air flames in ψ-shaped mesoscale combustors

Fuel 241 (2019) 138–154

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Full Length Article

Experimental study on soot formation, evolution and characteristics of diffusion ethylene/air flames in ψ-shaped mesoscale combustors

T

Mingfei Chena,b, Dong Liua,b, , Yaoyao Yinga,b, Kai Leia,b, Minye Luoa,b, Guannan Liua,b, Rui Zhanga,b, Bo Jianga,b ⁎

a

MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China b Advanced Combustion Laboratory, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Soot Mesoscale combustor Characteristics Reactivity Scale effect

Soot formation, evolution and characteristics of diffusion ethylene/air flames in ψ-shaped mesoscale combustors of two different diameters with the variations of excess air ratio and flow rate were experimentally investigated. The variation in nanostructure and oxidation reactivity of soot was compared based on the results of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Results demonstrated that with an increasing excess air ratio and flow rate, the unventilated flames with bifurcated shapes were observed in both types of combustors due to the deteriorated mixing process in large flow velocity. Different effects on characteristics of soot from two combustors with the same variation of flow rate were found. For the variation in ethylene flow rate of 60–100 ml/min at excess air ratio of 0.5, the oxidation reactivity of soot from the combustor with d = 4 mm first decreased and then slightly increased, while it decreased all the time as for the soot from the combustor with d = 6 mm. Moreover, the significant distinctions in soot nanostructure due to the scale effect were observed. The soot from the combustor with d = 4 mm exhibited the partial amorphous structure aggregated by a large amount of PAHs with high reactivity. Whereas among the

⁎ Corresponding author at: MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China. E-mail address: [email protected] (D. Liu).

https://doi.org/10.1016/j.fuel.2018.12.023 Received 24 July 2018; Received in revised form 13 November 2018; Accepted 7 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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soot from the combustor with d = 6 mm, a typical fullerene-like structure which represented the simultaneous existence of PAHs and graphitic parts was found. The soot graphitization degree and production increased notably with the enlargement of combustor size because the higher combustion temperature and longer residence time were simultaneously obtained, which both were beneficial to soot formation and growth rate. Significantly, the lower combustion efficiency was obtained in the combustor of d = 6 mm than 4 mm at α = 0.5 because of the larger soot production and more existence of unburned gas. But the higher combustion efficiency was found for the 6 mm than 4 mm at α = 1 in the case of almost no soot generation, which was due to the longer residence time.

1. Introduction

blow-off limit in a planar micro-combustor were numerically compared [18]. The results showed that the blow-off limit using the triangular bluff body was less than semicircular bluff body duo to the flame stretching variation. In the past few years, some studies on the soot formation characteristics in micro or mesoscale combustion were also conducted [19–21]. Dubey et al. [19] experimentally investigated the different sooting behaviors among methane, ethane, propane and n-butane flames in a micro flow reactor. They observed four kinds of flames and soot responses namely FREI, flame, flame with soot and soot, which were similar to the results reported by Nakamura et al. [20]. Nakamura et al. [20] found that the sooting limits depended on the equivalence ratio and residence time. More soot particles could be formed with the high equivalence ratio and low flow velocity for the constant temperature profile. Similar conclusions were also obtained by Dubey et al. [21]. In that study [21], soot was only observed in the flat temperature region at 1300 K, which implied that the high temperature was also a key factor of soot formation in micro or mesoscale combustion. However, these studies above focused on the micro or mesoscale premixed combustion and there were very few researches on detailed characteristics of soot formed in micro or mesoscale diffusion flames. It is well known that soot is the product of incompletely combustion of hydrocarbon fuels and does harm to human health and environment [22]. The increasingly demand for the reduction of soot emission caused the extensive researches on the formation process and physicochemical characteristics of soot formed in various conditions such as premixed flat flames [23,24], co-flow diffusion flames [25–27], counterflow diffusion flames [28,29]. At present, the consensus amongst the most researchers is that the benzene and PAH (polycyclic aromatic hydrocarbons) are important precursors during the soot formation process [30,31] but there are still many controversies on its specific mechanism [32]. Furthermore, in order to increase the combustion efficiency and decrease the pollutant emission of combustion-based micro power equipment, it is necessary to conduct some researches on the formation and characteristic of soot from the combustion process

Micro-combustion is a new research field of combustion community with a history of just more than ten years. The development of microcombustion has a great significance on not only the combustion fundamentals but also the engineering purposes [1]. Due to the much higher energy densities of hydrocarbon fuels than conventional batteries, the combustion-based micro power generator is considered as an appropriate power source of the micro-electromechanical system (MEMS) and micro-power devices [2]. Therefore, the combustion process occurred in a small channel has attracted extensive attentions in recent years. Compared to the combustion process in conventional macroscale, the micro combustion has its unique adversities such as the shortened residence time of the fuel/oxidant mixture and the increased heat loss ratio, which result in many fantastic flame structures with dynamic behaviors [2–11] and the researches of stabilization method [12–18]. For instance, the flame with repetitive extinction and ignition (FREI) and cyclic oscillatory motions were experimentally observed by Maruta et al. [3] in a quartz channel of d = 2 mm and then were theoretically analyzed [4]. Xu et al. [2] observed the “flame street” composed of multiple flamelets at mesoscale diffusion flames with comparatively low mixture flow velocity and high wall temperature. Kim et al. study designed a new “Swiss-roll” combustor which could utilize the recirculated heat of exhausted gas and it was demonstrated that the flame stabilization was significantly improved. In addition, Li et al. [14] applied the combined effects of catalyst segmentation and cavities to enhance combustion efficiency in a microchannel. Recently, Ning et al. [16] experimentally studied the impact of fibrous porous media addition on diffusion ethylene/air flames in Y-shaped mesoscale combustors. The results demonstrated that the flame stability and flammability were notably improved by the application of fiber porous media. Fan et al. [17] experimentally investigated the combustion characteristics of lean hydrogen/air mixture in a planar micro-channel with/without bluff body. Furthermore, the effects of various bluff body shapes on the

Fig. 1. Schematic diagram of experimental set up. 139

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mesoscale combustor which had already been applied to non-premixed mesoscale combustion in previous study [16]. For obtaining a better mixing effect of fuel and oxidant and more stable combustion in small scale, another channel of transporting oxidant is designed. In this study, the effects of various factors such as the excess air ratio, inlet flow rate and combustor diameter (scale effect) are taken into account. Special attentions are focused on the flame structures, combustion temperatures, soot productions, soot formation characteristics, soot nanostructure, soot reactivity and exhaust gases analyses. The variation in nanostructure and oxidation reactivity of soot was compared based on the results of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA).

Table 1 Flame conditions for the ψ-shaped mesoscale combustors. Flame conditions

Combustor channel diameter

Excess air ratio α

D4-80-0.4 D4-80-0.5 D4-80-0.6 D4-80-0.7 D4-60-0.5 D4-100-0.5

d = 4 mm

D6-80-0.4 D6-80-0.5 D6-80-0.6 D6-80-0.7 D6-60-0.5 D6-100-0.5

d = 6 mm

Gas flow rate (ml/min)

Mean flow velocity (m/ s)

C2H4

Air

0.4 0.5 0.6 0.7 0.5 0.5

80 80 80 80 60 100

457.0 571.2 685.4 799.7 428.4 714.0

0.71 0.86 1.02 1.17 0.65 1.08

0.4 0.5 0.6 0.7 0.5 0.5

80 80 80 80 60 100

457.0 571.2 685.4 799.7 428.4 714.0

0.30 0.38 0.45 0.52 0.29 0.48

2. Experimental methodologies 2.1. Experimental set up and flame conditions

occurred in a small scale. This work attempts to conduct a comprehensive investigation on detailed soot physicochemical properties and exhaust gases analyses in non-premixed mesoscale combustion. It not only provides the additional information on soot from a new formation condition (limited small place) but also can contribute a new research direction and diagnostic method in micro or mesoscale combustion subject which is mainly focused on flammable limits [2–11], combustion stabilizations [12–18], new combustor designs [12,33,34], models and simulations [35–37], combustion characteristics of new liquid fuels [38,39] currently. Besides that, it is worth mentioning that a novel design of ψshaped combustor is utilized to obtain mesoscale diffusion flames in the present study. This kind of combustor is similar with the Y-shaped

As shown in Fig. 1, the experimental system is composed of ψshaped mesoscale combustor, soot collection system, mass flow controllers (Sevenstar, CS200A), CCD camera, air and fuel. Fuel is ethylene with purity of 99.95% (Nanjing Gases). The ψ-shaped mesoscale combustor consists of three inlet channels and a straight channel, corresponding to the fuel steams (ethylene) ejected from the middle inlet channel, the oxidizer streams (air) ejected from the bilateral inlet channels. The combustor is made of quartz glass with a wall thickness of 1 mm. Two different channel inner diameters (d = 4 mm and 6 mm) are selected to investigate the scale effect of the combustor. The inner diameters and wall thicknesses of the inlet channels and the straight channel are kept same. The more detailed geometric dimensions of ψ-shaped mesoscale combustor are shown in

Fig. 2. Images of diffusion flames in the combustor of d = 4 mm at various inlet flow rates and excess air ratios. 140

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Fig. 3. Images of diffusion flames in the combustor of d = 6 mm at various inlet flow rates and excess air ratios.

Fig. 4. Flame temperature profiles for the variation of excess air ratios at QC2H4 = 80 ml/min in the combustors of (a) d = 4 mm and (b) d = 6 mm, respectively.

Fig. S1. As shown in Fig. 1, a dismountable soot collection system was linked to the outlet of the straight channel. The soot collection system was composed of a replaceable quartz fiber filter (50 mm diameter and 0.7 μm pore size) and an openable filter holder. To simultaneously guarantee enough soot captured and prevent the blockage of the whole system, the quartz filter was replaced with a new one after 10 min of collection. The soot particles were then peeled off from the filter for the subsequent analyses. This collection method was similar with the previous study [40]. Besides that, the exhaust gases was also sampled at the outlet of collection system by a gas collecting bag (E-Switch 1L) and analyzed by a gas chromatography (GC) (Agilent 7890B). During the whole sampling process, no obvious disturbance of the flame was

observed. The detailed experimental conditions were summarized in Table 1. According to the previous research [19,20], the excess air ratio and inlet flow rate both affected soot formation and combustion characteristic, thus the experimental conditions were classified into the independent variation of the two parameters in the present study. Here, the excess air ratio was defined as the ratio of actual air supply to the theoretical stoichiometric air flow rate [16]. To investigate the influence of excess air ratio, the values of excess air ratios (α) were set as 0.4, 0.5, 0.6 and 0.7 at a fixed ethylene flow rate (QC2H4) value of 80 ml/ min. And three different ethylene flow rates of 60 ml/min, 80 ml/min and 100 ml/min at the same α = 0.5 were selected to determine the effect of flow rate. The above working condition were examined and 141

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Fig. 5. Flame temperature profiles for the variation of flow rate at α = 0.5 in the combustors of (a) d = 4 mm and (b) d = 6 mm, respectively.

Fig. 6. Flame peak temperatures: (a) For the variation of excess air ratio at QC2H4 = 80 ml/min; (b) for the variation of ethylene flow rate at α = 0.5.

tested in both types of combustors with channels of d = 4 mm and 6 mm, respectively. Furthermore, the mean mixture flow velocities were calculated by the formula vmix = 4(QC2H4 + QAir)/πd2 and the experimental conditions here were expressed as abbreviations for convenience. For example, the experimental condition for combustor channel diameter of 4 mm, ethylene flow rate of 80 ml/min and excess air ratio of 0.5 (d = 4 mm, QC2H4 = 80 ml/min, α = 0.5) was abbreviated as D4-80-0.5. The cold-state velocity fields and C2H4 concentration distributions of some representative experimental conditions were shown in Figs. S2–S11.

E (T )= ( )

C1 5

exp

C2 T

(1)

2

where Eλ (W·m ) is the radiative energy, ε is the emissivity, λ (m) is the wavelength, C1 and C2 are the Planck constants with a value of 3.742 × 10−16 W·m2 and 1.4388 × 10−2 m·K, respectively. T (K) is the temperature. So the monochromatic radiative intensity can be expressed as the following formula,

I (T )=

1

( ,T )

C1 5

C2 T

exp

(2)

According to chromatic theory, the flame images photographed by a digital single lens reflex (DLSR) camera can be represented as three primary colors: red (R), green (G) and blue (B) with their respective wavelength (λr = 7.00 × 10−7 m, λg = 5.46 × 10−7 m, λb = 4.35 × 10−7 m). Coefficients Kr, Kg and Kb were used for the calibration of three primary colors R, G and B. From equation (2), we have:

2.2. Flame temperature measurement Since it was quite difficult and rough to measure flame temperatures using thermocouples due to such small channel size, a two-color temperature measurement method was adopted in this study. This method was widely applied for temperature measurement in the combustion system [41–44]. It should be noted that the temperature distribution measured here should be the flame surface temperatures rather than inner temperatures according to the two-color method principle, but these temperatures were still useful to provide the temperature levels and compare different combustion processes for different flame conditions based on the same standard. Some important aspects of the twocolor temperature measurement method were given below. In most combustion process, the wavelength of flame radiation is between 300 nm and 1000 nm and the corresponding combustion temperature ranges from 800 K to 2000 K [41]. In this situation, Planck’s law of radiation can be substituted by Wien’s law as follows,

I r (T )= Kr R =

I g (T )= K g G =

I b (T ) = K b B =

1

1

1

( r ,T )

C1 5 r

( g ,T )

( b,T )

exp

C1

5 exp g

C1 5 b

exp

C2 rT

(3)

C2 gT

(4)

C2 bT

(5)

where R, G and B refer to the intense values of red, green and blue in each pixel, respectively. Therefore, the flame temperature can be 142

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Fig. 7. Soot response to excess air ratio and mass flow rate between the combustors: (a) In the combustor of d = 4 mm; (b) in the combustor of d = 6 mm.

obtained by calculating the ratio of the monochromatic radiative intensity at two different and adjacent wavelengths. In this experiment, red (R) and green (G) were selected for the calculation. Furthermore, in the consideration of the high visible light transmittance of quartz glass (≥90%) [45], it is reasonable that the influence of the channel wall on flame radiative intensity can be negligible. Generally, the emissivity of soot with high temperature can be approximately considered to be similar due to the narrow range of flame temperature and wavelength variation [42]. In combination of equation (3) and (4), the combustion temperature expression can be deduced as follows:

T=

C2

1

1

r

g

/In

Kr R Kg G

5 r 5 g

Fig. 8. Average weights of soot for different parameters variation in both types of combustors: (a) variation of excess air ratio at QC2H4 = 80 ml/min; (b) variation of ethylene flow rate at α = 0.5.

2.3. Soot characterizations In this study, the morphologies and nanostructures of soot samples were analyzed by an FEI Titan G2 60-300 transmission electron microscope with a magnification of 75,000× and 1,000,000×, respectively. For structural analyses, the soot samples peeled off from the filter were ultrasonically dissolved in ethanol liquid for 30 min, and one or two drops of the suspension was then dropped to the carbon film coated lacey grids (200-mesh). This approach was same with the previous research [25]. According to the definition in [48], fringe length was a measurement of the physical size of the lattice fringe, and fringe tortuosity was a value of fringes curvature, which expressed as the ratio of the fringe length to its straight-line distance within carbon atom. In this study, the crystallite structures of soot, including fringe length and fringe tortuosity, were calculated through lattice fringe analysis of quantifying HRTEM images. The process was completed by means of MATLAB software using the algorithms in previous literature [48,49]. The graphitization degree of soot was evaluated by X-ray diffraction. In this study, a D8 Advance X-ray diffractometer (Cu-Ka source, Bruker, Karlsruhe, Germany) was utilized to scan the soot samples with the 2θ range of 10−70°. The step size was 0.05 and scan speed was 0.2 s/step. To evaluate the soot oxidation reactivity, an STA 449 F3 Jupiter thermogravimetric analyzer from NETZSCH with recording software was utilized in the study. Before the isothermal test, the soot sample

(6)

where Kr/Kg is the only unknown parameter that needs to be measured, which can be given in equation (7):

Kr = Kg

5 g 5 r

G exp R

C2 (1/

r

T

1/ g ) (7)

In this study, the value of Kr/Kg was calibrated by using a block furnace. To be specific, it can be calculated by the known block furnace temperature and the corresponding radiation images’ RGB value obtained by MATLAB software. Thus the flame temperature profiles were achieved by this method. Furthermore, DLSR camera need to be set the appropriate parameters to prevent the flame images overexposure. The two color method mainly included theoretical error and random error because of the emissivity assumption and camera noise, respectively. To minimize the noise of camera, every experiment was repeated for three times. According to the examination results in our previous research [46,47], the absolute error of this method was less than 30 K. It was hard to examine the error by thermocouples in the present study due to the operated difficulty but it was reasonable to conduct some comparisons based on the same standard. 143

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Fig. 9. Representative TEM images of soot for different excess air ratios at QC2H4 = 80 ml/min in both types of combustors: (a) D4-80-0.4; (b) D4-80-0.5; (c) D4-800.6; (d) D6-80-0.4; (e) D6-80-0.5; (f) D6-80-0.6.

with initial loading of 5.0 ± 0.2 mg was spread evenly in a quartz crucible. During the test, the samples were initially heated up in the atmosphere of Ar with flow rate of 100 ml/min from 30 °C to 300 °C. The temperature was then remained constant for an hour to remove volatile organic fraction (VOF) content among soot samples [29,50]. Subsequently, the soot samples continued to be heated from 300 °C to 500 °C at a rate of 10 °C/min. After that, the atmosphere of Ar flow was replaced with 22% O2 and 78% Ar of total flow rate 100 ml/min. Meanwhile, the soot sample was oxidized and the isothermal oxidation process lasted for 150 min. Finally, the mass loss of each sample during isothermal oxidation was normalized by calculating the formula (mtmf)/(mi-mf) × 100%, where mi was the initial mass of soot sample in the beginning of oxidation, mt was the residue mass at the current time during the oxidation, mf was the ultimate mass after the oxidation. The comparison between each normalized TG curve of soot sample was to determine oxidation reactivity. According to our previous examination [50–52], the result uncertainty was ± 4.7% with 95% accuracy. Every experiment was repeated two times to ensure the reproducibility of the results.

C2H2, H2, CO, CO2 and N2) mole fractions of the exhaust gases were YCH4, YC2H4, YC2H2, YH2, YCO, YCO2 and YN2 respectively. HCH4, HC2H4, HC2H2, HH2 and HCO were the low heat values of CH4, C2H4, C2H2, H2 and CO, respectively, KJ/m3. The calculated efficiency might be a little higher than actual result due to the neglect of heat value of soot and other hydrocarbons, but their content were quite small compared with the total amount. It was safe and reasonable to conduct some qualitative comparisons in the present study. 3. Results and discussion 3.1. Flame structures Figs. 2 and 3 illustrated the flame structures for all experimental conditions in both types of combustors. The flame images were taken by a CCD (charge coupled device) camera with the exposure time of 22,000 and the gain value of 5. The combustion process occurred in the burner was non-premixed because the fuel and oxidizer were transported in separated channels before the flame formation. Results demonstrated that with the increase of the inlet flow rate and excess air ratio, the flame structure was gradually deformed from center symmetry to bifurcated shape. As shown in Figs. 2 and 3, the unventilated flame with irregular shape especially occurred in the cases of large excess air ratio (α = 0.7). Besides that, it was also more obvious in the higher ethylene flow rate of QC2H4 = 100 ml/min than 80 ml/min at α = 0.6, which could be attributed to the large disparity in flow rates between ethylene and air. The mixing process became more deteriorated at a larger inlet velocity and thus affected the flame structures, which could be further explained as follows. For diffusion flames, the mixing process is mainly determined by Peclet number (Pem), which can be approximately calculated according to the following formulas [54,55],

2.4. Exhaust gases analyses To obtain more information on soot formation and combustion characteristic in non-premixed mesoscale combustion, the composition of the exhaust gases between the two combustors were analyzed by a GC (95% accuracy) at several representative conditions. A standard calibration gas was used for the quantification of the main components. Furthermore, the combustion efficiency ηc could be approximately evaluated as follows [53]: c

= 1

Heat value of unburned components Total heat value of input fuel

(QC 2H 4 =

(8)

Pem =

Qgas Ygas )HC 2H 4

Q gas (YCH 4 HCH 4 + YC 2H 2 HC 2H 2 + YH 2 HH 2 + YCO HCO ) QC 2H 4 HC 2H 4

diff

where QC2H4 was the flow rate of ethylene, Qgas was the flow rate of exhaust gas (removed water vapor), which could be approximately calculated by mass balance of N2. Some main components (CH4, C2H4,

conv

144

= =

diff conv 2 ldiff

(9)

D

(10)

L U

(11)

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Fig. 10. Representative HRTEM and the corresponding extracted skeleton images of soot for the variation of excess air ratio at QC2H4 = 80 ml/min in both types of combustors: (a) D4-80-0.4; (b) D4-80-0.5; (c) D4-80-0.6; (d) D6-80-0.4; (e) D6-80-0.5; (f) D6-80-0.6.

where τdiff and τconv, refer to the characteristics time of molecular diffusive mixing and convective mixing, respectively. ldiff is the characteristic scale of molecular diffusion in the straight channel. D is the molecular diffusion coefficient. L is the characteristic combustor length and U is the mean flow velocity of the channel. In general, low Peclet number signifies uniform mixing process because the diffusion mixing time is much less than the convective mixing time, which represents that the concentration gradient cannot be maintained for a long time and the flow field will be soon fully mixed [55]. Now, it is evident that the Peclet number is proportional to the flow velocity and thus the mixing effect becomes worse at a larger inlet flow velocity, which results in the observation of many unventilated flames with irregular structures. Moreover, by comparing Fig. 2 with Fig. 3, it could be obviously found that the larger and brighter flames can be obtained in the

combustor of d = 6 mm than d = 4 mm, which implied the higher combustion temperatures and larger soot production. The quantitative results about the flame temperatures and mass of formed soot will be given in the Sections 3.2 and 3.3, respectively. 3.2. Flame temperature profiles The flame temperature profiles for the excess air ratio variation of 0.4–0.7 at QC2H4 = 80 ml/min in both types of combustors were shown in Fig. 4. As the excess air ratio increased, the overall temperature distribution increased at first and the high temperature zone approximately reached a constant value at QC2H4 = 80 ml/min, α = 0.5 for different excess air ratios in the combustor of d = 4 mm. Whereas the temperature always increased but the growth rate slowed down in the combustor of d = 6 mm with the same excess air ratio variation. It 145

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Fig. 11. Representative TEM images of soot particles for the variation of flow rate at α = 0.5 in both types of combustors: (a) D4-60-0.5; (b) D4-80-0.5; (c) D4-1000.5; (d) D6-60-0.5; (e) D6-80-0.5; (f) D6-100-0.5.

3.3. Soot formation characteristic

could be explained as follows: in the beginning, the higher temperature profiles were found as the excess air ratio increased, which was due to the participation of more fuel and larger heat release amount. Subsequently, there was a large amount of unburned mixture and the maximum heat release amount was limited with the continued increasing excess air ratio, because the restricted space worsened the mixing process and confined the reaction zone. Besides that, larger heat amount was taken away by the increasing flow velocity. These detrimental factors both impeded the temperature rising. For the same flow rate of ethylene and air, the enlargement of combustor corresponded to a larger maximum heat release amount and less unburned mixture due to the extended reaction zone and the reduced mixture flow velocity, which resulted in the higher attainable temperature in larger combustor. The flame temperature profiles for the ethylene flow rate variation of 60–100 ml/min at α = 0.5 in two combustors were shown in Fig. 5. Different temperature altering tendency between the two combustors could also be observed. The whole temperature profiles in the combustor of d = 4 mm were almost the same for the ethylene flow rate of 80 and 100 ml/min at α = 0.5 but it slightly increased with the same variation in the combustor of d = 6 mm, which was similar to the condition of the excess air ratio augment. This inconsistent temperature variation between both types of combustors was also a result of the different reaction zone and mixing effect as explained before. Moreover, the peak temperatures were all located in the center of flame and the corresponding values were showed in Fig. 6. The error bar in Fig. 6 was the standard deviation of our multiple experimental data. According to Fig. 6 and the aforementioned overall temperature distribution, it was noted that the obviously higher peak temperatures were obtained in the larger combustor, which could be attributed to the two main reasons. The first one was that the reduction of combustor size led to a lager surface-area-to volume ratio, which implied the increased heat-loss ratio (heat loss to the ambient/heat release in combustion) [16]. The other one was that the less heat dissipation could be achieved due to the reduction of flow velocity. Considering these two aspects, the higher combustion temperatures could be achieved with the enlargement of combustor size.

In the experiment of soot collection process, it was found that soot particles could only be collected on the quartz fiber filter for certain conditions, which were plotted on excess air ratio-ethylene flow rate plane, as summarized in Fig. 7. In this figure, the red dot, blue block and “X” point represented that much, little and almost none soot could be collected within 10 min, respectively. The dividing amount between “much” and “little” was 0.5 mg in this study. As shown in Fig. 7(a), the wider excess air ratio ranges of much soot (red dot in the figure) were observed with the increase of ethylene flow rate. For instance, much soot can be produced at α = 0.5–0.6 when the ethylene flow rate was 40 ml/min but the excess air ratio range extended to 0.3–0.6 at QC2H4 = 100 ml/min. Besides that, the larger limit of collectible soot (red dot and blue block in the figure) can be found in the combustor of d = 6 mm than d = 4 mm. To be specific, for the ethylene flow rate of 10–100 ml/min, soot couldn’t be collected on the quartz fiber filter in the combustor of d = 4 mm when the excess air ratio was larger than 0.7. But in Fig. 7(b), there was almost none soot being collected at α > 0.8 in the combustor of d = 6 mm for the same range of ethylene flow rate. Several reasons may be responsible for this phenomenon. First of all, the mixture residence time obviously decreased as the excess air ratio increased during the mesoscale combustion process, which allowed less time for soot inception and growth [56]. Secondly, the generated primary soot particles were oxidized due to the high oxygen contents and temperatures. Thus the soot couldn’t be collected by the filter even though it was formed in flames when the excess air ratio exceeded a certain value. In the case of the same ethylene flow rate and excess air ratio, the enlargement of combustor size led to a longer residence time and higher combustion temperature (according to the result in Section 3.2), both of which can accelerate the soot growth rate and contribute to the soot formation [56]. Meanwhile, the better mixing process due to the reduction of flow velocity resulted in a decreasing content of unburned oxygen and thus the less soot particles were oxidized. In consequence, the larger limit of collectible soot was obtained in the larger combustor. Fig. 8(a) depicted the average weight of collected soot versus the excess air ratio at QC2H4 = 80 ml/min and Fig. 8(b) showed the average weight versus the ethylene flow rate at α = 0.5, for both types of combustors within 10 min. As seen in Fig. 8(a), for the variation of 146

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Fig. 12. Representative HRTEM and the corresponding extracted skeleton images of soot for the variation of flow rate at α = 0.5 in both types of combustors: (a) D460-0.5; (b) D4-80-0.5; (c) D4-100-0.5; (d) D6-60-0.5; (e) D6-80-0.5; (f) D6-100-0.5.

excess air ratio from 0.3 to 0.6 at QC2H4 = 80 ml/min, the soot productions increased and then declined for both types of combustors, which were consistent with the results in Fig. 7. Similar to the previous explanations, more fuel participated in combustion as the excess air ratio increased at first, which caused a gradual increase in soot production. Subsequently, a part of the generated soot particles were oxidized by the excess oxygen when most fuels were consumed, which reduced the mass of collected soot. As shown in Fig. 8(b), for the variation of ethylene flow rate of 40–100 ml/min at α = 0.5, the mass of soot increased with the ethylene flow rate for both types of combustors, because of a growing number of carbons participating in the combustion process. Furthermore, significantly more soot production were achieved in the combustor of d = 6 mm than that in the combustor of d = 4 mm for the same flow rate, because when the combustor size increased, the lager reaction

zone, higher combustion temperature and longer residence time can be simultaneously achieved which all were beneficial to the soot formation and growth rate. As a result, the soot production made a distinct difference for these two types of combustors although the fuel flow rate was identical in the combustion process. 3.4. Soot nanostructure Representative TEM images of the soot for different excess air ratios at QC2H4 = 80 ml/min between the two combustors were illustrated in Fig. 9. The most soot exhibited the appearance of chain-like or tufted aggregates composed of hundreds of monomers or spherules with vague margin. This kind of structure was not obvious in Fig. 9(e) and (f) because of the close aggregation of soot particles. Besides that, there were no obvious differences by visual observations of soot for various excess 147

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parameters with standard deviations were listed in Table 2. Fig. 13 depicted the fringe length histograms by the HRTEM image analysis algorithms in all tested conditions. The fringe lengths all primarily concentrated in the range of 0.6–0.8 nm but with the different distribution proportion. Among Fig. 13(a)-(c) and (g)-(i), the peak range of fringe length (0.6–0.8 nm) accounts for approximately 40% and few fringe length exceed 2 mm for soot from the combustor of d = 4 mm. However, among Fig. 13(d)-(f) and (j)-(l), regarding the soot from the combustor of d = 6 mm, there were still a small amount of fringes longer than 2 mm with a peak range of about 30%. In addition, the fringe length of soot from larger combustor significantly had wider distributions. The rankings of mean value of fringe length for the variation of excess air ratio and flow rate corresponded to D6-80-0.6 (0.935) > D6-80-0.5 (0.926) > D6-80-0.4 (0.889) > D4-80-0.6 (0.828) > D4-80-0.5 (0.825) > D4-80-0.4 (0.802), and D6-100-0.5 (0.944) > D6-80-0.5 (0.925) > D6-60-0.5 (0.875) > D4-80-0.5 (0.825) > D4-100-0.5 (0.807) ≈ D4-60-0.5 (0.806), respectively. Fig. 14 depicted the fringe tortuosity histograms for all experimental conditions. The fringe tortuosity mainly concentrated in the range of 1.0–1.2, which were the short side for overall distribution. Among Fig. 14(a)-(c) and (g)-(i), the fringe tortuosity peak range (1.10–1.15) accounts for about 15% in the case of soot from the combustor of d = 4 mm. Whereas regarding the soot from combustor of d = 6 mm, the fringe tortuosity had more centralized distribution with a peak range of more than 20%. The mean value of the fringe tortuosity for the variation of excess air ratio and flow rate showed a sequence of D6-80-0.6 (1.252) < D6-80-0.4 (1.255) < D6-80-0.5 (1.275) < D480-0.6 (1.360) < D4-80-0.5 (1.379) < D4-80-0.4 (1.397) and D6100-0.5 (1.268) < D6-80-0.5 (1.275) < D6-60-0.5 (1.302) < D4-800.5 (1.379) < D4-100-0.5 (1.383) ≈ D4-60-0.5 (1.383), respectively. According to the result listed in Table 2, it could be found that the rank tendency of the fringe length and fringe tortuosity was generally opposite. Larger fringe lengths were usually associated with the smaller fringe tortuosity, which was consistent with the previous study [50–52]. Soot with large fringe tortuosity prevented developments of stacked layers and expressed the fast oxidation reactivity, which could be attributed to the curvature of five-membered rings weaken C–C bonds with an increasing of sp3 character [59]. As shown in Table 2, in addition to the distinction in fringe length and fringe tortuosity of soot from different combustors, it was noted that the fringe length and fringe tortuosity varied differently for the same flow rate variation. To be specific, regarding the soot from the combustor of d = 6 mm, the mean fringe length increased from 0.875 to 0.944 and the fringe tortuosity decreased from 1.302 to 1.268 as the ethylene flow rate increased from 60 ml/min to 100 ml/min at α = 0.5. However, the fringe length first increased from 0.806 to 0.825 and then dropped to 0.807, the fringe tortuosity decreased from 1.383 to 1.379 and then rose to 1.383 with the same flow rate variation for the soot from the combustor of d = 4 mm. This inconsistent tendency between the different combustors was also found in the TGA results, which will be introduced in the Section 3.6.

Table 2 Mean values of fringe length and fringe tortuosity with standard deviations across all experimental conditions. Flame condition

Fringe length (nm)

Fringe tortuosity

D4-80-0.4 D4-80-0.5 D4-80-0.6 D6-80-0.4 D6-80-0.5 D6-80-0.6

0.802 0.825 0.828 0.889 0.926 0.935

± ± ± ± ± ±

0.029 0.010 0.028 0.028 0.020 0.014

1.397 1.379 1.360 1.255 1.275 1.252

± ± ± ± ± ±

0.026 0.008 0.019 0.021 0.031 0.013

D4-60-0.5 D4-80-0.5 D4-100-0.5 D6-60-0.5 D6-80-0.5 D6-100-0.5

0.806 0.825 0.807 0.875 0.925 0.944

± ± ± ± ± ±

0.020 0.010 0.014 0.007 0.020 0.028

1.383 1.379 1.383 1.302 1.275 1.268

± ± ± ± ± ±

0.025 0.008 0.029 0.015 0.031 0.031

air ratios in the same combustor. To further explore soot structural characteristics, HRTEM images and the corresponding extracted skeleton images were shown in Fig. 10. The soot nanostructures were composed of carbon lamellae with various fringe orientations. The parallel fringes showed the existence of graphitization segments, while the curved fringes represented a large amount of PAHs [57]. There were no significant differences in HRTEM images for different excess air ratios in the same combustor. However, notable distinctions could be found between the two combustors. The soot from the combustor of d = 4 mm exhibited the partial amorphous structure with much short and disordered lamellae while the soot from the combustor of d = 6 mm showed a typical fullerene-like structure composed of much curved lamellae with a small radii, suggesting the simultaneous existence of graphitic parts and PAHs [58]. The soot carbonization degree increased notably with the enlargement of combustor size because the higher combustion temperature and longer residence time were simultaneously obtained, which both were beneficial to the formation of soot with higher maturity. The HRTEM results well illustrated the soot nanostructure variation for the scale effect of combustor. Representative TEM morphology images of the soot particles for the variation of ethylene flow rate at α = 0.5 were shown in Fig. 11. Among Fig. 11(a)-(c), soot produced under small and large flow rate (QC2H4 = 60 ml/min and 100 ml/min, respectively) in the combustor of d = 4 mm appeared to be film-like overlayer with an irregular shape and a vague margin while the soot under moderate flow rate (QC2H4 = 80 ml/min) exhibited a chain-like structure. However, among Fig. 11(d)-(f), the soot samples all showed a chain-like structure for the same flow rate in the combustor of d = 6 mm, which indicated that the flow-induced difference in soot morphology images did not occur. This phenomenon was presumably due to the different temperature and residence time variation for the same growth amount of flow rate between two different combustors. More detailed discussion about the effect of flow rate will be introduced in Section 3.7. Fig. 12 exhibited the HRTEM and corresponding extracted skeleton images for different ethylene flow rates in both types of combustors. Similar with case of excess air ratio variation, the difference of soot nanostructure was mainly caused by the combustor size (scale effect) rather than flow rates. For the ethylene flow rate of 60–100 ml/min at α = 0.5, the soot samples from the combustor of d = 4 mm all appeared the partial amorphous structures similarly with results in Fig. 10, while the soot from the combustor of d = 6 mm seemed to be composed of more graphitic parts with extended and organized fringe. The results suggested that the soot nanostructure variation due to the scale effect was not influenced by different flow rates. To investigate the quantification information on HRTEM images in details, the further analysis of extracted skeleton images using statistical metric was carried out to acquire fringe parameters, including fringe length and fringe tortuosity. The mean values of fringe

3.5. Soot XRD analysis The X-ray patterns of soot samples were exhibited in Fig. 15 and the corresponding extracted peak diffraction angles were listed in Table 3. In Fig. 15(a), the deficiency of D4-80-0.6 curve was due to the relative low soot yield at this condition, which was difficult for soot to be peeled off from the filter and conduct the XRD test. A clearly major peak at ∼25° could be observed at all patterns, which was 002 reflection. The 002 reflection was an indicative of the existence of crystalline graphitic carbon [60] and its asymmetric characteristic represented the presence of aliphatic chain on PAH [61]. According to the diffraction angles variations listed in Table 3, for the conditions of an increasing excess air ratio (D4-80-0.4, D4-80-0.5 and D6-80-0.4, D6-80-0.5, D6-80-0.6), the soot diffraction peak shifted to right in both combustors, which 148

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Fig. 13. Fringe length distribution from the corresponding images under different flame conditions: (a) D4-80-0.4; (b) D4-80-0.5; (c) D4-80-0.6; (d) D6-80-0.4; (e) D6-80-0.5; (f) D6-80-0.6; (g) D4-60-0.5; (h) D4-80-0.5; (i) D4-100-0.5; (j) D6-60-0.5; (k) D6-80-0.5; (l) D6-100-0.5.

suggested that the augment of excess air ratio led to the higher soot graphitization degree. For the conditions of an increasing flow rate in the combustor of d = 4 mm (D4-60-0.5, D4-80-0.5 and D4-100-0.5), the soot diffraction peak first shifted right and then left slightly. However, for the conditions of the same flow rate variation in the combustor of d = 6 mm (D6-60-0.5, D6-80-0.5, D6-100-0.5), the diffraction peak shifted right all the time. Moreover, the peak diffraction angles obviously increased with the enlargement of combustor in the case of same excess air ratio and flow rate, which represented the enhancement

of soot graphitization degree. Meanwhile, a distinct difference in the peak intensity of soot from different combustors could be observed, which also confirmed the structural changes among soot samples [56]. 3.6. Soot reactivity analysis Figs. 16 and 17 exhibited the normalized mass loss curves of soot samples during the isothermal oxidation process. In Fig. 16, there was an absence of D4-80-0.6 mass loss curve because it was quite difficult to 149

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Fig. 14. Fringe tortuosity distribution from the corresponding images under different flame conditions: (a) D4-80-0.4; (b) D4-80-0.5; (c) D4-80-0.6; (d) D6-80-0.4; (e) D6-80-0.5; (f) D6-80-0.6; (g) D4-60-0.5; (h) D4-80-0.5; (i) D4-100-0.5; (j) D6-60-0.5; (k) D6-80-0.5; (l) D6-100-0.5.

peel off soot from the filter as a result of the relative low soot yield at this condition. The overall mass loss curves displayed the similar trend but the distinct oxidation rates. The higher oxidation rate of soot (slope of mass loss curve) corresponded the stronger reactivity because it represented the soot was easier to be oxidized. In Fig. 16, the slopes of normalized mass loss curves ranked as D6-80-0.6 < D6-80-0.5 < D680-0.4 and D4-80-0.5 < D4-80-0.4, which suggested that the soot oxidation reactivity decreased as the excess air ratio increased. This trend was same for soot from both types of combustors. However, the distinct inconsistent tendency between both types of combustors with

the same flow rate variation could be observed. In Fig. 17, the slopes of normalized mass loss curves increased in the following order: D6-1000.5 < D6-80-0.5 < D6-60-0.5 and D4-80-0.5 < D4-100-0.5 ≈ D460-0.5 (the curves of D4-60-0.5 and D4-100-0.5 are very close but the distinction between them and D4-80-0.5 could be distinguished). It indicated that with the increase of ethylene flow rate (60-100 ml/min) at α = 0.5, the oxidation reactivity of soot from the combustor of d = 6 mm decreased all the time, but it showed a downward trend first and then increased as for soot for the d = 4 mm. This phenomenon was similar to the flow-induced difference of TEM images observed in 150

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Fig. 15. XRD spectra of soot particles for the variation of different parameters in both types of combustors: (a) variation of excess air ratio in the combustor of d = 4 mm; (b) variation of excess air ratio in the combustor of d = 6 mm; (c) variation of flow rate in the combustor of d = 4 mm; (d) variation of excess air ratio in the combustor of d = 6 mm. Table 3 The peak diffraction angles of soot from different flame conditions. Flame condition

θ002 (degree)

D4-80-0.4 D4-80-0.5 D6-80-0.4 D6-80-0.5 D6-80-0.6

23.465 23.906 23.973 24.011 24.282

D4-60-0.5 D4-80-0.5 D4-100-0.5 D6-60-0.5 D6-80-0.5 D6-100-0.5

23.634 23.906 23.702 24.007 24.011 24.041

Fig. 16. TGA curves of soot particles for the variation of excess air ratio at QC2H4 = 80 ml/min in both types of combustors.

Fig. 11. Furthermore, it could be found that the slopes of normalized mass loss curves of soot for d = 6 mm were always much less than them for the d = 4 mm, which indicated that the soot from a smaller combustor exhibited the stronger oxidation reactivity. The TGA results further confirmed the distinct different soot properties caused by the scale effect.

3.7. Detailed discussions on influences of excess air ratio and flow rate on soot characteristics According to the results described above, it can be found that the 151

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Fig. 17. TGA curves of soot particles for the variation of flow rate at α = 0.5 in both types of combustors.

Fig. 20. Main component concentrations of the exhaust gases between the two combustors at QC2H4 = 80 ml/min and α = 1.

research [50–52,59]. Moreover, it has been demonstrated that the excess air ratio and flow rate affected the soot characteristics and the influence of flow rate was different due to scale effect. The reason can be further summarized as follows. The nanostructure and oxidation reactivity of soot is closely linked with the oxygen content, temperature and residence time in its formation conditions [56,62]. The impact of these factors may be more sensitive at the limited small space. In mesoscale combustion, the increase of flow rate and excess air ratio had two opposite effects. On the one hand, the increase of flow rate and excess air ratio can lead to a larger heat release and higher oxygen content in the combustion process, which contributed to the formation of soot with higher carbonization degree. However, on the other hand, the mixture flow velocity became larger due to an increasing flow rate and excess air ratio, which notably decreased the mixture residence time in the mesoscale combustion and conversely caused a large amount of relatively low mature soot which still needed more time to grow further. Therefore, the effects of excess air ratio and flow rate on soot were determined by both aspects above. In the case of same flow rate, the flow velocity in the combustor of d = 4 mm was approximately 2.4 times larger than that in the combustor of d = 6 mm based on parameters listed in Table 1, which led to the obvious different residence time. As a result, in the combustor of d = 6 mm, the combustion temperature and oxygen content played a dominant role affecting soot characteristics rather than residence time due to the relatively low flow velocity, which led to the fact that the soot with higher carbonization degree were formed as the excess air ratio and flow rate increased. Inversely, regarding the combustor of d = 4 mm, the significantly reduced residence time became the major factor influencing soot characteristics instead of the combustion temperature, and thus the increasing flow rate reduced the soot maturity in the case of large inlet velocity. This was the reason that the slopes of TG curves were ranked as D4-80-0.5 < D4-100-0.5 ≈ D4-60-0.5. Furthermore, the two opposite impacts of flow rate were also reflected in TEM morphology images. As shown in Fig. 11(a) and (c), the soot appeared to be film-like material with an irregular shape and a vague margin, which were different from other TEM images and only appeared in the combustor of d = 4 mm. This was presumably because the experimental conditions in Fig. 11(a) and (c) correspond to a comparatively low combustion temperature and short residence time, respectively. These two aspects both reduced the soot inception and growth rate and thus caused a chemical condensation of heavy PAHs (film-like overlayer) being observed. However, no film-like deposition was found in other TEM images because the soot particles going through oxidation region during the growth period with the moderate

Fig. 18. Main component concentrations of the exhaust gases between the two combustors at QC2H4 = 80 ml/min and α = 0.5.

Fig. 19. Main component concentrations of the exhaust gases between the two combustors at QC2H4 = 80 ml/min and α = 0.8.

soot characteristics from HRTEM, XRD and TGA analyses have good consistency. The disordered nanostructure of soot particles with shorter fringe length exhibited the higher oxidation reactivity, corresponding to the lower graphitization degree. This correlation between the soot nanostructure and oxidation reactivity has been confirmed by previous 152

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temperature and residence time.

detailed impacts were as follows. The larger soot production and limit of collectible soot were obtained with the enlargement of combustor, which was due to the higher combustion temperature and longer residence time. A notable distinction in nanostructure of soot from different combustors was observed. The soot samples from the combustor of d = 4 mm all showed partial amorphous structure with disordered and short planar lamellae, while a typical fullerene-like structure was found in the soot from the combustor of d = 6 mm. The soot samples with higher carbonization degree were obtained with the enlargement of combustor. Furthermore, different effects on soot characteristics between both types of combustors with the same variation of flow rate were found. For the same variation in ethylene flow rate of 60–100 ml/min at α = 0.5, the oxidation reactivity of soot from the combustor of d = 6 mm decreased all the time, while it showed a downward trend first and then increased as for the soot from the combustor of d = 4 mm. The above result was due to the different temperature and residence time increment for the same growth amount of flow rate between the two combustors as mentioned in discussions. Significantly, the lower combustion efficiency was obtained in the combustor of d = 6 mm than 4 mm at α = 0.5 because of the larger soot production and more existence of unburned gas. But the higher combustion efficiency was found for the 6 mm than 4 mm at α = 1 due to the longer residence time. The study here presented some characteristics of soot from a new formation condition (limited mesoscale space), which enriched the research of soot formation and might be helpful in increasing combustion efficiency and reducing soot emission of combustion-based micro power generator. In practical applications, the result also could provide some useful information on the size and working condition when designing mesoscale combustor with high efficiency and cleanliness.

3.8. Exhaust gases analyses Fig. 18 exhibited the main component concentrations of the exhaust gases between the two combustors at QC2H4 = 80 ml/min and α = 0.5. With the generation of much soot, a large number of soot precursors and unburned gases such as C2H2, CH4, C2H4, CO and H2 were also detected at this condition. The higher concentrations of C2H2, CH4, C2H4 were found in the combustor of d = 6 mm than 4 mm, which was presumably due to the larger soot production for the 6 mm as mentioned in Fig. 8. The concentrations of N2 and H2 were very close in both types of combustors but the higher content of CO could be observed in the combustor of d = 4 mm. Furthermore, the combustion efficiencies in the combustor of d = 4 mm and 6 mm were 61.33% and 57.68%, respectively. The combustion efficiency for the 6 mm was less than it for the 4 mm, which was due to the much more soot production and the more unburned gas existence such as CH4, C2H4 and C2H2. The main components of exhaust gases at QC2H4 = 80 ml/min and α = 0.8 were shown in Fig. 19. According to the result of Fig. 7, very little/no soot generated under this case, which was consistent with the result in Fig. 19 that almost no C2H2, CH4 and C2H4 were detected. Compared Fig. 19 with Fig. 18, the marked increase of CO2 concentration and lower CO content also signified the more complete combustion at α = 0.8 than 0.5 but there were still some H2 and CO remained in both combustors. At this condition, the combustion efficiency for the 4 mm and 6 mm were 96.05% and 95.97%, respectively. They were quite close with each other and much higher than the efficiencies at α = 0.5. Fig. 20 exhibited the main component concentrations of the exhaust gases between the two combustors at QC2H4 = 80 ml/min and α = 1, which corresponded to the condition of theoretically complete combustion. It could be found that only CO2 and N2 were detected in the combustor of d = 6 mm but there were still a few unburned gases such as C2H4, CO and H2 existed in the combustor of d = 4 mm. At this condition, the combustion efficiencies for the 4 mm and 6 mm were 95.90% and 99.95%, respectively. Different with the previous two cases, the obvious higher combustion efficiency was obtained in the d = 6 mm than 4 mm. This was because with a continued increasing excess air ratio, soot was not produced in both combustors and the inlet velocity increased correspondingly. The faster inlet velocity notably reduced the residence time and mixing effect of fuel and air in the combustor of d = 4 mm, which resulted in an incomplete combustion and the decrease of combustion efficiency [7]. However, this tendency didn’t reflect in the combustor of d = 6 mm because of the much larger residence time and smaller increment of inlet velocity. In conclusion, the above result of exhaust gases had good agreement with the soot formation characteristic and well reflected the different impacts of excess air ratio on combustion efficiency caused by the scale effect. The lower combustion efficiency was obtained in the combustor of d = 6 mm than 4 mm at α = 0.5 because of the larger soot production and more existence of unburned gas. But the higher combustion efficiency was found for the 6 mm than 4 mm at α = 1 in the case of almost no soot generation, which was due to the longer residence time.

Acknowledgments This work was supported by the National Natural Science Foundation of China (51776181, 51822605) and 333 Program of Jiangsu Province (BRA2017428). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.12.023. References [1] Nakamura Y, Gao J, Matsuoka T. Progress in small-scale combustion. J Therm Sci Technol 2017;12. 0001-1. [2] Xu B, Ju Y. Studies on non-premixed flame streets in a mesoscale channel. Proc Combust Inst 2009;32:1375–82. [3] Maruta K, Kataoka T, Kim NI, et al. Characteristics of combustion in a narrow channel with a temperature gradient. Proc Combust Inst 2005;30:2429–36. [4] Maruta K. Nonlinear dynamics of flame in a narrow channel with a temperature gradient. Combust Theor Model 2007;11:187–203. [5] Kumar S, Maruta K, Minaev S. Pattern formation of flames in radial microchannels with lean methane-air mixtures. Phys Rev E 2007;75:016208. [6] Kumar S, Maruta K, Minaev S. On the formation of multiple rotating Pelton-like flame structures in radial microchannels with lean methane–air mixtures. Proc Combust Inst 2007;31:3261–8. [7] Fan AW, Minaev S, Kumar S. Experimental study on flame pattern formation and combustion completeness in a radial microchannel. J Micromech Microeng 2007;17:2398–406. [8] Fan AW, Maruta K, Nakamura H, Kumar S, Liu W. Experimental investigation on flame pattern formations of DME–air mixtures in a radial microchannel. Combust Flame 2010;157:1637–42. [9] Fan AW, Nakamura H, Maruta K, Liu W. Experimental investigation of flame pattern transitions in a heated radial micro-channel. Appl Therm Eng 2012;47:111–8. [10] Fan AW, Wan JL, Maruta K, Nakamura H, Yao H, Liu W. Flame dynamics in a heated meso-scale radial channel. Proc Combust Inst 2013;34:3351–9. [11] Min JL, Kim NI. Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels. Combust Flame 2014;161:2361–70. [12] Kim NI, Kato S, Kataoka T, Yokomori T, Maruyama S, Fujimori T. Flame stabilization and emission of small Swiss-roll combustors as heaters. Combust Flame 2005;141:229–40.

4. Conclusions Soot formation, evolution, characteristics from diffusion ethylene/ air flames in ψ-shaped mesoscale combustors of two different diameters (d = 4 mm and 6 mm) were experimentally investigated under various excess air ratios and flow rates. Results demonstrated that with an increasing excess air ratio and flow rate, the flame structure was gradually deformed from center symmetry to bifurcated shape in both types of combustors due to the deteriorated mixing process in large flow velocity. Besides that, the scale effect (combustor size) significantly affected the soot formation, property and combustion efficiency. The 153

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