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A comparison study on the combustion and sooting characteristics of base engine oil and n-dodecane in laminar diffusion flames
Applied Thermal Engineering 158 (2019) 113812
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Research Paper
A comparison study on the combustion and sooting characteristics of base engine oil and n-dodecane in laminar diffusion flames
T
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Hu Wang, Yixuan Li, Zohaib Iqbal, Yang Wang , Chunshan Ma, Mingfa Yao State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China
H I GH L IG H T S
tendencies of n-dodecane and base engine oil were compared experimentally. • Sooting LaminarSMOKE code was adopted to simulate the PAHs’ formations. • The mechanism of n-dodecane was revised to simulate combustion of engine oil. • Reaction • Formation of PAH species in engine oil flames were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: LaminarSMOKE Engine oil Dodecane Soot
With the introduction of more strict emission legislations and advanced engine technologies, the contribution of engine oil to soot emissions from engines becomes more and more significant. In order to understand the sooting tendency of engine oil, experiments and chemical kinetic analysis were performed on a laminar diffusion flame of the base engine oil, together with a similar n-dodecane flame for comparison. Distributions of polycyclic aromatic hydrocarbon (PAH) and soot in the flames were measured using the method of laser-induced fluorescence and two-color planar laser induced incandescence respectively in the two flames. A reduced n-dodecanePAH mechanism was adopted to simulate the PAHs’ formations in the n-dodecane flame with the laminarSMOKE code. To simulate the four-ring aromatic species (A4), a precursor of soot in simulations, in the base engine oil flame, the reaction rate of a formation reaction of the A4 species, A2R5 + C4H2 = > A4, was enhanced by 3.6 times to better predict the PAH formation process of base oil. With this modification, the n-dodecane-PAH mechanism can well predict the A4 species in the base engine oil flames. Based on the experimental measurements and numerical simulations, the laminar diffusion flames of n-dodecane and base engine oil were analyzed and compared.
1. Introduction With the development of the automobile industry, emissions, especially particulate matters, pose a great burden to the atmosphere [1]. Stringent emission regulations were introduced to reduce the emissions. A great deal of research work were reported to meet the requirements of the increasingly stringent emission regulations and to improve the air quality, such as optimizing the combustion process, improving the fuel quality [2–4], and developing new combustion modes [5–8], etc. With the adoptions of these new technologies, the particulate matters in the engine exhaust were reduced dramatically. However, it has been reported that the soot emission could still be measured even using natural gas as the fuel in a homogenous charge compression ignition engine [9]. Since natural gas is generally believed
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to be a soot free fuel, the soot emission can be possible to partially originate from the burning of engine oil that is widely used for lubrication in engines. With the implementation of the cleaner combustion technologies, the contribution of engine oil to the particle emissions will be more and more significant. However, the combustion of engine oil and its contribution to engine pipe out soot emission is still one of the unsolved problems in the combustion field. Generally, engine oil is mainly burned in the engine cylinders [10]. As the engine speed increases, more engine oil will be splashed into the cylinder and then be burned [11]. Brandenberger et al. [12] reported that engine oil was the main source of polycyclic aromatic hydrocarbons (PAHs) in the emissions of an engine fueled with diesel. Kytö et al. [13] reported that the particulate emissions from a heavy-duty diesel engine fueled with diesel+2% engine oil blend was about two
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.applthermaleng.2019.113812 Received 10 November 2018; Received in revised form 24 April 2019; Accepted 22 May 2019 Available online 23 May 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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times higher than that without engine oil. Taylor et al. [14] investigated the influence of volatility of engine oil on the particle emissions from a diesel engine and reported that the light-weight components of engine oil played an important role in particulate formations. Kahandawala et al. [15] conducted experiments in a modified single pulse reflected shock tube filled with diesel, engine oil, oxygen, and helium to investigate the influence of pressure on the particulate emissions. They reported that the addition of engine oil increased the particulate yields at higher pressures, and that the increase was more evident with higher engine oil concentrations. Andrews et al. [16] added calcium in engine oil as tracers to clarify the contribution of engine oil on the particulate emissions from an engine fueled with diesel. They reported that the contribution of engine oil to the soot formation decreased in the exhaust as the engine oil combustion efficiency increased. Kleeman et al. [17] reported that the reactions of engine oil were the reason of particle formation in the gasoline engine. Froelund et al. [18] reported that increasing the engine oil viscosity could significantly reduce the particle emission. All of the above investigations indicated that engine oil contributed significantly to the formations of particle emissions in the engine. However, the combustion mechanism of engine oil has not been explored to reveal the contribution engine oil combustion on soot formation and emission. Engine oil is a complex mixture of the base engine oil, mainly straight-chain alkanes (n-alkanes) ranging from C20 to C70, and about 10% of other additives [19]. In the chemical kinetic studies of the hydrocarbons, Sarathy et al. [20] developed a combustion mechanism of 2-methylalkanes to C20, and Westbrook et al. [21] developed detailed mechanism for n-alkanes up to C16. Due to the complex composition of the base engine oil, it is not possible to develop a detailed combustion mechanism for it. However, the combustion characteristics of the nalkanes, such as the burning velocity, tend to converge as the carbon number increases. Therefore, in order to simplify the investigation, the base engine oil was studied in this work, and n-dodecane was chosen as a representative to study the sooting tendencies of engine oil, due to the similarity of the structures between n-dodecane and base engine oil, and a suitable combustion mechanism size that favors the simulation and analysis. Therefore, in this work, the sooting tendency of base engine oil and n-dodecane were investigated experimentally and numerically in laminar diffusion flames. The laser-induced fluorescence (LIF) and twocolor planar laser induced incandescence (TC-PLII) [22] were used to measure the concentrations of PAH and soot, respectively. The four-ring aromatic species (A4, mainly pyrene) were used to demonstrate the formation trend of soot in these two flames, since A4 is usually considered to be an important soot precursors, as proposed by Vishwanathan and Reitz [23]. Moreover, the diffusion flames were simulated with the laminarSMOKE code. Based on the experimental measurements and numerical simulations, the combustion and sooting behaviors of n-dodecane and base engine oil in laminar diffusion flames were compared and analyzed.
Fig. 1. Schematic diagram of the experiment setup. Table 1 The boundary conditions of the experiments.
Fuel Initial velocity/mL·min−1 Ambient pressure/MPa Ambient temperature/K
Flame a
Flame b
n-dodecane 0.15 0.1 300
engine oil 0.15 0.1 300
uncertainties within ± 5%. The flow rates of the fuels were accurately measured and controlled by a mass flow meter. After ignition, a steady flame was formed above the quartz burner at ambient pressure. The LIF was used to detect the PAHs volume fraction by using 266 nm excitation and the TC-PLII was employed for measuring the soot volume fraction by using the second harmonics at 532 nm. For the LIF measurements, the fourth harmonics at 266 nm from an Nd:YAG laser was used to excite the PAHs. The resulting fluorescence signal was detected using an ICCD camera equipped with a lens. The PAHs can be detected independently by the selecting different fluorescence spectral bands. The monocyclic aromatics (mainly benzene, A1) were measured with the 315 nm bandpass filter. The double cyclic aromatics (mainly naphthalene, A2) and three-ring aromatics (mainly phenanthrene, A3) were measured with 400 nm low-pass filter which allows the spectrum wavelength in the range of 350–400 nm. The four-ring aromatics (mainly pyrene, A4) were measured with the 492 nm low-pass filter which allows the spectrum wavelength in the range of 400–480 nm. The soot particles were excited by an Nd:YAG laser sheet to emit the incandescence, with the energy of the laser pulse being kept constant at 25 mJ and a pulse-duration of 8 ns. After filtration by an optical filter (Andover, CA), the incandescence signal was also recorded by the ICCD camera. When the strong 266 nm beam was employed for the excitation of PAHs, there were some influence on both the PAHs fluorescence and soot incandescence in the signal detected. In order to distinguish between the above effects, two measurements were carried out, i.e., delayed detection (50 ns after the laser pulse has passed) to avoid the detection of the LIF signal from the fluorescence signal of the PAHs, due to the short lifetime of the former, and prompt detection (synchronized with the laser pulse) involving the contributions from both LII and LIF. For both the PAHs LIF and soot LII measurements, 1000 single shots were recorded and averaged. The signal intensity of the 1000 singleshot measurements showed variations of 7% (1σ). The temperatures of the flames were measured by the two-color method.
2. Experimental methods The experiments were carried out on the diffusion combustion platform in the State Key Laboratory of Engines at Tianjin University. The schematic diagram of the experimental system is shown in Fig. 1. Details of the experimental setup were reported in [22]. In brief, the experimental apparatus consisted of three parts, i.e., a laminar diffusion flame quartz burner, an Nd:YAG laser, and an intensified charge-coupled device (ICCD) camera (DH720i, Andor, U.K.). The inner-diameter of the quartz burner was 4 mm and the outside-diameter was 6 mm. Since the base engine oil and n-dodecane are liquid in room temperature (300 K), a heating vaporization section was placed before the quartz burner outlet. As shown in Table 1, the mean velocity of the vaporized fuel was controlled at 0.15 mL·min−1 in both flames. The fuel flow rates were regulated by a syringe pump with the absolute
3. Simulation methods The laminarSMOKE code [24–29] was used to simulate the laminar diffusion flames. The laminarSMOKE code is a framework for 2
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numerically modeling the multicomponent, compressible, thermallyperfect mixtures of gases by coupling with the related chemical mechanisms. The laminarSMOKE code takes the radiation of the major flame species and the Fickian, thermal diffusion into consideration. The transport properties (mass diffusion coefficients, viscosity and thermal conductivity) of the flame species were taken from the CHEMKIN transport database. In laminar flame conditions, the laminarSMOKE code uses an operator-splitting technique to solve the conservation equations of total mass, momentum, mixture energy, and individual species mass fractions. Because the flames investigated in this work are axisymmetric, the numerical calculations were performed on a nonequispaced, stretched, two-dimensional, rectangular domain, with a length of 100 mm and a width of 40 mm. A mesh with 18,000 computational cells (150 × 120) was fine enough for the purposes of the present work [27]. Along the centerline, a finer numerical grid near the exit of the burner was adopted, with an average mesh size of about 0.5 mm. The flame region is described using the uniform cell spacing along the radial direction. The reduced n-dodecane-PAH kinetic model of Wang et al. [30] was adopted in this work. Basically, the reduced mechanism merged the reduced mechanism of n-dodecane [21] and that of PAHs [31]. The reduced n-dodecane-PAH mechanism includes 100 species and 432 reactions. The thermodynamic and transport properties of the mechanism were taken from the detailed n-alkane and PAH mechanisms of Westbrook et al. [21] and Slavinskaya et al. [32], respectively. The reduced n-dodecane-PAH mechanism could well predict the formation of soot in a constant volume vessel based on the authors’ previous study [30]. 4. Results and discussion 4.1. Experiment results of n-dodecane and base engine oil flames
Fig. 3. Comparisons of the soot concentrations between the laminar diffusion flames of n-dodecane and base engine oil: a., Radius concentrations at h = 25 mm; b., Axial concentrations in the central line.
Fig. 2 shows the experimentally measured soot distributions in the n-dodecane and base engine oil flames. The horizontal axis is the radius of the flame and the vertical axis is the flame height. Due to the axisymmetric structure of the flames, only half of the experimental result is presented. It can be observed that the peak soot concentration in the base engine oil flame is significantly higher than that in the n-dodecane flame. Fig. 3a shows the radial soot concentration profiles in these two flames. The horizontal axis is the radial distance (r) and the vertical axis is the soot concentration at a height above the burner outlet (h) of 25 mm. It can be seen that the radial soot concentrations both peak at about r = 1.5 mm in two flames. The radial peak soot concentrations at h = 25 mm are about 480 ppb and 190 ppb in the base engine oil and n-
dodecane flames, respectively. The radial peak soot concentration at h = 25 mm in the base engine oil flame is about 2.5 times higher than that in the n-dodecane flame. Fig. 3b shows the axial soot concentration in the n-dodecane and base engine oil flames. The horizontal axis is the flame height above the burner outlet and the vertical axis is the concentration in the central line of the flames. The axial soot concentrations both peak at h = 30 mm in the two flames. The peak soot concentrations at the central line of the flames are about 150 ppb and 80 ppb in the base engine oil and n-dodecane flames, respectively. The peak soot concentration at the central line in the base engine oil flame is about 1.9 times higher than that in the n-dodecane flame. Fig. 4a–c show the experimental PAHs distributions in the base engine oil and n-dodecane flames at h = 25 mm, respectively. It can be seen the concentration of A1 is the highest among the four species, with a peak concentration of around 10 ppm at r = 2 mm in the base engine oil flame. The concentration of A2 and A3 is the sum of both species. The position of radial peak concentration of A4 is at around r = 1.8 mm. The trend of radial concentration of these two flames is that the concentration slightly increased as extending outward along the radius, and then dropped rapidly after the peak position. The radial peak A4 concentration at h = 25 mm in the base engine oil flame is about 3.0 times higher than that in the n-dodecane flame. Fig. 4d–f show the experimental axial concentration of PAHs. It can be seen that the concentrations of A1, A2 and A3, A4 peak at h = 19 mm, 23 mm and 28 mm in both flames, respectively. The position of peak concentration of these species move further toward the burner outlet and the concentration are also decreased as PAH molecular weight increased, which is consistent with the trend in Jia’s work [33]. The peak A4 concentrations at the central line of the flames are about 6 ppm and 2.6 ppm in the base engine oil and n-dodecane flames, respectively. The
Fig. 2. The experimental soot distributions in the laminar diffusion flames of ndodecane (a) and base engine oil (b). 3
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Fig. 4. Comparisons of the PAHs concentrations between the laminar diffusion flames of n-dodecane and base engine oil: a., b., c., Radius concentrations at h = 25 mm; d., e., f., Axial concentrations in the central line.
peak A4 concentration at the central line in the base engine oil flame is about 2.3 times higher than that in the n-dodecane flame.
r = 1.8 mm and h = 28 mm, and the trend and magnitude of the simulated results are in accordance with the experimental results in Fig. 4. Since A4 is generally considered to be the soot precursor which has great influence on soot formation, it is important to observe similar results in the simulations, which also enables the possibility to explore the underlying PAH formation reaction pathways in these two flames. Fig. 6 shows the reaction pathways analysis of the PAHs in the laminar diffusion flames of n-dodecane. It can be seen that around 76.3% of A1 in the n-dodecane flame is produced from the reactions between the C2 and the C4 species,
4.2. Combustion reaction pathway analysis of n-dodecane flame Fig. 5a shows the simulated mole fraction profiles of the PAHs at h = 25 mm and Fig. 5b shows the simulated axial results in the central line of the n-dodecane flame. It can be seen that the positions of the radial and axial peak mole fractions of A4 are also at around
C4H5 + C2H2 = A1 + H
(R1)
C4H4 + C2H3 = A1 + H
(R2)
Combination of C3H3 radicals accounts for about 17.6% of A1 formation, C3H3 + C3H3 = A1
(R3)
As for the consumption, about 65.9% of A1 is consumed via the reaction with C4H5 to form A2, C4H5 + A1 = A2 + H2 + H
(R4)
H-abstraction by H atom to produce phenyl radical (A1-) accounts
Fig. 5. The simulated mole fraction profiles of PAHs in the laminar diffusion flame of n-dodecane: a., Radius results at h = 25 mm; b., Axial results in the central line.
Fig. 6. Flow rate analyses of PAHs in the laminar diffusion flames of n-dodecane and base engine oil. Normal text, n-dodecane; italic text, base engine oil. 4
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for 28.0% of A1 consumption, A1 + H = A1- + H2
(R5)
Reactions of A1-lead to the formation of C2H2 and C4H3 or C3H3, A1- = C4H3 + C2H2
(R6)
A1- + H = C3H3 + C3H3
(R7)
R6 and R7 account for about 70.0% and 7.4% of A1-consumption, respectively. A2 is overwhelmingly produced from the reaction between A1 and C4H5 (R4). Similar to A1, A2 can also be consumed by Habstraction to form naphthyl radical (A2-), but with a much higher branching ratio of 90.8%, A2 + H = A2- + H2
(R8)
In addition to R8, A2 can also react with diacetylene (C4H2) to form A3, accounting for about 3.0% of A2 consumption, A2 + C4H2 = A3
Fig. 7. Comparisons of the simulated mole fraction profiles of A4 in the laminar diffusion flames of n-dodecane and base engine oil by the reduced n-dodecanePAH mechanism with and without the revision of R16 (A2R5 + C4H2 = > A4): a., Radial results at h = 25 mm; b., Axial results in the central line.
(R9)
The radical produced in R8, A2-, can either give a C2H2 to yield 1(2-ethynyl)phenyl radical (A1C2H-), or react with a C2H2 to produce ethynylnaphthalene (A2R5), with branching ratios of 50.5% and 40.5%, respectively, A2- = A1C2H-+C2H2
(R10)
A2- + C2H2 = A2R5 + H
(R11)
A3- = A2- + C4H2 A3- may also reversely transform to A3 and A4.
4.3. Combustion reaction pathway analysis of base engine oil flame
The radical produced in R10, A1C2H-, is mainly transformed to ethynylbenzene (A1C2H), accounting for 78.9% of A1C2H- consumption, A1C2H- + HR = A1C2H + R
In this work, the reduced n-dodecane-PAH mechanism is used to simulate the formation of soot precursors, A4, in the n-dodecane flame. As described in Section 4.1, the experiment results of A4 radial concentration in the base engine oil flame is about 3.0 times of that in the n-dodecane flame, and the simulation result of A4 should correspond to the experiment results, thus the simulated A4 mole fraction should also be about 3.0 times higher than that in the n-dodecane flame. According to the rate of production analysis for A4 in Fig. 6, R16 is the most important reaction to the formation of A4. To simulate the formation of A4 in the diffusion flame of base engine oil, the reduced n-dodecanePAH mechanism with the revision of R16 was used in this work, and the reaction rate constant of R16 was increased by 3.6 times to increase the A4 production (by increasing the pre-exponential factor). The constantArrhenius rate of R16 after revision was that K = ATnexp(-Ea/RT), A = 0.8687E + 3, n = 2.2313, Ea = −0.1131E + 4. The simulated mole fraction profile of A4 with this updated mechanism is shown in Fig. 7, together with the simulated results of the n-dodecane flame with the original mechanism (without any adjustment). It can be seen from Fig. 7 that the predicted radial peak mole fractions of A4 are around 2.2 × 10−6 and 6.7 × 10−6 in the n-dodecane and base engine oil flames, respectively. It should be pointed out that the axial peak mole fraction is also increased by about 2.3 times. The reaction pathway analysis with the revision of R16 is shown in Fig. 6. It can be seen that, with this revision, the contribution of R16 to the formation of A4 is increased from 64.8% to 89.4%, with the branching ratio of the competing reaction, R10, being decreased from 27.0% to 9.4%. Since A3- is mainly produced from A4, the branching ratio of R22, the main consumption reaction of A3-, is also increased slightly, from 78.1% to 87.8%. Thus, by increasing the reaction rate constant of R16, the radial peak mole fraction of A4 is improved by about 3.0 times. With such adjustment, the predicted A4 concentration profile agrees well with the experimental results, as shown in Fig. 4c. Therefore, the updated ndodecane-PAH mechanism can reasonably well reproduce the PAH formation characteristics of base engine oil..
(R12)
HR may be any molecule containing an H atom in R12. A1C2H is mainly consumed by H-addition/acetylene-abstraction reaction to yield A1-, with a branching ratio of 79.9%, A1C2H + H = A1- + C2H2
(R13)
A1C2H may also react with C4H2 to form A2R5, with a branching ratio of 8.8%, A1C2H + C4H2 = A2R5
(R14)
A2R5 may react with C2H2 to form A3, or with C4H2 to form A4, with branching ratios of 27.0% and 64.8%, respectively, A2R5 + C2H2 = A3
(R15)
A2R5 + C4H2 = > A4
(R16)
Again, similar to A1 and A2, A3 and A4 may also be consumed by Habstraction. In the case of A3, it is almost completely consumed by Habstractions, A3 + H = A3- + H2
(R17)
A3 + OH = A3- + H2O
(R18)
While reactions of A4 with H atom or OH radical lead to different products, A4 + H = A3- + C2H2
(R19)
A4 + H = A1C2H + A1C2H-
(R20)
A4 + OH = A3- + CH2CO
(R21)
(R22)
The current mechanism shows that almost all A3 and A4 come from the reactions of A2R5 (R15 and R16). The radical produced via R17, R18, R19, and R21, A3-, is mainly consumed by decomposition, accounting for 78.1%,
5
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5. Conclusions
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In this work, experimental and theoretical investigations have been conducted to explore the PAH and sooting characteristics of base engine oil and n-dodecane in laminar diffusion lames. Distributions of polycyclic aromatic hydrocarbon (PAHs) and soot in these two flames were measured using the method of laser-induced fluorescence and two-color planar laser induced incandescence, respectively. The reduced combustion mechanism of n-dodecane-PAH was used to analyze the PAHs formation reaction pathways. Based on the comparison between experiment and simulation, the reaction rate constant of (A2R5 + C4H2 = > A4) was revised to better predict the formation of the four-ring aromatic hydrocarbons (A4) in the base engine oil flame. The major conclusions of the studies can be summarized as follows. 1. The axial and radial PAHs and soot distribution profiles in the laminar diffusion flames of the base engine oil and n-dodecane both show some characteristics of the Gaussian curve. The radial peak concentration is higher than the axial one. 2. The radial and axial peak A4 concentration in the laminar diffusion flame of base engine oil is about 3.0 and 2.3 times of that of ndodecane respectively. 3. The radial peak soot concentration in the laminar diffusion flame of base engine oil is about 2.5 times of that of n-dodecane. The peak positions of the soot concentrations coincide with that of the simulated A4 mole fraction in the laminar diffusion flames. A4 can be used to characterize combustion chemistry process of the soot formation of the base engine oil in the laminar diffusion flames. 4. A4 is mainly produced by the reaction, A2R5 + C4H2 = > A4. Multiplying the reaction rate constant of this reaction by 3.6 in the reduced n-dodecane-PAH mechanism can improve the predictions of A4 in the diffusion flame of base engine oil. This revision may be used to predict the formation of A4 and demonstrate the trend of sooting process in laminar diffusion flames of base engine oil. Acknowledgment This work is funded by the Major Research Plan of the National Natural Science Foundation of China (No. 91541205 and No. 51506145). References [1] P. Pant, R.M. Harrison, Estimation of the contribution of road traffic emissions to particulate matter concentrations from field measurements: a review, Atmos. Environ 77 (2013) 78–97. [2] B. Rajesh kumar, S. Saravanan, Effect of exhaust gas recirculation (EGR) on performance and emissions of a constant speed DI diesel engine fueled with pentanol/ diesel blends, Fuel 160 (2015) 217–226. [3] L. Labecki, L.C. Ganippa, Effects of injection parameters and EGR on combustion and emission characteristics of rapeseed oil and its blends in diesel engines, Fuel 98 (2012) 15–28. [4] J.L. Liu, H. Wang, Y. Li, et al., Effects of diesel/PODE (polyoxymethylene dimethyl ethers) blends on combustion and emission characteristics in a heavy duty diesel engine, Fuel 177 (2016) 206–216. [5] Y. Liu, R.D. Reitz. Optimizing HSDI diesel combustion and emissions using multiple injection strategies. SAE Paper 2005; 2005-01-0212. [6] D.A. Splitter, R.D. Reitz, Fuel reactivity effects on the efficiency and operational
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
A comparison study on the combustion and sooting characteristics of base engine oil and n-dodecane in laminar diffusion flames
T
⁎
Hu Wang, Yixuan Li, Zohaib Iqbal, Yang Wang , Chunshan Ma, Mingfa Yao State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China
H I GH L IG H T S
tendencies of n-dodecane and base engine oil were compared experimentally. • Sooting LaminarSMOKE code was adopted to simulate the PAHs’ formations. • The mechanism of n-dodecane was revised to simulate combustion of engine oil. • Reaction • Formation of PAH species in engine oil flames were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: LaminarSMOKE Engine oil Dodecane Soot
With the introduction of more strict emission legislations and advanced engine technologies, the contribution of engine oil to soot emissions from engines becomes more and more significant. In order to understand the sooting tendency of engine oil, experiments and chemical kinetic analysis were performed on a laminar diffusion flame of the base engine oil, together with a similar n-dodecane flame for comparison. Distributions of polycyclic aromatic hydrocarbon (PAH) and soot in the flames were measured using the method of laser-induced fluorescence and two-color planar laser induced incandescence respectively in the two flames. A reduced n-dodecanePAH mechanism was adopted to simulate the PAHs’ formations in the n-dodecane flame with the laminarSMOKE code. To simulate the four-ring aromatic species (A4), a precursor of soot in simulations, in the base engine oil flame, the reaction rate of a formation reaction of the A4 species, A2R5 + C4H2 = > A4, was enhanced by 3.6 times to better predict the PAH formation process of base oil. With this modification, the n-dodecane-PAH mechanism can well predict the A4 species in the base engine oil flames. Based on the experimental measurements and numerical simulations, the laminar diffusion flames of n-dodecane and base engine oil were analyzed and compared.
1. Introduction With the development of the automobile industry, emissions, especially particulate matters, pose a great burden to the atmosphere [1]. Stringent emission regulations were introduced to reduce the emissions. A great deal of research work were reported to meet the requirements of the increasingly stringent emission regulations and to improve the air quality, such as optimizing the combustion process, improving the fuel quality [2–4], and developing new combustion modes [5–8], etc. With the adoptions of these new technologies, the particulate matters in the engine exhaust were reduced dramatically. However, it has been reported that the soot emission could still be measured even using natural gas as the fuel in a homogenous charge compression ignition engine [9]. Since natural gas is generally believed
⁎
to be a soot free fuel, the soot emission can be possible to partially originate from the burning of engine oil that is widely used for lubrication in engines. With the implementation of the cleaner combustion technologies, the contribution of engine oil to the particle emissions will be more and more significant. However, the combustion of engine oil and its contribution to engine pipe out soot emission is still one of the unsolved problems in the combustion field. Generally, engine oil is mainly burned in the engine cylinders [10]. As the engine speed increases, more engine oil will be splashed into the cylinder and then be burned [11]. Brandenberger et al. [12] reported that engine oil was the main source of polycyclic aromatic hydrocarbons (PAHs) in the emissions of an engine fueled with diesel. Kytö et al. [13] reported that the particulate emissions from a heavy-duty diesel engine fueled with diesel+2% engine oil blend was about two
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.applthermaleng.2019.113812 Received 10 November 2018; Received in revised form 24 April 2019; Accepted 22 May 2019 Available online 23 May 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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times higher than that without engine oil. Taylor et al. [14] investigated the influence of volatility of engine oil on the particle emissions from a diesel engine and reported that the light-weight components of engine oil played an important role in particulate formations. Kahandawala et al. [15] conducted experiments in a modified single pulse reflected shock tube filled with diesel, engine oil, oxygen, and helium to investigate the influence of pressure on the particulate emissions. They reported that the addition of engine oil increased the particulate yields at higher pressures, and that the increase was more evident with higher engine oil concentrations. Andrews et al. [16] added calcium in engine oil as tracers to clarify the contribution of engine oil on the particulate emissions from an engine fueled with diesel. They reported that the contribution of engine oil to the soot formation decreased in the exhaust as the engine oil combustion efficiency increased. Kleeman et al. [17] reported that the reactions of engine oil were the reason of particle formation in the gasoline engine. Froelund et al. [18] reported that increasing the engine oil viscosity could significantly reduce the particle emission. All of the above investigations indicated that engine oil contributed significantly to the formations of particle emissions in the engine. However, the combustion mechanism of engine oil has not been explored to reveal the contribution engine oil combustion on soot formation and emission. Engine oil is a complex mixture of the base engine oil, mainly straight-chain alkanes (n-alkanes) ranging from C20 to C70, and about 10% of other additives [19]. In the chemical kinetic studies of the hydrocarbons, Sarathy et al. [20] developed a combustion mechanism of 2-methylalkanes to C20, and Westbrook et al. [21] developed detailed mechanism for n-alkanes up to C16. Due to the complex composition of the base engine oil, it is not possible to develop a detailed combustion mechanism for it. However, the combustion characteristics of the nalkanes, such as the burning velocity, tend to converge as the carbon number increases. Therefore, in order to simplify the investigation, the base engine oil was studied in this work, and n-dodecane was chosen as a representative to study the sooting tendencies of engine oil, due to the similarity of the structures between n-dodecane and base engine oil, and a suitable combustion mechanism size that favors the simulation and analysis. Therefore, in this work, the sooting tendency of base engine oil and n-dodecane were investigated experimentally and numerically in laminar diffusion flames. The laser-induced fluorescence (LIF) and twocolor planar laser induced incandescence (TC-PLII) [22] were used to measure the concentrations of PAH and soot, respectively. The four-ring aromatic species (A4, mainly pyrene) were used to demonstrate the formation trend of soot in these two flames, since A4 is usually considered to be an important soot precursors, as proposed by Vishwanathan and Reitz [23]. Moreover, the diffusion flames were simulated with the laminarSMOKE code. Based on the experimental measurements and numerical simulations, the combustion and sooting behaviors of n-dodecane and base engine oil in laminar diffusion flames were compared and analyzed.
Fig. 1. Schematic diagram of the experiment setup. Table 1 The boundary conditions of the experiments.
Fuel Initial velocity/mL·min−1 Ambient pressure/MPa Ambient temperature/K
Flame a
Flame b
n-dodecane 0.15 0.1 300
engine oil 0.15 0.1 300
uncertainties within ± 5%. The flow rates of the fuels were accurately measured and controlled by a mass flow meter. After ignition, a steady flame was formed above the quartz burner at ambient pressure. The LIF was used to detect the PAHs volume fraction by using 266 nm excitation and the TC-PLII was employed for measuring the soot volume fraction by using the second harmonics at 532 nm. For the LIF measurements, the fourth harmonics at 266 nm from an Nd:YAG laser was used to excite the PAHs. The resulting fluorescence signal was detected using an ICCD camera equipped with a lens. The PAHs can be detected independently by the selecting different fluorescence spectral bands. The monocyclic aromatics (mainly benzene, A1) were measured with the 315 nm bandpass filter. The double cyclic aromatics (mainly naphthalene, A2) and three-ring aromatics (mainly phenanthrene, A3) were measured with 400 nm low-pass filter which allows the spectrum wavelength in the range of 350–400 nm. The four-ring aromatics (mainly pyrene, A4) were measured with the 492 nm low-pass filter which allows the spectrum wavelength in the range of 400–480 nm. The soot particles were excited by an Nd:YAG laser sheet to emit the incandescence, with the energy of the laser pulse being kept constant at 25 mJ and a pulse-duration of 8 ns. After filtration by an optical filter (Andover, CA), the incandescence signal was also recorded by the ICCD camera. When the strong 266 nm beam was employed for the excitation of PAHs, there were some influence on both the PAHs fluorescence and soot incandescence in the signal detected. In order to distinguish between the above effects, two measurements were carried out, i.e., delayed detection (50 ns after the laser pulse has passed) to avoid the detection of the LIF signal from the fluorescence signal of the PAHs, due to the short lifetime of the former, and prompt detection (synchronized with the laser pulse) involving the contributions from both LII and LIF. For both the PAHs LIF and soot LII measurements, 1000 single shots were recorded and averaged. The signal intensity of the 1000 singleshot measurements showed variations of 7% (1σ). The temperatures of the flames were measured by the two-color method.
2. Experimental methods The experiments were carried out on the diffusion combustion platform in the State Key Laboratory of Engines at Tianjin University. The schematic diagram of the experimental system is shown in Fig. 1. Details of the experimental setup were reported in [22]. In brief, the experimental apparatus consisted of three parts, i.e., a laminar diffusion flame quartz burner, an Nd:YAG laser, and an intensified charge-coupled device (ICCD) camera (DH720i, Andor, U.K.). The inner-diameter of the quartz burner was 4 mm and the outside-diameter was 6 mm. Since the base engine oil and n-dodecane are liquid in room temperature (300 K), a heating vaporization section was placed before the quartz burner outlet. As shown in Table 1, the mean velocity of the vaporized fuel was controlled at 0.15 mL·min−1 in both flames. The fuel flow rates were regulated by a syringe pump with the absolute
3. Simulation methods The laminarSMOKE code [24–29] was used to simulate the laminar diffusion flames. The laminarSMOKE code is a framework for 2
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numerically modeling the multicomponent, compressible, thermallyperfect mixtures of gases by coupling with the related chemical mechanisms. The laminarSMOKE code takes the radiation of the major flame species and the Fickian, thermal diffusion into consideration. The transport properties (mass diffusion coefficients, viscosity and thermal conductivity) of the flame species were taken from the CHEMKIN transport database. In laminar flame conditions, the laminarSMOKE code uses an operator-splitting technique to solve the conservation equations of total mass, momentum, mixture energy, and individual species mass fractions. Because the flames investigated in this work are axisymmetric, the numerical calculations were performed on a nonequispaced, stretched, two-dimensional, rectangular domain, with a length of 100 mm and a width of 40 mm. A mesh with 18,000 computational cells (150 × 120) was fine enough for the purposes of the present work [27]. Along the centerline, a finer numerical grid near the exit of the burner was adopted, with an average mesh size of about 0.5 mm. The flame region is described using the uniform cell spacing along the radial direction. The reduced n-dodecane-PAH kinetic model of Wang et al. [30] was adopted in this work. Basically, the reduced mechanism merged the reduced mechanism of n-dodecane [21] and that of PAHs [31]. The reduced n-dodecane-PAH mechanism includes 100 species and 432 reactions. The thermodynamic and transport properties of the mechanism were taken from the detailed n-alkane and PAH mechanisms of Westbrook et al. [21] and Slavinskaya et al. [32], respectively. The reduced n-dodecane-PAH mechanism could well predict the formation of soot in a constant volume vessel based on the authors’ previous study [30]. 4. Results and discussion 4.1. Experiment results of n-dodecane and base engine oil flames
Fig. 3. Comparisons of the soot concentrations between the laminar diffusion flames of n-dodecane and base engine oil: a., Radius concentrations at h = 25 mm; b., Axial concentrations in the central line.
Fig. 2 shows the experimentally measured soot distributions in the n-dodecane and base engine oil flames. The horizontal axis is the radius of the flame and the vertical axis is the flame height. Due to the axisymmetric structure of the flames, only half of the experimental result is presented. It can be observed that the peak soot concentration in the base engine oil flame is significantly higher than that in the n-dodecane flame. Fig. 3a shows the radial soot concentration profiles in these two flames. The horizontal axis is the radial distance (r) and the vertical axis is the soot concentration at a height above the burner outlet (h) of 25 mm. It can be seen that the radial soot concentrations both peak at about r = 1.5 mm in two flames. The radial peak soot concentrations at h = 25 mm are about 480 ppb and 190 ppb in the base engine oil and n-
dodecane flames, respectively. The radial peak soot concentration at h = 25 mm in the base engine oil flame is about 2.5 times higher than that in the n-dodecane flame. Fig. 3b shows the axial soot concentration in the n-dodecane and base engine oil flames. The horizontal axis is the flame height above the burner outlet and the vertical axis is the concentration in the central line of the flames. The axial soot concentrations both peak at h = 30 mm in the two flames. The peak soot concentrations at the central line of the flames are about 150 ppb and 80 ppb in the base engine oil and n-dodecane flames, respectively. The peak soot concentration at the central line in the base engine oil flame is about 1.9 times higher than that in the n-dodecane flame. Fig. 4a–c show the experimental PAHs distributions in the base engine oil and n-dodecane flames at h = 25 mm, respectively. It can be seen the concentration of A1 is the highest among the four species, with a peak concentration of around 10 ppm at r = 2 mm in the base engine oil flame. The concentration of A2 and A3 is the sum of both species. The position of radial peak concentration of A4 is at around r = 1.8 mm. The trend of radial concentration of these two flames is that the concentration slightly increased as extending outward along the radius, and then dropped rapidly after the peak position. The radial peak A4 concentration at h = 25 mm in the base engine oil flame is about 3.0 times higher than that in the n-dodecane flame. Fig. 4d–f show the experimental axial concentration of PAHs. It can be seen that the concentrations of A1, A2 and A3, A4 peak at h = 19 mm, 23 mm and 28 mm in both flames, respectively. The position of peak concentration of these species move further toward the burner outlet and the concentration are also decreased as PAH molecular weight increased, which is consistent with the trend in Jia’s work [33]. The peak A4 concentrations at the central line of the flames are about 6 ppm and 2.6 ppm in the base engine oil and n-dodecane flames, respectively. The
Fig. 2. The experimental soot distributions in the laminar diffusion flames of ndodecane (a) and base engine oil (b). 3
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Fig. 4. Comparisons of the PAHs concentrations between the laminar diffusion flames of n-dodecane and base engine oil: a., b., c., Radius concentrations at h = 25 mm; d., e., f., Axial concentrations in the central line.
peak A4 concentration at the central line in the base engine oil flame is about 2.3 times higher than that in the n-dodecane flame.
r = 1.8 mm and h = 28 mm, and the trend and magnitude of the simulated results are in accordance with the experimental results in Fig. 4. Since A4 is generally considered to be the soot precursor which has great influence on soot formation, it is important to observe similar results in the simulations, which also enables the possibility to explore the underlying PAH formation reaction pathways in these two flames. Fig. 6 shows the reaction pathways analysis of the PAHs in the laminar diffusion flames of n-dodecane. It can be seen that around 76.3% of A1 in the n-dodecane flame is produced from the reactions between the C2 and the C4 species,
4.2. Combustion reaction pathway analysis of n-dodecane flame Fig. 5a shows the simulated mole fraction profiles of the PAHs at h = 25 mm and Fig. 5b shows the simulated axial results in the central line of the n-dodecane flame. It can be seen that the positions of the radial and axial peak mole fractions of A4 are also at around
C4H5 + C2H2 = A1 + H
(R1)
C4H4 + C2H3 = A1 + H
(R2)
Combination of C3H3 radicals accounts for about 17.6% of A1 formation, C3H3 + C3H3 = A1
(R3)
As for the consumption, about 65.9% of A1 is consumed via the reaction with C4H5 to form A2, C4H5 + A1 = A2 + H2 + H
(R4)
H-abstraction by H atom to produce phenyl radical (A1-) accounts
Fig. 5. The simulated mole fraction profiles of PAHs in the laminar diffusion flame of n-dodecane: a., Radius results at h = 25 mm; b., Axial results in the central line.
Fig. 6. Flow rate analyses of PAHs in the laminar diffusion flames of n-dodecane and base engine oil. Normal text, n-dodecane; italic text, base engine oil. 4
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for 28.0% of A1 consumption, A1 + H = A1- + H2
(R5)
Reactions of A1-lead to the formation of C2H2 and C4H3 or C3H3, A1- = C4H3 + C2H2
(R6)
A1- + H = C3H3 + C3H3
(R7)
R6 and R7 account for about 70.0% and 7.4% of A1-consumption, respectively. A2 is overwhelmingly produced from the reaction between A1 and C4H5 (R4). Similar to A1, A2 can also be consumed by Habstraction to form naphthyl radical (A2-), but with a much higher branching ratio of 90.8%, A2 + H = A2- + H2
(R8)
In addition to R8, A2 can also react with diacetylene (C4H2) to form A3, accounting for about 3.0% of A2 consumption, A2 + C4H2 = A3
Fig. 7. Comparisons of the simulated mole fraction profiles of A4 in the laminar diffusion flames of n-dodecane and base engine oil by the reduced n-dodecanePAH mechanism with and without the revision of R16 (A2R5 + C4H2 = > A4): a., Radial results at h = 25 mm; b., Axial results in the central line.
(R9)
The radical produced in R8, A2-, can either give a C2H2 to yield 1(2-ethynyl)phenyl radical (A1C2H-), or react with a C2H2 to produce ethynylnaphthalene (A2R5), with branching ratios of 50.5% and 40.5%, respectively, A2- = A1C2H-+C2H2
(R10)
A2- + C2H2 = A2R5 + H
(R11)
A3- = A2- + C4H2 A3- may also reversely transform to A3 and A4.
4.3. Combustion reaction pathway analysis of base engine oil flame
The radical produced in R10, A1C2H-, is mainly transformed to ethynylbenzene (A1C2H), accounting for 78.9% of A1C2H- consumption, A1C2H- + HR = A1C2H + R
In this work, the reduced n-dodecane-PAH mechanism is used to simulate the formation of soot precursors, A4, in the n-dodecane flame. As described in Section 4.1, the experiment results of A4 radial concentration in the base engine oil flame is about 3.0 times of that in the n-dodecane flame, and the simulation result of A4 should correspond to the experiment results, thus the simulated A4 mole fraction should also be about 3.0 times higher than that in the n-dodecane flame. According to the rate of production analysis for A4 in Fig. 6, R16 is the most important reaction to the formation of A4. To simulate the formation of A4 in the diffusion flame of base engine oil, the reduced n-dodecanePAH mechanism with the revision of R16 was used in this work, and the reaction rate constant of R16 was increased by 3.6 times to increase the A4 production (by increasing the pre-exponential factor). The constantArrhenius rate of R16 after revision was that K = ATnexp(-Ea/RT), A = 0.8687E + 3, n = 2.2313, Ea = −0.1131E + 4. The simulated mole fraction profile of A4 with this updated mechanism is shown in Fig. 7, together with the simulated results of the n-dodecane flame with the original mechanism (without any adjustment). It can be seen from Fig. 7 that the predicted radial peak mole fractions of A4 are around 2.2 × 10−6 and 6.7 × 10−6 in the n-dodecane and base engine oil flames, respectively. It should be pointed out that the axial peak mole fraction is also increased by about 2.3 times. The reaction pathway analysis with the revision of R16 is shown in Fig. 6. It can be seen that, with this revision, the contribution of R16 to the formation of A4 is increased from 64.8% to 89.4%, with the branching ratio of the competing reaction, R10, being decreased from 27.0% to 9.4%. Since A3- is mainly produced from A4, the branching ratio of R22, the main consumption reaction of A3-, is also increased slightly, from 78.1% to 87.8%. Thus, by increasing the reaction rate constant of R16, the radial peak mole fraction of A4 is improved by about 3.0 times. With such adjustment, the predicted A4 concentration profile agrees well with the experimental results, as shown in Fig. 4c. Therefore, the updated ndodecane-PAH mechanism can reasonably well reproduce the PAH formation characteristics of base engine oil..
(R12)
HR may be any molecule containing an H atom in R12. A1C2H is mainly consumed by H-addition/acetylene-abstraction reaction to yield A1-, with a branching ratio of 79.9%, A1C2H + H = A1- + C2H2
(R13)
A1C2H may also react with C4H2 to form A2R5, with a branching ratio of 8.8%, A1C2H + C4H2 = A2R5
(R14)
A2R5 may react with C2H2 to form A3, or with C4H2 to form A4, with branching ratios of 27.0% and 64.8%, respectively, A2R5 + C2H2 = A3
(R15)
A2R5 + C4H2 = > A4
(R16)
Again, similar to A1 and A2, A3 and A4 may also be consumed by Habstraction. In the case of A3, it is almost completely consumed by Habstractions, A3 + H = A3- + H2
(R17)
A3 + OH = A3- + H2O
(R18)
While reactions of A4 with H atom or OH radical lead to different products, A4 + H = A3- + C2H2
(R19)
A4 + H = A1C2H + A1C2H-
(R20)
A4 + OH = A3- + CH2CO
(R21)
(R22)
The current mechanism shows that almost all A3 and A4 come from the reactions of A2R5 (R15 and R16). The radical produced via R17, R18, R19, and R21, A3-, is mainly consumed by decomposition, accounting for 78.1%,
5
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5. Conclusions
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In this work, experimental and theoretical investigations have been conducted to explore the PAH and sooting characteristics of base engine oil and n-dodecane in laminar diffusion lames. Distributions of polycyclic aromatic hydrocarbon (PAHs) and soot in these two flames were measured using the method of laser-induced fluorescence and two-color planar laser induced incandescence, respectively. The reduced combustion mechanism of n-dodecane-PAH was used to analyze the PAHs formation reaction pathways. Based on the comparison between experiment and simulation, the reaction rate constant of (A2R5 + C4H2 = > A4) was revised to better predict the formation of the four-ring aromatic hydrocarbons (A4) in the base engine oil flame. The major conclusions of the studies can be summarized as follows. 1. The axial and radial PAHs and soot distribution profiles in the laminar diffusion flames of the base engine oil and n-dodecane both show some characteristics of the Gaussian curve. The radial peak concentration is higher than the axial one. 2. The radial and axial peak A4 concentration in the laminar diffusion flame of base engine oil is about 3.0 and 2.3 times of that of ndodecane respectively. 3. The radial peak soot concentration in the laminar diffusion flame of base engine oil is about 2.5 times of that of n-dodecane. The peak positions of the soot concentrations coincide with that of the simulated A4 mole fraction in the laminar diffusion flames. A4 can be used to characterize combustion chemistry process of the soot formation of the base engine oil in the laminar diffusion flames. 4. A4 is mainly produced by the reaction, A2R5 + C4H2 = > A4. Multiplying the reaction rate constant of this reaction by 3.6 in the reduced n-dodecane-PAH mechanism can improve the predictions of A4 in the diffusion flame of base engine oil. This revision may be used to predict the formation of A4 and demonstrate the trend of sooting process in laminar diffusion flames of base engine oil. Acknowledgment This work is funded by the Major Research Plan of the National Natural Science Foundation of China (No. 91541205 and No. 51506145). References [1] P. Pant, R.M. Harrison, Estimation of the contribution of road traffic emissions to particulate matter concentrations from field measurements: a review, Atmos. Environ 77 (2013) 78–97. [2] B. Rajesh kumar, S. Saravanan, Effect of exhaust gas recirculation (EGR) on performance and emissions of a constant speed DI diesel engine fueled with pentanol/ diesel blends, Fuel 160 (2015) 217–226. [3] L. Labecki, L.C. Ganippa, Effects of injection parameters and EGR on combustion and emission characteristics of rapeseed oil and its blends in diesel engines, Fuel 98 (2012) 15–28. [4] J.L. Liu, H. Wang, Y. Li, et al., Effects of diesel/PODE (polyoxymethylene dimethyl ethers) blends on combustion and emission characteristics in a heavy duty diesel engine, Fuel 177 (2016) 206–216. [5] Y. Liu, R.D. Reitz. Optimizing HSDI diesel combustion and emissions using multiple injection strategies. SAE Paper 2005; 2005-01-0212. [6] D.A. Splitter, R.D. Reitz, Fuel reactivity effects on the efficiency and operational
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