Numerical investigation into the effects of oxygen concentration on flame characteristics and soot formation in diffusion and partially premixed flames

Fuel 268 (2020) 117398

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Numerical investigation into the effects of oxygen concentration on flame characteristics and soot formation in diffusion and partially premixed flames

T

Yang Huaa, Liang Qiua, , Fushui Liub, Yejian Qiana, Shun Menga ⁎

a b

School of Automotive and Transportation Engineering, Hefei University of Technology, Hefei, Anhui 230009, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China

ARTICLE INFO

ABSTRACT

Keywords: Oxygen concentration Diffusion flame Partially premixed flame Particle growth Particle oxidation

Oxygen-enriched combustion is an important way to improve the combustion efficiency of hydrocarbon fuels and reduce soot formation. In this work, the difference of the effects of oxygen concentration on flame characteristics and soot formation in diffusion and partially premixed flames were comprehensively analyzed. The results showed that both the peak temperature and the OH increase monotonically whether oxygen is added into co-flow or fuel flow. In diffusion flame, the flame height decreases linearly with the increasing oxygen, while the addition of oxygen into fuel flow has little effect on the flame height. As the oxygen in co-flow increases, the peak soot volume fraction (SVF) increases monotonically due to the synergistic increase of HACA and PAHs condensation rates. As the oxygen in fuel flow increases from 0% to 20%, the peak SVF first decreases and then increases, reaching a minimum at 15%, which attributes to the competition of the decreased HACA rate and the increased PAHs condensation rate. With the addition of oxygen into fuel stream, the SVF along the flame centerline increases at the second stage, which is quite different from that of diffusion flame, whose SVF remains stable after inception and before complete oxidation. In the diffusion flames, as the oxygen concentration increases, the peak values of O2 oxidation and OH oxidation increase linearly. However, in the partially premixed flames, the OH oxidation rate is insensitive to the oxygen concentration, and the O2 oxidation rate has a tolerance limit of 15%.

1. Introduction Soot (carbon black) is the main by-product of condensation phase that results from incomplete combustion of hydrocarbon fuels [1]. In most cases, the formation of soot indicates poor combustion conditions, in which not all fuel molecules can be completely oxidized to CO2 and H2O for maximum thermal energy utilization [2,3]. More importantly, the emission of soot particle into the atmosphere will also pose a serious threat to human health and the environment [4]. In order to meet the global demand for energy conservation and emission reduction, it is necessary to develop advanced combustion technologies to improve energy utilization and reduce soot emissions. Oxygen-enriched combustion is a new combustion technology in which fuel is burned in a high concentration oxygen condition (higher than the oxygen concentration in the atmosphere) [5]. This method can effectively improve the combustion efficiency, achieve higher temperatures, faster combustion speed, higher thermal efficiency and lower soot emissions, so it has high practical application value [6,7]. Oxygen-enriched combustion has been recognized as one of the

⁎

important ways to improve combustion performance and reduce emissions of internal combustion engine. Gong et al. [8] numerically studied the effects of intake oxygen enrichment on combustion and emission during the cold start condition of DISI engine. They found that the concentration of OH, O and H radicals increased significantly with the increase of intake oxygen concentration, the chain reaction of methanol combustion accelerated, and the emission of unburned methanol decreased. Seong et al. [9] investigated the effect of intake oxygen concentration (21–27%) on particle oxidation reactivity and soot performance of diesel engine under low and high loads. They observed that the effect of oxygen enrichment is more pronounced at high loads than that at low loads, reflecting the complexity of the effect of engine load on oxygen enrichment. In view of the complexity of soot emissions from internal combustion engines, Bi et al. [10] studied the effect of ambient oxygen concentration (21%, 18%, 15%, 12%) on the formation and oxidation process of soot during diesel combustion in constant volume chamber. They found that as the ambient oxygen concentration decreased, the acetylene, soot precursors and soot mass concentration increased first and then decreased, reaching the peak at 15%. They

Corresponding author. E-mail address: [email protected] (L. Qiu).

https://doi.org/10.1016/j.fuel.2020.117398 Received 29 November 2019; Received in revised form 2 February 2020; Accepted 13 February 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

Fuel 268 (2020) 117398

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Nomenclature fv T ρ v u Nia

Ni p

soot volume fraction temperature density radial velocity axial velocity number of the ith sectional particle aggregates per unit mass of the mixed gas

Dia VTs,r VTs,z β kB Av

pointed out that the increase of soot is attributed to the strong soot formation mechanism and the inhibition of oxidation mechanism, while the decrease of soot at low ambient oxygen concentration was due to the inhibition of inception reaction. Zhao et al. [11] also found the nonmonotonic behavior of soot with decreasing ambient oxygen concentration in the numerical simulation of ABE combustion in a constant volume chamber, and pointed out that this is attributed to the shift of the dominant role of soot oxidation mechanism and the formation mechanism. Zhu et al. [12] experimentally and numerically studied the combustion and emission characteristics of B30 (30% n-butanol and 70% diesel) under different intake oxygen concentrations (21%, 19%, 17% and 15%), and observed that as the intake oxygen concentration decreased, the soot emission first increased and then decreased. They pointed out that with the decrease of intake oxygen concentration, the formation of soot precursors (C2H2, A1 and A4) is obviously delayed, the increase of soot emission is caused by the decrease of oxidation, and the decrease is mainly caused by the decrease of production rate. The studies in the internal combustion engine environment provide a visual observation of the effects of intake oxygen concentration on combustion and soot emissions, but the results include complex combustion behavior and engine operating conditions, which is not conducive to the analysis of the mechanism of oxygen action. A well-controlled laminar diffusion flames provide a more manageable environment for experimental measurements and mechanism analysis [13]. At present, some basic research has been conducted to understand the soot behavior at different oxygen concentrations in oxidants. Vandsburger et al. [14] measured particle size, number density and volume fraction in ethylene and propane counterflow diffusion flames, and report that the SVF in both flames increases as the oxygen concentration in the oxidant stream increases. Fuentes et al. [15] studied the effect of oxygen concentration (17–35%) in the oxidant stream on the formation and oxidation of soot in ethylene laminar diffusion flame. They observed that the maximum SVF increased with increasing oxygen concentration over the entire oxygen concentration range. They concluded that the evolution of soot behavior with oxygen concentration is attributed to the competition between an increase in the soot production rate due to elevated flame temperatures and a decrease in the soot residence time due to shorter flame lengths. Sun et al. [16] investigated the effect of oxygen concentration in oxidant stream (16.8–36.8 vol%) on the formation characteristics of soot in ethylene laminar co-flow diffusion flame using LII technology. It was found that as the oxygen concentration increased, the local and total soot concentrations increased and the primary particle size decreased. The study further confirmed that the SVF has a strong correlation with the flame temperature change, and the primary particle size has a good correlation with the soot residence time. The effect of oxygen concentration (21%, 30%, 40% and 50%) on the soot formation in C2H4/(O2-CO2) laminar diffusion flame was studied by Wang et al. [17] numerically. The results showed that with the increase of oxygen concentration, the flame temperature, polycyclic aromatic hydrocarbons (PAHs) concentration and particle inception rate showed a logarithmic growth trend, and the surface growth rate and soot oxidation rate showed an exponential growth trend. In addition, some studies have investigated the effects of high

number of the ith sectional primary particles per unit mass of the mixed gas diffusion coefficient radial thermophoretic velocity of particles axial thermophoretic velocity of particles nucleation efficiency Boltzmann constant Avogadro constant

oxygen concentration ranges and found synergistic effects of oxygen concentrations. Lee et al. [18] studied the effect of oxygen concentration (air, 50% oxygen, 100% oxygen) on the soot formation in methane laminar diffusion flame using laser extinction and thermophoresis sampling-transmission electron microscopy, and found that as the oxygen content in the oxidant stream increased, the SVF increased first and then decreased. Shaddix et al. [19] also found that the formation of soot was decreased when the oxygen concentration increased from 50% to 100% in a methane turbulent diffusion flame. Wang et al. [20] investigated the effects of oxygen index (OI, 21–100% molar fraction of oxygen in oxidant) on the radiation and soot characteristics of natural gas and propane turbulent jet flames by using laser extinction method. They found that the amount of soot increased first and then decreased with increasing oxygen content and peaked at an OI of about 40%, and pointed out that fuel type and jet velocity primarily affect soot formation by affecting residence time. Glassman et al. [21] found that the oxygen concentration has two different competitive effects on the soot behavior of the jet flame. Specifically, an increase in oxygen concentration increases the flame temperature, which in turn increases the rate of fuel pyrolysis and soot formation, and on the other hand increases the rate of combustion of the particles. The former is dominant at high oxygen concentration (> 24 mol%), while the latter is dominant at low oxygen concentration (< 24 mol%). Jain et al. [22] studied the effect of OI (21–76.3%) on the soot formation in methane laminar diffusion flame experimentally and numerically. They reported that as the oxygen index increases, the maximum soot concentration increases first and then decreases. The increase of OI will increase the flame temperature, resulting in a higher soot production rate, as well as shorten the soot residence time in the flame area, thereby reducing the soot formation time. It is this competitive effect that leads to the initial increase and subsequent decrease of the maximum soot production. Merchan-Merchan et al. [23] observed the synergistic effect of oxygen content in the oxidant on soot in the biodiesel laminar diffusion flame. They point out that the initial increase in oxygen content raises the flame temperature, resulting in an increase in the rate of fuel pyrolysis, which significantly increases the formation of soot precursors in the lower half of the flame, resulting in higher density soot particles in the upper half of the flame. However, the further increase of oxygen content limits the residence time of soot aggregates, and the competition of pyrolysis rate and residence time ultimately inhibits soot formation in the flame. Other studies have also indicated that in some cases, the oxygen-rich conditions in oxidants have less impact on soot formation processes. Jung et al. [24] studied the effects of oxygen concentration (21%, 27%, 33%, and 40%) on the primary particles, soot evolution, and particle spatial distribution in the inverse diffusion flame (IDF). They report that as long as the temperature remains high enough, the small PAH produced by fuel pyrolysis could flow downstream and grow into large PAHs and soot. Oh et al. [25] investigated the effect of oxygen concentration in the oxidant on the formation and growth of soot in the propane laminar diffusion flame by combining TIRE-LII and TEM methods. The results showed that the specific growth rate (mass deposition rate per unit particle surface area) is relatively weakly dependent on the change in oxygen concentration in the laminar diffusion flame. 2

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The effect of oxygen concentration on the oxidizer side (diffusion state) in the diffusion flame is mainly the competitive effect of flame temperature and residence time, while the effect of the oxygen added to the fuel flow (premixed state) will be more complex. Hura et al. [26] studied the effect of adding a small amount of oxygen into the fuel flow on soot formation in counterflow diffusion flames of ethylene, propylene, propane, isobutane and n-butane. It was found that the addition of 10% oxygen into the ethylene flame increased the SVF by more than 100%. Other fuels such as propylene, propane, isobutane and n-butane have little change. They explained the difference between ethylene and other fuels based on the chemical effects of H radical concentrations. Du et al. [27] determined the effects of oxygen addition on soot inception, growth and burnout by measuring the soot inception limit in the counterflow flame and the integral SVF in the co-flow flame. They noted that the effects of oxygen addition to the fuel side and the oxidant side were different. When oxygen is added to the fuel side, the inception limit suddenly increases, indicating an accelerated inception chemistry. Gülder et al. [28] studied the effects of oxygen addition to fuel flow on soot formation in laminar diffusion flames of methane, propane and nbutane. They found that two counter-chemical effects occur when oxygen was added to the fuel flow in diffusion flame. On the one hand, oxygen promotes the pyrolysis of fuel, resulting the formation of hydrocarbon group and H atoms, thereby enhancing the formation of soot. On the other hand, aromatic groups and key aliphatic hydrocarbon groups are removed by reaction with molecular oxygen and oxygen atoms. The net chemical effect is the difference between the two offsets. Hwang et al. [29] investigated the effect of oxygen and propane addition on soot formation in counterflow diffusion flame of ethylene. They found that the SVF and PAH concentration increased with the addition of a small amount of oxygen or propane in the fuel stream, and pointed out that propane or oxygen addition enhances the formation of the initial aromatic ring through the C3H3 recombinant chemical reaction, which in turn promotes the formation of soot. Leusden et al. [30] compared the effect of oxygen addition to the fuel and oxidant side on soot formation in counterflow diffusion flame, and pointed out that the increase of SVF caused by oxygen addition in fuel flow was due to the chemical catalytic effect caused by the formation of benzene from C3 and C4 substances, while the increase of soot concentration caused by oxygen addition in oxidant flow was due to the higher flame temperature. Camacho et al. [31] analyzed the effect of molecular oxygen on the oxidation of new soot in premixed flames of ethylene, n-heptane and toluene, and found that the oxidation rate of soot is first-order dependent on O2 concentration. In addition, there are some similarities between the formation of soot in oxygen enriched diffusion flame and premixed flame. Kumfer et al. [32] found that the formation of soot particles is closely related to the flame structure. The local C/O ratio is the control parameter of soot inception in the diffusion flame, and the critical C/O ratio is similar to the global C/O ratio of the premixed flame in number. By measuring temperature and PAH fluorescence, they pointed out that the most favorable position for soot inception is near C/O = 0.6, and soot inception in oxygen enriched diffusion flame is due to competition between formation and oxidation, just like in

premixed flame. The above studies in the coaxial co-flow diffusion flames and counterflow diffusion flames have shown that the effect of oxygen concentration in the oxidant is mainly the competitive effect of flame temperature and residence time, while the effect of oxygen concentration in fuel flow is more complex chemical effect. However, the difference of the effects of oxygen in diffusion and premixed modes on soot formation and the dynamics mechanism are still unclear. The counterflow diffusion flame is beneficial to the addition of oxygen in the oxidant and fuel streams, but its research is limited by one-dimensional space factors, which is not conducive to the analysis of microscopic processes such as particle inception, growth and oxidation. In this work, the difference of the effect of oxygen on flame characteristics and soot formation in diffusion and premixed modes are comprehensively analyzed by changing the oxygen concentration in co-flow of diffusion flame and the oxygen concentration in fuel flow of partially premixed flame. Furthermore, the microscopic processes of particle inception, surface growth and oxidation were simulated and the dynamic mechanism was revealed. Considering the cost of oxygen enriched combustion and the significant range of oxygen, this work only investigates the flame with moderately increased oxygen concentration. The results of this research are help to understand the effects of oxygen concentration on soot formation under different combustion modes, which is of great significance for organizing combustion mode reasonably, controlling the soot particle formation during combustion and developing efficient and clean combustion equipment. 2. Soot model and numerical method In this chapter, soot formation theory, soot model development and numerical calculation methods were introduced, as well as soot models including processes such as gas phase chemistry, particle inception, growth, and oxidation. Then, the experimental setup was described, and the accuracy of the numerical method and the soot model was verified based on the experimental measurement results and literature results in ethylene–air laminar diffusion flame. 2.1. Soot formation theory Soot is the core component of particulate matter formation, usually accounting for 50–80% of the total particulate matter. Soot is produced by incomplete combustion of hydrocarbon fuel, and its main component is carbon. After it is produced, it forms the final particulate matter by absorbing various soluble organic matter, metal substances and water. The formation of soot is a very complex chemical and physical process, including complex gas-phase reaction process, phase transition process from gas precursor to solid particle inception, as well as particle growth and oxidation process [33]. From the view of kinetic mechanism, the entire formation process of soot involves gas-phase chemical reaction kinetics and solid particle dynamics, which can be represented by the structure diagram shown in Fig. 1. The formation and growth of gaseous PAHs originates from the

Fuel

Condensation CH3 C2 H2 C3 H3 Pyrolysis C H 4 i Oxidation C H 5 5 ……

A1

Precursors

Inception

Nucleation PAHs growth

Primary particles

Coagulation

HACA

Surface growth Oxidation Oxidation

Inert products

Gas-phase chemical kinetics

Soot particle dynamics

Fig. 1. Structure diagram of soot formation. 3

Aggregates

Agglomeration

Oxidation

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chemical reaction of fuel. The first ring is formed from the pyrolysis and oxidation of fuel, then the large ring aromatic hydrocarbons are formed through the growth reaction. The formation of the first ring (A1) is considered as a rate-limiting step, and its main pathways include C1 + C5, C2 + C4, and C3 + C3 [34–36]. Subsequently, according to the different molecular structure of the fuel, macrocyclic aromatic hydrocarbons may be formed by the mechanisms of methyl addition/cyclization (MAC) [37], hydrogen absorption-carbon addition (HACA) [38], propargylic addition [39], PAH radical addition reaction [40,41], and other mechanisms. At present, chemical kinetic simulations of PAHs growth usually employ one or more mechanisms. The nucleation of soot is a phase change process in which the PAHs continue to grow and form soot inception. At present, the exact mechanism of nucleation is still unclear. There are three main theoretical routes: (1) growth into curved fullerene-like structure [42]; (2) physical coalescence [33]; (3) chemical reaction into cross-linked structure [43,44]. Recently, Johansson et al. [1] proposed the mechanism of clustering of hydrocarbons by radical-chain reactions (CHRCR), which is initiated and propagated by the resonantly stabilized radicals (RSR) continuously generated by the free radical chain reactions including acetylene or vinyl. These RSR can cluster various hydrocarbons, including radicals, stable PAHs, and unsaturated aliphatic substances through radical-chain reactions, thus forming soot inception. After nucleation, the initial soot particles will undergo coagulation and surface growth. Surface growth methods include HACA mechanism [45] and PAHs condensation (physical combination of PAHs and the existing soot particle surface) [33]. Such growth will not increase the amount of soot, but only increase its particle size and mass. Primary particles can also collide to form larger primary particles or chain-like particle aggregates. In addition, since the generated soot is in a hightemperature and oxygen-containing environment, the soot will always be accompanied by an oxidation reaction, and it may be completely oxidized and disappeared at any time during the combustion. Among them, O2 and OH plays a major role in oxidation [46]. Particle aggregates may also undergo oxidation-driven fragmentation, resulting in smaller aggregates [47].

z

2.2. Soot model 2.2.1. Model development Soot models mainly include three categories of empirical model, semi-empirical model and detailed model [48]. Empirical model is empirical relationship formulas based on the experimental results of correlation between soot formation and combustion conditions, which usually contains two equations for solving soot mass and particle number density, respectively [49,50]. This kind of model is easy to understand and has a small amount of calculation, but it cannot understand the process of soot formation in depth, and lacks of wide applicability. Semi-empirical model describes the basic processes of fuel pyrolysis, soot nucleation, coagulation, surface growth, and oxidation with a small number of components and multi-step reactions, such as the 9-Step reaction model proposed by Kazakov et al. [51] and Tao et al. [52], and the two-equation model coupled chemical mechanism developed by Fairweather et al. [53]. These models ignore the aggregate structure and polydispersity of particles and do not use detailed chemical reaction kinetics mechanism, so they cannot provide detailed soot formation characteristics. Detailed model starts from the actual process of soot particle formation and provides an in-depth understanding of the soot formation and oxidation micro-processes. In the detailed model, the gas-phase reaction process of the fuel is described by a detailed chemical kinetics mechanism, and the particle growth and evolution process is described by the particle dynamic equations solved with advanced numerical methods. Typical numerical methods include the moment method used by Frenklach et al. [33], the stochastic method used by Balthasar et al. [54], and the sectional method used by Smooke et al. [55]. Among them, sectional models can provide soot average characteristics and size distribution. The initial sectional models can only solve one variable per section, such as soot mass fraction, which is not enough to model the structure of particle aggregates. Later, Park et al. [56] developed an advanced sectional model that can solve the equations of aggregate number density and primary particle number density in each section, which can predict the particle size distribution and structure. Thomson et al. [57] have implemented it as an executable flame code, CoFlame.

Zero gradient Computational domain

Symmetry condition

9 8 7

Free-slip

HAB (cm)

Flame

6 5 4 3

Fuel(+O2)

r

O2+N2

2 1

Φ10.9 mm

0

Φ89 mm

(a) Computational model Fig. 2. Computing domain and grid. 4

0

1

2

3

4

r (cm) (b) Computational grid

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Y. Hua, et al.

Pyrolysis

the radial and axial directions. Studies have shown that further refinement of the grids has little effect on the results [59]. The centerline of the flame is considered to be a symmetrical boundary condition, the downstream outlet boundary of the computational domain is considered to be a zero gradient condition, and the outer boundary on the right is considered to be a free-slip boundary condition.

Oxidation

C2H4

O2 +O2

C2H2

OH +C2H4

CH2HCO

C4H6

+C2H2

+CH3CHO

C4H4

CH3CHO

CH2O

CH3CO

2.2.3. Gas phase chemical kinetics Chernov et al. [60] developed a modified chemical kinetic model for the formation and growth of polycyclic aromatic hydrocarbons to pentacyclic aromatic hydrocarbons for laminar co-flow diffusion flames of methane-air, ethane-air and ethylene-air, including 102 species and 831 reactions. The mechanism has been widely verified based on laminar burning velocity, the concentration of key growth components and aromatic. The main pyrolysis and oxidation reaction pathways of ethylene is shown in Fig. 3. It can be seen that ethylene grows into monocyclic aromatics (A1) via the reactions of R1-R4, and is oxidized to CH2HCO through reaction R(5), and finally to CO via the reactions of R6 and R7.

HCO

+C2H2

A1

CO Fig. 3. Pyrolysis and oxidation pathways of ethylene.

2.2.2. Numerical method In this work, the soot formation in coaxial symmetrical laminar diffusion flame is modeled numerically by using the widely validated CoFlame code [57]. This code couples a detailed chemical kinetics model with a fixed sectional soot particle dynamics model, and solves the conservation equations of mass, momentum, species, energy, aggregate number density, and primary particle number density in a twodimensional axisymmetric system. These governing equations in cylindrical coordinates are discretized by the finite volume method. In this fixed sectional soot model, particle mass is divided into 35 discrete sections (that is, i = 1,2,…,35) in the form of logarithm with the partition interval coefficient of 2.35 (that is, the representative quality of the current section is 2.35 times that of the previous one). Previous studies have shown that 35 sections are sufficient to ensure that the average soot morphology parameter does not change as the number of sections further increases [57]. Each particle aggregate is assumed to consists of spherical primary particles of the same size with a fractal dimension of 1.8. The number density equations of aggregate (Nia ) and primary particle (Ni p ) are solved in the ith section, as follows [57]: Aggregate number density: v

Nia Na + u i r z Na 1 = r Dia i + r r r z +

Nia t

+ nu

(i = 1, 2,

Nia t

Dia +

co

Nia z Nia t

1 (r Nia VTs, r ) r r + sg

Nia t

+ ox

Nia t

z

( Nia VTs, z )

(1)

=

N 1 r Dia i r r r +

Nip t

(i = 1, 2,

+ nu

, 35)

+ Nip t

z

Dia +

co

Ni z Nip t

p

1 (r Nip VTs, r ) r r + sg

Nip t

+ ox

Nip t

(R2)

C4H6 = C4H4 + H2

(R3)

C4H4 + C2H2 = A1

(R4)

C2H4 + O2 = CH2HCO + OH

(R5)

CH3CO(+M) = CH3 + CO(+M)

(R6)

CH2O + M = H2 + CO + M

(R7)

Nia t

p

p

C2H2 + C2H4 = C4H6

N1a t

fr

, 35)

N N v i + u i r z

(R1)

2.2.4. Soot inception, growth and oxidation Soot inception is simulated by assuming the collision and adhesion of PAHs. The inception reaction connects the species of the gas phase soot precursors and the soot particles in the smallest section. The inception reaction in this work is based on the collision among three PAHs of benzo[a]pyrene (BAPYR), secondary benzo[a]pyrenyl (BAPYR*S) and benzo(ghi)fluoranthene (BGHIF) [57]. The inception rate is calculated according to the following formulas:

Primary particle number density: p

C2H4 + M = C2H2 + H2 + M

z

= nu

= nu

N1p t

nu

Nip t

nu

=

8 kB T (rA + rB ) 2Av2 [A][B] µAB

= 0, i = 2, 3,

, 35

(3)

(4)

where β is the nucleation efficiency with the value of 0.0001 [61], kB is the Boltzmann constant, μAB is the converted mass of two collision PAHs, rA and rB are the radii of two collision PAHs, and Av is Avogadro constant, [A] and [B] are two PAHs concentrations involved in collision. The surface reactions of soot particles include PAHs condensation, HACA surface growth and oxidation. The PAHs condensation model is based on the transition and continuum collision theory between particle aggregates and PAHs, with the collision condensation efficiency of 1.0 [57]. The mobility diameter of PAHs is taken as Lennard-Jones diameter. The HACA surface growth and oxidation model is based on the soot surface reaction mechanism developed by Frenklach et al. [33], as shown in Table 1. The calculation of HACH growth reaction rate is based on the surface site concentration of soot, including saturation sites (Csoot-H) and dehydrogenation sites (Csoot). The oxidation model takes into account the oxidation of O2 and OH components. The oxidation of soot by O2 is based on the phenyl oxidation model proposed by Frenklach et al. [33], while the oxidation of OH uses the collision model of Neoh et al. [62], with the collision efficiency of 0.13.

( Nip VTs, z )

fr

(2)

where ρ is the average density, v is the radial speed, u is the axial speed, Nia is the number of particle aggregates per unit mass of the mixed gas in the ith section, Ni p is the number of primary particles, respectively, Dia is the diffusion coefficient of particle aggregates in the ith section, VTs,r and VTs,z are the radial and axial thermophoretic velocity of particles, and the subscripts nu, co, sg, ox, and fr refer to the source of changes caused by soot inception, coagulation, surface growth, particle oxidation and fragmentation. The computational domain is a two-dimensional region above the burner exit [58], and its size and grid are shown in Fig. 2. The computational domain is divided into 80(r) × 250(z) non-uniform grids in 5

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premixed modes are comprehensively analyzed by changing the oxygen concentration in the co-flow or fuel. Furthermore, the differences of micro dynamic processes including inception, surface growth and oxidation were analyzed and revealed.

Table 1 Soot surface growth and oxidation mechanism. Number

Reaction

A/(cm3/mol·s)

S1 S2 S3 S4 S5 S6

Csoot-H + H = Csoot + H2 Csoot-H + OH = Csoot + H2O Csoot + H → Csoot-H Csoot + C2H2 → Csoot-H + H Csoot + O2 → 2CO + product Csoot-H + OH → CO + product

4.2 × 1013 0 13.0 1.0 × 1010 0.73 1.43 2.0 × 1013 0 0 8.0 × 107 1.56 3.8 2.2 × 1012 0 7.5 Neoh model, γOH = 0.13

b

Ea/(kcal/ mol)

3.1. Comparison of experimental and numerical SVF The experimental and calculated soot concentration distribution with different oxygen addition in the co-flow is compared in Fig. 6. It should be noted that the experimental results are qualitative distributions of soot concentration (incandescent intensity), which were measured by Sun et al. [16] using the LII method. The fuel inlet velocity in the experiment was 3.98 cm/s. Although there is a difference in the flame height results between the calculation and the literature due to the difference in fuel flow rate, these experimental data can still be used to qualitatively verify the computational model. The experimental and numerical results consistently show that as the oxygen concentration in the co-flow increases, the soot concentration gradually increases, and the flame height and flame radius decrease. In addition, the experimental and calculated soot distribution showed similar characteristics, and the high concentration of soot is mainly located in the middle wing region of the flame. The soot first appeared above the fuel pipe wall, and the initial height of soot formation decreases with the increase of oxygen concentration in the co-flow, which indicates that the particle inception and growth rate accelerated with the increase of oxygen concentration. The above comparisons shows that the calculation of this work can successfully capture the effect of O2 addition on the soot formation in the flame. Therefore, the soot model can be used to analyze the particle dynamics mechanism of the influence of oxygen concentration on soot formation in diffusion and partially premixed state.

2.3. Experiment and model verification In order to verify the numerical model, the SVF distribution in the ethylene laminar diffusion flame produced by the Gülder burner was first measured by using the TC-LII method. The burner consists of two concentric steel pipes, the inner tube is used to provide ethylene fuel, and the outer tube is used to provide air. The flow of ethylene and air was adjusted by a two-way gas control device. The flow of ethylene was set to 167 mL/min (at 273 K, 1 atm) and the air flow was 200 L/min (at 273 K, 1 atm). The air was provided by an air compressor. Under these conditions, an ethylene laminar diffusion flame with a visible flame height of about 60 mm can be obtained. The TC-LII measurement system displayed in Fig. 4 mainly comprises the Nd: YAG laser (532 nm, 10 Hz), image doubler device, intensified charge coupled device (ICCD), two narrowband filters (425 and 590 nm), reflecting mirrors, light sheet system, and LaVision data acquisition system. The detial measurement methods has been described in our previous research [58,63]. Fig. 5 shows the comparisons of experimental and numerical SVF in the ethylene co-flow diffusion flame. Apart from the experimental results of this work, the measurement though 2D-LOSA method in Literature [64] was also employed for the model validation. It can be seen that the distribution characteristics of the numerical SVF is similar to the experimental measurement results, with high concentration of soot in the flame located along the two wings of the middle flame, and the flame height can also be well predicted. From the specific numerical values, the SVF is in the range of 0–8 ppm, the peak value of the SVF measured by TC-LII in this experiment is 7.58 ppm, and that calculated in the current work is 7.32 ppm. Therefore, the numerical method and the soot particle model are accurate and effective, which can be used for subsequent calculations.

3.2. Effects of oxygen concentration on flame characteristics In order to understand the effect of oxygen concentration on flame structural characteristics, the distribution of temperature and OH concentration in the ethylene laminar diffusion flames under different diffusion oxygen concentrations is first shown Fig. 7. Since both the OH radical concentration and temperature are related to the intensity of combustion reaction, there is a close correlation between the temperature and OH distribution in each flame. The high concentration area of OH and the high temperature area are both located in the two wing regions upstream of the flame. The temperature and OH mole fraction in the two wings of the flame upstream and the central region of the flame top increase with increasing oxygen concentration. This is mainly because as the oxygen content in the co-flow oxidant increases, the entrainment of oxygen by the flame increases, and the concentration of oxygen near the axis of the flame increases, promoting the combustion reaction downstream. However, compared with the distribution of the

2.4. Flame conditions In this study, two types of flames, a co-flow diffusion flame and a partially premixed flame, were used. The fuel used was ethylene and the co-flow was a mixture of O2 and N2. Under co-flow diffusion flame conditions, the fuel stream is pure ethylene and the co-flow is a mixture of five different ratios of O2 and N2. Considering the cost of oxygen addition to the actual combustion device, the oxygen concentration under oxygen enriched condition is usually less than 27%, only a small proportion of oxygen concentration including 17%, 19%, 21%, 23%, and 25% by mole fraction is studied here to investigate the effect of oxygen concentration in diffusion mode, as shown in Table 2. Ethylene inlet flow rate is set to be consistent with the experiment. Under partially premixed flame conditions, the fuel flow is a mixture of five different proportions of ethylene and oxygen with the flow rate of ethylene kept constant, and the co-flow is air (21%O2). The O2 mixing ratio was set at 0%, 5%, 10%, 15%, and 20% to investigate the effect of oxygen concentration in premixed mode, as shown in Table 2. 3. Results and discussion In this chapter, the difference of the effects of oxygen concentration on flame characteristics and soot formation under diffusion and

Fig. 4. TC-LII measurement system. 6

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6

6 5

3 2 1

0

HAB (cm)

7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5

4

HAB(cm)

8

6

4

5

3

4 3

2

2

1 0

0.6

r(cm)

C2H4 laminar diffusion flame

7

HAB (cm)

5

0

fv (ppm)

fv (ppm)

(a) Numerical results in this work

1 0

0 0.6 r (cm)

(c) Experimental results in literature

(b) Experimental results in this work

Fig. 5. Verification of numerical model.

concentration zone of OH is located at the height of 0.1 ~ 1.5 cm. The trend of the peak temperature and the OH peak mole fraction as a function of the oxygen concentration in co-flow is shown in Fig. 8. It can be seen that in the laminar diffusion flame, the peak temperature and the OH peak mole fraction show a linear increasing trend with the increase of the oxygen concentration in the co-flow. When the oxygen concentration is 17%, 19%, 21%, 23% and 25%, the peak temperatures is 1872.2, 1967.4, 2052.6, 2137.8 and 2210.6 K, respectively, and the peak mole fraction of OH is 0.00359, 0.00463, 0.00565, 0.00681 and 0.00796, respectively. Therefore, the increase of oxygen concentration in the coflow can effectively increase the temperature of the flame, resulting in an intensification of chemical reactions. In the ethylene partially premixed flames, the distribution of temperature and OH concentration under different premixed oxygen concentrations is shown in Fig. 9. It can be noted that the temperature and OH on the downstream axis of the flame do not show a monotonous increase trend as the diffusion flame, but increase when the oxygen addition is 0–10%, and then gradually decrease. The temperature and

Diffusion flame

C2H4

17%O2 19%O2 21%O2 23%O2 25%O2

83%N2 81%N2 79%N2 77%N2 75%N2

To investigate the effect of oxygen concentration in diffusion mode

Partially premixed flame

100%C2H4 + 0%O2 95%C2H4 + 5%O2 90%C2H4 + 10%O2 85%C2H4 + 15%O2 80%C2H4 + 20%O2

21%O2 + 79%N2

To investigate the effect of oxygen concentration in premixed mode

+ + + + +

high temperature area, the radial distribution of OH is narrower, the axial distribution height is lower. Specifically, the high temperature zone of the flame is mainly located at the height of 0.1 ~ 3 cm, while the high OC: 16.8% 18.9% 10

21%

25%

28.9%

16.8%

36.8%

9 8

6

HAB (cm)

HAB (cm)

7

5 4 3 2 1 0

0 1.0 0 1.0 r(cm) r(cm)

0 1.0 r(cm)

0 1.0 r(cm)

0 1.0 0 1.0 r(cm) r(cm)

18.9%

8

8

28.9%

36.8%

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21%

25%

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4 3

4 3

4 3

4 3

8

HAB (cm)

Purpose

HAB (cm)

Co-flow inlet

HAB (cm)

Fuel inlet

HAB (cm)

Flame type

HAB (cm)

Table 2 Flame type and initial conditions.

4 3

2

2

2

2

2

2

1

1

1

1

1

1

0 0 0.6 r (cm)

0 0 0.6 r (cm)

0 0 0.6 r (cm)

0 0 0.6 r (cm)

0 0 0.6 r (cm)

0 0 0.6 r (cm)

(b) Numerical result

(a) Experimental result

Fig. 6. Experimental and computational soot distributions as a function of oxygen concentration in the co-flow. 7

fv 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5

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7

7

6

6

6

6

6

5

5

5

5

5 4 3

2

2

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2

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1

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1

1

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0 0.5 1 r (cm)

0

0

0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

(a) Temperature field

8

8

17%

7

7

8

21%

8

23%

7

7

5

5

5

5

5

3

4 3

4 3

HAB (cm)

6

HAB (cm)

6

HAB (cm)

6

4 3

2

2

2

1

1

1

1

1

0

0 0.5 1 r (cm)

0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

(b) OH concentration

0.0062 0.0058 0.0054 0.005 0.0046 0.0042 0.0038 0.0034 0.003 0.0026 0.0022 0.0018 0.0014 0.001 0.0006 0.0002

3

2

0

OH

4

2

0

8

25%

7

6

4

2250

OH Peak mole fraction

Peak Temperature (K)

2050

0.006

2000 1950

0.005

1900

0.004

1850 18

19

20

7

7

7

6

6

6

6

6

5

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5

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5

4 3

4 3

4 3

4 3

3

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2

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2

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0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600

4

2

0 0.5 1 r (cm)

T (K)

0

0 0.5 1 r (cm)

8

0%

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5%

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10%

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15%

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1

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1

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0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

0

0 0.5 1 r (cm)

OH 0.0062 0.0058 0.0054 0.005 0.0046 0.0042 0.0038 0.0034 0.003 0.0026 0.0022 0.0018 0.0014 0.001 0.0006 0.0002

4

2

0 0.5 1 r (cm)

20%

0

0 0.5 1 r (cm)

(b) OH concentration

temperature and the OH peak mole fraction as a function of the oxygen concentration in the fuel stream is shown in Fig. 10. When the oxygen concentration is 0%, 5%, 10%, 15% and 20%, the peak temperatures is 2052.6, 2073.6, 2097.4, 2116.9 and 2141.1 K, respectively, and the OH peak mole fraction is 0.00565, 0.00583, 0.00605, 0.00625 and 0.00652, respectively. It can be seen that in the partially premixed flame, the flame peak temperature and the OH peak mole fraction also show a consistent linear monotonous increase trend with the increase of the premixed oxygen concentration, which is consistent with the influence of the oxygen concentration in the diffusion flame, but the increase is smaller than that in the diffusion flame. This shows that in the diffusion combustion mode, increasing the concentration of premixed oxygen will further improve the flame temperature and intensify the chemical reaction to a certain extent, but the degree of influence is limited. Flame height and the flame visible height are very important characteristic parameters of flame structure, which can reflect the combustion and soot formation in the flame, and also qualitatively represent the residence time experienced by the soot particles from inception to complete oxidation. The flame height is usually defined as the position of the temperature peak or OH concentration peak on the

0.007

17

7

20%

Fig. 9. Computed distribution of temperature and OH in the partially premixed flames under different oxygen concentrations in fuel flow.

0.008

2100

1800 16

8

0.009

Peak Temperature OH Mole Fraction

2150

15%

7

0

0 0.5 1 r (cm)

Fig. 7. Computed distributions of temperature and OH in the diffusion flames under different oxygen concentrations in co-flow.

2200

8

(a) Temperature field

6

HAB (cm)

HAB (cm)

8

19%

10%

7

0

0 0.5 1 r (cm)

8

HAB (cm)

0

0 0.5 1 r (cm)

5%

HAB (cm)

0

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600

8

HAB (cm)

3

T

0%

HAB (cm)

3

4

8

HAB (cm)

4

25%

8

HAB (cm)

3

HAB (cm)

HAB (cm)

3

23%

8

HAB (cm)

7

4

21%

HAB (cm)

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8

HAB (cm)

7

4

19%

HAB (cm)

8

HAB (cm)

8 17%

HAB (cm)

HAB (cm)

Y. Hua, et al.

21

22

23

24

Oxygen concentration in coflow (%)

25

0.003 26

Fig. 8. Computed trend of peak temperature and OH peak mole fraction in diffusion flames with oxygen concentration in co-flow.

OH concentration distribution of partially premixed flame and diffusion flame also show similar characteristics. The temperature and OH mole fraction in the two wings of upstream flame gradually increase with the increase of oxygen concentration. Specifically, the trend of the peak 8

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Peak Temperature OH Mole Fraction

2120

2080

0.0060

2060

0.0058

2040

0.0056 0

5 10 15 Oxygen concentration in fuel (%)

Height (cm)

0.0062

Hvisable

7.0

0.0064

2100

(a) Diffusion flame

7.5

0.0066

OH Peak mole fraction

Peak Temperature (K)

2140

HOH HT

6.5 6.0 5.5 5.0

20

4.5 16

Fig. 10. Computed trends of peak temperature and OH peak mole fraction in partially premixed flames with oxygen concentration in fuel flow.

17

18

19

20

21

22

23

24

25

26

Oxygen concentration in coflow (%) flame center line [65,66], while the flame visible height is usually determined by the radiation luminescence of soot particles. Fig. 11 shows the trend of height defined by soot, peak temperature and OH in the diffusion flame and partially premixed flame studied in this paper as a function of oxygen concentration in the co-flow or fuel. It can be found that there are differences among the three heights under the same oxygen concentration, but follow the similar trend with the increase of oxygen concentration. In the diffusion flame, when the oxygen concentration in the co-flow increases from 17% to 25%, the flame height based on OH gradually decreases from 7.24 to 5.05 cm, indicating that the residence time of the soot in the axis direction decreases gradually. This is because as the oxygen content in the co-flow increases, the entrainment rate of oxygen by the fuel stream increases, the reaction rate increases, and the flame temperature increases, so that the residence time of OH shortens and the flame height decreases. In addition, since the oxygen concentration in the reaction zone increases, the peak concentration of OH increases, resulting in an increase in the oxidation rate of the soot particles and a decrease in the visible height of the soot. In the partially premixed flame, the height of the reaction zone decreases from 5.99 to 5.81 cm when the oxygen concentration in the fuel stream increases from 0% to 10%. However, when the oxygen concentration increases from 10% to 20%, the flame height increases from 5.81 cm to 6.09 cm. In this study, since the volume flow of ethylene in the fuel stream is constant, after adding oxygen, the flow rate of the fuel flow is increased naturally. On the other hand, the burning velocity is accelerated due to the increased temperature. The competition between them determines the final flame height. When the oxygen concentration is less than 15%, the change of burning velocity caused by O2 addition is dominant, so the flame height decreases, while when the oxygen concentration is more than 15%, the effect of the flow velocity is relatively larger, so the flame height and the visible flame height increase. However, the effect of oxygen addition into the fuel flow on the flame height is relatively small on the whole under the constant fuel mass flow rate. In order to understand the relationship between flame height and local temperature distribution, the computed radial distribution curves of temperature in the four flames at four heights of 10, 20, 30, and 50 mm above the burner is displayed in Fig. 12. The four flames include two diffuse flames (17% or 21% O2 in co-flow) and two partially premixed flames (5% or 10% O2 in fuel). It can be seen the local temperature monotonically increases as more oxygen is blended in the coflow or fuel stream throughout the flame, which is consistent with the trend of 2D temperature distribution shown in Figs. 7a and 9a. In addition, as the height above the burner increases, the peak temperature positions of all flames gradually move from the flame periphery towards the flame center, and the peak value gradually decreases. Specifically, at a height of 10 mm above the burner, the peak temperatures

(b) Partially premixed flame

7.5

Hvisable

Height (cm)

7.0

HOH HT

6.5 6.0 5.5 5.0 4.5

0

5 10 15 Oxygen concentration in fuel (%)

20

Fig. 11. The trend of height as a function of oxygen concentration in the coflow and fuel flow.

of the four flames are located at about 5.5 mm in the radial direction, and the peak values are 1859.6, 2051.6, 2071.6, and 2094.7 K, respectively. At a height of 50 mm, the peak flame temperature is about 1.5 mm in the radial direction, and the peak values are 1580.5, 1653.2, 1683.8, and 1731.1 K, respectively. In the diffusion combustion mode, the increase of the oxygen concentration in the co-flow will increase the local flame temperature, and the addition of premixed oxygen will further increase the temperature. To further understand the details of the flame structure, the trend of normalized concentration of soot and key components and temperature at different height above burner (HAB) on the centerline of ethylene diffusion flame (21% O2 in co-flow) and partially premixed flame (10% O2 in fuel and 21% O2 in co-flow) are displayed in Fig. 13. On the centerline, the order of the peak value of the key components and soot in the diffusion flame and partially premixed flame is the same, which are C4H4 > C2H2 > A1 > A5 > Soot. It should be noted that the reaction pathway in the kinetic model that the monocyclic aromatic hydrocarbon (A1) is mainly produced via the reaction C4H4 + C2H2 = A1, so C4H4 and C2H2 first reach the peak value. Subsequently, A1 gradually grows into soot precursor A5, which leads to inception and initial formation of soot particles. Therefore, the region where the concentration of A5 decreases rapidly corresponds to the region where the soot rises rapidly, and the position in which the A5 is lowered to zero corresponds to the position at which the soot reaches the peak. It can be seen from the temperature curve that the high 9

Fuel 268 (2020) 117398

T (K)

T (K)

Y. Hua, et al.

2200 2000 1800 1600 1400 1200 1000 800 600 400 200

2200 2000 1800 1600 1400 1200 1000 800 600 400 200

(a) HAB=10 mm

0

2

4

6

8

10

(c) HAB=30 mm

0

2

4

6

r (mm)

8

10

2200 2000 1800 1600 1400 1200 1000 800 600 400 200 2200 2000 1800 1600 1400 1200 1000 800 600 400 200

(b) HAB=20 mm

0

2

4

6

8

10

0%O2 in fuel-17%O2 in co-flow 0%O2 in fuel-21%O2 in co-flow 5%O2 in fuel-21%O2 in co-flow 10%O2 in fuel-21%O2 in co-flow

(d) HAB=50 mm

0

2

4

r (mm)

6

8

10

Fig. 12. Computed radial distribution curves of temperature in the four flames at four heights above the burner: (a) 10 mm, (b) 20 mm, (c) 30 mm, and (d) 50 mm.

concentration zone of soot is mainly located at about 1600 K. In the area of high concentration of soot, the temperature tends to decrease for a period of time, which is due to the formation of a large number of soot in this area, resulting in an increase of radiation heat loss. The position of OH peak is very close to that of temperature peak, and corresponds to the position where soot is reduced to zero.

(a) Diffusion flame Soot C4H4 C2H2 OH A1 A5 T

0.8 0.6

1600 1400

Temperature (K)

1.0

Normalized value

1800

1200

3.3. Effects of oxygen concentration on soot formation

1000

0.4

800

0.2

In terms of the effect of oxygen concentration on soot formation, Fig. 14a shows the linear increase of the peak SVF in the ethylene diffusion flame with the oxygen concentration in the co-flow. The change of SVF on the centerline of ethylene diffusion flame at different oxygen concentrations is shown in Fig. 14b. As the oxygen concentration increases, the peak value of the SVF on the centerline gradually increases, which is consistent with the trend of soot peak in the global flame. Under different oxygen concentrations, the SVF on the flame center line shows a three-stage trend with the increase of height, that is, the fast rising area, the stable area and the fast falling area. The fast rise zone is dominated by particle formation and growth reactions. In this region, the flame temperature increases gradually with the increase of height above burner promoting the inception and growth of soot. As the height above burner increases, the amount of oxygen entrainment increases (as shown in Fig. 15), resulting in the balance stage of soot formation and oxidation in the stable zone. Finally, the flame radial size is significantly reduced and more oxygen is involved in the flame (Fig. 15), the dominant oxidation causes a rapid decrease in the volume fraction of soot. As the oxygen concentration in the co-flow oxidant increases, the reaction time of particle inception, formation and oxidation is shortened due to the enhancement of combustion intensity and therefore a decrease in flame height. On the other hand, as the flame temperature increases, the rate of soot inception and growth gradually increases, and the peak value of soot increases. The variation of the peak SVF in the partially premixed flame with the oxygen concentration in the fuel stream is shown in Fig. 16a. It can

600 400

0.0 0

1

2

3

4 5 HAB (cm)

6

7

8

9

200

(b) Partial premixed flame 1800

Soot C4H4 C2H2 OH A1 A5 T

0.8 0.6

1600 1400 1200 1000

0.4

800

0.2

Temperature (K)

Normalized value

1.0

600 400

0.0 0

1

2

3

4 5 HAB (cm)

6

7

8

9

200

Fig. 13. Computed trends of normalized concentration of soot and key components and temperature with height above burner on the flame centerline.

10

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7.4

11

Peak soot volume fraction (ppm)

Peak soot volume fraction (ppm)

(a) Peak soot volume fraction

(a) Peak soot volume fraction

12

(11.12)

10 9

(9.15)

8 7

(7.32)

6

(5.78)

5

(4.27)

4 16

17

18 19 20 21 22 23 24 Oxygen concentration in coflow (%)

25

(7.32)

7.2 7.0 6.8

(6.67)

6.6

(6.33)

6.4

(6.38)

6.2

(6.15)

6.0

26

0

5 10 15 Oxygen concentration in fuel (%)

(b) Soot along the centerline

Increased O2

2.0 1.6

(b) Soot along the centerline

5

17% 19% 21% 23% 25%

Soot volume fraction (ppm)

Soot volume fraction (ppm)

2.4

1.2 0.8 0.4 0.0

20

0% 5% 10% 15% 20%

Increased O2

4 3 2 1 0

0

1

2

3

4 5 HAB (cm)

6

7

8

9

0

Fig. 14. Computed soot volume fraction in diffusion flames at different oxygen concentrations.

8

0.25

(a)

7

HAB (cm)

0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02

5 4 3 2 1 0

O2 mole fraction

O2 mole fraction

6

1

2

4 5 HAB (cm)

(b)

21%O2 in coflow-0%O2 in fuel

0.20 0.15

Increased HAB

0.10 HAB=0.2 cm HAB=1 cm HAB=3 cm HAB=5 cm HAB=7 cm

0.05

0.0

6

7

8

Fig. 16. Computed soot volume fraction in partially premixed flames at different oxygen concentrations.

0.00 0 0.5 1 r (cm)

3

0.2

0.4

0.6

r (cm)

0.8

1.0

Fig. 15. Radial profiles of oxygen mole fraction at various axial heights above the burner.

11

1.2

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8

8

0%

8

15%

6

6

6

6

6

5

5

5

5

5

4 3

4 3

4 3

HAB (cm)

7

HAB (cm)

7

HAB (cm)

7

4 3

20%

4 3

2

2

2

2

2

1

1

1

1

1

0 0 0.6 r (cm) 8

0 0 0.6 r (cm) 8

0 0 0.6 r (cm) 8

0 0 0.6 r (cm) 8

0 0 0.6 r (cm) 8

7

7

7

7

7

6

6

6

6

6

5

5

5

5

5

3

4 3

15%

4 3

HAB (cm)

4

10%

HAB (cm)

3

5%

HAB (cm)

4

HAB (cm)

HAB (cm)

In order to further understand the mechanism of oxygen concentration affecting particle formation and growth, the variation of particle inception, PAHs condensation and Hydrogen-abstractioncarbon-addition (HACA) rate with oxygen concentration need to be compared and analyzed in ethylene laminar diffusion flame and ethylene partially premixed flame. It should be noted that the rates described herein refer to the contribution of each reaction to the mass of the particles. The 2D distribution of HACA rate and PAHs condensation rate in ethylene partially premixed flame under different premixed oxygen concentration is shown in Fig. 17. When the premixed oxygen concentration is 0%, it is an ethylene diffusion flame. It can be seen that the surface growth rate in the diffusion flame and the partially premixed flame exhibit similar distribution characteristics. However, the distribution characteristics of HACA and PAHs condensation are obviously different. HACA mainly occurs in the two-wing area inside the flame surface, which plays an important role in the soot growth on the two wings of the flame, and conforms to the two-wings distribution characteristics of high-concentration SVF. PAHs condensation mainly occurs in the “bell-shaped” region of the flame center, which plays an important role in the particle growth on both the two-wings and the center of the flame. As the concentration of premixed oxygen increases, the distribution area of HACA and PAHs condensation gradually decreases. However, as the premixed oxygen concentration increases, the peak HACA rate on the two wings of the flame gradually decreases, and the peak PAHs condensation rate at the center of the flame increases significantly, which explains to some extent the different trends of the soot peaks on the two wings of the flame and on the centerline as a function

8

10%

7

0%

3.4. Effects of oxygen concentration on particle growth

8

5%

7

HAB (cm)

HAB (cm)

be seen that this is significantly different from the effect of adding oxygen to the co-flow oxidant. As the amount of oxygen added to the fuel stream increases, the SVF decreases first and then increases. This reflects the difference in the effect of increased concentrations of diffused oxygen and premixed oxygen on the soot formation process. When oxygen is added to the co-flow oxidant, the higher flame temperature is the main reason for the increase of soot concentration, while the effect of oxygen addition in the fuel stream is mainly based on chemical effect [30]. On the one hand, oxygen addition promotes the pyrolysis reaction of the fuel (such as R1-R4), thereby enhancing the formation of soot precursors. On the other hand, fuel molecule, aromatic, and critical aliphatic hydrocarbon groups are removed by enhanced oxidation reactions, so that the carbon in them cannot form final soot. The final net chemical effect is the competition result of the two effects [28]. Fig. 16b shows the variation of SVF along the centerline of the flame under different premixed oxygen concentrations. As the concentration of premixed oxygen in the fuel increases, the peak value of the SVF on the centerline increases monotonously, which is different from the non-monotonic tendency exhibited by the overall peak soot of the flame shown in Fig. 16a. This indicates that the increase of premixed oxygen concentration has different effects on the particle growth mechanism in different regions (wings and center) of the flame. In addition, it is also noted that as the concentration of premixed oxygen increases, the central SVF varies with height. Under the condition of low concentration premixed oxygen (0–10%), the central SVF has the same trend as the diffusion flame (as shown in Fig. 14b), that is, the central SVF presents three stages: rapid rising zone, stable zone and rapid falling zone with the increase of height. However, under the condition of high concentration of premixed oxygen (15–20%), the central SVF does not show a stable zone with the increase of height, but evolves into a slowly rising zone. To a certain extent, this indicates that in the main growth zone of soot, the promotion effect of premixed oxygen on the growth of soot particles is enhanced, so that the equilibrium point of growth reaction and oxidation reaction moves downstream of the flame.

20%

4 3

2

2

2

2

2

1

1

1

1

1

0 0 0.6 r (cm)

0 0 0.6 r (cm)

0 0 0.6 r (cm)

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Fig. 17. Computed distributions of HACA rate and PAHs condensation rate in ethylene partially premixed flames with different oxygen concentration in fuel flow.

of the premixed oxygen concentration (as shown in Fig. 16). Further, the trends of the peak rate of each reaction affecting the increase in particle mass with the amount of oxygen added are compared, as shown in Figs. 18 and 19. In the ethylene laminar diffusion flame, the soot inception rate, HACA rate and PAHs condensation rate show a monotonous increase with the increase of oxygen concentration in the co-flow. When the oxygen concentration is 17%, 19%, 21%, 23% and 25%, the peak inception rates are 4.2183 × 10-8, 5.2582 × 10-8, 6.171 × 10-8, 7.2643 × 10-8, and 8.4055 × 10-8 g/cm3s, respectively, the peak HACA rate are 0.000707, 0.000966, 0.00123, 0.00157 and 0.00193 g/cm3s, respectively, and the peak PAH condensation rate are 0.000162, 0.000214, 0.000256, 0.000293 and 0.000320 g/cm3s, respectively. The contribution of the inception to the growth of soot mass is not significant. However, since inception generates initial nucleus, and increases the particle number density, providing active sites for the subsequent surface growth, thus it also has a very important indirect impact on the contribution of particle mass growth. In the surface growth reaction, both the HACA rate and the PAHs condensation rate increase with the increase of oxygen concentration, which directly leads to the monotonous increase of the SVF in the ethylene diffusion flame with the increase of oxygen concentration. However, the value of HACA rate is greater than the PAHs condensation rate, which is the main mechanism for controlling the mass growth of particles in ethylene 12

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Fig. 19. Computed trends of soot inception and surface growth rate with oxygen concentration in ethylene partially premixed flames.

diffusion flame. In the ethylene partially premixed flame, as the oxygen concentration in the fuel stream increases, the soot inception rate and HACA rate decrease gradually, while the PAHs condensation rate increases gradually. It can be concluded that the increase of premixed oxygen has an opposite effect on the growth mechanism of HACA and PAHs condensation. As to the specific values, when the oxygen concentration increases from 0% to 15%, the decrease of HACA rate is greater than the increase of PAHs condensation rate, but when the oxygen concentration increases from 15% to 20%, the decrease of HACA rate (0.00002 g/ cm3s) is less than the increase of PAHs condensation rate (0.00062 g/ cm3s). The decrease in HACA rate may be mainly due to the decrease in the concentration of dehydrogenated soot surface sites, while the increase in PAHs condensation rate is mainly due to that the premixed oxygen to some extent promotes the pyrolysis reaction pathways of the fuel, thereby increases the formation of PAHs.

located in the two-wing of the middle flame, which corresponds well to the high concentration zone of soot. Thus it can be inferred that in the ethylene partially premixed flame and diffusion flame, soot particles will be firstly oxidized by OH after initial formation above the pipe wall, then by O2 and OH together, and finally mainly completely oxidized by O2 after developing to the upstream. Fig. 21 shows the variation of O2 oxidation and OH oxidation rate peaks with oxygen concentration in laminar diffusion flame and partially premixed flame, respectively. It can be seen that in both flames, the contribution of O2 oxidation to particle mass reduction is greater than that of OH oxidation. It can be seen from Fig. 21a that in the ethylene laminar diffusion flame, as the oxygen concentration increases, the peak values of O2 oxidation rate and OH oxidation rate increase approximately linearly. The oxidation rate of O2 shows similar growth characteristics to the HACA growth rate shown in Fig. 18b, and soot formation is mainly a process in which the two compete with each other. It can be seen from Fig. 21b that in the partially premixed flame of ethylene, the premixed oxygen concentration has little effect on the oxidation rate of OH, and there is a tolerance limit for the effect on O2 oxidation rate. When the oxygen concentration increases from 0% to 15%, the increase rate of O2 oxidation rate is small, and when the oxygen concentration increases from 15% to 20%, the O2 oxidation rate increases rapidly. Combined with the results of particle surface growth rate in Fig. 19, it is known that when the premixed oxygen concentration is less than 15%, the monotonous decrease of SVF with increasing oxygen concentration (as shown in Fig. 16a) is mainly due to the decrease of inception rate and HACA rate. When the oxygen concentration is higher than 15%, the increase of SVF with the increase of the oxygen concentration is the result of the competition between the

3.5. Effects of oxygen concentration on particle oxidation Fig. 20 displays the two-dimensional distributions of O2 oxidation rate and OH oxidation rate in the partially premixed flame of ethylene as a function of oxygen concentration. It can be seen that the oxidation regions of O2 and OH are distributed along the flame face and continue until the particles at the flame tip are completely oxidized. However, the main region of O2 and OH oxidation are different. O2 oxidation starts in the middle of the flame, with its peak zone mainly located in the downstream and the flame tip, which plays an important role in the final burnout of particles. However, the OH oxidation starts from the upstream of the flame near outlet of the burner, and its peak region is 13

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linearly with increasing oxygen concentration in the co-flow. In the case where the fuel mass flow rate remains constant, the effect of oxygen addition to the fuel stream has less effect on the flame height. As the oxygen concentration in the co-flow increases, the peak SVF increases monotonously in the diffusion flame due to the synergistic increase of HACA rate and PAHs condensation rate. However, the increase of oxygen in the fuel stream has opposite effect on the particle growth mechanism HACA and PAHs condensation in different regions of the flame. As the premixed oxygen concentration in the fuel stream increases, the peak SVF decreases first and then increases, which is attributed to the competitive effect of the decrease in HACA rate and the increase in PAHs condensation rate. However, the soot peak along the flame centerline increases monotonously due to the increase of PAHs condensation rate. In the diffusion flame, as the concentration of oxygen in the co-flow increases, the peak of O2 oxidation rate and the peak of OH oxidation rate increase linearly. In the partially premixed flame, the oxidation rate of OH is insensitive to changes in premixed oxygen concentration, while the O2 oxidation rate has a tolerance limit of 15% O2 for premixed oxygen concentration.

Fig. 20. Computed distributions of O2 oxidation rate and OH oxidation rate in the partially premixed flames with different oxygen concentration in fuel flow.

PAHs condensation rate and the O2 oxidation rate. It can be inferred that the premixed oxygen can promote the pyrolysis reaction pathway of the fuel to a certain extent, thus increasing the formation of PAHs and triggering the competition effect of PAHs condensation and HACA, resulting in a non-monotonic trend of the peak soot with the increase of premixed oxygen. 4. Conclusions This work numerically investigated the soot formation process in laminar diffusion flames and partially premixed flames. The difference of the effects of oxygen concentration on flame characteristics and soot formation under diffusion and premixed modes are comprehensively analyzed by changing the oxygen concentration in the co-flow or fuel. Furthermore, the differences of micro dynamic processes such as inception, surface growth and oxidation were analyzed and revealed. In the laminar diffusion flame, both the peak temperature and OH mole fraction show a monotonous increasing trend with increasing oxygen concentration in the co-flow. In the partially premixed flame mode, increasing the concentration of premixed oxygen in the fuel stream will further increases the flame temperature and intensify the chemical reaction. Under diffusion flame conditions, the flame height decreases

CRediT authorship contribution statement Yang Hua: Conceptualization, Methodology, Investigation, Writing - original draft. Liang Qiu: Software, Resources, Data curation. Fushui Liu: Validation, Visualization. Yejian Qian: Supervision. Shun Meng: Writing - review & editing.

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Declaration of Competing Interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The current work is based on research supported by the National Natural Science Foundation of China (Grant No. 51676062). Any findings from the current work are those of the authors and do not necessarily reflect the views of the funding organization. References [1] Johansson KO, Head-Gordon MP, Schrader PE, et al. Resonance-stabilized hydrocarbon-radical chain reactions may explain soot inception and growth. Science 2018;361(6406):997–1000. [2] Wang Y, Chung SH. Soot formation in laminar counterflow flames. Prog Energy Combust Sci 2019;74:152–238. [3] Sepehri A, Sarrafzadeh MH. Effect of nitrifiers community on fouling mitigation and nitrification efficiency in a membrane bioreactor. Chem Eng Process-Process Intensification 2018;128:10–8. [4] Kennedy IM. The health effects of combustion-generated aerosols. Proc Combust Inst 2007;31(2):2757–70. [5] Wu KK, Chang YC, Chen CH, et al. High-efficiency combustion of natural gas with 21–30% oxygen-enriched air. Fuel 2010;89(9):2455–62. [6] Song J, Zello V, Boehman AL, et al. Comparison of the impact of intake oxygen enrichment and fuel oxygenation on diesel combustion and emissions. Energy Fuels 2004;18(5):1282–90. [7] Poola RB, Sekar R. Reduction of NOx and particulate emissions by using oxygenenriched combustion air in a locomotive diesel engine. J Eng Gas Turbines Power 2003;125(2):524–33. [8] Gong C, Li J, Peng L, et al. Numerical investigation of intake oxygen enrichment effects on radicals, combustion and unregulated emissions during cold start in a DISI methanol engine. Fuel 2019;253:1406–13. [9] Seong HJ, Boehman AL. Impact of intake oxygen enrichment on oxidative reactivity and properties of diesel soot. Energy Fuels 2011;25(2):602–16. [10] Bi X, Liu H, Huo M, et al. Experimental and numerical study on soot formation and oxidation by using diesel fuel in constant volume chamber with various ambient oxygen concentrations. Energy Convers Manage 2014;84:152–63. [11] Zhao Z, Wu H, Wang M, et al. Computational investigation of oxygen concentration effects on a soot mechanism with a phenomenological soot model of acetone–butanol–ethanol (ABE). Energy Fuels 2015;29(3):1710–21. [12] Zhu J, Huang H, Zhu Z, et al. Effect of intake oxygen concentration on diesel–nbutanol blending combustion: An experimental and numerical study at low engine load. Energy Convers Manage 2018;165:53–65. [13] Smooke MD, Long MB, Connelly BC, et al. Soot formation in laminar diffusion flames. Combust Flame 2005;143(4):613–28. [14] Vandsburger U, Kennedy I, Glassman I. Sooting counterflow diffusion flames with varying oxygen index. Combust Sci Technol 1984;39(1–6):263–85. [15] Fuentes A, Henríquez R, Nmira F, et al. Experimental and numerical study of the effects of the oxygen index on the radiation characteristics of laminar co-flow diffusion flames. Combust Flame 2013;160(4):786–95. [16] Sun Z, Dally B, Alwahabi Z, et al. The effect of oxygen concentration in the co-flow of laminar ethylene diffusion flames. Combust Flame 2020;211:96–111. [17] Wang Y, Liu X, Gao Y, et al. Numerical simulations on effects of oxygen concentration on the structure and soot formation in a two-dimensional axisymmetric laminar C2H4/(O2–CO2) diffusion flame. J Therm Anal Calorim 2019;137(2):689–702. [18] Lee KO, Megaridis CM, Zelepouga S, et al. Soot formation effects of oxygen concentration in the oxidizer stream of laminar coannular nonpremixed methane/air flames. Combust Flame 2000;121(1–2):323–33. [19] Shaddix CR, Williams TC. The effect of oxygen enrichment on soot formation and thermal radiation in turbulent, non-premixed methane flames. Proc Combust Inst 2017;36(3):4051–9. [20] Wang L, Endrud NE, Turns SR, et al. A study of the influence of oxygen index on soot, radiation, and emission characteristics of turbulent jet flames. Combust Sci Technol 2002;174(8):45–72. [21] Glassman, Yaccarino P. The effect of oxygen concentration on sooting diffusion flames. Combust Sci Technol 1980;24(3–4):107–14. [22] Jain A, Das DD, McEnally CS, et al. Experimental and numerical study of variable oxygen index effects on soot yield and distribution in laminar co-flow diffusion flames. Proc Combust Inst 2019;37(1):859–67. [23] Merchan-Merchan W, McCollam S, Pugliese JFC. Soot formation in diffusion oxygen-enhanced biodiesel flames. Fuel 2015;156:129–41. [24] Jung Y, Oh KC, Bae C, et al. The effect of oxygen enrichment on incipient soot particles in inverse diffusion flames. Fuel 2012;102:199–207. [25] Oh KC, Shin HD. The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames. Fuel 2006;85(5–6):615–24. [26] Hura HS, Glassman I. Fuel oxygen effects on soot formation in counterflow diffusion

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