Large eddy simulation of plasma-assisted ignition and combustion in a coaxial jet combustor

Energy 199 (2020) 117463

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Large eddy simulation of plasma-assisted ignition and combustion in a coaxial jet combustor Ming Dong *, Jinglong Cui , Ming Jia , Yan Shang , Sufen Li Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian, 116024, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 July 2019 Received in revised form 29 February 2020 Accepted 23 March 2020 Available online 31 March 2020

To study the effect of plasma O3 on the combustion-supporting process for a coaxial jet combustor, large eddy simulation (LES)-partially stirred reactor (PaSR) simulation of a methane/air turbulent diffusion combustion was carried out based on OpenFOAM open-source software platform. The prediction of the turbulent diffusion flame was verified, and the results are in good agreement with the experiment data. Then the effect of air discharge product (i.e., O3) on the methane ignition and combustion process was investigated using plasma-assisted combustion model by 341 steps detailed reaction mechanism. The results show that the addition of O3 can increase the speed of flame propagation and accelerate the ignition process of methane combustion. It is also found that the vortex structure with O3 is more continuous in the recirculation zone, and the flame recirculation zone with O3 is closer to the inlet. The effect of O3 on enhanced combustion is more obvious in the low-temperature region, while the axialvelocity ratio with O3 is considerably improved in the high-temperature region. Besides, the plasma O3 will reduce the fluctuation of vx0 vx’, especially at the peak point, which will tend to stabilize the recirculation zone. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Coaxial jet combustor Plasma O3 LES Ignition Enhanced combustion

1. Introduction With the rapid growth of global energy demand, natural gas and renewable energy have become leaders in the growth of primary energy consumption. As a kind of clean energy, natural gas has a higher combustion calorific value. Therefore, research of the instability and pollutant emissions during ignition and combustion of natural gas plays an important role in citizens’ lives and industrial applications, and plasma has a broad prospect in this field. Plasma-assisted ignition and combustion is a new type of ignition assisted combustion technology to improve ignition capability and combustion efficiency [1]. In the processes of ignition and combustion by discharge, the change of gas molecule is mainly dominated by thermal and non-thermal mechanisms. These mechanisms, together or separate, can provide the additional combustion control [2,3]. Plasma assisted-combustion (PAC) generally includes the following aspects: (1) dielectric barrier

* Corresponding author. School of Energy and Power Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian, Liaoning, 116024, China. E-mail address: [email protected] (M. Dong). https://doi.org/10.1016/j.energy.2020.117463 0360-5442/© 2020 Elsevier Ltd. All rights reserved.

discharge (DBD), (2) pulse discharge, (3) microwave discharge, and (4) non-equilibrium plasma. Tang et al. [4,5] designed a coaxial cylindrical DBD reactor to study the combustion enhancement process by activating the propane and air. The results show that there is no significant change for the flame physical properties in terms of propane activation, while air activation changes significantly. Tang et al. [6] found a negative linear correlation between the flame transfer functions (FTF s)’ gain and perturbation frequency on the logarithmic plot by particle tracing method. By generating active species (i.e., ionized, dissociated, and electronically excited species) in air or air/fuel mixtures, the nanosecond high-pressure pulses can significantly reduce ignition delay time and stabilize the flame. In addition, it can widen the flammability limit and reduce nitrogen oxides (NOx) emissions [7,8]. Nagaraja et al. [9] attempted to simulate in a self-consistent manner, multiple nanosecond pulses, and the energy coupling, gas heating, and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBDs) in air was analyzed. By comparison with conventional combustion, Wang [10] tested a repetitive pulsed plasma actuator and found that pulse discharge causes wrinkles on the thin mixing layers because the

2

M. Dong et al. / Energy 199 (2020) 117463

deposited energy enhances the chemical reaction for combustion. Cui et al. [11] used a repetitively pulsed plasma to stabilize a lean premixed turbulent swirl flame under flow pulsations. It was found that when the discharge pulses occur before or during the flow pulsation, the plasma extends the lean flammability limit of the swirl flame. Focus on the turbulent microwave assisted combustion, Wei et al. [12] used a microwave PAC facility to study the methane/air mixtures. The results indicated that a U-shaped ignition curve of plasma power versus fuel equivalence ratio was observed in premixed PAC, whereas an approximately linear increasing line occurred in non-premixed PAC. Yamamoto et al. [13] performed the experiment of spray combustion by the superposition of nonequilibrium plasma using microwave and investigated the characteristics of combustion and reformation of plasma-assisted combustion. Moreover, Larsson [14] utilized a skeletal reaction mechanism in a reduced electric field E/N below the critical field strength (of around 125 Td) for the formation of a microwave breakdown plasma. The results show that the laminar flame speed (SL) increases more than the turbulent flame speed, but the radical pool created by the microwave irradiation significantly increases the lean blow-out limits of the turbulent flame. The non-equilibrium plasma accelerates chain-branching reactions, thereby can ignite or aid the ignition of a fuel/air mixture [15,16]. Ombrello et al. [17] integrated a non-equilibrium gliding arc plasma discharge with a counterflow flame burner to study the effect of a plasma discharge on methane/air diffusion flames. Furthermore, Gong et al. [18,19] simulated the effects of ozone (O3) addition from 0 to 7000 ppm in the intake manifold on combustion characteristic in a direct injection spark ignition methanol engine. The model results show that the maximum cylinder pressure increases with increasing O3 addition for cold start and steady state modes, and O3 addition can also significantly reduce ignition delay. Ehn et al. [20] obtained the results of different O3 enrichment concentrations in a low-swirl CH4/air flame at lean conditions through laser diagnosis and large eddy simulation (LES). The results indicate that the CH2O increases in the low-swirl flame as small amounts of O3 is supplied to the CH4/air stream upstream of the flame. Gao et al. [21] found that SL enhancement increased significantly at elevated pressures. Elevated pressure both promotes O3 decomposition, suppresses diffusion of H, which respectively provides O atoms and reduces the influence of the O3þH]OH þ O2 reaction. For turbulent combustion, Owen et al. [22] studied the effects of inlet air swirl, pressure and fuel/air velocity on the time-averaged and fluctuating flow field for a turbulent diffusion flame combustor using natural gas as fuel. Pierce et al. [23] used three different combustion models to simulate the diffusion combustion of a coaxial jet combustor without swirl by using LES. The results show that the progress-variable approach appears to be an effective method for capturing the basic realistic flame behavior. Recently, Paul et al. [24] analyzed the ability of LES for predicting nonpremixed turbulent combustion. The turbulent-radiation interactions (TRI) in one-dimensional and two-dimensional nonpremixed flames have been studied [25,26]. The results indicate that the radiant heat loss is always underestimated. But for a threedimensional combustion chamber, in the case of a large number of grid nodes, the above methods cannot be universally applied. Zhang et al. [27] modeled HCCI engine combustion to study the effects of turbulent chemical interactions (TCL). The results showed that TCL had little effect on the ignition process under low swirl conditions. Considering the structure and initial conditions of this paper, in the large eddy simulation, the influence of TRI and TCL was not considered. From the above review, it can be found that the plasma-assisted

combustion is mostly studied through discharge. Because the discharge mechanism is rather complex, it is impossible to study the combustion-supporting mechanism of a single plasma component directly. To eliminate the interference of other factors, such as electric field force, it is necessary to use the simulation method to study the plasma combustion-supporting process. However, there is few research on the simulation of plasma combustion process at present. In particular, the employment of LES turbulence model with high precision for studying the plasmaassisted combustion is rather scarce. This is related to the following limiting factors. First, the LES method requires a finer mesh than the RANS method, lower magnitudes for time-step and long integral times for analysis of time-averaged parameters and statistics of turbulence [28]. Second, the reaction between plasma, fuel and oxidant must be considered to study the effect of plasma on combustion, so it is necessary to select a reaction mechanism with at least several decade steps. Third, in order to make the combustion process converge, it takes a long physical time to advance. O3, as one of the main products of dielectric barrier discharge and gliding arc plasma discharge, has a long active lifetime, so it has great significance for the study of combustion-supporting problem. To investigate the ignition-assisted combustion effect of plasma O3 on methane combustion for a coaxial jet combustor, the LES turbulence model and partially stirred reactor (PaSR) combustion model in the OpenFOAM platform coupling with the detailed chemical mechanism are employed in this paper.

2. Mathematical and numerical modelling In the present study, the simulation is divided into two parts, i.e., model verification and plasma (O3) assisted combustion. The influence of O3 on the methane combustion was predicted by adding air discharge product O3 at the air inlet.

2.1. Turbulence model To determine the instantaneous variation of scalar, we used the LES turbulence model in this study. Because the RANS model cannot well predict the transient mixing of the fuel and oxidizer, the LES model is more suitable for the unsteady calculation. In the LES, the large scale effect of turbulent motion is simulated, and the small scale effect and the interaction with the large scale effect are modeled. The LES equations are obtained by spatially Favrefiltering the unsteady, compressible Navier-Stokes equations. The governing equations include mass equation, momentum equation, species equation, and enthalpy equation [29]:

vr v þ ðruei Þ ¼ 0 vt vxi

(1)

i  vp ~i vru v  v h þ Suj þ ru~ i u~ j þ ¼ tij  rtSGS ij vxi vxj vxi vt

(2)

~ Þ   vðrY v k ~ Þ ¼ v V Y  rðug ~ Þ þ u· ~i Y ~i Y þ ðru i Yk  u k k k vt vxi vxi k;i k

k ¼ 1; N (3)

  ~s Þ vðrh v ~ s Þ ¼ Dp þ v l vT  rðug ~ s Þ þ t vui ~ ~i h þ h ðru h  u s i i ij vt vxi Dt vxi vxi vxj

M. Dong et al. / Energy 199 (2020) 117463

v  vxi

r

N X

! Vk;i Yk hs;k

·

þ u T þ Sh

(4)

k¼1

~i u ~ j Þ is the unwhere tij is the viscous stress tensor; tSGS ¼ ðug i uj u ij resolved Reynolds stress; Suj is the source of momentum; ~ Þ is the unresolved species flux; u· ~i Y ðug i Yk u k is the filtered k · chemical reaction rate; u T is the filtered reaction term due to heat ~ s Þ is the unresolved enthalpy flux; V Y ¼  ~i h release; ðug i hs u k;i k ~k vY vT vT~ are the filtered laminar diffusion fluxes; and rDk vxi and l vxi ¼ l vx i Sh is the source of energy. For the simulation of plasma-assisted ignition and combustion, we analyzed the distribution of temperature, composition, and velocity inside the combustor. We did not focus on the distribution of variables near the wall. To avoid massive consumption of computing resources, we selected Smagorinsky model to close the subgrid scale. The Smagorinsky subgrid-scale model in the above LES equations are defined as:

tSGS ij 

dij 3

tSGS kk ¼  yt



~ j 2 vu ~ i vu ~k vu þ  d vxi vxi 3 ij vxk 



dij Sij  ~ S ¼  2 yt ~ 3 kk

(5)

where yt ¼ ðCS DÞ2 ð2Sij Sij Þ1=2 is the eddy viscosity, D is the filter width, CS ¼ 0:167 is the model coefficient, and S is the resolved shear stress. The unresolved scalar fluxes are often expressed using a gradient assumption:

~ ~ ¼  yt vY k ~i Y ui Yk  u k Sck vxi

(6)

~ ~ s ¼  yt vhs ~i h ui hs  u Prsgs vxi

(7)

where Sck ¼ 1 is the subgrid-scale Schmidt number and Prsgs ¼ 0:9 is the subgrid-scale Prandtl number. Due to the significant dissipation of eddy viscosity at the wall, the wall damping is corrected by the Van Driest function [30]. Finally, the yplus ¼ 0.2e1.8 of the whole wall is obtained, and the corresponding calculation equations are as shown in Ref. [31]. 2.2. Combustion model and reaction mechanism The combustion model used in this paper is the Partially Stirred Reactor (PaSR) model. In the PaSR model, each cell is divided into two parts: a reacting zone, modeled using the PSR (perfectly stirred reactor) approach and a non-reacting zone. A reactive volume fraction kappa function of the computational time step (Dt) and time scales related with chemical (tch) and mixing (tmix) processes has been proposed [32]:

kappa ¼

Dt þ tch Dt þ tch þ tmix

(8)

where tmix is the local mixing time computed according to

tmix ¼ Cmix

~ε ¼ Cm

~1:5 k l

rffiffiffiffiffiffiffiffi

meff r~ε

(9)

(10)

~ and The dissipation ~ε is a function of turbulent kinetic energy k characteristic length of the domain l. For turbulent combustion, the

3

model constant Cmix is generally between 0.001 and 0.3. In this paper, the value of Cmix is 0.01. We used reactingFoam solver for chemical reactions in open-source OpenFoam platform with detailed chemical reaction mechanism. The detailed chemical reaction mechanism with 341 steps reactions [33,34] was used for plasma-assisted combustion to consider the interaction between ozone, fuel and oxidant. The detailed chemical reaction mechanism is based on GRI3.0 [33] and 16 steps reaction mechanism between O3 and CH4 [34]. The detailed mechanism includes 54 species and has been applied in previous cases [35,36]. 2.3. Physical configuration and computational setup The configuration is based on the experiment without swirl condition conducted by Owen et al. [22], as plotted in Fig. 1. In the coaxial jet combustor, the methane is injected into the combustor through a central tube, and the air is injected into the combustor through an annulus surrounding the central tube. The air consists of 77% N2 and 23% O2. The fuel is maintained at room temperature of Tfuel ¼ 300 K, and the air is preheated to Tair ¼ 750 K. The pressure in the combustor is P ¼ 3.8 atm. The physical configuration and initial conditions are introduced in Refs. [22] in detail. For the plasma-assisted combustion, the similar physical configuration and initial conditions are introduced, as in Table 1. The pressure in the combustor was set to 1 atm. To study the effect of plasma O3 on combustion under atmospheric pressure, 0, 0.5% O3 was added to the air inlet as air discharge product, respectively. Accordingly, the mass fraction of O2 was decreased to 23%, 22.5%, respectively. The computational domain started at a distance of 60 mm upstream of the combustor and ended at a distance of 400 mm downstream of the combustor. The center point of the backwardfacing step inlet was taken as the origin point, and the flow direction was along the X-axis direction. A structured o-grid with 981,600 cells was used to simulate turbulent combustion as shown in Fig. 1. The size of the grid in the combustor was 120  81  80 points in the axial, radial, and azimuthal directions, respectively. The inlet section of the combustor was divided into 30 cells in the axial direction. The yplus along the wall of combustor was in the range of 0.2e1.8, and the smallest grid size is smaller than 1 mm. The zeroGradient boundary condition at the outlet was selected. Adiabatic, no slip and impermeable boundary conditions were selected for all of the solid walls. Discretization and solving of the partial differential equations were carried out using the finite volume method (FVM) based on the OpenFOAM Cþþ library [37]. The compressible Pressure-based Implicit Splitting of Operators (PISO) was selected to manage the pressure-velocity-density coupling. The convection term was discretized by limited Gaussian linear difference scheme, and the explicit Gaussian linear orthogonal difference scheme was used for the diffusion term. In general, the whole discrete scheme has second-order numerical accuracy. In order to make the case converge quickly, the max Courant number was set to 0.5 and self-adaptive time step was used for the time step. We used a total of 16 processors of Intel Xeon E5-2620v4 in this calculation. For the combustion-supporting process, about 15,360 processor-hours were used per simulation case for a physical time 0.4 s. 3. Results and discussion In this section, the computational model was first verified based on the measurements from Ref. [22]. After the model was verified in Section 3.1, the plasma assisted combustion was discussed focusing on the ignition and enhanced combustion in Sections 3.2, 3.3 by comparison with conventional combustion.

4

M. Dong et al. / Energy 199 (2020) 117463

Fig. 1. Schematic of physical configuration and computational mesh.

Table 1 Physical configuration and initial conditions in different cases. Item

Conventional combustion

Plasma-assisted combustion

R1 (mm) R2 (mm) R≡R3 (mm) R4 (mm) TCH4 (K) Tair (K) VCH4 (m/s) Vair (m/s) P (atm) Component in fuel inlet Component in air inlet

30 30.2 45 60 300 800 1 20 1 CH4 77%N2þ23%O2

30 30.2 45 60 300 800 1 20 1 CH4 77%N2þ22.5%O2þ0.5%O3

3.1. Modelling verification To verify the model, a suitable computational mesh is required. Two kinds of mesh with 1.47 and 0.98 million cells were tested for this purpose to simulate the non-premixed methane flame. To show a tendency of stabilization with the mesh, the results for the time-averaged temperature and axial-velocity ratio u= Vair of different profiles are presented in Fig. 2. As shown in Fig. 2 (a1), (a2), and (a3) there is a slight difference upstream of the combustor for Y/ R < 0.8. And for axial-velocity ratio profiles, there is a slight difference near the peak value. But the overall trend is well predicted. Considering the efficiency and accuracy of the simulation process, the mesh with 0.98 million elements is selected to simulate the following cases. The above experiment was verified by using the detailed chemical reaction mechanism with 341 steps reactions. With the full development of turbulent combustion, the instantaneous distributions of methane combustion were obtained in Fig. 3 at t ¼ 0.4 s. As can be seen, the temperature gradient in the recirculation zone is very large, and the fluid disturbance increases, which accelerates the mixing of heat flow air and cold flow methane to achieve the purpose of enhancing combustion. To verify the correctness of the solver, the time-averaged temperature, axial-velocity ratio (VX =Vair ), and the axial-velocity qffiffiffiffiffithe ffi 0 rms ( vX2 =Vair ) of different profiles were compared with the experimental and the simulated data from Refs. [22,23] in Fig. 2. As

can be seen from Fig. 2 (a1), (a2), and (a3), the trend of present study is consistent with the fast-chemical model in Refs. [23], but the prediction of present study is closer to the experimental data than the prediction of fast-chemical model. For this paper, the PaSR combustion model is based on the fast-chemical model to optimize the interaction between turbulence and reaction, so the trend is the same. The main difference between present study and experiment is at the upstream of the combustion chamber. For Y ¼ 0.6 at the profile X/R ¼ 0.89 the prediction of present study is about 400 K lower than the experimental value, and for Y ¼ 0.75 at the profile X/ R ¼ 0.89 the predicted of present study is about 420 K higher than the experimental value. This indicates that there are some errors in the prediction of recirculation zone because the temperature gradient and the component gradient near the recirculation zone vary greatly. Although the uncertainty was not analyzed in the experiment [22], one of the researchers pointed out that a rather large, invasive, and dynamically unresponsive thermocouple probe was used [23]. This leads to considerable uncertainty, especially in regions with large temperature fluctuations. Besides, radiation heat transfer could also affect the temperature prediction. For Fig. 2 (b1), (b2), and (b3) the predicted axial-velocity ratio of the present study is in good agreement with the measurements at the profiles of X/R ¼ 0.14,X/R ¼ 0.38, and X/R ¼ 1.27 which illustrates that the turbulence model and grid settings are reasonable for the test case. However, the rms of axial-velocity is underestimated at the upstream of the combustion chamber near the

M. Dong et al. / Energy 199 (2020) 117463

Fig. 2. (a1), (a2), (a3) temperature profiles; (b1), (b2), (b3) axial-velocity ratio:u=Vair and axial-velocity rms profiles:

5

pffiffiffiffiffiffiffi u02 =Vair .

Fig. 3. Instantaneous temperature distribution of methane combustion, t ¼ 0.4 s.

outer flame of Y/R ¼ 0.8. A significant part of the disagreement may be due to that pipe, and annular inflow conditions with uniform cross-section velocity were estimated in the simulations, whereas in the experiment, flow conditioning devices were located a certain distance upstream of the jet orifice. 3.2. Ignition Fig. 4. Instantaneous distribution of temperature: (a) without O3; (b) with O3.

When the turbulent combustion is simulated by the LES method, the distribution of the instantaneous field in the combustion chamber can be obtained. The process of plasma combustion was simulated by 341 steps mechanism, and the two processes without O3 and with 0.5% O3 were calculated. The instantaneous distribution of temperature during combustion is shown in Fig. 4. The flame structure is almost the same at t  0.04 s for the case with and without O3 addition. It indicates that the addition of O3 has little effect on the propagation of flame before 0.04 s. This is because combustion occurs at the fuel inlet and diffuses into the burners through the backward-facing step, while O3 is added to the

air inlet. So the fuel inlet can't be affected by O3 at the beginning of the ignition. However, as the flame propagates downstream, the flame development with O3 is faster than that without O3 at t  0.06 s. This is related to two effects of O3 on the plasma assisted combustion. First, the O3 molecules, as an air discharge product, has higher energy than O2. The temperature in the chamber is increased by the thermal effect, which accelerates all temperaturedependent reactions. Second, the O3 molecules and O atoms produced by decomposition in the gas mixture accelerate the chain-

6

M. Dong et al. / Energy 199 (2020) 117463

branching reactions, then increases the propagation of flame through the reactions including O3 þ H⇔O þ HO2 , O3 þ OH⇔O2 þ HO2 , O3 þ H2 O⇔O2 þ H2 O2 , and O þ CH3 ⇔H þ CH2 O. To describe the effect of plasma on ignition clearly, the change of the instantaneous temperature at point A (0.36, 0, 0) downstream is monitored, as shown in Fig. 5. As can be seen, the temperature at point A rises rapidly from the initial temperature of 800 K to about 2400 K after 0.05 s, then decreases, and finally vibrates between 1400 K and 1800 K. This is because as the flame develops toward point A, the outer flame passes through point A first, and the outer flame temperature is as high as 2400 K. As the flame develops forward, the inner flame passes through point A, so the temperature at point A begins to decrease and oscillates at 1400e1800 K. The temperature at point A with O3 is first to reach the peak at 0.098 s, while the temperature at point A without O3 reaches the peak at 0.11 s. This is consistent with the phenomenon that flame propagates faster under plasma assisted combustion. There is a relatively stable pulsation from about 0.3 s, so it can be concluded that t  0.3 s is fully developed for turbulent combustion, thus the variables in the flow field are averaged from 0.3 s for a duration of 0.1 s. 3.3. Enhanced combustion To study plasma-enhanced combustion, we discuss further the flame structure and field variables. The second order invariant Q of velocity gradient to extract instantaneous iso-surface of vorticity is used. Here the second invariant of the velocity gradient tensor is defined by

Q¼

 1  Sij Sij þ Wij Wij 2

(11)

Here, Sij and Wij represent the symmetric and asymmetric parts of the velocity gradient tensor, respectively, and are given by

1 vui vuj Sij ¼ þ 2 vxj vxi

!

1 vui vuj  Wij ¼ 2 vxj vxi

(12) !

Fig. 5. Instantaneous temperature at point A.

(13)

As shown in Fig. 6 (a), the vortex structure is relatively full with the size of Q ¼ 50,000. The vortex structure with O3 is continuous in the recirculation zone, while the vortex structure without O3 is broken in the recirculation zone. This is because the reaction mechanism of O3 with fuel and oxidant enhances the disorder of fluid in this area, which makes the vortex structure continuous. Besides, we selected the instantaneous iso-surface of vorticity Q ¼ 100,000 since the vortex structure is sensitive to the isosurface. As shown in Fig. 6 (b), with the increase of the instantaneous iso-surface of vorticity Q, the vortex structure becomes sparse, but the vortex structure with O3 in the recirculation zone remains continuous. From the color of vortex structure, it can be seen that velocity of vortices in this region is relatively large, and plays an important role in the mixing of fluids. Fig. 7 (a) shows the distribution of the time-averaged temperature. The 1000 K isotherm is located at the bottom of the flame recirculation zone, 1500 K isotherm is at the top of the recirculation zone, and 2000 K isotherm is located in the outer flame region. By comparing the isotherms, we can find that 1000 K and 1500 K isotherms with O3 move forward. It indicates that flame recirculation zone with O3 is closer to the inlet. Meanwhile, the 2000 K isotherm area with O3 is greater than that without O3. For the timeaveraged temperature, the maximum temperature without O3 is 2183 K, while the maximum temperature with O3 combustion is 2201 K, which is about 18 K higher than conventional combustion. The increase in temperature is related to the higher energy of the O3 molecule, as discussed in Section 3.2. Besides, the comparison of the time-averaged temperature in different profiles is shown in Fig. 7 (b). The flow field variables near the recirculation zone are different greatly and play an important role in the combustion process, so X/R ¼ 1 and X/R ¼ 2 profiles are selected for analysis. The addition of O3 increases the temperature near the central axis of the combustor. The temperature rises by 135 K at the central axis of profile X/R ¼ 1, and the temperature rises by 175 K at the central axis of profile X/R ¼ 2. In addition, by comparing the two profiles, we can observe a slight increase in temperature at Y/R > 1. It shows that the enhanced combustion effect of O3 on the low-temperature region is more obvious. The low-temperature region is shown in the yellow mark area in Fig. 7 (a) which is close to the recirculation zone. As described in Ref. [20]: revealing the O3 decomposition and the reaction with H (early in the flame preheat zone) producing O and OH, respectively, from which O react rapidly with CH4, producing additional OH. The subsequent reaction of OH with CH4 and fuel fragments, such as CH2O, provided chemical heat-release at lower temperatures. Besides, due to extra energy carried by O3 molecules, the flame temperature will have a slight increase [34]. So the enhanced combustion effect of the high energy active molecule of O3 in the low-temperature region is more obvious, which also makes the distribution of the temperature field in the chamber more uniform. To deeply study the effect of O3 on the low-temperature region, the comparison of field time-averaged components in different profiles was analyzed in Fig. 8. As shown in Fig. 8 (a1) and (a2), at the profile X/R ¼ 1, the O3 mass fraction with O3 is about 2.1E-5 at the maximum, while at the profile X/R ¼ 2, the O3 mass fraction decreases to 7.6E-7 which indicates the rapid reaction of O3 near the low-temperature region. It can be seen from Fig. 8 (b1) and (b2), the CH4 mass fraction decreases by 0.07481 at the central axis of profile X/R ¼ 1, and the CH4 mass fraction decreased by 0.11126 at the central axis of profile X/R ¼ 2 which indicates the reaction rate of CH4 with O3 is higher than that without O3 in the lowtemperature region. Besides, the enhanced effect of O3 on the combustion in the low-temperature region can also be reflected in product mass fraction (YPro ¼ YH2O þ YCO2), as is shown in Fig. 8 (c1) and (c2). The product mass fraction increases by 0.02378 at the

M. Dong et al. / Energy 199 (2020) 117463

7

Fig. 6. Instantaneous iso-surface of vorticity (a) Q ¼ 50,000; (b) Q ¼ 100,000: t ¼ 0.4 s, color by U. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. Distribution and profiles of the time-averaged temperature.

central axis of profile X/R ¼ 1, and the product mass fraction increased by 0.01498 at the central axis of profile X/R ¼ 2. This indicates that in the low-temperature region, O3 and CH4 react violently in the low-temperature region during the plasma-assisted combustion process, more products are generated, and this process will release chemical heat. The result shown in Fig. 8 is a strong support for chemical heat-release in the low-temperature region in

Ref. [20]. Thus, the enhanced combustion effect of O3 on the lowtemperature region is more obvious. Fig. 9 (a1) and (a2) shows the distribution of the time-averaged axial-velocity ratio (V x =Vair ) at different profiles. The increasing trend of the axial-velocity ratio is different from that of temperature. In the high-temperature region, i.e., the high-speed region, the axial-velocity ratio is obviously increased. The axial-velocity ratio

8

M. Dong et al. / Energy 199 (2020) 117463

Fig. 8. (a1), (a2) O3 mass fraction profiles; (b1), (b2) CH4 mass fraction profiles; (c1), (c2) product mass fraction profiles.

increases by 5.05% at the profile X/R ¼ 1, and the axial-velocity ratio increases by 4.85% at the profile X/R ¼ 2. This is related to O3 molecules and the O atoms produced by O3 decomposition, which increases the flame propagation speed by accelerating chainbranching reactions, as discussed in Section 3.2. For the turbulence combustion, the statistics of turbulence is an important aspect to discuss the effect of O3 on the enhanced combustion. The prime2Mean belonging to the fieldAverage1 functions in OpenFOAM has been selected to calculate the statistics of turbulence (v0 YO3 0 vx 0 vx 0 ) during 0.3 se0.4 s. As shown in Fig. 9 (b1) and (b2), the statistics of turbulence (v0 YO3 0 ) with O3 is only presented. This is because there is no O3 component in conventional combustion under initial conditions, and the magnitude of v0 YO3 0 without O3 is two orders of magnitude smaller than the magnitude of v0 YO3 0 with O3. At the profile X/R ¼ 1, the value of v0 YO3 0

increases at Y/R > 0.9, which indicates that O3 flows into the combustor in this region. At the profile X/R ¼ 2, the value of v0 YO3 0 decreases by two orders of magnitude, and the peak point moves towards the wall. Thus v0 YO3 0 is helpful for increasing the axialvelocity ratio by accelerating the chain reaction. Besides, the statistics of turbulence (vx 0 vx 0 ) is presented in Fig. 9 (c1) and (c2). At the profile X/R ¼ 1, the value of vx 0 vx 0 increases at Y/ R > 1 and the value of vx 0 vx 0 with O3 is 16.32% smaller than that without O3 at the peak. While at the profile X/R ¼ 2, the value of vx 0 vx 0 increases in wave shape and rises rapidly to a peak at Y/R > 1.1. The value of vx 0 vx 0 with O3 is 50.89% smaller than that without O3 at the peak. The trend of vx 0 vx 0 is related to the fluctuation of velocity, and the plasma O3 will reduce this fluctuation, especially at the peak point. As described in Refs. [22], a reduction in large-scale inhomogeneities in the initial mixing regions, thereby preserving

M. Dong et al. / Energy 199 (2020) 117463

9

Fig. 9. (a1), (a2) axial-velocity ratio profiles: u=Vair ; (b1), (b2) the statistics of turbulence: v0 YO3 0 ; (c1), (c2) the statistics of turbulence: vx 0 vx 0 .

the separation of the reactants for a greater axial distance. Thus the introduction of inlet air swirl or increased combustor pressure, at constant swirl number, tends to stabilize the recirculation zone due to reducing large-scale motions by decreasing vx 0 vx 0 in the turbulent velocity field. Through this analysis, it can be found that the addition of plasma O3 will also tend to stabilize the recirculation zone by decreasing vx 0 vx 0 in the turbulent velocity field.

4. Conclusions In the present work, the LES numerical simulation of plasmaassisted ignition and combustion is performed in a coaxial jet combustor. The main effects of plasma (O3) are divided into two parts, i.e., ignition and enhanced combustion. The following conclusions can be drawn:

1. For the ignition process, the flame development with O3 is faster than that without O3. The time to reach the maximum temperature at the location with O3 is earlier than that without O3. This is mainly caused by two aspects: the active molecule O3 has higher energy and O3 molecules and the O atoms produced by decomposition accelerate chain-branching reactions. 2. For the enhanced combustion, the vortex structure with O3 is more continuous in the recirculation zone due to the increase of fluid disorder, which enhances the mixing of the cold and hot fluids. In the meanwhile, the flame recirculation zone moves forward. 3. The enhanced combustion effect of O3 in the low-temperature region is more obvious, which makes the distribution of the temperature field in the combustor more uniform. In the hightemperature region, the axial-velocity ratio with O3 is obviously increased.

10

M. Dong et al. / Energy 199 (2020) 117463

4. For the statistics of turbulence, the value of v0 YO3 0 increases where O3 flows into the combustor and v0 YO3 0 is helpful for increasing the axial-velocity ratio by accelerating the chain reaction. Besides, the plasma O3 will reduce the fluctuation of vx 0 vx 0 , especially at the peak point, which will tend to stabilize the recirculation zone. The above studies explain the phenomena and the combustionsupporting mechanism of methane combustion assisted by O3 in turbulent diffusion flame, and play a guiding role in optimizing the combustion of natural gas. Declaration of competing interest There are no conflicts of interest. CRediT authorship contribution statement Ming Dong: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Jinglong Cui: Validation, Formal analysis, Visualization, Software, Data curation. Ming Jia: Formal analysis, Visualization, Writing - review & editing. Yan Shang: Resources, Writing - review & editing, Supervision. Sufen Li: Resources, Writing - review & editing, Supervision. Acknowledgments The authors acknowledge the National Natural Science Foundation of China (No. 51876031). References [1] Madani A, Khanehzar A. Numerical assessment of MILD combustion enhancement through plasma actuator. Energy 2019;183(15):172e84. [2] Starikovskiy A, Aleksandrov N. Plasma-assisted ignition and combustion. Prog Energy Combust Sci 2013;39(1):61e110. [3] Wu Y, Li YH. Progress in research of plasma-assisted flow control, ignition and combustion. High Volt Eng 2014;40(7):2024e38. [4] Tang J, Zhao W, Duan Y. In-depth study on propaneeair combustion enhancement with dielectric barrier discharge. IEEE Trans Plasma Sci 2010;38(12):3272e81. [5] Tang J, Zhao W, Duan Y. Some observations on plasma-assisted combustion enhancement using dielectric barrier discharges. Plasma Sources Sci Technol 2011;20(4):045009. [6] Tang Y, Zhuo J, Cui W, et al. Non-premixed flame dynamics excited by flow fluctuations generated from Dielectric-Barrier-Discharge plasma. Combust Flame 2011;204:58e67. [7] Pilla G, Galley D, Lacoste DA, Lacas F, Veynante D, Laux CO. Stabilization of a turbulent premixed flame using a nanosecond repetitively pulsed plasma. IEEE Trans Plasma Sci 2006;34(6):2471e7. [8] Lee DH, Kim KT, Kang HS, Song YH, Park JE. NOx reduction strategy by staged combustion with plasma-assisted flame stabilization. Energy Fuels 2012;26(7):4284e90. [9] Nagaraja S, Yang V, Adamovich I. Multi-scale modelling of pulsed nanosecond dielectric barrier plasma discharges in plane-to-plane geometry. J Phys D Appl Phys 2013;46(15):155205. [10] Wang CC. Numerical simulation of combustion enhancement through a repetitive pulsed plasma actuator. J Thermophys Heat Tran 2015;30(1):1e7. [11] Cui W, Ren YH, Li SQ. Stabilization of premixed swirl flames under flow pulsations using microsecond pulsed plasmas. J Propul Power 2019;35(1): 190e200. [12] Wu W, Fuh CA, Wang C. Comparative study on microwave plasma-assisted combustion of premixed and nonpremixed methane/air mixtures. Combust Sci Technol 2015;187(7):999e1020.

[13] Yamamoto T, Tsuboi T, Iwama Y, Tanaka R. Combustion and reformulation enhancement characteristics of plasma-assisted spray combustion by microwave-induced non-equilibrium plasma. Energy Fuels 2016;30(4): 3495e501. [14] Larsson A, Zettervall N, Hurtig T, Nilsson EJK, Ehn A, Petersson P, et al. Skeletal methane-air reaction mechanism for large eddy simulation of turbulent microwave-assisted combustion. Energy Fuels 2017;31(2):1904e26. [15] Campbell CS, Egolfopoulos FN. Kinetics paths to radical-induced ignition of methane/air mixtures. Combust Sci Technol 2005;177(12):2275e98. [16] Starikovskiy A. Physics and chemistry of plasma-assisted combustion. Phil Trans 2015;373(2048):20150074. [17] Ombrello T, Qin X, Ju Y, Gutsol A, Fridman A, Carter C. Combustion enhancement via stabilized piecewise nonequilibrium gliding arc plasma discharge. AIAA J 2006;44(1):142e50. [18] Gong CM, Yu JW, Wang K, Liu JJ, Huang W, Si XK, et al. Numerical study of plasma produced ozone assisted combustion in a direct injection spark ignition methanol engine. Energy 2018;153(15):1028e37. [19] Gong CM, Si XK, Wang K, Wei FX, Liu FH. Numerical analysis of carbon monoxide, formaldehyde and unburned methanol emissions with ozone addition from a direct-injection spark-ignition methanol engine. Energy 2018;144(1):432e42. n M, Fureby C, et al. Plasma assisted [20] Ehn A, Zhu JJ, Petersson P, Li ZS, Alde combustion: effects of O3 on large scale turbulent combustion studied with laser diagnostics and large eddy simulations. Proc Combust Inst 2015;35(3): 3487e95. [21] Gao X, Zhang Y, Adusumilli S, Seitzman J, Sun W, Ombrello T, et al. The effect of ozone addition on laminar flame speed. Combust Flame 2015;162(10): 3914e24. [22] Owen FK, Spadaccini LJ, Bowman CT. Pollutant formation and energy release in confined turbulent diffusion flames. Symposium on Combustion 1977;16(1):105e17. [23] Pierce CD, Moin P. Progress-variable approach for large-eddy simulation of non-premixed turbulent combustion. J Fluid Mech 2004;504(504):73e97. [24] Paul SC, Paul MC, Jones WP. Large eddy simulation of a turbulent nonpremixed propane-air reacting flame in a cylindrical combustor. Comput Fluids 2010;39(10):1832e47. [25] Deshmukh KV, Modest MF, Haworth DC. Direct numerical simulation of turbulence-radiation interactions in a statistically one-dimensional nonpremixed system. J Quant Spectrosc Ra 2008;109(14):2391e400. [26] Li G, Modest MF. Application of composition PDF methods in the investigation of turbulence-radiation interactions. J Quant Spectrosc Ra 2002;73(2e5): 461e72. [27] Zhang YZ, Kung EH, Haworth DC. A PDF method for multidimensional modeling of HCCI engine combustion: effects of turbulence/chemistry interactions on ignition timing and emissions. Proc Combust Inst 2005;30(2): 2763e71. [28] Georgiadis NJ, Rizzetta DP, Fureby C. Large-eddy simulation: current capabilities, recommended practices, and future research. AIAA J 2009;48(8): 1772e84. [29] Poinsot T, Veynante D. Theoretical and numerical combustion. second ed. Philadelphia: Edwards; 2005. p. 125e82. [30] Vandriest ER. On turbulent flow near a wall. J Aeronaut Sci 1956;13(11): 1007e11. [31] Mao J, Zhang KL, Liu K. Comparative study of different subgrid-scale models for large eddy simulations of magnetohydrodynamic turbulent duct flow in OpenFOAM. Comput Fluids 2017;152:195e203. [32] Marzouk OA, Huckaby ED. A comparative study of eight finite-rate chemistry kinetics for CO/H2 combustion. Eng Appl Comp Fluid 2010;4(3):331e56. [33] Gregory P, Smith DMG, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al. http://combustion.berkeley.edu/gri-mech/. n M, et al. Investigation of com[34] Wang ZH, Yang L, Li B, Li ZS, Sun ZW, Alde bustion enhancement by ozone additive in CH4/air flames using direct laminar burning velocity measurements and kinetic simulations. Combust Flame 2012;159(1):120e9. [35] Ji SB, Lan X, Lian J, Xu HM, Wang YQ, Cheng Y, et al. Influence of ozone on ignition and combustion performance of lean methane/air mixture. Energy Fuels 2017;31(12):14191e200. [36] Liu YZ, Wang ZH, Li L, Wan KD, Cen KF. Reaction mechanism reduction for ozone-enhanced CH4/Air combustion by a combination of directed relation graph with error propagation, sensitivity analysis and quasi-steady state assumption. Energies 2018;11(6):1470. [37] Weller HG, Tabor G, Jasak H, Fureby C. Atensorial approach to CFD using object oriented techniques. Comput Phys Commun 1997;12(6):629.