in Fig. 6a. For the two kinetic models, the formyl decomposition reaction, HCO + M = H + CO + M, is the second most sensitive chain-branching reaction. As shown in Fig. 8a, formyl radical mainly forms through the H abstraction from formaldehyde (CH2O) by H and OH radicals. Therefore, methyl and formyl radicals also play an important role in the high temperature DME oxidation. In addition, the sensitivity coefficients of R1 and R2 decrease with the increase of DME blending ratio. CO + OH = CO2 + H is the third important reaction and most of the CO2 is produced through it. In

43

H. Yu et al. / Fuel 136 (2014) 37–45

Pu=0.10MPa

2.1

Tu=303K

(a)

DME 20/80 DME/CH4

Lb/mm

40/60 DME/CH4 1.8

60/40 DME/CH4

1.5

80/20 DME/CH4 CH4

1.2 0.9 0.6 0.3 0.0 0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Equivalence ratio φ 0.30 0.25

Pu=0.10MPa

(b)

DME 20/80 DME/CH4

Tu=303K

3.4. Markstein lengths and hydrodynamic instability

40/60 DME/CH4

0.20

Lu/mm

60/40 DME/CH4 0.15

CH4

0.10 0.05 0.00 -0.05 0.6

0.7

0.8

0.9

1.0

1.1

1.2

decreases with the increase of DME, the large increase in CH2O and small increase in CH3 are mainly resulted from an increased DME concentration through R250. As shown in Fig. 8c, C2H6 forms mainly via R54, and the small increase is mainly from the small increase of CH3.Previous studies revealed there exists a correlation between maximum radical concentration and laminar flame speeds [36–39]. Chen et al. [39] and Hu et al. [38] studied the methane/ethylene/air premixed flames and methane/hydrogen/ air premixed flames, respectively. They found that laminar flame speeds presented linear correlation with maximum H + OH mole fraction. Fig. 9 gives the H + OH mole fraction at maximum H position (HMAX + OH) versus the DME blending ratio. It is found that the mole fraction of HMAX + OH increases monotonically with the increase of XDME. From the point of chemical kinetics, the monotonous increase of radical mole fraction contributes to the monotonous increase of S0u . Fig. 10 gives the relationship of laminar flame speed to HMAX + OH mole fraction at pressure of 0.1 MPa and temperature of 303 K. Results also show the linear correlation between laminar flame speed and HMAX + OH mole fraction.

1.3

1.4

1.5

Equivalence ratio φ Fig. 11. Markstein lengths versus equivalence ratio at different DME blending ratios.

addition, CH3OCH3 + H = CH3OCH2 + H in Zhao model [26] is more sensitive than in NUIG Aramco Mech 1.3 [25] and has the negative sensitivity coefficient. 3.3. Kinetic analysis As discussed above, Zhao model [26] gives good prediction on experimental data. Hence, analysis on flame structure is given for better understanding of the effect of DME substitution on the flame structure and laminar flame speed. Flame structures of CH4–air premixed mixtures with 0%, 40%, 80%, 100% DME substitution at / = 1.0 are given in Fig. 7. Maximum concentrations of the intermediate species (O, OH, H) are increased with the increase of XDME while formaldehyde (CH2O) increases largely. Methyl (CH3) and ethane (C2H6) are slightly increased when DME is added in the CH4–air laminar premixed flame. Fig. 8a–c gives the ROP analysis of CH2O, CH3, and C2H6 for 40DME/60CH4 blend. As shown in Fig. 8a, CH2O forms in a remarkable amount via the following reactions:

In premixed flames, wrinkled flame front undergoing selfacceleration due to cellular instabilities could cause the turbulence of unburned mixtures and could be one of the major reason for gas explosion [40–42]. Besides, flame instability is an important effect even for turbulent combustion [43,44]. Practically speaking, wrinkled flame could potentially promote engine knock. Therefore, the flame instability is also important and can hardly be overemphasized. Parameters characterizing the intrinsic flame instability were analyzed in this study for the hydrodynamic instability. Hydrodynamic instability known as Landau–Darrieus instability is caused by the density disparity across the flame front. Its intensity can be characterized by the flame thickness (d), and the density ratio of unburned gas to burned gas (r) [45,46]. Flame thickness can be evaluated through various definitions, and in this study it was calculated via the gradient method [47] using the temperature curve of the simulated flame structure, defined as

d¼

T ad  T u ðdT=dxÞmax

ð7Þ

The Markstein length represents the sensitivity of flame speeds to the stretch rate and reflects flame instability. The burned Markstein length, Lb, and unburned Markstein length, Lu, correspond to

0.6 0.5

Present CH4

Present DME

Lowry et al. CH4 [8]

Vries et al. DME [10]

Chen et al. CH4 [7]

Chen et al. DME [7] Present 20/80 DME/CH 4

0.4

Lu/mm

2.4

Lowry et al. 20/80 DME/CH [8] 4 Chen et al. 20/80 DME/CH [7] 4

0.3 0.2 0.1

CH3 OCH2 ¼ CH2 O þ CH3

ðR250Þ 0.0

CH3 þ O ¼ CH2 O þ H

ðR50Þ

The CH3OCH2 forms mainly through the hydrogen abstraction from DME by OH, H, O and CH3 radicals. As shown in Fig. 8b, CH3 forms in a notable amount through R250 and H abstraction from CH4 by OH, O and H radicals. Since the concentration of CH4

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Equivalence ratio φ Fig. 12. Comparison of unburned Markstein lengths for pure DME, 20/80 DME/CH4 and pure CH4 at pressure of 0.1 MPa and temperature of 303 K.

44

H. Yu et al. / Fuel 136 (2014) 37–45

the burned and unburned flame propagation speeds respectively. Definition of Lu is similar to Lb and follows the behavior of flame speeds to the unburned gas, in terms of Su ¼ S0u   Lu j [47]. Lu can be calculated from Lb through Lu ¼ r1 Lb  a  r1 d by employing the continuity across the flame front and taking into account of the mass accumulation within the flame [21,47], where R a ¼ 01 ðq=qu Þdðx=dÞ. Variation of Markstein length reflects the variation of flame instability. Fig. 11a and b gives the burned and unburned Markstein lengths at pressure of 0.1 MPa and temperature of 303 K for. The burned and unburned Markstein lengths show the similar behavior. The Markstein lengths of methane and DME exhibit an opposite trend, suggesting the balanced effect on the flame instability of blend. Markstein lengths of DME decrease monotonically with the increase of equivalence ratio, especially large increase in Markstein lengths is presented with small amount of DME. Markstein lengths increase with the increase of DME blending ratio at fuellean side and decrease with the increase of DME blending ratio at fuel-rich side. This phenomenon was also observed by Chen et al. [7] and Lowry et al. [8]. When the blending ratio of DME is larger than 20%, the Markstein lengths of DME/CH4 binary fuel decrease monotonically with the increase of equivalence ratio, demonstrating the same instability tendency to that of DME. At 20% DME substitution, the Markstein lengths show an decrease and then increase behavior with the increase of equivalence ratio. Fig. 12 gives the comparison of the unburned Markstein lengths with the previous results in Refs. [7,8,10] at pressure of 0.1 MPa

8.5

(a)

Density ratio σ

8.0

7.5

7.0

DME 20/80 DME/CH 4

6.5

40/60 DME/CH 4 60/40 DME/CH 4 CH4

6.0 0.6

0.8

1.0

1.2

1.4

1.6

Equivalence ratio φ 0.08

(b)

CH4 20/80 DME/CH4

0.07

The effects of DME addition on laminar flame characteristics of DME/methane–air mixtures were experimentally and numerically studied. The diluted laminar flame speeds were also provided. Kinetic analysis was performed using the Zhao model. Main conclusions are as follows: (1) Laminar flame speed increases almost linearly with the increase of DME blending ratio and decreases almost linearly with the increase of dilution ratio. Sensitivity of laminar flame speed to pressure is decreased with the increase of DME blending ratio. (2) Zhao model presents good prediction on laminar flame speed of DME/CH4–air mixtures, whereas NUIG Aramco Mech 1.3 has over-prediction compared with those of experimental data. Formaldehyde, methyl radical and formyl radical play an important role in the high temperature DME oxidation. (3) Large increase in formaldehyde and small increase in methyl is mainly from the increase of DME. Small increase in ethane is mainly from the small increase of methyl. Laminar flame speed presents a linear correlation with H + OH mole fraction at maximum H mole fraction position. (4) Burned and unburned Markstein lengths give the similar behavior and they change greatly when small amount of DME is added. (5) Flame intrinsic hydrodynamic instability is increased with the increase of DME blending ratio because of the decreased flame thickness and increased density ratio. Conflict of interest

60/40 DME/CH4

0.06

δ /cm

4. Conclusions

The authors declare no competing financial interest.

40/60 DME/CH4 DME

Acknowledgements This study is supported by the National Natural Science Foundation of China (51306144 and 51136005), the National Basic Research Program (2013CB228406) and the State Key Laboratory of Engines at Tianjin University (SKLE201302). The support from the Fundamental Research Funds for the Central Universities is also appreciated.

0.05

0.04

0.03

0.6

and temperature of 303 K. It was found that the values of this study capture the trend in qualitatively, but in quantitatively, the difference between them is presented. In general, the values of this study are close to those of Lowry et al. [8] and Vries et al. [10], but have the difference to those of Chen et al. [7]. Fig. 13 depicts the density ratio and flame thickness versus equivalence ratio at pressure of 0.1 MPa and temperature of 303 K for different DME blending ratios. The density ratio and flame thickness exhibit an inverse trend. Flame thickness shows the decrease and then increase behavior with the increase of equivalence ratio for all mixtures. Density ratio increases monotonically with the increase of DME blending ratio and presents rapid increase at small DME blending ratios. Flame thickness decreases monotonically with the increase of DME blending ratio and presents rapid decrease at small DME blending ratios. The large change at small DME blending ratios is mainly resulted from the large increase in adiabatic temperature. The trends of density ratio and flame thickness indicate that flame intrinsic hydrodynamic instability will increase with the increase of DME blending ratio.

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Equivalence ratio φ Fig. 13. Density ratio and flame thickness versus equivalence ratio at different DME blending ratios at pressure of 0.1 MPa and temperature of 303 K.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2014.07.032.

H. Yu et al. / Fuel 136 (2014) 37–45

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