Experimental and Numerical Investigations of Soot Formation in

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Experimental and Numerical Investigations of Soot Formation in Laminar Coflow Ethylene Flames Burning in O2/N2 and O2/CO2 Atmospheres at Different O2 Mole Fractions yindi zhang, Fengshan Liu, and Lou Chun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04069 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Energy & Fuels

Experimental and Numerical Investigations of Soot Formation in Laminar Coflow Ethylene Flames Burning in O2/N2 and O2/CO2 Atmospheres at Different O2 Mole Fractions

Yindi Zhanga,b, Fengshan Liub,∗, Chun Louc∗ a School of Petroleum Engineering, Yangtze University, Wuhan 430100, Hubei Province, People’s Republic of China b Measurement Science and Standards, National Research Council Canada, Building M-9,1200 Montreal Road, Ottawa, Ontario K1A OR6, Canada c State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074 Hubei, People’s Republic of China

A paper submitted to Energy & fuels

∗

Corresponding author, Fax: +1-613-993-9470. E-mail address: [email protected] (F. Liu). ∗ Corresponding author, Fax: +86-27-8754-5526. E-mail address: [email protected] (Chun Lou). 1

ACS Paragon Plus Environment

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ABSTRACT

This paper presents an experimental and numerical study of the distributions of temperature and soot volume fraction in laminar coflow ethylene diffusion flames burning in O2/N2 and O2/CO2 atmospheres with the O2 mole fraction varying from 21% to 50% in both atmospheres. The fuel flow rate was maintained constant in all the experiments and simulations. The two-color flame emission method based on the response spectrum of R and G bands of a color 3-CCD camera was applied to measure temperature and soot volume fraction. Numerical calculations were conducted using the C2 chemistry model [ABF, Appel et al. (2000)] with formation of PAHs up to pyrene and a soot model incorporating the dimerization of two pyrene molecules as the soot inception step and hydrogen-abstraction acetylene addition mechanism and PAH condensation as the surface growth processes. Numerical results are in qualitative agreement with the experimental measurements when the oxidizer stream is air. The numerical model predicts well temperature but overpredicts soot volume fraction in oxygen-enriched flames in both O2/N2 and O2/CO2 atmospheres. With increasing the oxygen mole fraction in the oxidizer stream, the flame becomes brighter and shorter, the peak temperature zone shifts from the flame wing to the upper part, and the peak soot volume fraction moves from the flame wing to flame center. The soot loading grows rapidly with increasing the oxygen mole fraction in the oxidizer stream. Under the same oxygen mole fraction, the temperature and soot volume fraction in O2/N2 atmosphere are always higher than those in O2/CO2 atmosphere due to the higher heat capacity of CO2 and soot formation suppression by CO2. The chemical effect of CO2 may promote O and OH which enhance the oxidation of the critical soot formation species, including H, C2H2, C6H6 and C16H10. The primary pathway for the chemical effect of CO2 is its competition for H radical to form CO and OH, i.e., CO2+HCO +OH. Soot formation in these flames is affected by two primary reactions: CO2+HCO + OH and H+O2 O + OH. Keywords: Soot formation; Laminar diffusion flame; Oxygen enrichment; CO2 addition; Flame emission measurement; Numerical simulation. 2


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1. INTRODUCTION Soot and carbon dioxide emissions from combustion systems have been identified to be key contributors to global warming.1 In addition, the ultrafine soot aerosols emitted from combustion devices and biomass burning have harmful health and environmental effects.2 Development of effective and clean combustion technologies to reduce CO2 and soot emissions remains an active research topic. Various technologies have been proposed and implemented for these purposes, such as the oxy-fuel combustion technology for CO2 capture and storage (CCS), the exhaust gas recirculation (EGR) technology for diesel and gasoline engines, and the flue gas recirculation (FGR) technology for combustors and furnaces. In oxy-combustion chambers, with the increase of O2 concentration and addition of CO2 in the oxidizer stream, the distributions of flame temperature and soot volume fraction will be changed compared with traditional air-combustion.1-3 Such changes are likely to have significant influences on combustion efficiency and CO2 emission. Therefore, investigations of flame temperature and soot volume fraction are essential for improved understanding of the combustion characteristics and for maximizing the potentials of the oxy-combustion technology. In order to understand the influence of CO2 addition on oxygen enriched diffusion flames, a number of studies of flame temperature and soot volume fraction have been investigated.4-7 According to Du et al.8 and Gülder and Baksh,9 CO2 addition affects the flame properties and soot formation through the following three aspects: dilution, thermal, and chemical. Liu et al.6 revealed in counterflow ethylene diffusion flames that the temperature and acetylene concentration decreased and OH radical concentration increased by addition of CO2, leading to suppressed soot inception. Oh et al.10 experimentally identified a reduced primary soot particle number concentration in the soot inception regions of a laminar propane diffusion flame as a result of lowered flame temperature and enhanced OH concentration due to CO2 addition to the oxidizer. Guo and Smallwood 7 found that CO2 addition to the fuel side of coflow ethylene/air diffusion flame suppresses inception and surface growth rates of soot through its chemical influence. Several earlier investigations

9,11

also

speculated that addition of CO2 suppresses soot formation by chemically affecting soot inception and soot oxidation rates. The dominant pathway for the chemical effects of CO2 addition was concluded to be through CO + OH ↔ CO 2 + H .6,7 According to Haynes and Wagner12 and Fuentes et al.,13 investigation of soot formation and its impact on local temperature and thermal radiation should usually focus on diffusion flames. Soot is primarily formed on the fuel side of reaction 3


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regions in diffusion flames. Soot particles are oxidized mainly by O2 and OH in flames under high temperature conditions in the outer region of soot layer since soot is transported downstream mainly through convection.14-16 The important effect of oxygen concentration in the oxidizer stream on flame structure, soot formation, and the related radiation was revealed by Merchan-Merchan et al.17 The increase of oxygen concentration in the oxidizer stream accelerates fuel pyrolysis process, enhances flame temperature, and consequently promotes greatly the soot formation reactions. In addition, the overall oxidation rates of soot and combustion in general are also significantly accelerated because of the higher amount of oxygen available, 18,19

which shortens the flame heights. The total soot volume fraction is the competed

result by the formation and oxidation. Hence, it is important to investigate soot formation and combustion property from both technological and fundamental points of view, which will provide comprehensive understanding of the characteristics of oxygen-enriched combustion. Many researchers have conducted experimental investigations of temperature and soot volume fraction from the R, G, and B bands of color cameras based on the two-color method.20-22 The two-color method utilizes thermal radiative emission from particulates in flames at two wavelengths in the visible spectrum to calculate flame temperature and soot volume fraction. For laminar non-smoking diffusion flames fueled with gaseous or vaporized liquid hydrocarbon fuels, the size of soot particles is usually much smaller than the wavelengths in the visible spectrum. As such, the radiative properties of soot particles can be described by the Rayleigh approximation. Therefore, the variation of soot emissivities with wavelength should be considered. In recent years, imaging systems combined with two-color principle and charge coupled device (CCD) cameras have been developed to measure soot temperature and volume fraction.23-27 Fu et al.26 have also conducted flame temperature measurement from the R, G, and B broad bands by three-color pyrometry. The effects of oxygen enrichment on soot formation and radiative properties in diffusion flames have been experimentally investigated,28,29 which reveal that increasing the oxygen concentration can result in higher radiation emission rate. Also, there has been a significant progress towards understanding soot formation in hydrocarbon flames.30 Appel et al.31 proposed that soot surface growth is mainly through the hydrogen abstraction acetylene addition (HACA) mechanism, which is especially suitable for ethylene flame. Veshkini et al.32 proposed an improved model to take into account the soot surface aging effect. Liu et al.33 have evaluated the dependence of the soot surface growth rate on soot surface area (soot surface area per unit volume). They found that the square-root dependence performed better in the predicted soot distribution. 4


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Although the effects of CO2 addition on soot formation in oxygen-enriched diffusion flames have been investigated in several previous studies,6-10,17-19 there is still a lack of robust gas-phase/soot formation models that perform equally well under different flame conditions and help gain clear understanding of soot formation in oxygen-enriched diffusion flames burning in O2/CO2 atmospheres of different oxygen mole fractions. In this paper, soot temperature and volume fraction distributions in laminar coflow C2H4/(O2/N2) and C2H4/(O2/CO2) diffusion flames with varying O2 mole fraction from 21% to 50% were experimentally measured using the two-color principle with the spectral band measurement technique based on a color CCD camera. Meanwhile, these flames were also numerically simulated using a detailed gas-phase C2 chemistry mechanism and a soot model based on PAH inception and HACA mechanism for surface growth. O2/N2 and O2/CO2 were chosen as the oxidizer due to their abundance and relevance to CCS, EGR, FGR and oxy-combustion technologies. This study was motivated to investigate how CO2 and O2 influence on flame and soot formation in laminar coflow C2H4 diffusion flames over a range of O2 mole fraction from 21% to 50%.

2. EXPERIMENTAL METHOD 2.1. Experimental Setup A co-flow laminar diffusion flame burner studied in this article is the same as that reported by Snelling et al.,34 as shown in Figure 1. The fuel tube is made of stainless steel and has a 10.9 mm inner diameter and a thickness of 0.9 mm. The oxidizer flows through the co-annular region between the fuel tube and an outer nozzle with an 88 mm inner diameter and 100 mm outer diameter. Glass beads and porous metal disks were used to provide a uniform flow in the oxidizer stream. Electronic mass flow controllers control the flow rates of all gases, which are delivered to the burner at room temperature and atmospheric pressure (293 K, 1 atm). Table 1 summarizes the test conditions for both O2/N2 and O2/CO2 atmospheres. For each set of conditions, the flow rate of fuel (ethylene) is constant at 194 ml/min, and the total flow rate of oxidizer co-flow is 284 l/min. The flow rates were controlled by using mass flow controllers with accuracy of 1%. To analyze the effects of oxygen concentration and CO2 replacement of N2 on temperature and soot volume fraction distributions, the mole fraction of oxygen in the oxidizer stream, XO2, was increased from 21% up to 50%.

5


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Figure 1.

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A schematic diagram of the experimental setup.

A 3-color CCD camera (Type: CV-M9CL), which has three CCD sensors with a size of 4.8 (h, horizontal) × 3.6 (v, vertical) mm and a resolution of 1024 (h) × 768 (v) pixels, was used to image the flames. Using the detected radiation images of R, G, and B by the camera, the temperature and radiative properties distributions of soot particles in the flames can be simultaneously reconstructed. Further details of experimental methodology have been given in Refs. 20 and 21. Table 1. Summary of laminar diffusion ethylene flames in O2/N2 and O2/CO2 atmosphere for measurement Flame 21 O2/N2 30 O2/N2 40 O2/N2 50 O2/N2 30 O2/CO2 40 O2/CO2 50 O2/CO2

X O2 21% 30% 40% 50% 30% 40% 50%

QC2 H4

Qair

QO2

QCO2

(ml/min)

(l/min)

(l/min)

(l/min)

194 194 194 194 194 194 194

284 251 215 179 / / /

0 32 68 104 85 113 142

/ / / / 198 170 142

2.2. Two-color Principle with Spectral Band Measurement for a 3-color CCD Camera The raw data obtained by the R, G, and B bands of a color 3-CCD camera represent thermal radiation of flame in the visible spectrum. According to the characteristics of the color digital camera, the spectral response curves are broad. Assuming the spectral response curves of R, G, and B bands are described by 6


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functionsη R (λ ) ,ηG (λ ) , and η B (λ ) , the emissive powers ER , EG , and EB received by the R, G, and B bands of the camera can be written as below λ2

ER = ∫ η R (λ ) ⋅ ε R (λ ) ⋅ Eb (λ , T )dλ λ1

λ4

EG = ∫ ηG (λ ) ⋅ ε G (λ ) ⋅ Eb (λ , T )dλ ,

(1)

λ3

λ6

EB = ∫ η B (λ ) ⋅ ε B (λ ) ⋅ Eb (λ , T )dλ λ5

where Eb (λ ,T ) is spectral emissive power of blackbody governed by Planck’s radiation law, ε (λ ) is spectral emissivity, λ is wavelength, and T is absolute temperature. A blackbody furnace can be used to calibrate the relationship between the absolute emissive powers and the raw data of R, G, and B bands of the camera. For processing flame images in the visible spectrum, which are attributed to line-of-sight integrated flame emissions, the thermal radiation can be considered from soot only, since radiation from gaseous components such as CO2 and H2O contribute negligibly. However, in the spectral response bands of the camera from 400 nm to 700 nm, the chemiluminescent emission from CH at 430 nm also contributes to the B band of the camera, especially in hydrocarbon flames.35 Besides, many experimental investigations have shown that the B band signal is weak and might suffer from random noise. 20, 36 So, the emissive powers received by the R and G bands will be used to derive soot temperature and volume fraction. On the other hand, for a non-gray emissivity model, the Hottel and Broughton’s empirical emissivity model will be substituted into Eq. (1), so flame temperature T and KL could be calculated from λ2

α

ER = ∫ η R (λ ) ⋅ Eb (λ , T ) ⋅ (1 − e − KL / λ )dλ λ1

λ4

α

EG = ∫ ηG (λ ) ⋅ Eb (λ , T ) ⋅ (1 − e − KL / λ )dλ

,

(2)

λ3

where KL is the optical thickness of the flame along the line-of-sight under consideration, and α is a constant, given by the mature soot value of 1.34.37 The average flame emissivities in the response spectrum of R and G bands will then be calculated as λ2

εR

∫ = λ

1

λ2 − λ1

λ4

εG =

α

(1 − e − KL / λ )dλ

∫λ

α

(1 − e − KL / λ )dλ

.

3

λ4 − λ3

7


(3)

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The above expressions are applicable to paths of uniform temperature and soot volume fraction with negligible emission absorption along the path. In the CCD camera image processing of this study, the respective emissive powers of the R and G bands were first reconstructed to obtain radically resolved local emissive powers by using a standard Abel inversion procedure. Then the local soot temperature and volume fraction were retrieved following the algorithm described by Snelling et al.34 The experimental uncertainties of the two-color spectral band measurements were estimated to be 5% for soot temperature and 10% for soot volume fraction.

3. NUMERICAL MODEL AND COMPUTATIONAL DETAILS The co-flow laminar diffusion flames simulated in this article are the same as the experimental cases. The computational domain is 104.6 mm (in the streamwise direction)×47.1 mm (in the radial direction) using 210(z)×88(r) control volumes. A non-uniform mesh was used with a resolution of 0.05 mm in the r-direction and 0.066 mm in the z direction near the burner exit. The fully elliptic governing equations for mass, momentum, energy, species in the low-Mach number limit and in axisymmetric cylindrical coordinates were solved. The transport equations can be found in several previous studies.38-40 Radiative heat transfer was calculated using the discrete-ordinates method (DOM) coupled with a statistical narrow-band correlated-k (SNBCK) based wide-band model for the absorption coefficients of CO, CO2, and H2O.39 The Rayleigh expression40 approximates the absorption coefficient of soot. Further details of the numerical method are given in Refs. 38-40. The thermal and transport properties of gaseous species and the chemical reaction rates were obtained using Sandia’s CHEMKIN 41 and TRANSPORT 42 libraries and the databases associated with the ABF mechanism.31 The ABF gas-phase reaction mechanism containing PAHs up to pyrene (A4)31 was used in this paper. The physical and chemical mechanisms governing the transition from large PAH molecules to soot particles i.e., the soot inception processes, are currently poorly understood. Soot inception step is simplified as the collision of two A4 (pyrene) molecules in the free-molecular region39 in this study. The subsequent soot surface growth was simulated by the HACA mechanism and PAH (A4) condensation and oxidation by O2 and OH following.31,43 The sectional soot model was used to model the interactions among different sized particles. The flame code used in this study has been extensively described in previous studies, e.g.,38-40 where further details of the numerical method and soot model can be found. Numerical calculations were conducted for laminar coflow C2H4/(O2/N2) and C2H4/(O2/CO2) diffusion flames. In all the cases considered, the mean inlet ethylene velocity is 3.465 cm/s, the average oxidizer stream velocity is 77.0 cm/s. The boundary conditions used in the present numerical calculations were similar to those described in Ref. 40. A parabolic and uniform velocity profile were imposed at the 8


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fuel and oxidizer inlet, respectively. Both the fuel and oxidizer were delivered at 300 K. Table 2 provides a summary of the oxidizer compositions for all the cases of simulation. Conditions at other boundaries can be found in Ref. 40. All numerical simulations were conducted using 16 CPUs. Table 2. Summary of laminar diffusion ethylene flames in O2/N2 atmosphere and O2/CO2 atmosphere for calculation Flame

X O2

Oxidizer stream compositions (mole fraction)

21 O2/N2 30 O2/N2 40 O2/N2 50 O2/N2 21 O2/CO2 30 O2/CO2 40 O2/CO2 50 O2/CO2

21% 30% 40% 50% 21% 30% 40% 50%

wO2 = 0.21, wN2 = 0.79 wO2 = 0.30, wN2 = 0.70 wO2 = 0.40, wN2 = 0.60 wO2 = 0.50, wN2 = 0.50 wO2 = 0.21, wCO2 = 0.79 wO2 = 0.30, wCO2 = 0.70 wO2 = 0.40, wCO2 = 0.60 wO2 = 0.50, wCO2 = 0.50

4. RESULTS AND DISCUSSION 4.1. Visible Flame Appearance Figure 2 shows the visible flame appearances in still flame photography of the ethylene flames in O2/N2 and O2/CO2 atmospheres as XO2 is increased from 21% to 100%. These flame photos were taken using a commercial digital camera with identical settings. As displayed in Figure 2, the flame shape and brightness are strongly affected by the oxygen mole fraction. With increasing XO2, the flame becomes increasingly shorter and brighter, regardless of the balancing composition in the oxidizer stream (N2 or CO2). Comparing the flame photos between O2/N2 and O2/CO2 atmosphere, at a given XO2, the flames are shorter and brighter, and the flame luminosity appears closer to the burner exit in the O2/N2 atmosphere. It is obvious from Figure 2 that soot inception is delayed and less soot is produced in the O2/CO2 atmosphere. Because pure oxygen is rarely used as oxidizer in practical combustion systems, the case of 100% O2 will not be further considered.

4.2. Comparison of Temperature and Soot Volume Fraction between Measurement and Numerical Simulation in Ethylene/air Flame In this section, the results of two-color R and G spectral band measurements will be compared with those of CARS (coherent anti-Stokes Raman spectroscopy for temperature)/LOSA (light-of-sight attenuation for soot volume fraction) methods,34 and numerical calculation for the C2H4/air flame. Figure 3 shows the distributions of temperature and soot volume fraction in the ethylene/air diffusion flame by the 9


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two-color spectral band method, CARS/LOSA methods, and simulation.

Figure 2. Photos of the ethylene diffusion flame burning in O2/N2, (a), and in O2/CO2, (b).

From Figure 3, it can be seen that the simulation captures the primary features of temperature and soot volume fraction (SVF) in the C2H4/air flame, albeit the simulation fails to predict SVF in the flame centerline region. The distributions of flame temperature obtained by all three methods are similar. However, the two-color spectral band method missed the high-temperature region low in the flame, likely because of the absence of soot or very low soot concentrations. In addition, the peak temperatures in the spectral band method and simulation are about 100 K lower than the CARS method. The predicted peak SVF is slightly higher than the two measurements. Overall, the predicted flame shape characterized by temperature and SVF distributions is closer to that based on the CARS/LOSA measurements than the two-color spectral band method. To further verify the two-color spectral band measurements and numerical results against the CARS/LOSA data, Figure 4 compares temperature and SVF distributions at three heights z = 20, (a), 30, (b), and 50 mm, (c), in the C2H4/air flame. From Figure 4, the distributions of temperature and SVF are well predicted, except in the centerline region where the predicted SVFs are much lower than measurements. The measurements of the two-color spectral band method are also in reasonable agreement with the CARS/LOSA method for both temperature and SVF distributions at the three heights. Figures 3 and 4 serve as validation of the two-color spectral band method and numerical model benchmarked by the CARS/LOSA results in the case of C2H4/air flame. The imaging processing measurement and simulation methods are used to investigate the effects of oxygen enrichment. 10


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z(mm)

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1990

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0 -5

0

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-5

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r(mm)

r(mm)

(b) 21O2/N2, LOSA fv,max=8.66ppm

50

21O2/N2,Two-color method fv,max=8.38ppm

21 O2/N2, Simulation fv,max=9.02ppm

60

60

7.78

7.42

7.18 50

6.80

6.58

6.18

5.98

z(mm)

3.71

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r(mm)

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0 0

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CARS 21O2/N2,Tmax=2168.0 K

z(mm)

(a)

0

0

5

-5

r(mm)

`

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r(mm)

Figure 3. Distributions of temperature (a) and soot volume fraction (b) in ethylene/air diffusion flame.

(b)

(c) 2200

10

Simulation 2-Color Measurement CARS/LOSA Measurement

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Simulation 2-Color Measurement CARS measurement

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Simulation 2-Color Measurement CARS/LOSA Measurement

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Energy & Fuels

z(mm)

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0

1

2

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r(mm)

4

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r(mm)

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Figure 4. Temperature and soot volume fraction distributions at flame height z = 20 mm, (a), 30 mm, (b), and 50 mm, (c), in C2H4/air flame. 11


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4.3 Effect of O2 Enrichment on Diffusion Flame 4.3.1 Soot volume fractions in C2H4/(O2/N2) flames Figure 5 displays the measured, (a), and simulated, (b), SVF distributions in C2H4/(O2/N2) flames for different XO2 in the oxidizer stream ranging from 21 to 50%. As demonstrated in Figure 5, the distributions of SVF are strongly affected by the oxygen content in oxidizer stream. The sooting region or the visible flame is drastically shortened and the soot inception starts earlier (at a location closer to the burner exit) with increasing XO2. Meanwhile, the peak SVF increases modestly in the measurements, but increases significantly by the simulation. The reason for the overprediction of SVF is likely due to the neglect of soot aging effect as demonstrated recently by Soussi et al.16 Nevertheless, the simulation captures the main effects of increasing XO2, such as reduced visible flame height, earlier soot inception, and increased peak SVF. However, the underprediction of SVFs in the centerline region persists, regardless of the level of XO2. 21O2/N2,Two-color Method fv,max=8.38ppm

(a)

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8.74

7.18

8.04

8.03

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9.40 9.36

50

8.58

7.30

6.70

5.39

50 O2/N2 fv,max=11.13

60

8.08

6.58

40

60

40 O2/N2 fv,max=9.57

7.78

5.98

7.80

6.57

40

5.84

40

7.02 6.24

30

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2.99

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6

30

30

4 3.5

20

3

20

2.5 2 1.5

10

10

1 0.5 0

0 -5

0

r(mm)

5

0 -5

0

0

1.56 0.78 0.00

5

-5

0

5

r(mm)

40 O2/N2 fv,max=14.267ppm 60

60 9

50

2.34

r(mm)

30 O2/N2 fv,max=12.56

21 O2/N2, Simulation fv,max=9.02ppm 60

3.12

0

r(mm)

r(mm)

3.90

20

10 0.73

0 -5

5

10

0.67

0

0

(b)

2.92

20

1.20

0.00

0

4.68

2.19

10

5.46

30

3.65

2.01

0.60

-5

4.38

1.80

50 O2/N2 fv,max=15.334ppm 60

14

15 14.5 14 13.5 13 12.5 12 11.5 11 10.5 10 9 8 7 6 5 4 3 2 1 0

13.5

50

13

50

12.5 12 11.5

40

40

11 10.5

z(mm)

z(mm)

5.11

z(mm)

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

Page 12 of 26

r(mm)

9

30

8 7

20

6

20

5 4

10

3

10

2 1

0 -5

5

10

30

0

0 r(mm)

5

0 -5

0

5

r(mm)

Figure 5. SVF distributions from measurement, (a), and simulation, (b), in C2H4/(O2/N2) flames. 12


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

Energy & Fuels

4.3.2. Soot volume fractions in C2H4/(O2/CO2) flames Figure 6 shows the measured and predicted SVF distributions from measurement (a) and simulation (b) in C2H4/(O2/CO2) flames with increasing XO2 from 21 to 50% in the oxidizer stream consisting of O2 and CO2. As illustrated in Figure 6, the visible flame height decreases with increasing XO2, which is similar to the effect of XO2 in C2H4/(O2/N2) flames shown in Figure 5. In addition, the simulations capture qualitatively the variation of peak SVF with XO2, though discrepancies still exist. For example, the model overpredicted the peak soot volume fraction and failed to capture the significant soot volume fractions along the flame centerline except at

XO2 = 50%.

Compare Figures 5 and 6, it is evident that at a given level of oxygen enrichment, the soot inception location is higher, the visible flame is taller, and the peak SVF is lower in the C2H4/(O2/CO2) flame than those in the corresponding C2H4/(O2/N2) flame, indicating that soot formation in C2H4/(O2/CO2) flames is suppressed as N2 is replaced with CO2. The results shown in Figures 3, 5, and 6 indicate that the simulation predicts reasonably well the SVF levels in the C2H4/air flame and also captures qualitatively the effect of oxygen content in the oxidizer stream on SVF. However, the simulation overpredicts SVF in oxygen enriched flames, in both O2/N2 and O2/CO2 atmosphere. It is also noticed from Figures 5 and 6 that the replacement of N2 by CO2 in the oxidizer leads to a significantly less delay of soot inception in the simulation than that observed from the measurements. Although such discrepancies could be attributed to both the deficiencies in the soot model and the sensitivity issue in the present two-color method, it is believed that the model deficiencies in the inception submodel is the main cause for the discrepancies. In the present soot inception model, A4 (pyrene) collision was assumed to lead to soot inception, instead of considering larger (such as 5-ring PAHs) PAHs as the inception species. Therefore, this inception model tends to predict earlier appearance of soot.

4.3.3 The maximum values of temperature and soot volume fraction in oxygen-enrichment diffusion flames Variations of the maximum temperature and SVF with XO2 from both measurements and simulations are compared in Figure 7. From Figure 7(a), the maximum temperature increases with increasing XO2 in both C2H4/(O2/N2) and C2H4/(O2/CO2) flames, as expected. The peak temperatures by measurement and simulation are in fairly good agreement over the entire range of XO2 considered with the maximum discrepancy between simulation and measurement being only about 84 K. The peak flame temperatures in O2/CO2 atmosphere are significantly lower than 13


Energy & Fuels

those in O2/N2 atmosphere mainly due to the higher heat capacity of CO2 than N2 and the chemical and radiative properties of CO2 to a lesser degree. Figure 7(b) shows that the simulation overpredicts the peak SVFs under oxygen-enrichment conditions, regardless of the compositions in the oxidizer stream, though the model predicts reasonably good peak SVF when the oxidizer is air. As has already been displayed in Figure 5, the simulation also reproduces the qualitative increasing trend of the peak SVF with increasing XO2, though the simulation predicts a significantly faster increase in peak SVF than the measurements. The overprediction of peak SVF is attributed to the neglect of the soot aging effect, which was shown to be important at higher XO2 but not at XO2= 0.21 (oxidizer is air).16 However, the model predicts the correct trend of peak SVF variation with XO2 compared to the measurements. 21O2/N2,Two-color method fv,max=8.38ppm

(a)

60

60

30 O2/CO2 fv,max=4.17

7.78 7.18 50

6.58

50

5.39

40

40

z(mm)

5.90

2.56

5.37

40

2.24

4.19 3.59

6.44

3.52 50

1.92

30

30

1.60

2.99 2.39

20

1.28 20

1.80 1.20 10

0.60

10

0.00 -5

0

0

9 8.5

6.5

5.5

6

5

5.5

4.5

5

30

4.5

30

4

3

2.5

2.5

2

2

1.5

1.5

10

20

10

1

1 0

0 -5

0

r(mm)

5

60 10.5 10 9.5 9 8.5 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

50

40

30

20

10

0.5

0.5

0 -5

0

0

-5

0 -5

5

r(mm)

0 r(mm)

0

5

r(mm)

3

3.5

0.00

5

3.5

4

20

0

60

6

40

0.61

0

40 O2/CO2 fv,max=10.487ppm

7

z(mm)

40

1.23

0.00

6.5

7.5

1.84

r(mm)

7

50

2.45

20

0.54

7.5

8

3.07

10

-5

60

50

3.68

2.68

10

5

30 O2/CO2 fv,max=7.6 ppm

60

4.29

3.22 30

1.07

r(mm)

21 O2/N2, Simulation fv,max=9.02ppm

(b)

3.76

0 -5

5

r(mm)

4.91

0.64

0

6.13 5.52

1.61

0.00

0

6.75 50

4.29 40

0.96

0.32

7.36

4.83

2.15

20

50 O2/CO2 fv,max=8.22

60

3.82

2.88

4.79 30

40 O2/CO2 fv,max=7.53

60

3.20

5.98

z(mm)

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

Page 14 of 26

5

50 O2/CO2 fv,max=11.12ppm 11 10.5

50

10 9.5 9 8.5

40

8 7.5 7

30

6.5 6 5.5

20

5 4.5 4 3

10

2 1

0 -5

0

0 r(mm)

5

Figure 6. Measured, (a), and predicted, (b), SVF distributions in C2H4/(O2/CO2) flames. 14


Page 15 of 26

20

3000

Simulation,O2/N2 Measurement,O2/N2 Simulation,O2/CO2 Measurement,O2/CO2

(a)

2600

2400

293K

2200

199 K

Simulation, O2/N2 Measurement,O2/N2 Simulation, O2/CO2 Measurement,O2/CO2

18

The maximum soot volume fraction,ppm

The maximum temperature, K

2800

290 K

2000

1800

16

(b)

14 12

3.78ppm in 30O2/N2

4.96ppm 10 8 6

4.61ppm

4

3.43ppm in 30O2/CO2

2 0

21%O2

30%O2

40%O2

21%O2

50%O2

30%O2

O2 mole fraction, x%

40%O2

O2 mole fraction,

50%O2

x%

Figure 7. Variations of the maximum temperature, (a), and SVF, (b), with x O 2 for both measurements and simulations.

4.3.4. Temperature distributions at different flame heights in oxygen-enrichment diffusion flames Figure 8 displays the predicted radial profiles of temperature at three heights of z = 4, 6, and 10 mm for different XO2 in both C2H4/(O2/N2) and C2H4/(O2/CO2) flames. These results again reveal that oxygen-enrichment increases temperatures and replacement of O2 in the oxidizer by CO2 decreases temperature. From Figures 7-8, it can be concluded that although the flame model overpredicts SVFs with increasing XO2, it captures the main features of the effect of oxygen content in oxy-fuel combustion, such as increased temperatures, reduced visible flame height, and increased peak SVF. 3000

3000

3000

2400

Temperature(K)

2000

21 O2/N2 30 O2/N2 40 O2/N2 50 O2/N2 30 O2/CO2 40O2/CO2 50O2/CO2

2700

z=4mm

1500

2100

(b)

21 O2/N2 30 O2/N2 40 O2/N2 50 O2/N2 30 O2/CO2 40 O2/CO2 50 O2/CO2

2700

z=6mm 2400

Temperature(K)

(a)


2500

Temperature(K)

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

Energy & Fuels

1800 1500 1200

2100

(c) z=10 mm

1800 1500 1200

1000 900

900

600

500 0

1

2

3

4

r(mm)

5

6

7

600 0

1

2

3

4

5

6

7

r(mm)

0

1

2

3

4

5

6

7

r(mm)

Figure 8. Calculated radial temperature distributions for O2 mole fraction increasing from 21% to 50% at the height z = 4, 6, and 10 mm.

4.4. The Chemical Effect of CO2 on Temperature and Soot Volume Fraction 4.4.1. Flame shapes and temperature distributions Figure 9 shows the measured (top row) and simulated (bottom row) temperature distributions in C2H4/air, C2H4/(30%O2/70%N2) and C2H4/(30%O2/70%CO2) flames. The measurements and simulations display an overall similar trend in the effect of oxidizer compositions on flame shape: the flame becomes shorter with oxygen enrichment but it is taller in the O2/CO2 atmosphere. Nevertheless, there are some 15


Energy & Fuels

differences between the prediction and measured results. The measurements fail to capture the high-temperature regions near the burner exit, where soot concentrations are low. Also the two-color measurements display high temperatures in the flame tip region, Figure 9(a), while the predictions do not show this characteristic. This is likely caused by the experimental error since the soot concentrations are low in the flame tip region. The results illustrated in Figure 9 indicate that increasing O2 content in the oxidizer stream increases temperature, but the presence of CO2 in the oxidizer stream decreases the temperature. This is simply because increasing O2 reduces the inert concentration in the oxidizer, leading to higher flame temperatures. When CO2 is used (a) 21O2/N2,Tmax=2168.0 K

30O 2/N 2 Two-color Method 21O2/N2,Tmax=2060.0 K 60 Tmax=2305K 60 1990

60

30 O2/CO2 Tmax=2012K

1956 1918

50

50

50

40

40

30

30

20

20

10

10

1880 1842 40

z(mm)

1766 30

1728

z(mm)

1804

1690 1652

20

1614 1576 10 1538 1500

0

0

0 -5

0

-5

5

(b) 60

0

-5

5

Simulation 21 O2/N2,Tmax=2048.3K

60

0

5

r(mm)

r(mm)

r(mm)

30 O2/N2 Tmax=2248K

60

30 O2/CO2 Tmax=1958K

1960

50

1950

50

50

40

40

30

30

20

20

10

10

1900

40

z(mm)

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

Page 16 of 26

1850 1800

30

1750 1700

20

1650 1600

10 1550 1500

0

0

0 -5

0

5

-5

0

5

-5

r(mm)

r(mm)

0

5

r(mm)

Figure 9. Comparison of measured, (a), and simulated, (b), temperature distributions C2H4/air, C2H4/(30%O2/70%N2) and C2H4/(30%O2/70%CO2) flames.

16


Page 17 of 26

to replace N2 in the C2H4/(30%O2/70%N2) flame, the maximum temperature declines sharply by almost 300 K from both measurements and simulations. This is because CO2 has a strong influence on combustion reactions and flame temperature caused mainly by its higher heat capacity and to a lesser degree by its chemical and radiative effects.

4.4.2. Radial distributions of temperature soot volume fraction The radial distributions of temperature and SVF at z = 14 mm are compared in Figure 10. From Figure 10(a), the measured peak temperatures are 2058, 2125 and 1970

K

for

the

C2H4/(30%O2/70%CO2)

C2H4/(21%O2/79%N2), flames,

respectively.

C2H4/(30%O2/70%N2)

The

corresponding

and

simulation

temperatures are 2040, 2060, and 1945 K, respectively. Comparing the C2H4/(21%O2/79%N2) and C2H4/(30%O2/70N2) flames at z = 14 mm, the latter temperature is higher, and the peak value of the temperature shifts towards the flame centerline, which is associated with the reduced flame height. When nitrogen in the oxidizer stream is replaced by CO2, the peak temperature is significantly reduced by about 120~150 K and the flame becomes taller. The reasons for such influences of CO2 replacement of N2 in the oxidizer stream have been discussed above. From Figures 5-6, it can also be seen that the SVF distributions are strongly affected by both the O2 and CO2 concentrations in the oxidizer stream. 12

3.0

(a)

2200

(b) z=14mm

21 O2/N2, Simulation 21 O2/N2, Measurement 30 O2/CO2, Simulation 30 O2/CO2, Measurement 30 O2/N2, Simulation 30 O2/N2, Measurement

10

1800 1600 1400

21 O2/N2,Simulation 30 O2/N2, Simulation 30 O2/CO2, Simulation 21 O2/N2,Measurement 30 O2/N2,Measurement 30 O2/CO2,Measurement

1200 1000


2000

800

8

6

2.5

2.0

1.5

z=14mm

4

1.0

2

0.5

0

0

1

2

3

4

5

0.0 0

6


2400

Temperature(K)

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

Energy & Fuels

1

r(mm)

2

3

4

5

6

r(mm)

Figure 10. Comparisons of measured and simulated radial distributions of temperature, (a), and soot volume fraction, (b), at z = 14 mm in C2H4/air, C2H4/(30% O2/70%N2) and C2H4/(30% O2/70%CO2) flames.

Figure 10(b) shows the radial distributions of SVF at z = 14 mm in C2H4/(21%O2/79%N2), C2H4/(30%O270%N2) and C2H4/(30%O270%CO2) flames. Comparing the C2H4/(21%O2/79%N2) and C2H4/(30%O2/70%N2) flames, the latter has a higher peak SVF than the former and the peak value moves towards the flame centerline, again due to reduced flame height. When N2 in the oxidizer stream of the 17


Energy & Fuels

C2H4/(30%O2/70%N2) flame is replaced by CO2, the peak value of SVF is significantly reduced. This is because CO2 addition lowers the flame temperature and has a strong chemical effect on soot formation suppression. According to literatures,6,7 CO2 suppresses soot formation mainly by its thermal and chemical effects.

4.5. Analysis of Soot Formation in Oxygen-enrichment Diffusion Flames 4.5.1. Soot volume fraction distributions at different flame heights The measured radial profiles of SVF at z = 4 and 10 mm are compared in Figure 11. These results again indicate that oxygen-enrichment in general enhances soot formation, but replacement of N2 in oxidizer by CO2 suppresses soot formation. From Figures 7 and 11, it can also be found the maximum value of SVF (Figure 7) and the radial distribution of SVF at the height z = 4 mm and z = 10 mm (Figure 11) increase with the oxygen concentration in oxidizer stream of C2H4/(O2/CO2) flames. 10

8

(a)


6

(b)

Measurement, z=10mm

8


Measurement, z=4mm Soot volume fraction(ppm)

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

Page 18 of 26

4

2

0


6

4

2

0 0

1

2

3

4

5

0

6

1

2

3

4

5

6

r(mm)

r(mm)

Figure 11. Measurement soot volume fraction distributions for O2 mole fraction increasing from 21% to 50% at the height z = 4 mm, (a), 10 mm, (b).

4.5.2. Soot formation in ethylene diffusion flame at the different conditions Oxygen affects flame and soot formation both physically and chemically.10,44 Increasing oxygen concentration decreases the flame height, leading to a shorter residence time for soot growth. Meanwhile, it enhances heat release rate and flame temperature, leading to higher gas-phase reaction rates as well as soot inception and surface growth rates. On the other hand, increasing oxygen will also prompt OH concentrations, which in turn accelerates soot oxidation. To gain insights into the effects of oxygen-enrichment and CO2 replacement of N2 in the oxidizer on soot inception, surface growth, and oxidation processes, the calculated maximum mole fractions of H, OH, C2H2, C6H6 (A1), and C16H10 (A4) and peak SVF and temperature in the C2H4/(O2/N2) and C2H4/(O2/CO2) flames are summarized in Table 3. From Table 3, it can be found that the maximum mole 18


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

Energy & Fuels

fractions of all the five species, the peak SVF and temperature increase with increasing O2 both in C2H4/(O2/N2) and C2H4/(O2/CO2) flames. Comparing the C2H4/(O2/N2) and C2H4/(O2/CO2) flames with the same O2 component in oxidizer stream, there are significant decreases for the peak mole fractions of all the five species, the peak SVF and peak temperature when N2 is replaced by CO2. The decrease of the peak mole fractions of these species can be attributed to both the thermal and chemical effects of CO2. In this study, the particle inception is assumed to be due to the collision/dimerization of two pryene (A4) molecules. The rate of inception depends on temperature and pyrene concentration. Surface growth is caused by PAH condensation and by acetylene (C2H2) addition through the HACA mechanism. The HACA surface growth rate mainly depends on temperature, surface area, and concentrations of C2H2 and radical H. Table 3 shows that the concentration of A4 is significantly lower in C2H4/(O2/CO2) flames. The lower A4 concentration and lower temperature are responsible for the reduced soot inception rates in C2H4/(O2/CO2) flames. The lower concentrations of H radical, C2H2, and A4 in C2H4/(O2/CO2) flames imply that soot growth rates through PAH condensation and HACA mechanism are all lower in C2H4/(O2/CO2) flames, resulting in lower SVFs in these flames. Table 3. Calculated Maximum Mole Fractions of H, OH, C2H2, A1(C6H6) and A4(C16H10), Soot Volume Fraction(SVF), and Temperature

O2/N2

O2/CO2

Oxygen mole fraction, χ O (%)

Maximum mole fraction/

Flame

2

temperature/SVF(ppm)

21

30

40

50

H

0.0038

0.0056

0.00757

0.00923

OH

0.0058

0.0121

0.0193

0.0245

C2H2

0.051

0.0631

0.0704

0.0785

A1

0.00032

0.000391

0.000430

0.000498

A4

2.059e-007

2.731e-007

3.802e-007

4.354e-007

Temperature (K)

2048.3

2248

2502

2688

SVF(ppm)

9.020

12.775

14.267

15.334

H

0.00186

0.00225

0.00320

0.00450

OH

0.00261

0.00627

0.01100

0.0170

C2H2

0.03682

0.04842

0.05768

0.06848

A1

0.000147

0.000214

0.000367

0.000460

A4

1.4609e-007

1.599e-007

2.0e-007

2.521e-007

Temperature (K)

1756

1958

2284

2450

SVF (ppm)

2.3

7.6

10.487

11.12

19


Energy & Fuels

It can be seen that at the same oxygen concentration, using CO2 to replace N2 will lower the maximum value of H and OH in Table 3. This is because CO2 addition will affect the primary pathway for the chemical effect of CO2, i.e., CO 2 + H → CO + OH . The increase in CO2 prompts the forward reaction, which lowers the H concentration. Furthermore, reaction H + O 2 → O + OH also plays a vital role in combustion systems. The decreased H radical due to consumption by CO2 lowers OH production by H + O 2 → O + OH . The net change of OH formation rate due to the addition of CO2 depends on the relative importance of the two reactions.6,7 We also found that with the increase of oxygen concentration, the H and OH radical maximum values increase in both C2H4/(O2/N2) and C2H4/(O2/CO2) flames in Table 3. This is because increasing the oxygen concentration accelerates reaction H + O 2 ↔ O + OH . Figures 12-13 display the profiles of OH and H mole fraction at two axial heights above the burner exit. These two heights cover the main soot formation region in the studied flames. It can be found that the peak values of H and OH increase with increasing the O2 content in both C2H4/(O2/N2) and C2H4/(O2/CO2) flames. From Figure 13, we can also observe that the H mole fraction decreases when N2 is replaced by CO2. This is the same reason as that discussed in the last paragraph. As described above, we can conclude that soot formation in C2H4/(O2/CO2) flames is affected by the two primary reactions, which are CO2+HCO + OH and H+O2 O + OH. 0.010

0.4

0.012 0.2 0.1

0.020

0.0

z=4mm

0.015

-0.1 -0.2

0.010

0.010

0.006 0.004 0.002

0.006 0.000 0.004 -0.002

-0.3 0.005

0.008

0.008

z=4mm

H mole fraction

0.025

0.3

OH mole fraction

0.030


0.014


H mole fraction

0.035

OH mole fraction

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

Page 20 of 26

0.002

z=10mm

z=10mm -0.004

-0.4

0.000

0.000

-0.006

-0.5 0

1

2

3

4

5

0

6

1

2

3

4

5

6

r(mm)

r(mm)

Figure 12. Calculated OH mole fraction distributions from 21O2 to 50O2 at the flame height z=4,10 mm.

Figure 13. Calculated H mole fraction distributions from 21O2 to 50O2 at the flame height z=4,10 mm.

Figures 14-16 show the mole fraction of C2H2, C6H6, C16H10, respectively, at the two axial heights. Figures 14-16 give the consistent trend that with increasing the O2 mole fraction the peak mole fractions of C2H2, C6H6 and C16H10 all increase. In O2/N2 flames, the distributions of C2H2 and C6H6 mole fraction display a regular and consistent trend, and the peak value moves from the wing to the center of flame. However, in O2/CO2 flames, the peak values of these two species show a weakened 20


Page 21 of 26

regularity. From Figure16, it can be found that the peak value of A4 decreases with the increasing O2 at the flame height z = 4 mm, but the peak value of A4 increases with O2 at the flame height z = 10 mm.

0.04 0.02

0.14


z=10mm

0.12 0.10

0.00

0.08

-0.02

0.06

z=4mm

0.0002

A1 mole fraction

0.06

0.0004

0.04

-0.04

0.0008

0.0006

0.0000 0.0004

A1 mole fraction


C2H2 mole fraction

z=10mm

C2H2 mole fraction

0.0010

0.16

0.08

-0.0002 0.0002

z=4mm 0.02

-0.06

-0.0004 0.0000

0.00

-0.08 0

1

2

3

4

5

0

6

1

2

3

4

5

6

r(mm)

r(mm)

Figure 14. Calculated C2H2 mole fraction distributions

Figure 15. Calculated A1 (C6H6) mole fraction

from 21O2 to 50O2 at the flame height z=4,10 mm.

distributions from 21O2 to 50O2 at the flame height.

1.00E-007 2.40E-007


6.00E-008 4.00E-008 2.00E-008

2.00E-007

1.60E-007

1.20E-007 0.00E+000 8.00E-008

z=10mm

-2.00E-008

z=4mm

-4.00E-008

A4 mole fraction

8.00E-008

A4 mole fraction

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4.00E-008

-6.00E-008 0.00E+000 -8.00E-008 0

1

2

3

4

5

6

r(mm)

Figure 16. Calculated A4 (pyrene, C16H10) mole fraction distributions from 21% O2 to 50% O2 at the flame height z = 4 and10 mm.

It should be pointed out that the lower concentration of radical H in the lower region of the CO2 addition flame is also a reason for the lower pyrene concentration there. This is because pyrene is closely related to A1. Figure 15 illustrates the profile of A1, they give a basic and consistent regularity. The lower concentrations of radical H and A1 cause the lower formation rate of pyrene (as Figure16 shown), due to the HACA growth mechanism of PAH. Therefore, the decreasing H decreases both the inception rate and surface growth rate in O2/CO2 flames.

5. CONCLUSIONS Experimental measurements and numerical calculations were conducted in laminar diffusion ethylene flames burning in O2/N2 and O2/CO2 atmospheres with the O2 mole fraction in oxidizer varied from 21% to 50% to investigate how oxygen-enrichment and replacement of N2 by CO2 affect temperature and soot formation. The fuel and oxidizer flow rates are maintained constant in all the 21


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experiments and simulations. The two-color method using the R and G spectral band measurements based on the response spectrum of R, G, and B bands of a color 3-CCD camera was applied. The simulated results were obtained using the ABF mechanism and a soot model consisting of inception by collision of two pyrene molecules and HACA and pyrene condensation for surface growth. Numerical results reproduced the main features of the experimental results and observations. However, the numerical simulation overpredicts soot volume fraction in oxygen-enriched flames. The following conclusions can be drawn based on the experimental and numerical results: 1. Increasing the oxygen mole fraction in the oxidizer stream yields brighter and shorter flames and higher flame temperatures, and moves the peak temperature and soot volume fraction zones from the flame wing to the top center. 2. Soot volume fraction is increased rapidly with increasing oxygen in both the O2/N2 and O2/CO2 atmospheres. The model overpredicts the maximum soot volume fraction in oxygen enrichment flames in comparison to measurements. 3. CO2 is effective to suppress soot formation at the O2 concentration from 21% up to 50% under the present conditions mainly due to its thermal effect and additional chemical effect. 4. At the same oxygen mole fraction in the oxidizer, the flame height and soot volume fraction burning in O2/N2 atmosphere are always shorter and higher, respectively, than those in O2/CO2 atmosphere. 5. The replacement of N2 in oxidizer by CO2 lowers the concentrations of critical soot formation species, including H, C2H2, C6H6 and C16H10. 6. The primary pathway for the chemical effect of CO2 is its competition for H radical to form CO and OH, i.e., CO2+HCO +OH. Soot formation in C2H4/(O2/CO2) flames is affected by the two reactions, namely CO2+HCO + OH and H+O2 O + OH.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (F. Liu). Tel.: +1-613-993-9470. *E-mail: [email protected] (Chun Lou). Tel.: +86-27-8754-5526.

ORCID Yindi Zhang: 0000-0003-2355-1537 Fengshan Liu: 0000-0002-1790-6381 Chun Lou: 0000-0003-3302-7210

Notes The authors declare no competing financial interest. 22


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ACKNOWLEDGMENT The authors acknowledge the financial supports of the National Overseas Study Foundation of China, the National Natural Science Foundation of China (Nos.51306022, 51676078 and 51176059), the Science and Technology Innovation Foundation of PetroChina (No. 2015D-5006-0603) and the Yangtze Youth Talents Fund (No. 2015cqt01). The authors also acknowledge the help from the Black Carbon Metrology Research Group, Measurement Science and Standards, National Research Council Canada.

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Experimental and Numerical Investigations of Soot Formation in

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