Measurements of laminar flame speeds and flame instability analysis of E30-air premixed flames at elevated temperatures and pressures

Fuel 259 (2020) 116223

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Measurements of laminar flame speeds and flame instability analysis of E30air premixed flames at elevated temperatures and pressures

T

⁎

Qing Yanga,b, Zechang Liua,b, Xinghe Houa,b, Xu Hea,b, , Magnus Sjöbergc, David Vuilleumierc, Cong Liua,b, Fushui Liua,b a

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China Collaborative Innovation Center for Electric Vehicles, Beijing, China c Sandia National Laboratories, Livermore, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Laminar burning velocity Flame instability Ethanol TRF Nitrogen dilution

Ethanol is regarded as one of the most promising alternative renewable fuels and as well as an oxygenate blending component in gasoline fuels, with widespread usage in many countries around the world. Laminar flame speeds can have strong influence on the stability and operability of Spark-Ignition combustion in certain operating regimes, and so the effects of different initial conditions on laminar combustion characteristics of E30 (gasoline blended with ethanol of 30% liquid volume) were analyzed in a constant-volume combustion vessel using the high-speed Schlieren method. This work presents results for equivalence ratios of 0.7–1.4, dilution ratios of 0%, 5%, 10%, and at different initial temperatures (408, 453 and 498 K) and initial pressures (1, 2 and 3 bar). It can be concluded that the laminar burning velocity has a positive correlation with initial temperature, but negative correlation with initial pressure and dilution ratio. The laminar burning velocity always reaches its maximum value at an equivalence ratio of 1.1 and does not change with varying initial conditions' the adiabatic flame temperature displays a similar variation with the initial conditions. The flame instability of E30-air mixture is enhanced as the initial pressure increases. Flame stability at lean and rich mixtures are exactly opposite at different initial temperature and dilution ratio. The laminar burning velocity was significantly promoted relative to gasoline and E10 by the addition of higher volume fractions of ethanol, highlighting one of the benefits of ethanol’s use as a blending component in gasoline fuels.

1. Introduction Currently, the issues of energy scarcity and environmental pollution are in need of urgent resolution all over the world. Vehicles are major consumers of fossil fuels and their emissions are a primary source of environmental pollution. At present, the transitional solution of the internal combustion engine community to the aforementioned problems is multifaceted, simultaneously developing new combustion modes, such as HCCI (Homogeneous Charge Compression Ignition) [1], Gasoline Compression Ignition [2], lean Spark Ignition [3], LTC (Low Temperature Combustion) [4], and supercritical fuel injection [5,6], while also finding and researching renewable clean alternative fuels, which in some cases can be synergistic to engine combustion systems [7–9]. As we know, alcohols [10–12], syngas [13], and hydrogen [14], can help reduce lifecycle carbon emissions by utilizing renewable energy (sunlight or wind) as a primary energy source rather than fossil fuels.

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Ethanol, as a low carbon-number alcohol with a higher oxygen content compared to gasoline and diesel, is considered one of the most important energy carriers to significantly reduce anthropogenic CO2 emissions, as it can be produced renewably from biomass by fermentation of sugar and starch crops [15,16]. Ethanol has high laminar flame speeds [17], and an extensive flammability limits, which is very beneficial to the combustion stability of lean or high EGR-ratio engine combustion [18]. However, there are some disadvantages of ethanol: (1) the calorific value of ethanol is only 60% of conventional gasoline by volume, (2) ethanol is highly hydroscopic and can easily dissolve in water, and (3) its material compatibility must be considered otherwise it tends to corrode the engine’s fuel system and results in higher production costs [19]. These disadvantages can be mitigated by using ethanol as a blending component rather than a neat fuel, as ethanol blends well with hydrocarbons [20], and gasoline-ethanol blends are commercially available in Africa [15], Europe [21] and the United States [22], especially in Brazil [23]. Ethanol can be added to gasoline

Corresponding author at: School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail address: [email protected] (X. He).

https://doi.org/10.1016/j.fuel.2019.116223 Received 19 July 2019; Received in revised form 13 September 2019; Accepted 16 September 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

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Nomenclature A α Lb P T ϕ Rf t Sb

S0 b ρu ρb SL S0 L σ Le δ ν β

Flame area Flame stretch rate Markstein length Initial pressure Initial temperature Equivalence ratio Flame radius Times Stretched propagation speed

Unstretched propagation speed Density of unburned gas Density of burned gas Stretched laminar burning velocity Unstretched laminar burning velocity Thermal expansion coefficient Lewis number Laminar flame thickness Kinetic viscosity of unburned mixture Diluent ratio

isooctane combustions, most likely due to the chemical kinetic effects of acetaldehyde, which is formed as an intermediate in the oxidation of ethanol. Broustail et al. [36,37] studied the laminar characteristics of isooctane-ethanol blends as a function of equivalence ratio at different initial pressure and ethanol liquid volume fraction. Their results demonstrated that the sensitivity to stretch rate and thermal-diffusive instability of ethanol-isooctane-air flames decrease as ethanol fraction increases. Van Lipzig et al. [38] determined the laminar burning velocity of an isooctane(1/2)-ethanol(1/2) mixture, an n-heptane(1/2)ethanol(1/2) mixture and an isooctane(1/3)-n-heptane(1/3)-ethanol(1/ 3) mixture by the heat flux method which can effectively reduce the uncertain of laminar flame speeds (total error ∓1 cm/s). Dirrenberger et al. [39] experimentally researched the laminar flame speeds with different ethanol volume fraction addition in commercial gasoline and a TRF gasoline surrogate. The results showed that the impact of ethanol addition (15% liquid volume fraction) was negligible, which is inconsistent with the findings of researchers which have already been discussed. Meng et al. [21] experimentally measured the laminar propagation speed of pure fuel components including isooctane, toluene, 1hexene, ethanol, and their quaternary blends at different initial conditions. They found that the effects of ethanol addition in gasoline with volume fractions up to 20% are negligible, especially for low temperatures and pressures; in addition, ethanol addition leads to the earlier occurrence of a cellular flame. Huang and co-works [40,41] studied flame instability and flame speeds of pentanol isomers, butanol isomers and n-propanol. The main conclusions are as follow: 1-pentanol has the highest flame speed and higher flame instability behavior compared with other pentanol isomers (1-, 2-, and 3-pentanol). Compared with other butanol isomers (sec-butanol, iso-butanol and tertbutanol), n-butanol has the highest laminar burning velocity. Propane had flame speed which was comparative with n-propanol. They also developed a new kinetic mechanism of n-propanol oxidation. Rau et al. [42] conducted experiments using a heat flux burner at atmospheric pressure and measured the laminar flame speeds of the isooctaneethanol-air (E10, E24, E40 and E85) premixed flame. This study increased the available data of isooctane-ethanol blends with 10, 24, 40 and 65, 85 vol% ethanol at 298 K and find that the relationship between ethanol addition volume percentage and laminar flame speeds is linear. Zhang et al. [43] explored the effect of ABE (acetone-butanol-ethanol) addition on the laminar flame speeds in a constant-volume combustion vessel. Eisazadeh et al. [44] utilized a mixture of 86% N2 and 14% CO2 to research the influence of the diluent gas on the laminar flame speeds of ethanol-air flames. The flame speeds of gasoline-ethanol mixtures significantly enhance with ethanol addition [45]; therefore the promoting effect of ethanol addition can be used to weaken the laminar burning velocity reduction because of EGR. Huang et al. [46–50] have carried out substantial and detailed investigations on the laminar flame speeds about various hydrocarbon and alcohol fuels such as ethanol, isooctane, methanol and methane at different initial conditions. Their results can be summarized as follows: ethanol addition can significantly enhance the laminar flame speeds of gasoline-ethanol mixture. The promoting effect of ethanol addition can decrease the laminar burning

to reduce the emissions of soot, CO, unburned HC and benzene with proper engine calibration [24,25]. Due to its higher octane number [26], ethanol addition can also reduce engine-knock in Spark-Ignition (SI) engines; hence the engines can operate at a higher compression ratio and supercharging ratio to enhance the power output and thermal efficiency [21,27]. Currently, the E25 fuel is already commercially available in many countries [28–30]. However, in ethanol producing countries, like EUA and Brazil, government initiatives are studying using E30 as fuel [31,32]. Compared with lower ethanol level blends (such as E5, E10), E30 offers significant advantages in terms of higher octane number (RON = 100), better fuel economy, desirable Reid vapor pressure and water tolerance characteristics [33,34]. In addition, E30 can significantly reduce the emission of HC, CO and NOx emissions at idle [35] and E30 is the main fuel of Co-Optima project sponsored by DOE (Department of Energy) EERE (Office of Energy Efficiency and Renewable Energy). Based on these considerations, the present study selects E30 as our research fuel. One of the main challenges of studying the characteristics of E30 is that the composition of the gasoline can vary significantly because of miscellaneous crude-oil sources and processing approaches. In order to maintain the reproducibility of the study through a well-defined gasoline composition, a TRF was used as a gasoline surrogate with a specific blend ratio of 1/3 iso-octane, 1/3n-heptane and 1/3 toluene on a volumtric bassis; such as blend can provided the similar laminar burning velocity to gasoline [28,29]. Although many previous researches have investigated the laminar combustion characteristics about gasoline blended with ethanol, they mainly focused on low-level ethanol-gasoline blends (E5-E20) while using PRFs (Primary reference fuels) or isooctane as gasoline surrogates, rather than the TRF using in this study, which better mimics the distribution of hydrocarbon classes found in modern reformulated gasoline. Only two studies using a TRF blended with ethanol are available [30,31]. Furthermore, previous studies did not focus on the study of flame cellular instabilities, which have implications for the turbulent flame propagation found in IC (internal combustion) engines. In addition, many previous works focused on the isooctane or PRF blends with ethanol with N2 dilution, rather than more complex fuels diluted by N2, such as TRF blends with ethanol. The laminar combustion characteristics measured in this study not only can develop and validate chemical kinetic mechanisms for gasoline-ethanol blends [32,33], simulate turbulent flame propagation [34], predict the performance and emissions of engines, but also can provide indispensable guidance for the design and optimization of fuels and engines. Therefore, the laminar combustion characteristics of E30 (30% volume ethanol blending with 70% volume gasoline) were studied in this work. There has been prior work investigating laminar flames of gasoline or gasoline surrogates blended with ethanol and other alcohols for flame speed measurement, kinetic study and flame instability analysis. The laminar flame speeds of isooctane-ethanol blends (with ethanol liquid volume fractions of 10% and 20%) in air was presented at different initial temperatures by Gülder [35]. The results suggested ethanol has a significant promoting influence on 2

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23.33%, 23.33%, respectively. The purity of the original fuel components was 99.99%. The test procedures were as follows: heating vessel to initial temperature, the upper valve of fuel inlet of the vessel was opened, and a micropipette was used to pour the E30 fuel into the closed chamber formed by the two valves. Then the upper valve was closed quickly, sequentially the second valve was opened to allow E30 to enter into the vessel and then was closed. The E30 evaporated rapidly in an environment with high temperature and near vacuum; the synthetic air and diluent gas were fed into the vessel and the mixture was allowed to stand for 10 min before ignition and absence of flow motion. At this point, the camera and ignition electrode were synchronously triggered to record the spherically expanding flame of E30air-nitrogen premixed mixture. Each condition was repeated three times to reduce experimental uncertainty.

velocity reduction because of EGR. Making clear the influence of equivalence ratio, dilution gas, and initial condition on the laminar flame speeds is an important prerequisite for further study about the application of E30. To the authors' best knowledge, the experimental data concerning the laminar combustion characteristics of E30 are still scarce and not sufficiently reproduced. Therefore, the purpose of this work is to use a spherical expansion flame to study the impact of equivalence ratio, initial pressure, initial temperature, and dilution on the laminar flame speeds and flame instability of E30 with high-speed Schlieren method. 2. Experimental setup and methodology 2.1. Experimental setup

2.3. Data processing

The experiment setup was described in detail in a previous study [14], and so only a brief description of the experimental setup is reiterated here. The experimental setup includes a constant-volume combustion chamber, the heating system, an ignition circuit, a highspeed Schlieren photography system, and the data acquisition system. The experimental setup is depicted schematically in Fig. 1. The combustion chamber was a stainless-steel sphere vessel with an inner diameter of 400 mm, which was beneficial to reduce pressure fluctuations as a function of absolute flame diameter and improve the overall accuracy of the test due to the low volume ratio of flame to chamber in early flame propagation. Two quartz windows were installed in the vessel to provide an effective optical channel with a diameter of 76 mm; the vessel was covered with heating tapes to realize a uniform temperature distribution. Two stainless ignition electrodes with a gap of 1.5 mm were placed in the center of the chamber. A Phantom v7.3 highspeed camera with 10,000 fps which can take pictures with 512*512 pixel was operated to record the Schlieren signal generated by the flame and the optical setup.

Previous research [51–53] has revealed that the ignition energy effect and pressure rise in the vessel influence can be neglected for the flame radii exceeding 5 mm and under more than 25 mm (corresponding to a 0.1% volume of the 400 mm inner-diameter spherical). Moreover, within the radius range of 25 mm, no flame front with apparently fully developed cellular structure was observed. Therefore, in present study, the flame images with flame radii ranging from 6 to 25 mm were selected for analysis in order to eliminate the impact of the ignition, increasing vessel pressure, and flame front instability. In order to assess the laminar combustion characteristics of the fueloxidizer mixtures, a number of parameters were calculated. The stretched flame propagation speed Sb can be calculated by:

Sb =

drf dt

(1)

The flame stretch rate α is defined as:

α= 2.2. Fuels preparation and test procedure

2 Sb rf

(2)

The linear relationship can calculate with [54]: The E30 mixture was prepared in advance by blending ethanol, nheptane, isooctane and toluene and it was allowed to stand for one hour to fully mix; the liquid volume ratio of the mixture was 30%, 23.33%,

Sb = Sb0 − Lb α

(3)

The unstretched flame propagation speed S0 b can be obtained with

Fig. 1. Schematic diagram of the experimental setup. 3

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the nonlinear extrapolation of Kelley and Law [55] as follows: 2

δSL =

2

⎛⎜ Sb ⎞⎟ ln ⎜⎛ Sb ⎟⎞ = −2 Lb α 0 0 Sb0 ⎝ Sb ⎠ ⎝ Sb ⎠

ρb ρu

n

Sb0

CSL =

(5)

ρu ρb

ν SL

(8)

2

⎜

⎟

(9)

where xi is any variable influencing the uncertainty of laminar burning velocity, ui is the error of variable xi. The error of equivalence ratio ϕ is mainly a function of the measurements of the partial pressure of vaporized fuel, synthetic air, and N2, and it can be calculated to be ± (1.0–2.4)%. The accuracy of the pressure gauge and thermocouples are ± 1 kPa and ± 1 K, respectively. Finally, the total uncertainty of laminar burning velocity was calculated to be ± (2–4) cm/ s.

(6)

The flame thickness is calculated as[56]:

δ=

⎟

∂SL (x i ) ⎞ ui ⎠ ⎝ ∂x i

∑⎛ i=1

where ρb is the burned gas density and ρu is the unburned gas density. The thermal expansion coefficient σ is defined as:

σ=

⎜

where M is the total number of experiments for the condition considered, TN−1 is the value of T-distribution with the freedom degree of N − 1 and 95% confidence interval, BSL is the standard deviation of laminar burning velocity. The systematic uncertainty CSL is calculated by:

(4)

where Lb is the Markstein length. According to the continuity of the flame front, the unstretched laminar burning velocity SL can express by:

SL =

TN − 1 BSL ⎞2 (CSL )2 + ⎛ N ⎠ ⎝

(7)

3. Results and discussion 2.4. Error analysis

3.1. Laminar flame propagation speed

The uncertainties of the laminar burning velocities measured and reported in this paper were calculated with Moffat’s theory [57].

Fig. 2 shows the variation of the stretched flame propagation speed as a function of equivalence ratios (ϕ) in range of 0.7–1.4. As shown in

Fig. 2. The relationship between stretched flame propagation speed and stretch rate. 4

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Fig. 2(a) in this paper, T = 453 K, P = 1 bar, β = 0% is regarded as the reference condition. Fig. 2(b) depicts results of a higher initial temperature (T = 498 K), while Fig. 2(c) shows the effects of initial pressure (P = 3 bar), and Fig. 2(d) illustrates the effects of changing dilution ratio (β = 5%). According to the Eq. (2), the sectorial area (sandwiched by two green lines) of the laminar flame data can be drawn in Fig. 2, which indicates the selected flame radii range from 6 to 25 mm. The data presented in Fig. 2 can be used to extrapolate the unstretched flame propagation speed. The most appropriate type of fitting method, either linear or nonlinear, depends on the slope of the flame propagation speed relative to the stretch rate, namely the absolute value of Markstein length [58–63] and in this paper both linear and nonlinear fitting methods are used to obtain the unstretched propagation speed S0 b. The nonlinear fitting method was adopted for absolute values of Lb larger than zero (when ϕ ≤ 1.1 as shown in Fig. 2(a)) and linear fitting is adopted in other conditions. Fig. 2, in addition to showing the raw experimental data, also overlays the curve fits which were used to calculate S0 b; in all cases, the experimental data display good agreement with the fitted lines.

becomes insensitive to equivalence ratio with increasing initial temperature. However, similar behavior is not found in the adiabatic flame temperature and Lewis number over the same range. Therefore, this

3.2. Laminar burning velocity Fig. 3(a) shows the laminar burning velocity with ϕ = 0.7–1.4 at T = 408, 453 and 498 K. The laminar burning velocity has a positive correlation with the initial temperatures (described by SL∝ T0m , m = 1.5–2.0), which is attributed to enhancing the adiabatic flame temperature. Because if the dependence of specific heat capacity and dissociation on temperature are not considered, an increase in the initial temperature will increase the temperature of the burned gas by the same value [64]. Fig. 3 (b) shows the laminar burning velocity under ϕ = 0.7–1.4, P = 1, 2 and 3 bar. The unstretched laminar burning velocity decreases as initial pressures increase and the reasons for this phenomenon can be explained from the following two aspects. First, from the perspective of combustion thermodynamics, the pyrolysis reactions increase as initial pressure decreases, the adiabatic flame temperature and the rate of chemical reactions increase, such that the laminar flame speed is expected to increase. Second, from the point of chain reactions, the pyrolysis reactions will be inhibited with the initial pressure increasing, resulting in suppressed chain reactions, which is expected to decrease the concentration of the active radicals, leading to the laminar flame speeds decrease. The trend of laminar burning velocity is the result of the combined effects of the above two factors, indicating that the active radical concentration plays the leading role, as burning velocities are observed to decrease with increasing initial pressure. Fig. 3(c) shows the laminar burning velocity for ϕ = 0.7–1.4 at β = 0%, 5% and 10%. The unstretched laminar burning velocity decreases with increasing dilutions because of the addition of diluent gas (N2) reduces the collision probability (dilution effect) of fuel molecules and oxygen molecules, and it also reduces the temperature of the flame reaction zone (thermal mass effect), both of which reduce the chemical reaction rates [65]. The decrease of flame temperature is also one of the dominant reasons for the reduced laminar burning velocity. The peak laminar burning velocities occur at ϕ = 1.1 at all initial conditions. Deviating from this equivalence ratio, the laminar burning velocity decreases. The decrease in laminar burning velocity of the lean mixture is because of the reduction in the amount of fuel per unit volume, leading to decreased adiabatic flame temperature and chemical reaction rates; the decrease in laminar burning velocity at rich mixture is mainly because of the amount of oxygen per unit volume is less than that is needed for complete combustion of the fuel, which also reduces the adiabatic flame temperature. Furthermore, comparing the change in magnitude of the laminar burning velocity on the rich and lean side of stoichiometric yields the observation that the laminar burning velocity is more sensitive to changes in equivalence ratio in lean mixture than rich mixture which agrees with results previously reported in the literature [66,67]. In addition, the unstretched laminar burning velocity

Fig. 3. The relationship between unstretched laminar burning velocity and equivalence ratio with T = 408, 453 and 498 K (a), P = 1, 2 and 3 bar (b), β = 0%, 5% and 10% (c). 5

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then displays a regular cellular structure; however, this effect only affects the stability of the flame propagation at large flame radius [75]. The hydrodynamic factors mainly depend on the thermal expansion coefficient and are reflected by the flame thickness. Combustion is an exothermic process, so thermal expansion is inevitable with premixed flames. The thermal expansion coefficient has a positive correlation

behavior may also be connected with the chemical kinetics of combustion, such as the active radical concentrations in the flame vicinity and elemental reaction rates, whose influences will be studied in future works. 3.3. Markstein length Markstein length, Lb, is an important parameter to characterize laminar combustion characteristics. The sensitivity of burning rate to flame stretch can be characterized by the Lb [14]. At Lb＞0, the flame propagation speed decreases with increasing stretch rate. The flame propagation speed of the protruding portion of the flame will be suppressed when the flame front appears cellular (stretch rate increases). Conversely, when the Lb reach negative values, which correspond to thermal-diffusive instabilities of the flame front and lead to a cellular flame structure [68], the flame speed will increase with increasing of stretch rate. Once the flame front appears cellular, the flame propagation speed will further increase, and increasing the instability of flame. Fig. 4(a) shows the Lb with ϕ = 0.7–1.4 at T = 408, 453 and 498 K. When the T = 408 K and 453 K, the Lb has a negative correlation with equivalence ratio. When the Lb increases to 1.4, the Lb changes from positive to negative, and the flame instability is enhanced. Lb is always positive and has a negative correlation with equivalence ratio when the initial temperature of 498 K, therefore the stability of flame decreases as equivalence ratio increases. Fig. 4(b) displays the relationship between Lb and equivalence ratio for P = 1, 2 and 3 bar. At a certain equivalence ratio, the Lb decreases as initial pressure increases, and correspondingly the stability of the flame front decreases. This is mainly because of the effect of the hydrodynamic instability because the Lewis number is insensitive to the initial pressure, indicating that the thermal-diffusive instability is insensitive to initial pressure [69]. In addition, with the increase of initial pressure, the flame thickness significantly decreases. The heat released by combustion creates a large thermal expansion which suggests that the hydrodynamic instability will always affect the evolution of the flame front. Under P = 1, 2 and 3 bar, the Lb decreases with increasing equivalence ratios. The Lb under all initial pressure conditions are negative at ϕ = 1.4, so the flame instability is strengthened. The variation of the Lb with equivalence ratio is in line with that for other fuels, such as ethanol [49], methane [47], and methanol [46,70], which exhibit decreasing Lb with increasing equivalence ratio and has similar results of Bechtoid et al. [71]. Fig. 4(c) shows the relationship between the Lb and the equivalence ratio at β = 0%, 5% and 10%. The Lb decreases as equivalence ratio increase. When the equivalence ratio increases to 1.4, the Lb changes from positive to negative, and so the instability of the flame is improved. The Lb increases with increasing dilution ratio at the lean mixture, and the dilution gas has a promoting effect on the flame stability at lean mixture. The Lb decreases as dilution ratio increases at rich mixture, and the diluent gas therefore has a weakening effect on the flame stability of rich mixtures. Similar conclusions have been drawn from previous studies [72,73]. In lean flames, the diluent gas replaces the fuel reduced the amount of fuel per unit volume decreases, leading to the promotion of flame stability, but in rich flames, the amount of oxygen per unit volume is less than what is needed for complete combustion of the fuel, result in sufficient burning and increasing flame instabilities. 3.4. Flame instability The thermal diffusion and hydrodynamic effects are two important aspects which can significantly affect the instability of the flame [74]. Thermal diffusion dominates the flame instability in small flame radius. As the development of the flame progresses, the influence of hydrodynamics becomes increasingly important. The hydrodynamic instability manifests in such a manner that the flame front wrinkled and

Fig. 4. Markstein length versus equivalence ratio with T = 408, 453 and 498 K (a), P = 1, 2 and 3 bar (b), β = 0%, 5% and 10% (c). 6

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Fig. 6(b), the flame thickness has an inverted U-shaped distribution with increasing equivalence ratio fort a certain initial pressure. This is consistent with the above conclusion regarding the peak laminar burning velocity. The flame thickness decreases with the initial pressure increasing for a certain equivalence ratio. Therefore, the hydrodynamic

with the hydrodynamic instability, and when the flame thickness is large, it reduces the density gradient between burned and unburned gas. Therefore, it is advantageous to suppress the occurrence of hydrodynamic instability. The thermal-diffusive instability is caused by the preferential diffusion of mass near the flame front with respect to heat. If there are more components with stronger diffusive tendencies, the preferential diffusion will stabilize the flame. The thermal-diffusive instability depends on the Lewis number of the mixture and will only occur when the Lb is negative [76]. For outwardly expanding flames, Le > 1 means that the development of wrinkles on the flame will be suppressed, making the flame front smooth and stable. Conversely, Le < 1 means that the flame front suffers the thermal-diffusive instability, and the instability of the flame will be enhanced and eventually transition to turbulent flame propagation. Fig. 5 shows the effective Lewis number, flame thickness and thermal expansion coefficient of the combustible mixture at the ϕ = 0.7–1.4 and at P = 1 bar and β = 0% and T = 408, 453 and 498 K. In Fig. 5(a), the effective Lewis number decreases as initial temperature increase, but is always greater than 1, indicating that the flame front preferential diffusion effect gradually decreases with the initial temperature increasing. For a certain initial temperature, the effective Lewis number of the mixture decreases as equivalence ratio increases, but is always greater than 1, indicating that the flame front preferential diffusion effect decreases as equivalence ratio increases. In Fig. 5(b), the flame thickness has an inverted U-shaped behavior with increasing equivalence ratios and reaches the minimum value at ϕ = 1.1. The thinner the flame thickness, the shorter the distance the temperature propagates to the preheating zone and the stronger the hydrodynamic instability [64]. The flame thickness decreases with increasing initial temperature when the flame at lean mixture. The flame thickness decreases with increasing initial temperature at rich mixture, but the magnitude of the change is small, indicating that the initial temperature has small effects on the flame thickness of rich mixtures. The viscosity of the mixture changes little with initial temperature, so the flame thickness changes contrary to the laminar burning velocity because the ν flame thickness can be described by δ = S . Dependent on whether a L mixture is rich or lean, the effect of the initial temperature on the flame thickness is different, and therefore the effect of the initial temperature on the flame hydrodynamics instability cannot be determined solely by the flame thickness. Fig. 5(c) displays the thermal expansion coefficient as a function of equivalence ratio at P = 1 bar, β = 0%. The thermal expansion coefficient has a negative correlation with initial temperature for a certain equivalence ratio. It suggested that the increase in the initial temperature significantly weakens the hydrodynamic instability because flame thickness is insensitive to the initial temperature. Regardless of the initial temperature, the thermal expansion coefficient has a U-shaped behavior as a function of equivalence ratio, peaking at ϕ = 1.1, which suggests that the hydrodynamic instability becomes strongest and is consistent with the conclusion that the flame thickness reaches a minimum at this equivalence ratio. In former section, we find that the laminar burning velocity is more sensitive to variations in stoichiometry on the rich side than the lean side. In Fig. 5(a), similar behavior can be observed in the computed Lewis number values. This behavior indicates that the mixture has similar diffusive flame characteristics in the rich regime, and thereby similar flame structures. Fig. 6 shows the effective Lewis number, flame thickness, and thermal expansion coefficient at the ϕ = 0.7–1.4 for T = 453 K, β = 0 and P = 1, 2 and 3 bar. In Fig. 6(a), with the initial pressure increasing, the effective Lewis number increases and is always greater than 1, indicating that the flame front preferential diffusion effect increases as increase initial pressure, which is beneficial to the stable propagation of the flame. At a given initial pressure, the effective Lewis number decreases as the equivalence ratio increase, indicating that the flame front preferential diffusion effect decreases as increase equivalence ratio, but it is always larger than 1, indicating the promotion effect of the preferential diffusion effect on flame front stability gradually decreases. In

Fig. 5. Effective Lewis number, flame thickness and thermal expansion coefficient of the mixture in different initial temperatures. (a) Lewis number, (b) Flame thickness, (c) Thermal expansion coefficient. 7

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Fig. 7. Effective Lewis number, flame thickness and thermal expansion coefficient of the mixture in different dilution ratios. (a) Lewis number, (b) Flame thickness, (c) Thermal expansion coefficient.

Fig. 6. Effective Lewis number, flame thickness and thermal expansion coefficient of the mixture in different initial pressures. (a) Lewis number, (b) Flame thickness, (c) Thermal expansion coefficient.

coefficient has a U-shaped distribution at the equivalence ratio in range from 0.7 to 1.4 and peaks at ϕ = 1.1, which indicates that the hydrodynamic instability peaks for ϕ = 1.1. Fig. 7(a) shows the effective Lewis number of the combustible mixture with ϕ = 0.7–1.4 at T = 453 K and P = 1 bar. It can be seen that the effective Lewis number of the mixture is insensitive to the

instability of the flame front is enhanced with initial pressure increasing. In Fig. 6(c), the thermal expansion coefficient increases as increase initial pressure, but the effect is weak, indicating the sensitivity of the thermal expansion coefficient to the initial pressure is low. The hydrodynamic instability has a positive correlation with the initial pressure. Regardless of the initial pressure, the thermal expansion 8

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and thereby slightly improved the engine performance and reduced the exhaust emissions relative to neat gasoline [78]. Compared with gasoline, E30 can decrease the emission of CO (75%) and HC (66%) in spark-ignition engines operated under low loads [79]. Fig. 9 shows the comparison of unstretched laminar burning velocity of E0, E10 and E30 under initial condition of 408 K and 1 bar. The laminar burning velocity

dilution ratio and is always greater than 1, indicating that at the same equivalence ratio, the diluent gas has minimal effects on the thermaldiffusion instability of the flame. The Lewis number increases as the dilution ratio increases at lean mixture. On the contrary, the Lewis number decreases as the dilution ratio increases in the rich mixture. This suggests that the effect of dilution ratio is insensitive to the Lewis number at ϕ = 0.7–1.4. Finally, a trend compared to the expected trend is opposite. Fig. 7(b) shows the flame thickness of the combustible with ϕ = 0.7–1.4 at β = 0%, 5% and 10%. At a given dilution ratio, the flame thickness first decreases and then increases as a function of equivalence ratio. The flame thickness has a positive correlation with dilution ratio at a certain equivalence ratio, thereby indicating that by increasing the dilution ratio the hydrodynamic instability of the flame front is weakened. Fig. 7(c) shows the thermal expansion coefficient of the combustible mixture for ϕ = 0.7–1.4 at β = 0%, 5% and 10%. Increasing the dilution ratio leads to the decrease of the thermal expansion coefficient for a certain equivalence ratio, but the magnitude of the change is extremely small, especially in the case of lean mixture. As the dilution ratio increases, the hydrodynamic instability is reduced, which is consistent with the previous conclusions regarding the influence of dilution ratio on flame thickness. Regardless of the dilution ratio, the thermal expansion coefficient first increases and then decreases with increasing equivalence ratio, reaching a maximum at ϕ = 1.1, which has the strongest hydrodynamic instability, consistent with the conclusion that the flame thickness reaches its minimum at this equivalence ratio. 3.5. Adiabatic flame temperature Fig. 8 shows the adiabatic flame temperature as a function of equivalence ratio for T = 408, 453 and 498 K, P = 1, 2 and 3 bar and β = 0%, 5% and 10%. The adiabatic flame temperature characterizes the heat release properties of combustible mixtures [77], which is important for laminar flame speeds. It affects the diffusive properties, and determines the temperature difference between the burned and unburned zones. The adiabatic flame temperature in this paper is calculated by Chemkin. In Fig. 8(a), the adiabatic flame temperature increases with the increase of the initial temperature. On the one hand, when the initial temperature increases and the initial pressure does not change, the fuel density per unit volume in the vessel decreases, and the decrease in the amount of the combustible mixture results in the decrease of the adiabatic flame temperature; In addition, the chemical reaction rate increases with the initial temperature increasing. According to the results, it is known that the dominant effect on adiabatic flame temperature with initial temperature increasing is due to the the promoting effect on the chemical reaction rate. In Fig. 8(b), in the vicinity of the equivalence ratio of 1.0 and 1.1, the adiabatic flame temperature has positive correlation with initial pressure. The adiabatic flame temperature changes little as the initial pressure increases at the combustible is too lean or too rich due to the balanced increase of total combustion heat released and system mass. In Fig. 8(c), the adiabatic flame temperature has negative correlation with the dilution ratio, which is the inevitable result of nitrogen dilution. The amount of combustible mixture per unit mass is reduced due to the replacement of the fuel-oxidizer mixture. The total calorific value per unit mass is reduced, and the addition of diluent gas increases the mixture heat capacity, which reduces the adiabatic flame temperature. In Fig. 8, the peak value of the adiabatic flame temperature appears at the equivalence ratio of 1.1 at all initial conditions, which is consistent with the conclusion that the above-mentioned laminar burning velocity and flame thickness reached an extreme value at ϕ = 1.1. 3.6. Performance of E0, E10, and E30

Fig. 8. Adiabatic Flame Temperature as a function of equivalence ratio at T = 408, 453 and 498 K (a), P = 1, 2 and 3 bar (b), β = 0%, 5% and 10% (c).

Previous studies have found that when ethanol is used as a gasoline blending component the resulting mixture enhanced the combustion 9

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E30 is significantly increased (up to 28.6%) relative to that of E0, elucidating engine studies which report improved combustion under lean or dilute conditions with ethanol addition. Acknowledgements The research performed by Qing Yang, Zechang Liu, Xu He and Magnus Sjoberg was financially supported by the the Co-Optima project sponsored by DOE EERE. Sandia National Laboratories is a multisession laboratory managed and operated by NTESS for NNSA under contract DE-NA0003525. References [1] Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog Energy Combust Sci 2009;35(5):398–437. [2] Yu L, Shuai S, Li Y, Li B, Liu H, He X, et al. An experimental investigation on thermal efficiency of a compression ignition engine fueled with five gasoline-like fuels. Fuel 2017;207:56–63. [3] Zhang Y, Shen T. Combustion variation feedback control approach for multi-cylinder spark ignition engines. IFAC-PapersOnLine 2018;51(31):105–10. [4] Agarwal AK, Singh AP, Maurya RK. Evolution, challenges and path forward for low temperature combustion engines. Prog Energy Combust Sci 2017;61:1–56. [5] De Boer C, Chang J, Shetty S. Transonic combustion – a novel injection-ignition system for improved gasoline engine efficiency. SAE Int. 2010. [6] Liu F, Gao Y, Zhang Z, He X, Wu H, Zhang C, et al. Study of the spray characteristics of a diesel surrogate for diesel engines under sub/supercritical states injected into atmospheric environment. Fuel 2018;230:308–18. [7] Sluder CS, Moriarty K, Jehlik F, West BH. Co-optimization of fuels & engines: misfueling mitigation. Golden, CO (United States): National Renewable Energy Lab. (NREL); 2017. [8] Chen L, Stone R, Richardson D. A study of mixture preparation and PM emissions using a direct injection engine fuelled with stoichiometric gasoline/ethanol blends. Fuel 2012;96:120–30. [9] Sjöberg M, He X. Combined effects of intake flow and spark-plug location on flame development, combustion stability and end-gas autoignition for lean spark-ignition engine operation using E30 fuel. Int J Engine Res 2016;146808741774029. [10] Yusri IM, Mamat R, Najafi G, Razman A, Awad OI, Azmi WH, et al. Alcohol based automotive fuels from first four alcohol family in compression and spark ignition engine: a review on engine performance and exhaust emissions. Renew Sustain Energy Rev 2017;77:169–81. [11] He X, Li Y, Sjöberg M, Vuilleumier D, Ding C-P, Liu F, et al. Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel. Proc Combust Inst 2019;37(4):4955–63. [12] Wang C, Li Y, Xu C, Badawy T, Sahu A, Jiang C. Methanol as an octane booster for gasoline fuels. Fuel 2019;248:76–84. [13] Xie Y, Wang X, Bi H, Yuan Y, Wang J, Huang Z, et al. A comprehensive review on laminar spherically premixed flame propagation of syngas. 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Effect of the valve timing and the coolant temperature on particulate emissions from a gasoline direct-injection engine fuelled with gasoline and with a gasoline–ethanol blend. Proc Inst Mech Eng D: J Automob Eng 2012;226(10):1419–30. https://doi.org/10.1177/ 0954407012444966. [21] Meng L. Chapter 11 – ethanol in automotive applications. In: Basile A, Iulianelli A, Dalena F, Veziroğlu TN, editors. Ethanol. Elsevier; 2019. p. 289–303. [22] Nadim F, Zack P, Hoag GE, Liu S. United States experience with gasoline additives. Energy Policy 2001;29(1):1–5. [23] Moreira JR, Goldemberg J. The alcohol program. Energy Policy 2007;27(4):229–45. [24] Rh HWC, Wu TL, Lin TH. Engine performance and pollutant emission of an SI engine using ethanol-gasoline blended fuels. Atmos Environ 2002;36(3):403–10. [25] Sen AK, Medina A, Curto-Risso PL, Hernández AC. Effect of ethanol addition on cyclic variability in a simulated spark ignition gasoline engine. Meccanica 2014;49(10):2285–97. 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Fig. 9. The unstretched laminar burning velocity of E0, E10 and E30 at P = 1 bar.

of E30 is 15.3 cm/s larger than E0, or 28.6% greater for an equivalence ratio of 1.0. Meanwhile, E10 is 5.0 cm/s larger than E0, or 9.3% greater for an equivalence ratio of 1.0. Ethanol can significantly enhance the laminar burning velocity of resulting ethanol-gasoline blends once the fraction of ethanol becomes significant; in the case of E10, the mole fraction of ethanol is 20.2%, while for E30, the mole fraction is 49.75%, which clearly has a significant impact on the combustion performance. The laminar burning velocity results provide insight into the significant improvements in engine performance which have been observed in some spark-ignition engine studies involving gasoline-ethanol blends; under conditions at which flame development time and flame stability are critical, gasoline-ethanol blends provide an advantage over neat gasoline. 4. Conclusions Experimental tests of laminar combustion characteristics of E30-air mixtures were carried out in a spherical constant-volume combustion vessel with high speed Schlieren images over a range of initial conditions spanning ϕ = 0.7–1.4, T = 408, 453 and 498 K, P = 1, 2 and 3 bar and β = 0% 5% and 10%. The main conclusions are: 1) The laminar burning velocity increases with increasing the initial temperature, decrease with increasing the initial pressure and with increasing the dilution ratio. Regardless of the initial temperature, initial pressure and dilution ratio, the peaks of laminar burning velocities always occur at the ϕ = 1.1. The initial conditions have the similar qualitative influence on the adiabatic flame temperature as on the laminar burning velocity. 2) With increasing initial pressures, the flame thickness decreases and the effective Lewis number and thermal expansion coefficient increase slightly. All these three parameters decrease with increasing initial temperature. With increasing dilution ratio, the effective Lewis number remains almost constant, but the flame thickness decreases and the thermal expansion coefficient increases. 3) The flame instability of E30-air mixtures enhances with the initial pressure increasing. The effects of initial temperature on the flame stability of lean and rich mixtures are exactly opposite. This conclusion also applies to the dilution ratio. Increasing the initial temperature promotes the flame stability of rich mixtures and increasing the dilution ratio enhances the flame stability of lean mixtures. 4) The effect of ethanol addition to gasoline has measurable impacts on laminar combustion characteristics. The laminar burning velocity of

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