Laminar Burning Characteristics of Two Rice-Husk-Derived Biofuels

Aug 27, 2018 - Unstretched flame speed of tested fuels versus equivalence ratio (T0, 358 K; P0, .... Comparison of experimental and simulated laminar ...
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Laminar Burning Characteristics of Two Rice-Husk-Derived Biofuels Cangsu Xu,† Kangquan Zhou,† Xiaolu Li,‡ Anhao Zhong,† Francis Oppong,† Hanyu Wang,† Siyuan Wu,† Wenhua Zhou,† and Chongming Wang*,§ †

College of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China College of Energy Engineering, China Jiliang University, Hangzhou, Zhejiang 310018, People’s Republic of China § School of Automotive Engineering, Coventry University, Coventry CV1 5FB, United Kingdom ‡

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S Supporting Information *

ABSTRACT: Biomass-derived fuels are emerging alternatives to fossil fuels because of their renewability and better carbon balance. Fuel researchers from Zhejiang University have developed and improved a catalytic method of converting rice husk to biofuels mainly composed of ethanol, ethyl acetate, and acetone. Optimization of the catalytic production process so that the final fuel has good combustion characteristics is something that requires detailed investigation. This study evaluates the laminar burning features of two fuels produced by the catalytic reaction. These two fuels are ETEAAC211 and ETEAAC121 (ETEAAC211, 50 vol % ethanol, 25 vol % ethyl acetate, and 25 vol % acetone; ETEAAC121, 25 vol % ethanol, 50 vol % ethyl acetate, and 25 vol % acetone). The experiment was conducted in outward propagating spherical flames at T0 of 358 K, P0 of 0.1 MPa, and equivalence ratios (ϕ) of 0.7−1.4. Moreover, the flame intrinsic hydrodynamic and thermal diffusion instabilities are assessed and discussed. It was noticed that the peak laminar burning velocity of the fuels occurred at ϕ of 1.1. The hydrodynamic instability reached its peak at ϕ of 1.1 as a result of the thin flame thickness and the high density ratio of burned/ unburned mixtures. The Markstein length decreased with the equivalence ratio. However, the Markstein length decreased below zero at ϕ of 1.4 for ethyl acetate and ETEAAC121, showing the increased thermal diffusion instability as the equivalence ratio increases.

1. INTRODUCTION Since it was invented in 1860, the internal combustion engine has made significant contributions to transportation. However, during the operation of the internal combustion engine, harmful pollutants, such as particulate matter, carbon monoxide, unburnt hydrocarbons, and nitrogen oxides, are produced.1 In addition,2 the limited fossil fuel reserves and greenhouse emissions are also key challenges for the transportation sector. Biomass-derived fuels are an important solution of reducing pollutant, greenhouse gas emission, and dependence upon fossil fuels. There are many biofuel candidates, for example, 2methylfuran (MF) and 2,5-dimethylfuran (DMF).3,4 DMF and MF have comparable physicochemical properties with gasoline, and they were successfully used in spark-ignition engines.5,6 There are also many other gasoline alternative candidates, such as alcohols, valerates, butyl ethers and levulinates, furanoids, and benzenoids. A comprehensive review and discussions regarding the catalytic production, engine combustion and emissions, and outlooks of these potential candidates are available in the literature.7 Recently, a rice husk biomass fast pyrolysis fuel was catalytically produced by fuel researchers from Zhejiang University.8,9 One of the fuels has been used in compression-ignition10 and spark-ignition11 engines, and it showed comparable and/or even better emissions than diesel and gasoline. The aforementioned fuel mainly consists of ethanol, ethyl acetate, and acetone, which are commonly additives/ components for gasoline and diesel fuels. The proportions of © XXXX American Chemical Society

these three major components vary depending upon the condition of the catalytic reaction. Ethanol has a high heat of vaporization, oxygen content, and octane rating, which have a positive influence on engine performance12,13 and emissions.14,15 Ethanol is also found to be effectively preventing and reducing injector deposit formation in direct-injection gasoline engines.16 Ethyl acetate, as an additive, can help to improve engine performance output as well as reduce smoke and NOx emissions.17 Ethanol and ethyl acetate were also diesel additives. A blend of 7 vol % ethyl acetate, 13 vol % ethanol, and 80 vol % diesel prevented phase separation, and the engine test results showed smoke emission reduction.18 In a study of spray combustion, acetone, as the main component of an acetone/butanol/ethanol blend, showed a reduced ignition delay and reduced soot formation when the fraction of acetone was increased.19 In addition, ethanol and ethyl acetate have been investigated in advanced combustion modes. Regression neural network analysis was used to investigate the performance of an ethanolfueled homogeneous charge compression ignition (HCCI) engine.20 The operational conditions of a wet-ethanol-fueled HCCI engine21 have been optimized using exhaust gas heat recovery. Ethyl acetate has also been studied in the HCCI combustion mode using numerical simulations, and the study disclosed that ethyl acetate misfiring and knock limits were higher than those of iso-octane.22 Received: April 23, 2018 Revised: August 27, 2018 Published: August 27, 2018 A

DOI: 10.1021/acs.energyfuels.8b01440 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Physicochemical Properties of Tested Fuels parameter

ethanol

ethyl acetate

acetone

ETEAAC211

ETEAAC121

density at 293 K (g/mL) lower heating value (MJ/kg) stoichiometric air/fuel ratio research octane number (RON)

0.798 26.8 9.0 108

0.897 23.4 7.8 118

0.785 29.6 9.5 110

0.820 26.5 8.8

0.844 25.6 8.5

Figure 1. Experimental setup.

It was found that, depending upon the catalytic production conditions, such as the temperature, pressure, catalysts, and reaction time, proportions of the three major components (ethanol, ethyl acetate, and acetone) vary. Optimization of the catalytic production condition is necessary to obtain a fuel that has preferred combustion features. Laminar burning velocity is used to evaluate and validate the chemical kinetics of fuel and can also affect engine performance and emissions.23,24 Furthermore, it also has a significant role to analyze the turbulent propagation speed.25 The motive of this work is to determine the laminar flame characteristics of the two fuels most likely produced from the catalytic reaction. These two fuels are ETEAAC211 and ETEAAC121 (ETEAAC211, 50 vol % ethanol, 25 vol % ethyl acetate, and 25 vol % acetone; ETEAAC121, 25 vol % ethanol, 50 vol % ethyl acetate, and 25 vol % acetone). Physicochemical properties of those fuels are listed in Table 1. The experimental investigations of this study were performed at T0 of 358 K, P0 of 0.1 MPa, and equivalence ratios (ϕ) of 0.7−1.4. The flame images were recorded with the Schlieren photographic system. The flame diffusive thermal and hydrodynamic instabilities were analyzed and discussed.

Figure 2. Flame front detection. front radius, rf, was calculated from the flame front and the optical window pixels using eq 1

rf =

N RW Nall

(1)

where N, Nall, and RW denote the flame front and the optical window pixels and the optical window actual radius. In this study, the flame image radius (8−25 mm) was chosen to calculate the laminar flame velocity. This radius range was chosen to avoid the interference from ignition energy and wall confinement.30 The maximum radius is also constrained by the pressure change in the vessel; the numerical method (linear and nonlinear extrapolation method) for calculating laminar flame velocity, which will be explained later, is only valid when the pressure does not change. Figure S1 of the Supporting Information shows an example of ethanol pressure history at the equivalence ratios of 0.7, 1, and 1.4 (T0, 358 K; P0, 0.1 MPa). It is found that, when the flame radius reached 25 mm, the pressure increase was lower than 1% of the maximum pressure rise (peak pressure minus initial pressure). Therefore, at the chosen flame radius range, it is safe to say that the pressure inside the vessel was kept the same.

2. EXPERIMENTAL AND NUMERICAL EVALUATIONS 2.1. Experimental Testing and Procedures. Figure 1 shows the schematic of the experimental testing apparatus, which includes the constant volume combustion vessel, Schlieren imaging system, ignition system, and data recording equipment. The total volume is 1.94 L, and the equivalent radius of the chamber is 77.4 mm. The experiment was performed at least 3 times for each test condition. A detailed description of the experimental apparatus and procedures are reported in the literature.26−29 2.2. Data Processing. Figure 2 shows an example of the flame front detection using the Adobe Photoshop software. The actual flame B

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Figure 3. Schlieren flame images of five fuels (ϕ, 1; T0, 358 K; and P0, 0.1 MPa). The flame propagation in spherically expanding flames is computed via

Sb =

drf dt

where Lb is the Markstein length of the burnt mixture. The linear extrapolation method is applicable only when the Lewis number is near unity.33 Law and Kelly proposed a nonlinear method.

ij S b yz ij S b yz jj zz lnjj zz = − 2 L bα jj 0 zz jj 0 zz S b0 k Sb { k Sb { 2

(2)

where t and Sb refer to the time after ignition and the stretched flame propagation speed, respectively. The flame stretch originates from the local flow strain and the curvature of the flame surface.30,31 The stretch rate (α) is defined as

α=

2 1 dA 1 d(4πrf ) 2 = = Sb A dt rf 4πrf 2 dt

Sb =

− L bα

(5)

Assuming a quasi-steady and quasi-planar flame, the laminar burning velocity, uL, is estimated from the flame front mass conversation

uL = (3)

ρb ρu

S b0

(6)

where ρb and ρu are the burnt and unburnt mixture densities, respectively. The density ratio is calculated in Chemkin. According to Miao et al.,34 the flame thickness, δT, is defined by

To determine the unstretched flame propagation speed, Sb0, extrapolations are used to calculate the flame propagation at zero stretch rate. The linear extrapolation is the most widely used technique32 S b0

2

δT =

(4) C

λ C pρu uL

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Figure 4. Extrapolated unstretched flame speed of blended fuels at ϕ, 0.8/1.4; T0, 358 K; and P0, 0.1 MPa. where λ is the thermal conductivity and Cp is the specific heat of unburned gas. The Markstein number (Ma) can be calculated via Ma =

Lb δT

3. RESULTS AND DISCUSSION 3.1. Flame Structure Analysis. Figure 3 shows the Schlieren images of the flame from five test fuels at ϕ of 1. Among the three pure fuels, ethanol showed the highest propagation speed, while ethyl acetate recorded the lowest propagation speed. The hydroxyl functional group (−OH) in ethanol leads to a higher flame propagation speed.37 The speeds of blend fuels ETEAAC211 and ETEAAC121 were between those of ethanol and ethyl acetate. The flame front was near-spherical at the initial stage as a result of the effect of reduced obstruction and reduced quenching achieved using thin spark wires. At the stoichiometric condition, the flame front surface was smooth and had no wrinkles, displaying a stable flame front. 3.2. Unstretched Flame Propagation Speed. Figure 4 presents the unstretched flame propagation speed of ETEAAC211 at a lean mixture and ETEAAC121 at rich conditions calculated from linear and nonlinear extrapolation methods. Both extrapolation methods have been extensively used in the literature.32,38,39 The nonlinear extrapolation method was preferable as a result of the nonlinearity relationship between the flame propagating speed and stretch rate, particularly at extreme lean or rich flames.30,40 Indicated by R2 values of the linear and nonlinear fittings in Figure 4, it is found that, for the positive Markstein length case, the nonlinear method is better, while for the negative Markstein length case, the linear method is better. The criterion of choosing a linear or nonlinear method is the fitting quality, as indicated by R2. Therefore, the nonlinear method was used when the Markstein length was positive, and the linear method was used when the Markstein length was negative. Figure 5 presents the unstretched flame propagation speed of the present investigated fuels at their respective equivalence ratios. The flame propagation speed increases from lean to stoichiometric mixtures, and the maximum propagation speed is at ϕ of 1.1. The speed decreases with the increase of the equivalence ratio. This can be attributed to the relationship between the adiabatic flame temperature and the equivalence ratio.41 With regard to the pure fuel components, the average unstretched flame propagation of acetone was 8% higher than that of ethyl acetate and was 18% lower than that of ethanol. In addition, ethyl acetate at ϕ of 0.7 could not be ignited. The

(8)

The laminar burning flux (f) can be calculated via

f = ρu uL

(9)

Chen pointed out that laminar flame propagation is influenced by radiation. Yu et al.36 have suggested a correlation to rectify the measured laminar burning speed 35

−1.14 ij T0 yzijj p0 yzz iu y jj zzjj zz uL0 = uL + 0.82uLjjj L zzz S * k { k T * {jk p * z{

−0.3

(10)

where the values of S*, T*, and p* are 1 cm/s, 298 K, and 0.1 MPa, respectively. Using eq 10, the radiation deviation (ΔUradi) is deduced by −1.14 i p y iu y jij T0 zyzjjj 0 zzz ΔUradi = 0.82jjj L zzz j zjj zz kS *{ k T * {k p * {

−0.3

(11)

Figure S2 of the Supporting Information shows the laminar flame speed of ethanol/air mixtures obtained from the present study and literature data.52−54 This figure is used for the validation of the Schlieren laminar flame speed measurement setup of the authors. 2.3. Experimental Uncertainties. The primary sources of the measurement uncertainty in the determination of laminar flame speed from a spherical outward expanding flame are (a) the initial experimental conditions (temperature and pressure), (b) radiation, (c) the precision of the instrument, and (d) the uncertainty of the flame radius. The thermocouple used in this study has a ±0.5% precision. Perturbation of the initial temperature results in an uncertainty of ∼0.8% (ΔUTini) in the estimation of the laminar burning velocity at 0.1 MPa. The pressure transducer accuracy is 0.0001 MPa. Thus, the initial pressure uncertainty is less than 0.1%. The measured laminar burning velocity is linked to the flame radii, which are calculated by determining the number of pixels inside the flame front of every image. The uncertainty in determining the area inside the flame front is approximately ±10 pixels, which leads to an overall uncertainty of ∼1% (ΔUradii). The global uncertainty of the laminar burning velocity ΔUglobal can be calculated by ΔUglobal =

ΔUini 2 + ΔUradi 2 + ΔUrepeat 2 + ΔUradii 2

(12) D

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the equivalence ratio of 1, instead of 1.1, which is possible because of the experimental errors, as shown in the error bars. Sarli et al.42 proposed a Le Chatelier’s rule-like formula that is based on the mole fraction for the laminar burning velocity of hydrogen/methane mixtures, and they obtained good estimations in lean and stoichiometric regions. The formula is expressed as uL,blend =

1 x n ∑i = 1 u i

L, i

(13)

where xi is the mole fraction of each component. Sileghem et al.43 have suggested a Le Chatelier’s rule-like formula that is based on the energy fraction for the laminar burning velocity of blended fuels

Figure 5. Unstretched flame speed of tested fuels versus equivalence ratio (T0, 358 K; P0, 0.1 MPa).

uL,blend =

unstretched flame propagation speeds of two blend fuels were between those of ethanol and ethyl acetate. ETEAAC211 had a lower flame speed than acetone in the lean region, while had a higher flame speed in the rich zone. The unstretched flame propagation speed of ETEAAC121 was 5% lower than that of ETEAAC211, because ETEAAC121 had more ethyl acetate and less ethanol content. 3.3. Laminar Burning Velocity. 3.3.1. Experimental Results. Laminar burning velocities of the tested fuel−air mixtures versus the equivalence ratio are presented in Figure 6.

1 e n ∑i = 1 u i L, i

(14)

where ei is the energy fraction of each component, which can be calculated as follows: ei =

ΔcHi0xi n

∑i = 1 ΔcHi0xi

(15)

ΔcH0i

where is the heat of combustion of each component. The formulas proposed by Sarli et al.42 and Sileghem et al.43 were applied to estimate the laminar burning velocity of the blended fuel based on the experimental data for pure fuels. Figure 7 shows the comparison between estimated results and experimental data. In general, the relative deviations of laminar burning velocities between the experimental value and calculation value by the mixing rules were less than 5% at stoichiometric and rich conditions and less than 10% at lean conditions. However, it should be noted that the experimental results contain uncertainty and errors, as shown in the error bars in Figure 6. Therefore, the agreement of experimental data and results from a Le Chatelier’s rule-like formula that is based on the mole fraction and a Le Chatelier’s rule-like formula that is based on the energy fraction will be affected. This is also the possible reason that results from the two formulas agree less in lean flames than rich flames. In comparison to the mole fraction rule, the energy fraction rule led to closer results to the experimental data, and the relative deviations could be reduced to less than 3% for stoichiometric and rich mixtures. This is because the flame temperature is the prevailing parameter for the laminar burning velocity of the blends.43 3.3.2. Chemical Kinetic Analysis. Numerical simulations of ethanol, ethyl acetate, and acetone are carried out in Chemkin. Three chemical kinetic models were chosen for simulation: the Olm model44 for ethanol, the Dayma model45 for ethyl acetate, and the Pichon model46 for acetone. Details about the chemical kinetic mechanism used in this work are summarized in Table 2. Figure 8 compares the experimental and simulated laminar burning velocities. For ethanol, the simulation results were close to the measured data, while the maximum absolute deviation was 2.5 cm/s at the equivalence ratio of 1.2. The discrepancies between the experiment and simulation for ethyl acetate and acetone were much larger than those for ethanol at examined initial conditions. Kinetic models overpredicted the burning velocity of ethyl acetate at the whole range of equivalence ratios, and the average absolute deviation was around 4.8 cm/s. Acetone experimental and simulated results

Figure 6. Laminar burning velocity of tested fuels (T0, 358 K; P0, 0.1 MPa).

The variation of the laminar burning velocity trend is consistent with that of the unstretched flame propagation speed. The laminar burning velocity of acetone is 8% higher than that of ethyl acetate and is 17% lower than that of ethanol. The laminar burning velocities of the blend fuels were between those of ethanol and ethyl acetone, and the blend fuel ETEAAC211 had a higher laminar burning velocity than ETEAAC121. The laminar burning velocity first increases with an increasing equivalence ratio and then decreases. Except for acetone, the other tested fuels obtained the peak burning velocities at ϕ of 1.1, which were 50.93, 39.86, 44.44, and 41.91 cm/s for ethanol, ethyl acetate, ETEAAC211, and ETEAAC121 respectively. Acetone recorded the maximum burning velocity (42.73 cm/s) at ϕ of 1.0. The maximum burning velocity is usually located at ϕ of 1.1 because the adiabatic temperature reaches the highest at this equivalence ratio. The maximum burning velocity of acetone is located at E

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Figure 7. uL of experimental data, Le Chatelier’s rule-like formula based on the mole fraction and energy fraction: (a) ETEAAC211 and (b) ETEAAC121 (T0, 358 K; P0, 0.1 MPa).

flow and jet-stirred reactors.44 However, the ethyl acetate and acetone mechanisms have not been updated for more than 5 years. This explains that the deviations of the simulation and experimental results from ethyl acetate and acetone are larger than those from ethanol. Therefore, further improvement is required for the above two mechanisms. Ethanol had the highest burning velocity, followed by acetone and ethyl acetate. The maximum OH + H and CH3 concentrations for each fuel in Figure 9 are used to explain this phenomenon. OH and H are important radicals, which can accelerate the consumption of fuels and their intermediates, while CH3 is a chain termination radical that will decelerate the flame speed. Ethanol produced a minimum CH3 concentration almost within the whole range of equivalence ratios and a maximum OH + H concentration for lean-fuel or stoichiometric conditions. Therefore, this underlines that ethanol had the highest burning velocity among the three fuels. Although ethyl acetate had a higher OH + H concentration than acetone for lean and stoichiometric conditions, its CH3 concentration was at a very high level; moreover, acetone produced the highest OH + H concentration for rich-fuel conditions; hence, ethyl acetate has the lowest burning velocity. Figure 10 describes the sensitivity analyses of ethanol, ethyl acetate, and acetone/air flames at lean, stoichiometric, and rich conditions. The effect of the constant rate of each reverse reaction on the flame speed is expressed by the sensitivity coefficient. The positive coefficient indicates that the reaction promotes the flame speed, while the negative coefficient indicates that the reaction suppresses the flame speed. For three different fuels, the most important reaction was the same: the oxidation of H by O2 to produce OH and O. This reaction consumed one active radical (H) but produced two radicals (O and OH), which can accelerate the process of combustion. Therefore, the equivalence ratio is used to increase its sensitivity coefficient. There were two other reactions having a positive impact on the flame speed of the fuels: the oxidation of CO to CO2 by OH, whose coefficient was reduced with the equivalence ratio, and HCO in H and CO decomposition, which could increase the quantity of active radicals (H) in the flame. Conversely, some reactions decreased H and OH radicals; therefore, they had negative influences on the flame

Table 2. Details about the Chemical Kinetic Mechanism Used in This Work chemical mechanism Olm et al. for ethanol41 Dayma et al. for ethyl acetate42 Pichon et al. for acetone43

number of species

number of reactions

47

251

232

1845

83

419

validation methodsa ST, RCM, LF, FR, and JSR ST, RCM, LF, and JSR ST and LF

a

ST, shock tube; RCM, rapid compression machine; LF, laminar flame; FR, flow reactor; and JSR, jet-stirred reactor.

Figure 8. Comparison of experimental and simulated laminar burning velocities (T0, 358 K; P0, 0.1 MPa).

are in good accordance at ϕ of 0.8−0.9; however, the difference was increased with the increase of equivalence ratios. Ethyl acetate and acetone experimental and simulated result deviations are larger than those of ethanol. The simulated laminar burning velocity depends upon the chemical kinetic mechanism. The ethanol kinetic mechanism has been constantly updated, and the mechanism used in this study was validated by a wide-ranging experimental data set, involving shock-tube ignition delay, rapid compression machines, laminar burning velocity, and species profiles in F

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Figure 9. Maximum OH and CH3 concentrations for ethanol, ethyl acetate, and acetone (T0, 358 K; P0, 0.1 MPa).

3.4. Flame Instability. The laminar premixed flame is susceptible to diffusive thermal instability, hydrodynamic instability, and buoyancy instability.47 In this work, the buoyancy instability had a limited impact on the flame stability as a result of the fast flame velocity, which can be proven by observing the flame morphology. 3.4.1. Diffusion Thermal Instability. The Lewis number (Le) governs the thermal diffusion instability, which describes the non-equidiffusive properties of the reactive mixture. Further, the diffusive thermal instability is related to the heat and mass exchange between the burned and unburned zones. The Lewis number is expressed as the fractional relation between a reactive mixture thermal diffusivity and mass diffusivity of the limiting reactant. It also explains the net energy inflow/outflow in the flame. If Le is less than 1, the heat transfer from the flame to the unburned zone is less than the mass transfer from the unburned zone to the flame, leading to a higher energy within the flame and a higher adiabatic flame temperature, which accelerates the flame speed and leads to flame instability. Le is reflected by the Markstein length, with a negative Markstein length indicating a Le of less than 1 and a positive Markstein length indicating a Le of larger than 1.48 Figure 11 presents the Markstein length and number of the five examined fuels. The Markstein length decreases with the increase of the equivalence ratio, showing the enhanced diffusive thermal instability tendency. At ϕ of 0.7−1.3, the Markstein lengths of the five examined fuels were similar, indicating that diffusion thermal instabilities were similar. The Markstein length of ethyl acetate and ETEAAC121 showed negative values at ϕ of 1.4, while those of other fuels maintained a positive value. If protuberance occurs at the flame front, the stretch rate increases at that local area, leading to a faster propagation speed and an unstable flame when the Markstein length is negative. The negative Markstein length indicates that the flame propagation speed increases with an increasing stretch rate, leading to an unstable flame. It can be found that the Markstein length of ethyl acetate is not the smallest of the five fuels at all of the equivalence ratios, except 1.4, but it decreases very much at ϕ of 1.4 and becomes less than zero. For a particular fuel, a large Markstein length at lean conditions does not mean that it also has a large Markstein length at rich conditions. For example, in the literature,23 gasoline had larger Markstein lengths than ethanol and

Figure 10. Sensitivity analyses of the flow rate for ethanol, ethyl acetate, and acetone/air flames at ϕ of 0.8/1.1/1.4 (T0, 358 K; P0, 0.1 MPa).

speed, such like the combination of CH3 and H, the combination of H and OH, and the combination of H and O2. G

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Figure 11. (a) Markstein length and (b) Markstein number of test fuels versus equivalence ratio (T0, 358 K; P0, 0.1 MPa).

Figure 12. (a) Flame thickness and (b) density ratio of test fuels versus equivalence ratio (T0, 358 K; P0, 0.1 MPa).

Figure 12. The trend of flame thickness was opposite the trend observed with the laminar burning velocity. The flame thickness of the tested fuels reached its minimum value at ϕ of 1.1. Among the fuels, ethanol had the lowest flame thickness and ethyl acetate had the highest flame thickness. The density ratio initially increases with the increase in the equivalence ratio and then decreases. The maximum value was observed at ϕ of 1.1. At the same equivalence ratio, the density ratios of five test fuels were close. All of the fuels had the highest hydrodynamic instability at ϕ of 1.1 as a result of the pattern of flame thickness and density ratio of burned and unburned gases. Because the density difference of tested fuels is minimal, the hydrodynamic instability difference of the tested fuels is mainly influenced by the flame thickness. Because of the smallest flame thickness of ethanol, ethanol addition in the blended fuel enhances the hydrodynamic instability. The total flame instability is the combined effects of diffusion thermal and hydrodynamic instabilities. In the literature, there is detailed analysis of the flame stability.50,51 Flame instability can be observed from the flame morphology. In this study, experiments are carried out at atmospheric pressure. Therefore, flame instability was only observed in rich flame morphology. In Figure 3, the flame surfaces from all test

cyclopentanone when ϕ is