Numerical study on electron energy distribution characteristics and evolution of active particles of methanol-air mixture by non-equilibrium plasma

Energy 193 (2020) 116881

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Numerical study on electron energy distribution characteristics and evolution of active particles of methanol-air mixture by nonequilibrium plasma Changming Gong a, Lin Yi a, Kang Wang b, c, Kuo Huang d, Fenghua Liu a, * a

College of Mechanical and Electronic Engineering, Dalian Minzu University, Dalian, 116600, China State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, 130022, China BMW Brilliance Automotive Ltd, Shenyang, 110021, China d Fluids & Thermal Engineering Research Group, Faculty of Engineering, University of Nottingham, NG7 2RD, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 February 2019 Received in revised form 8 November 2019 Accepted 30 December 2019 Available online 2 January 2020

The extent of combustion enhancement by non-equilibrium plasma strongly depends on the electric field and electron number density. The electron energy distribution characteristics and evolution of active particles of methanol-air mixture by non-equilibrium plasma were numerical simulated. As an introductory work in this field, this study only partially cited other people’s data for comparative verification under without experimental validation data. The results showed that the electron energy distribution is mainly affected by field intensity. The electron energy distribution is increased with the increase of mean electron energy. The field intensity directly affects the formation and development of radicals in methanol-air plasma discharge. The generation of radicals in plasma discharge mainly occurs in the electron collision reaction with the electron energy of 3e10 eV. The concentration of excited states matter is higher than that of ionized states matter. The concentration of O radical is higher than the concentration of H and CH2OH, and then the concentration of H and CH2OH is much than that of OH and CH3. Considering the demand of radical concentration and economics of the peak pulse voltage, the field intensity of 220 Td - 400 Td is selected as a reasonable range for generating radicals. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Non-equilibrium plasma Electron energy distribution Active particles Methanol-air mixture

1. Introduction Methanol is considered to be one of the most promising alternative fuels for automotive engines of the future in China [1]. However, the low vapor pressure and high latent heat of vaporization of methanol behaviors can also result in difficult mixture preparation of a spark ignition (SI) methanol engine [2]. Many methods have been adopted to improve mixture preparation [3] and broaden the lean-burn limit [4] of a SI methanol engine, such as methanol preheating [5], air preheating [6], ignition amelioration [7], oxygen-enriched [8], H2-enriched [9], and blend fuel [10], etc. [11]. Non-equilibrium plasma assisted ignition and combustion has shown great potential for improving cold start firing performance and controlling combustion processes in engines [12]. Nonequilibrium plasma has higher electron temperature and more

* Corresponding author. E-mail address: [email protected] (F. Liu). https://doi.org/10.1016/j.energy.2019.116881 0360-5442/© 2020 Elsevier Ltd. All rights reserved.

kinetically active due to the rapid production of active radicals and excited species vie electron impact dissociation, and subsequent energy relaxation compared with equilibrium plasma [13]. Nonequilibrium plasma could be used to ignite ultra-lean mixtures (excess air ratio of 2e3) at high pressures (up to 10 MPa) and to extend the limits of ultra-lean flame propagation [14]. The extent of combustion enhancement by non-equilibrium plasma strongly depends on the electric field and electron number density [15]. The amount of electron energy in plasma directly affects the result of electron collision, different electron energy will produce different electron collision excitation, dissociation and ionization reaction, and then affect the generation and development of radicals [16]. During the discharge of plasma, the electron energy is obtained from the external electric field. The mean electron energy between electrodes is directly determined by the field intensity (defined as the ratio of electric field and total gas number density, E/n, where E is the electric field and n is the gas density), and the electron collision reaction rate and pathways are directly affected by the electron energy [17]. The electron density determines the power

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density in a plasma. In discharges, the energy gained by electrons from external electric field is lost in collisions with neutral particles. Electron heating and the efficiency of electron impact excitation of various states is controlled by E/n and the composition of gaseous mixtures [18]. Generally, the energy is transferred to the electrons in plasma without causing large increases in the temperature of active particles, therefore leading to the electron temperature often significantly exceeds the gas temperature [19]. In non-equilibrium plasma, the electron energy distribution (EED) depends on the applied electric field, the gas density, the ionization degree, and the gas composition [20]. During the discharge of plasma, the electric field is the key factor affecting the flame dynamics and flame chemionization [21]. Jaggers et al. [22] found that a strong electric field induced flow motion due to the collisions between positive ions and neutral molecules. Lewis et al. [23] also found that a strong electric field enhanced significantly flame speed and improved stabilization owing to collisional energy transfer between electrons and neutral molecules. Ju et al. [12] found that the strong ionic wind induced by an electric field increased the flame instability and reduced the soot formation. Unfortunately, the coupled between ionic wind and electron heating and ion chemistry effects made it difficult to quantity the contribution s of individual enhancement pathways in the experiments. In recent experiment and numerical simulation, Stockman et al. [24] found that the flame speed of lean methane-air mixture increased approximately by 20% in microwave resonator, and microwave absorption by the electrons in the flame zone led to a flame temperature rise of only 200 K and thus accelerated flame propagation. However, in order to avoid microwaveeflame front interaction, the intensity of the electric field used had to the lower than the breakdown voltage. Many kinds of excited particles, ions and radicals are formed in the plasma reaction. Plasma affects ignition and combustion via three different pathways: thermal, kinetic, and transport [12]. In the kinetic pathway [25], plasma produced high energy electrons [26] and ions [27] will further produce active radicals via various kinetic reactions [28]. For the efficient production of a large amount of active particles in the air discharge, both efficient generation in the air discharge plasma and slow recombination during collisions with major mixture components are necessary. The ignition temperature had a substantial drop due to the plasma activation [29]. The production of radicals, especially atom oxygen, from the plasma can dramatically change the reaction pathways of CH4 and C3H8, thereby improving both ignition and extinction behaviors [30]. Most of the previous studies used non-equilibrium plasmas have been largely focused on investigating the mechanism of nonequilibrium plasma-assisted ignition and combustion of H2- [31], CH4- [32], and C2H6-air mixtures [33], etc. [34]. However, few study has been conducted to analyze the EED characteristics and evolution of active particles of methanol-air mixture. The aim of this paper was to numerically study the effects of the field intensity, the ionization degree, and excess air ratio on the EED, and analysis evolution process of active particles of methanol-air mixture by non-equilibrium plasma. As an introductory work in this field, this study only partially cited other people’s data for comparative verification under without experimental validation data. This study will determine the major factors that affect EED, select a reasonable range of electric field to produce free radicals, and provide insight into the evolution mechanism of active particles. These results are helpful to guide the development of non-equilibrium plasma experimental research and to overcome the disadvantages of experimental approaches. Hereby, these results will provide some guidelines for the non-equilibrium plasma-assisted ignition and combustion of methanol engine, and also lays a foundation for the

application of non-equilibrium plasma in methanol engine. 2. Modeling methodology 2.1. Establishment of plasma generator model A one-dimensional unipolar pulsed dielectric barrier discharge (DBD) plasma model was developed, and the model structure is shown in Fig. 1. Unipolar pulse DBD can produce high pressure and large volume plasma. The mean electron energy in the field of plasma is between 1 and 10 eV, and it satisfies the excitation and dissociation of most gas molecules, therefore, unipolar pulse DBD is a very suitable discharge mode to produce non-equilibrium plasma. The basic assumptions of unipolar pulsed DBD plasma model are as follows: (1) Methanol-air mixture is an ideal gas, and air is approximately 79% nitrogen and 21% oxygen. (2) Methanol and air are mixed evenly before entering the plasma generator. (3) The discharge of the mixture is uniformly distributed along the radial direction. The plasma generator is simplified to a one-dimensional model. The plasma model parameters are shown in Table 1. In this study, unipolar nanosecond pulse discharge was used to generate plasma, and its pulse width is 1.0 ms [35]. The peak pulse voltage depends on the size of the required electric field intensity E/n. Other parameters required in the plasma model, such as electron mobility, were determined by Boltzman solver. In the process of plasma discharge, the external electric field provides energy for the system, so that the electron density and electron energy in the system can be improved, and the electron collision excitation, ionization and dissociation reaction can be generated in the space to form plasma. The energy supplied to the system by the external electric field is expressed in terms of the ionization degree, it is defined as

a ¼ Ne =ðNe þ Na Þ

(1)

whereais the ionization degree, Ne is the electron density, and Na is the neutral particle density. 2.2. Dynamic model of plasma discharge During the discharge process of methanol-air plasma, there are very complex physical and chemical reactions among substances. It mainly includes the following processes: elastic collisions between electrons and neutral matter; excitation, dissociation, and ionization by an inelastic collision between an electron and a neutral

Fig. 1. Schematic diagram of plasma generator model.

C. Gong et al. / Energy 193 (2020) 116881 Table 1 Plasma model parameters. Mixture temperature (K)

1300

Mixture pressure (MPa) Initial electron density (#$m3) Discharge electrode gap (mm) Barrier thickness (mm) Barrier material

0.1 2.0Eþ16 10 1 Quartz

substance; electron adsorption and attachment; and quenching of excited matter, etc. The main active substances produced in the methanol-air plasma discharge process are: electron excited state O2 molecule, electron excited state N2 molecule, O atom, H atom, þ þ and radicals, electronic, Oþ produced by the 2 , N2 , and CH3OH dissociation of methanol molecules. The chemical effect of plasmaassisted combustion is considered, but the temperature rise effect is ignored in this study. The reaction and its reaction rate constant in the kinetic model used in this study are shown in Table 2. The kinetic model mainly includes the following reactions: electron impact dissociation, electron impact excitation, electron impact ionization, quenching of excited N2/O2, charge exchange, electron/ ion recombination, and other major reactions during discharge. We considered electron impact dissociation, excitation and ionization of neutral particles, quenching of electronically excited particles, charge exchange in collisions between ions and neutral particles, and dissociative electron/ion recombination, the dominant mechanism of electron loss in the discharge afterglow under the condition considered [36]. The main reactions considered in the plasma dynamics model are the collision reaction that causes the molecules to dissociate [36], the O2 high energy electron excited state O2 (4.5 eV), O2 (A3), and O2 (B3), and the N2 high energy electron excited state N2 (A3), N2 (B3), N2 (C3), and N2 (al). The effects of the vibrational excitation state and the excitation states of low energy electrons of O2 and N2 on the discharge process are neglected in this model. The reaction velocity constants of R1-R7 in the plasma dynamics model were calculated through the relevant electron collision cross section data, and from relevant databases and references [37e40]. The reaction velocity constants of R11 and R14 were estimated according to the literature of Kosarev et al. [33,36,41].

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The electron energy in plasma directly affects the result of electron collision. Different electron energies will produce different electron collision excitation, dissociation and ionization reactions, and directly affect the generation and development of free radicals. Different electron energies collide with particles may produce different products, such as rotational excited states, vibrational excited state, electron excited state, dissociation and ionization, etc. Among the excited states generated by primary particle excitation, the energy required for rotational excited states is the lowest. The rotational excitation of particles can be realized by 0.03 eV electron energy, rotational excitation of air discharge mainly includes: e þ O2 / e þ O2 (rot)

(R17)

e þ N2 / e þ N2 (rot)

(R18)

where O2 (rot) and N2 (rot) represent the rotational excited states of O2 and N2, respectively. The O2 (rot) and N2 (rot) are very unstable, they may undergo rotational - translational relaxation quenching by the collision of some neutral molecules, such as: O2 (rot) þ M / O2 þ M

(R19)

N2 (rot) þ M / N2 þ M

(R20)

where M is a neutral molecule. The time of rotational - translational relaxation is usually 0.5 ns. Due to the short lifetime and small energy of the rotational excited state, the chemical reaction rate cannot be changed in both time domain and energy domain. Therefore, the effect of rotational excitation state on the formation of active particles and gas heating was neglected in this study. Electrons with energy of 0.2e3 eV generate vibration excitation of particles in the process of gas discharge [47], vibration excitation of air discharge mainly includes: e þ O2 / e þ O2 (vib)

(R21)

e þ N2 / e þ N2 (vib)

(R22)

Table 2 Kinetic reaction process. Number Electron impact dissociation R1 R2 R3 Electron impact excitation R4 R5 Electron impact ionization R6 R7 R8 Quenching of excited N2/O2 R9 R10 R11 Charge exchange R12 R13 R14 Electron/ion recombination R15 R16

Reaction

Rate constant (cm3$s1)

Reference

e þ O2 / e þ O þ O e þ N2 / e þ N þ N e þ CH3OH / e þ CH2OH þ H

f (E/n) f (E/n) f (E/n)

See text See text See text

e þ O2 / e þ O2* e þ N2 / e þ N2*

f (E/n) f (E/n)

See text See text

e þ O2 / 2e þ Oþ 2 e þ N2 / 2e þ Nþ 2 e þ CH3OH / 2e þ CH3OH

f (E/n) f (E/n) f (E/n)

See text See text See text

N2 *þ O2 / N2 þ O þ O O2* þ CH3OH / O2 þ CH2OH þ H N2* þ CH3OH / N2 þ CH2OH þ H

2  1010 1010 5  1010

[42] [36] Estimate

þ Oþ 2 þ CH3OH / O2 þ CH3OH þ Nþ þ O / N þ O 2 2 2 2 þ Nþ 2 þ CH3OH / N2 þ CH3OH

5  1010 5  1011 109

[43] [44] Estimate

e þ Oþ 2 / O þ O e þ CH3OHþ / CH3 þ OH

2  107 (300/Te) 3  107 (300/Te)0.5

[45] [46]

þ

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where O2 (vib) and N2 (vib) represent the vibration excited states of O2 and N2, respectively. The vibrational excited state of different particles usually has several energy levels. In general, the energy of vibrational excited particles can be increased by electronic excited particles in the form of energy level transition. The electronic excited state of individual particles has quenching effect on the vibrational excited state particles, and the energy step can be realized by vibration excited state energy, such as: N2(A3) þ N2(vib) / N2(B3) þN2

(R23)

Electrons with energies between 3 and 10 eV may occur molecular electron excitation and dissociation reactions, such as: e þ N2 / e þ N2(B3)

(R24)

The generation of radicals in plasma discharge mainly occurs in the electron collision reaction with the electron energy of 3e10 eV.

Fig. 3. Effect of the field intensity on mean electron energy and energy percentage.

3. Simulation results and discussion 3.1. Electron energy distribution characteristics

2.3. Model validation The simplified mechanism of plasma kinetic reaction used in this study has been validated by a large number of experiments [33,36,38,40,41,48]. The process of the formation and evolution of the active substances obtained by using the simplified plasma mechanism simulated in this study is highly consistent with the experimental results under the same conditions. The electron collision cross section data and reaction rate constants used in the methanol-air plasma kinetic reaction simplification mechanism have been verified by a large number of experimental data. In this study, the air in the methanol-air mixture in the plasma was 87.7%, accounting for the majority. In order to verify the kinetic mechanism of methanol-air plasma discharge, the concentration of methanol-air mixture was changed to CH3OH:O2:N2 ¼ 0:0.21:0.79 in this study, the comparison between calculation results and experimental results is shown in Fig. 2 [49]. Calculation and experiment curves match well, with average error less than 10%. Therefore, the numerical model was validated as suitable calculate other parameters. The calculation conditions of methanol air mixture in this study are excess air ratio 1.0, pressure 0.1 MPa, and temperature 1300 K. The selection of these parameters is close to the actual operating parameters of the engine, so the results of this study have important reference value for studying the plasma-assisted combustion of methanol engine.

Fig. 2. Mole fraction of oxygen atoms in the air discharge plasma.

3.1.1. Effect of the field intensity on electron energy distribution Fig. 3 shows the effect of the field intensity on mean electron energy and energy percentage at excess air ratio 1.0 and ionization degree 1E-6. The mean electron energy increases with the increase of E/n. This is mainly due to the increase in the intensity of the external electric field, so that the electric field electrons can obtain more energy. When field intensity increases from 70 Td to 220 Td,

Fig. 4. Electron energy distribution under different mean electron energy.

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Fig. 5. Effect of ionization degree on electron energy distribution under different mean electron energy.

the mean electron energy of field intensity 220 Td is 200% higher than that of field intensity 70 Td, and energy percentage of 3e10 eV increases 2.24 time. Between field intensity of 70e220 Td, the energy percentage of 0e3 eV and above 10 eV decreases as the increase of field intensity. When field intensity further increases from 220 Td to 430 Td, the mean electron energy of field intensity 430 Td is 50% higher than that of field intensity 220 Td, and energy percentage of 3e10 eV decreases 18%. Between field intensity of 220e430 Td, the energy percentage of 0e3 eV and above 10 eV increases as the increase of field intensity. The variation trend of the average electron energy with the electric field intensity is basically consistent with calculation results by Liu et al. [42]. However, there are some differences in the calculation results of the electron energy percentage of 3e10 eV in the field intensity above 220 Td compared with calculation results by Liu et al. [42]. This may be mainly caused by the difference discharge medium of air and methanol-air. The generation of radicals in plasma discharge mainly occurs in the electron collision reaction with the electron energy of 3e10 eV [50]. Therefore, increasing the electron energy percentage of 3e10 eV will effectively improve the generation efficiency of radicals (such as O and H). Moreover, although the proportion of electrons energy above 10 eV is small, the collision between these electrons and molecules can ionize CH3OH, O2 and N2, and the electron/ion recombination can also generate radicals and other active particles.

Fig. 4 shows the EED under different mean electron energy. Fig. 4b is a partial enlargement of Fig. 4a. It can be seen from Fig. 4b that the EED decreases with the increase of the mean electron energy when the electron energy is between 0 and 3 eV. Most of the electrons are concentrated in the electron energy between 0 and 3 eV. When the electron energy is above 10 eV, the EED is increased with the increase of mean electron energy. Thus, high-energy electrons for the electron energy of 3e10 eV dominate the production of radicals.

3.1.2. Effect of ionization degree on electron energy distribution Fig. 5 shows the effect of ionization degree on the EED under different mean electron energy at excess air ratio of 1. It can be seen from these figures at constant mean electron energy, the EED increases with the increase of ionization degree. For example, at mean electron energy 3 eV, the EED increases from 2.51E-11 to 2.84E-08 when the ionization degree rises from 106 to 102 under electron energy 30 eV. This maybe that the electron number density in plasma increases with increasing ionization degree, and the EED increases. The ionization degree has a small effect on the EED for the electron energy of 0e10 eV, and the effect of ionization degree on the EED decreases gradually with the increase of mean electron energy. Fig. 6 shows the effect of ionization degree on ionization rate coefficient of O2 and N2. At constant mean electron energy for below 8 eV, the ionization rate coefficient of O2 and N2 increase

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3.2.1. Production and development of excited states and ionized states matter Fig. 8 shows the mole fraction of excited states and ionized states matter as a function of time at different field intensity at excess air ratio 1.0, pressure 0.1 MPa, and temperature 1300 K. In particular, the mole fraction of ionized state Nþ 2 is below 1.0E-08, so line of Nþ 2 doesn’t show up on the Fig. 8 (a). It can be seen from these data that the concentration of excited states matter is higher than that of ionized states matter. The concentration of excited state O2 is higher than that of excited state N2. The concentration of þ ionized state Oþ 2 is far higher than that of ionized state N2 . With the increase of time, both concentrations of the excited states and ionized states matter first increase and then decrease. And with the increase of field intensity, the peak concentrations of both excited states and ionized states matter are obviously increasing, and the time of peak concentrations of both excited states and ionized states matter is much earlier. Only O2 (4.5 eV), O2 (A3), O2 (B3), and N2 (A3), N2 (B3), N2 (C3), and N2 (al) are considered in the methanol-air plasma excitation state. Take O2 (4.5 eV) for example, the electron collision reaction equation is e þ O2 / e þ O2 (4.5 eV)

(R25)

The reaction path of other excited state molecules is similar to that of O2 (4.5 eV) or can be further formed by low-energy excited state molecules. The activation energy of these high-energy excited state molecules is between 4.5 and 13 eV. Where, the collision quenching reaction for the high-energy excited state of N2 with O2 and CH3OH maintains a high reaction rate, thus providing a source for the formation of free radicals O, H and CH2OH: N2 *þ O2 / N2 þ O þ O Fig. 6. Effect of ionization degree on ionization rate coefficient of O2 and N2.

N2* þ CH3OH / N2 þ CH3OH þ H

(R9) (R11)

with the increase of ionization degree. This will greatly speed up the ionization reaction. However, the ionization degree has almost no effect on ionization rate coefficient of O2 and N2 for mean electron energy above 8 eV. This is completely consistent with the results and trends shown in Fig. 5.

In addition, the high-energy excited state of O2 with CH3OH through the collision quenching reaction can also produce H and CH2OH:

3.1.3. Effect of excess air ratio on electron energy distribution Fig. 7 shows the effect of excess air ratio on the EED at ionization degree 106. For the mean electron energy of 3 eV and 5 eV, the EED increases with the increase of excess air ratio for electron energy above 15 eV, and the EED is essentially unchanged for electron energy below 15 eV. For the mean electron energy of 7 eV and 10 eV, the EED slightly increases with the increase of excess air ratio for electron energy above 25 eV, and the EED is also essentially unchanged for electron energy below 25 eV. The results are consistent with the effect of the increase of oxygen content on the EEDF by Zhang [51]. Therefore, the effect of the excess air ratio on the EED is negligible in this paper.

The quenching reaction of the excited state substance causes the concentration of the electronic excited state molecules of O2 and N2 to increase first and then decrease. The EED and peak concentration of high-energy excited state molecules increase with the increase of field intensity. Since the activation energy of O2 excited state is lower than that of N2 excited state, and the chemical reaction rate excited by the electron collision of O2 is relatively higher than that of N2, the concentration of O2 excited state is higher than that of N2 excited state. The ionized state molecules in methanol-air plasma are þ CH3OHþ, Oþ 2 , and N2 , and the activation energy of ionized state molecules is higher than that of excited state molecules:

3.2. Evolution process of active particles

e þ O2 / 2e þ Oþ 2

(R6)

e þ N2 / 2e þ Nþ 2

(R7)

e þ CH3OH / 2e þ CH3OHþ

(R8)

In the plasma-assisted combustion process, the concentration of radicals determines the speed of chemical chain reaction. The difference of EED will affect the formation and development of radicals in plasma discharge. The EED is mainly affected by field intensity, ionization degree and excess air ratio. However, the ionization degree is directly controlled by the external field intensity. Thus, the field intensity directly affects the formation and development of radicals.

O2* þ CH3OH / O2 þ CH2OH þ H

(R10)

Since the proportion of high-energy electrons corresponding to the collision ionization reaction is small, the concentration of ionized state molecules is smaller than that of excited state molecules. Although the number of ionized state molecules is only a

C. Gong et al. / Energy 193 (2020) 116881

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Fig. 7. Effect of excess air ratio on electron energy distribution.

small part of the total number of particles, ionized state molecules belong to high-energy state molecules. Moreover, the ionized state molecules are extremely easy combination with electrons to generate radicals by a quadratic combination, so ionized state molecules play an important role in the formation of radicals:

e þ Oþ 2 /O þ O

(R15a)

e þ CH3OHþ / CH3 þ OH (R16)

3.2.2. Production and development of radicals The main radicals in methanol-air plasma are O, H, CH2OH, CH3 and OH, etc. Fig. 9 shows the mole fraction of active particles as a function of time at different field intensity at excess air ratio 1, pressure 0.1 MPa, and temperature 1300 K. It can be seen from these data that the concentration of O is higher than the concentration of H and CH2OH, and then the concentration of H and CH2OH is much than that of OH and CH3. With the increase of time, the concentration of all radicals increase gradually at first, and then remains basically unchanged after reaching the peak concentration of radical. With the increase of the field intensity, the concentration of all radicals is increasing in different degrees. The radical O is the main active substance in the plasma discharge process, which can be obtained through electron impact dissociation of O2, quenching reaction of high-energy excited state

N2, ion/electron recombination and other reactions, such as: e þ O2 / e þ O þ O

(R1)

N2 *þ O2 / N2 þ O þ O (R9)

e þ Oþ 2 /O þ O

(R15b)

The activation energy of O2 in electron collision dissociation is 6 eV, which happens to be the region with high electron density. The dissociation reaction of O2 is easy to occur, so the concentration of radical O produced is higher than that of other radicals. The generation of radicals H and CH2OH is similar to that of radical O, which is mainly produced by electron impact dissociation of CH3OH and quenching reaction of high-energy excited state N2: e þ CH3OH / e þ CH2OH þ H

(R3)

N2* þ CH3OH / N2 þ CH3OH þ H (R11) The activation energy of CH3OH in electron collision dissociation is 7.96 eV, which is much higher than that of O2 in electron collision dissociation and the EEDF of CH3OH is lower that of O2, resulting in low concentration of CH2OH and H compared with the concentration of O. CH3 and OH can be generated through many paths in the methanol-air plasma discharge process, but the reaction is generally complex and the reaction rate is low, so this study only ion/

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Fig. 8. Mole fraction of excited states and ionized states matter as a function of time at different field intensity. Fig. 9. Mole fraction of active particles as a function of time at different field intensity.

electron recombination reaction is considered: e þ CH3OHþ / CH3 þ OH (R16) Due to the high activation energy required for CH3OH in ionization reaction and the small concentration of ionized state CH3OHþ, the concentration of CH3 and OH generated by R16 reaction is over an order of magnitude lower than that of O, H and CH2OH radicals. The peak concentration of O, H and CH2OH, and CH3 and OH for the field intensity 220 Td is 6.4 time, 6.4 time, and 7.3 time higher than the field intensity 70 Td, respectively. However, the peak concentration of O, H and CH2OH, and CH3 and OH for the field intensity 400 Td is 20.1 time, 24.6 time, and 12 time higher than the field intensity 70 Td, respectively. The peak pulse voltages of

generating field intensities 70 Td, 220 Td and 400 Td were 4 kV, 12 kV, and 22 kV, respectively. Considering the demand of radical concentration and economics of the peak pulse voltage, the field intensity between 220 Td and 400 Td is selected as a reasonable range for generating radicals. 4. Conclusions The electron energy distribution characteristics and evolution of active particles of O, H, CH2OH, CH3 and OH of methanol-air mixture by non-equilibrium plasma were numerical simulated. The major factors that affect EED, a reasonable range of electric field to produce free radicals, and the evolution mechanism of active particles of methanol-air mixture under the action of nonequilibrium plasma were studied in detailed. The main

C. Gong et al. / Energy 193 (2020) 116881

conclusions can be summarized as follows: (1) The difference of EED will affect the formation and development of radicals in plasma discharge. The EED is mainly affected by field intensity, ionization degree and excess air ratio. The EED is mainly affected by field intensity. The EED is increased with the increase of mean electron energy. The field intensity directly affects the formation and development of radicals in methanol-air plasma discharge. The generation of radicals in plasma discharge mainly occurs in the electron collision reaction with the electron energy of 3e10 eV. When field intensity increases from 70 Td to 220 Td, the mean electron energy of field intensity 220 Td is 200% higher than that of field intensity 70 Td. The effect of the excess air ratio on EED is negligible. (2) The concentration of excited states matter is higher than that of ionized states matter. The concentration of excited state O2 is higher than that of excited state N2. The concentration of þ ionized state Oþ 2 is higher than that of ionized state N2 . With the increase of time, both concentrations of the excited states and ionized states matter first increase and then decrease. And with the increase of field intensity, the peak concentrations of both excited states and ionized states matter are increasing. (3) The concentration of O radical is higher than the concentration of H and CH2OH, and then the concentration of H and CH2OH is much than that of OH and CH3. The peak concentration of O, H and CH2OH, and CH3 and OH for the field intensity 400 Td is 20.1 time, 24.6 time, and 12 time higher than the field intensity 70 Td, respectively. (4) Considering the demand of radical concentration and economics of the peak pulse voltage, the field intensity of 220 Td - 400 Td is selected as a reasonable range for generating radicals. (5) This study is only a simulation study under the condition close to the actual engine operating condition. If the research results are to be applied to the ignition and combustion improvement of methanol engine, the real operating conditions of the engine should be studied in the future work. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51176063 and 51676029). References [1] Zhen XD, Wang Y. An overview of methanol as an internal combustion engine fuel. Renew Sustain Energy Rev 2015;52:477e93. [2] Li J, Gong CM, Liu B, Su Y, Dou HL, Liu XJ. Combustion and hydrocarbon (HC) emissions from a spark-ignition engine fueled with gasoline and methanol during cold start. Energy Fuels 2009;23:4937e42. [3] Gong CM, Liu ZL, Su H, Chen YL, Li JB, Liu FH. Effect of injection strategy on cold start firing, combustion and emissions of a LPG/methanol dual-fuel spark-ignition engine. Energy 2019;178:126e33. [4] Gong CM, Li ZH, Yi L, Liu FH. Experimental investigation of equivalence ratio effects on combustion and emissions characteristics of an H2/methanol dualinjection engine under different spark timings. Fuel 2020;262:116463. ^lescu SCD, Vornicu LDL, R^ [5] D^ asca aducanu L. Improvement influence of methanol preheating temperature in direct injection engine. 2002. SAE Paper 200201-2689. [6] Zhang MM, Hong W, Xie FX, Liu Y, Su Y, Li XP, Liu HF, Fang KN, Zhu XB. Effects of diluents on cycle-by-cycle variations in a spark ignition engine fueled with methanol. Energy 2019;182:1132e40. [7] Shi C, Ji CW, Ge YS, Wang SF, Bao JH, Yang JX. Numerical study on ignition amelioration of a hydrogen-enriched Wankel engine under lean-burn condition. Appl Energy 2019;255:113800. [8] Subramanian KA, Nidhi. Experimental investigation on effects of oxygen enriched air on performance, combustion and emission characteristics of a methanol fuelled spark ignition engine. Appl Therm Eng 2019;147:501e8.

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