- Email: [email protected]
methanol-air-EGR Mixtures
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 4943 – 4948
The 8th International Conference on Applied Energy – ICAE2016
Dual Fuel Combustion of N-heptane/methanol-air-EGR Mixtures Siyuan Hu, Cheng Gong, Xue-Song Bai* Division of Fluid Mechanics, Lund University, 221 00 Lund,Sweden
Abstract
Numerical simulations are performed to study the combustion processes of n-heptane and methanol/air/EGR under hightemperature and high-pressure conditions relevant to a dual-fuel compression-ignition engine. Detailed chemical kinetic mechanism and transport properties are considered in the simulation. The simulations are carried out by performing threedimensional (3D) direct numerical simulation (DNS) in a cuboid constant volume enclosure and two-dimensional (2D) DNS in a constant volume domain corresponding to the 3D domain. The results reveal three combustion modes involved in dual fuel engines, the ignition of the n-heptane jet, the propagation of thin reaction fronts in the methanol/air/EGR mixture, and finally the onset of ignition of the entire mixture. In dual-fuel combustion under high initial temperature conditions (1000K or higher) the three different combustion modes occur rather quickly with two distinct peaks of heat release corresponding to the two ignition modes. At low temperatures (800K or lower) the ignition delay time of the mixture is longer and more complete mixing is achieved before the onset of a single ignition of the n-heptane/methanol mixture. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 S.Y. Hu, C. Gong, X.S. Bai. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: methanol/n-heptane combustion; openFoam; mixture-averaged; EGR; ignite-improving.
1.Introduction Petroleum-based fuels are widely used for internal combustion engines. The usage of petroleum-based fuels can however cause air pollution due to their inherent soot and NO x emissions. Thus, there is a great interest in finding alternative fuel sources and clean combustion solutions. To this end, methanol is an attractive fuel. It is a colorless, neutral and flammable liquid, which can be produced from not only the widely available fossil raw materials, such as coal, natural gas, coke-oven gas, but also from renewable fuel sources such as biomass etc [1]. Methanol has several physical and combustion properties similar to gasoline and diesel fuel. Compared with gasoline and diesel, the use of methanol can reduce the NOx formation owing to low flame temperature, and reduce the soot formation due to the higher stoichiometric fuel/air ratio [2]. Methanol can be used as a good quality fuel for spark ignition (SI) engines which exhibit better power performance due to its high anti-knock nature [3-5]. It can be also blended with gasoline, DME (dimethyl ether), water, and hydrogen etc., in SI engines to improve cold start and emissions [6-10]. However, it is difficult to use methanol directly to engines running at direct injection compression ignition (CI) conditions, since methanol has a high resistance to auto-ignition due to its low cetane number (CN<5). To address these challenges, the “dual-fuel” concept has been developed. In the duel-fuel approach, one fuel is port injected and another fuel is direct injected [11]. The port injected fuel and air are mixed and provided to the engine as in the SI engine, while instead of a spark ignition of the mixture, a high cetane number fuel (easily ignitable fuel, such as diesel or its surrogate n-heptane) is directly injected and ignited owing to the high pressure and temperature
* Corresponding author. Tel.: +46 46 222 48 60; fax: +46 46 222 47 17. E-mail address: [email protected].
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.986
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from the compression, which in turn ignites the methanol and air mixture. Previous studies have been focused on practical aspects of using dual-fuel in engines. Experimental results show that the dual-fuel strategy, whether both methanol and diesel are directly injected or mixing methanol with air in the chamber before the injection of diesel, can achieve clean combustion in CI engines [12]. Numerical studies have mostly been focusing on confirming the experimental findings [13]. There are several unclear fundamental questions on the dual-fuel combustion of methanol/diesel. For example, since the pressure is increased due to the combustion of diesel fuel, the combustion mode of the low CN methanol/air mixture is unclear. Does the mixture combust in the premixed flame propagation mode like a spark ignition engine, or in the auto-ignition mode like the HCCI engine? How changes in different operating conditions, such as the initial temperatures, ambient pressures and composition of surrounding mixture affect the combustion modes of methanol/air mixture? Detailed combustion model (e.g. DNS) is required to answer these questions. In this present study, DNS of dual-fuel combustion with methanol and n-heptane was carried out to gain deeper insights into the dual-fuel combustion process under CI engine conditions. More specifically, a homogeneous methanol/air/EGR mixture was assumed to exist in the computational domain at the start of the n-heptane injection. The simulation was performed at the start of the n-heptane injection. Combustion processes under various initial temperatures, ambient pressures and composition of surrounding mixture gas were investigated by performing three-dimensional (3D) and two-dimensional (2D) DNS. 2.Numerical setup and initial conditions A cuboid constant volume enclosure is chosen to mimic the condition of internal combustion engines with piston around the top-dead-center position. Figure 1 shows the schematic of the three dimensional (3D) computational domain. The domain has a dimension of 14mm, 12mm, and 2mm in the x, y, and z-directions, respectively. Wall boundary conditions are applied on all boundaries, except that in the boundaries of z=0 and z=h (h=2mm) where a periodic condition is used, and at y=0 boundary an inlet is placed at the center of the boundary, cf. Fig.1. The 3D simulation is performed to compare with a corresponding 2D simulation for the entire period of dual-fuel combustion, including the n-heptane injection, mixing, the first and second stage ignition, and until the appearance of different combustion modes of methanol/air/EGR mixture. During the early stage of dual-fuel combustion, the n-heptane mixing, ignition and combustion processes are found in the domain around the inlet as indicated by the dashed region in Fig.1. This inner domain has a dimension of 5mm, 10mm, and 2mm, in the x, y, and z-directions, respectively. In the 2D simulations six Cases are considered as listed in Table 1, with a range of initial temperature and pressure. The 2D simulations are performed in a constant volume domain with a dimension of 14mm and 12mm in x and y-directions, respectively.
Figure 1. A schematic illustration of the 3) domain. The injection flow is characterized by iso-surface of 2 ( 2 =2000).
A uniform grid is used for the 2D Cases with the cell size of 20 Pm and a total mesh cells of 420,000. For the 3D Case a locally refined grid system is employed. In the inner domain indicated by the dashed region a uniform grid is used with the same mesh as that in the 2D Cases; outside the inner domain a coarser grid is used since the main combustion events occur in the inner domain. The cell size in the coarse grid is 80 Pm . The total number of cells is about 13 million for the 3D Case. The conditions of the 3D Case, denoted as Case E, are identical to that of Case A. To compare with the 2D and 3D results one-dimensional (1D) simulations are also performed under the corresponding constant volume conditions. The initial distributions of the temperature and compositions of the 1D Cases are taken from the 3D Case across the n-heptane jet 0.5 mm downstream the jet inlet, at the instance of the end of injection. In the 2D and 3D Cases listed in Table 1, the initial mixture The simulations are performed using openFOAM [14] with an improved transport property. The original
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transport equations for the species and enthalpy in openFoam are based on two assumptions: a unity Schmidt number, and a unity Lewis number. The original solver was mainly developed for RANS (Reynolds averaged Navier-Stokes) simulations, where the molecular diffusion is negligible compared with the turbulent diffusion. In DNS, detailed transport properties are important since at the small scales (Kolmogorov scales) molecular transport is a dominant process. Thus, a moderate detailed account for the mass diffusion, namely the mixture-averaged approach, is implemented. The mixture-averaged transport properties are calculated using Cantera libraries [15]. The Soret and Dufour effects were neglected. A fourth-order Gaussian finite-volume scheme is used for the spatial discretization, together with a second-order implicit temporal integration scheme. A skeletal mechanism for n-heptane of Lu et al. [16], with 68 species and 283 reactions, is used. This mechanism is compared with a detailed methanol mechanism [17] in the 0D simulations. Table 1. Computational Cases and initial conditions. In all 2D and 3D Cases EGR to methanol/air ratio is 50:50 (volume basis), methanol/air equivalence ratio =0.6, n-heptane injection velocity v=45.2 m/s; n-heptane injection time tinj=0.1ms. EGR composition (on volume basis): CO2 (12.4%), H2O (15.6%), N2 (72%). N-heptane temperature in all Cases is 600K. Case No. A B C D E F G H
Initial T [K] 1000 900 800 1000 1000 1000 1000 1000
Initial P [bar] 42 42 42 24 42 42 42 42
phi 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
mixture n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air methanol/EGR/air n-heptane/methanol/EGR/air methanol/EGR/air
Specification 2D 2D 2D 2D 3D 2D 1D 1D
3. Results and discussion 3.1 Ignition time First, ignition delay times of various mixtures under homogeneous adiabatic zero-dimensional constant volume conditions are computed. Figure 2a shows a comparison of the results from a detailed methanol mechanism of Li et al. [22] with that from the methanol sub-mechanism in the n-heptane mechanism [21]. The results are nearly identical, thus suggesting the adequateness of the n-heptane mechanism for the ignition simulation of not only the n-heptane/methanol mixture but also for the pure methanol mixture. It is seen that the ignition delay time of n-heptane is shorter than that of methanol when temperature is below 1000K, however, at higher temperatures pure methane/air mixture can ignite quicker. The impact of n-heptane on the ignition delay of methanol is evidenced in the figure: by mixing 50% volume n-heptane into methanol/air, the ignition delay time of methanol is prolonged when the temperature is above 1000K whereas at lower temperatures n-heptane can shorten the ignition delay time of methane significantly. The ignition behavior of n-heptane/methanol/air mixture is similar to that of n-heptane/air mixture, exhibiting a NTC (negative temperature coefficient) regime. To examine the effect of n-heptane on methanol/air combustion under low temperature, the temporal evolution of combustor pressure from combustion of methanol/EGR/air mixture with n-heptane injection (Case A) and without n-heptane injection (Case F) is plotted in Fig.2b. Compared with pure methanol/air combustion, the addition of n-heptane to methanol/air leads to earlier ignition of the mixture. The pressure evolution of the 2D case (Case A) and the 3D case (Case E) are similar, indicating that the 2D case replicates the 3D case reasonably well. The two-stage ignition behavior due to the NTC behavior of n-heptane leads to the earlier pressure rise around 0.8ms. The pressure evolution in 1D cases is also shown in Fig.2b. The 1D results show that the pure methanol/EGR/air mixture (Case H) has a similar combustion progress as that with n-heptane addition (Case G) under low temperature. This contrasts with the results of the 2D/3D cases (Case A/E vs F). The difference between results of Case F and Case H indicates the importance of flow transport in the overall combustion process. Figure 3a shows the total heat release rate (HRR) for the 2D Cases. Two HRR peaks can be seen in Case A. The first peak occurs at about 0.8ms after the injection of n-heptane. The second peak can be observed at 1.2ms. The HRR value of second peak is much higher than that of the first peak. Case B has a longer ignition
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delay time due to the lower initial methanol temperature (900K) as compared with Case A (1000K), and the duration of the first ignition spreads to a wider time interval, from 1 ms to 2 ms, and the second peak comes at rather later time (at 2.3 ms). There is only one HRR peak, at a rather later time, about 3 ms, can be observed in Case C due to the initial temperature of this Case is 800K, which is the lowest initial temperature in all Cases. The heat release rate profile of Case D is similar to that of Case A due to the same initial temperature and compositions; the initial pressure in Case D is lower than that in Case A, which leads to the heat release rate peaks at a relatively late time owing to the pressure effect on the ignition reactions. The same phenomenon is also found in the pressure evolution shown in Fig.3b.
(a) (b) Figure 2. (a) Comparison of ignition delay time in various mixtures of n-heptane, methanol, and air in a constant volume condition with initial pressure P=42 bar and equivalence ratio of 1. The dashed blue lines are results from the methanol mechanism of Li et al. [22], while the rest results are from the mechanism of Lu et al. [21]. In the n-heptane/methanol/air mixture the molar ratio between the two fuels is 1. (b) Comparison of temporal evolution of pressure for methanol/EGR/air mixture ignited by n-heptane (Case A [2D], Case E [3D], Case G [1D]) and pure methanol/EGR/air mixture (Case F [2D] and Case H [1D]) with initial pressure P=42 bar and equivalence ratio of 0.6. Detailed description of the Cases are given in Table 1.
(a) (b) Figure 3. Temporal evolution of heat release rate (a) and pressure (b) for four 2D Cases under different initial methanol/air/EGR mixture temperature and pressure (Case A [1000K, 42 bar], Case B [900K, 42 bar], C [800K, 42 bar] , D [1000K, 24 bar]).
3.2 Detailed ignition zone structure To gain insightful information about the heat release rate process, the temporal evolution of temperature and heat release rate field in the chamber of Case A is shown in Fig.4. At 0.4ms, the low temperature region is where n-heptane jet is penetrated through, whereas the high temperature region with uniform temperature is where the methanol/air/EGR mixture locates. At 0.9 ms the onset of ignition is shown to begin in the n-heptane jet. The entire n-heptane jet is ignited at 1.2ms while the methanol/air/EGR mixture is now starting to ignite, which yields the second peak of heat release rate shown in Fig. 3a. This behavior is essentially the same as the HCCI combustion process, which burns out the whole methanol fuel in a short duration. After 1.5 ms the mixture of pure methanol/air/EGR is nearly burned out, while certain mixture with n-heptane is still undergoing ignition. The entire combustion process can be divided into three stages: (1) ignition of the n-heptane/methanol mixing at proximity of the ne-heptane jet, (2) propagation of the reaction front into the premixed methanol/air/EGR mixture, and (3) ignition of the entire mixture of methanol/air/EGR. Stage 2 is rather short in Case A owing to the relative high initial temperature in the charge. Case D has a similar combustion process as that of Case A, except that the ignition is slightly delayed due to the lower chamber pressure .
Siyuan Hu et al. / Energy Procedia 105 (2017) 4943 – 4948
Fig.4 Temperature (K) and heat release rate (J/s) distributions for Case A with Tu=1000K at different time.
Fig.5 Temperature (K) and heat release rate (J/s) distributions for Case C with Tu=800K at different time.
The heat release rate profile has only one peak and longer duration for Case C (cf. Fig. 3a), which is due to the difference in the relative importance of above discussed three combustion stages. To examine this phenomenon, the temporal evolution of temperature of Case C is plotted in Fig.5. The ignition time of Case C is the longest since it has the lowest initial temperature in all Cases. This allows for a better mixing of n-heptane with methanol/air/EGR. The low temperature region shown in Fig.5 at 2.4 ms of Case C corresponds to higher concentration of n-heptane. The onset of ignition, at 2.4 ms, is located at the downstream region of the nheptane jet, rather different from that in Case A. At 3.0 ms, more ignition sites can be observed in the high nheptane region, including the center of the large recirculation zone. The heat release rate is still located in thin layers from 2.4 ms to 3.2 ms, indicating that the combustion of methanol is likely due to the reaction front propagation (stage 2). This process is similar to premixed flame propagation (although with the assistance of ignition in the entire unburned mixture), and it is a rather slow process, which explains the long heat release duration as shown in Fig. 3a. This combustion characteristic is mainly due to the low initial temperature, which has rather long ignition delay time. The initial temperature of Case B is moderate so that the heat release rate behavior (cf. Fig.3) is due to both the HCCI combustion of methanol/air/EGR mixture mode and the premixed reaction front propagation mode. The heat release rate profile has only one peak and longer duration for Case C (cf. Fig. 3a), which is due to the difference in the relative importance of above discussed three combustion stages. To examine this phenomenon, the temporal evolution of temperature of Case C is plotted in Fig.5. The ignition time of Case C is the longest since it has the lowest initial temperature in all Cases. This allows for a better mixing of n-heptane with methanol/air/EGR. The low temperature region shown in Fig.5 at 2.4 ms of Case C corresponds to higher concentration of n-heptane. The onset of ignition, at 2.4 ms, is located at the downstream region of the nheptane jet, rather different from that in Case A. At 3.0 ms, more ignition sites can be observed in the high nheptane region, including the center of the large recirculation zone. The heat release rate is still located in thin layers from 2.4 ms to 3.2 ms, indicating that the combustion of methanol is likely due to the reaction front propagation (stage 2). This process is similar to premixed flame propagation (although with the assistance of ignition in the entire unburned mixture), and it is a rather slow process, which explains the long heat release duration as shown in Fig. 3a. This combustion characteristic is mainly due to the low initial temperature, which
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has rather long ignition delay time. The initial temperature of Case B is moderate so that the heat release rate behavior (cf. Fig.3) is due to both the HCCI combustion of methanol/air/EGR mixture mode and the premixed reaction front propagation mode. 4. Conclusions The dual-fuel combustion process can be summarized below. First the onset of ignition is always at the proximity of the n-heptane jet. The n-heptane fuel is heated up by the hotter methanol/air/EGR mixture, thus the onset of ignition is first in a mixture of n-heptane and methanol. The ignition delay time of the nheptane/methanol mixture is governed by the n-heptane mixture. Second, a reaction front propagation is established to burn the methanol/air/EGR mixture, which gives rise to increase of the chamber temperature and pressure, and finally triggers the onset of ignition in the entire methanol/air/EGR mixture. A higher initial temperature in the methanol/air/EGR mixture can therefore give rise to faster ignition of the n-heptane jet as well as onset of ignition in the methanol/air/EGR mixture; a too low initial temperature delays the ignition of the n-heptane jet, which can allow for more complete mixing of n-heptane with methanol/air/EGR before the onset of ignition of n-heptane as well as the methanol/air/EGR mixture. The combustion process in this Case relays on the propagation of premixed flame, which can be slow or even difficult if the mixture is fuel lean or the EGR level is too high. Acknowledgements This work was partly sponsored by Swedish Research Council (VR), and the Swedish Energy Agency (STEM) through the national center for combustion science and technologies (CeCOST) and Lund University Competence Center for Combustion Process (KC-FP). S.Y. Hu was sponsored by China Scholarship Council (CSC). References [1] X.D Zhen, Y. Wang. An overview of methanol as an internal combustion engine fuel. Renewable and Sustainable Energy Reviews. 52(2015)477–493. [2] Y.P Li, M. Jia, Y.D Liu, M.Z Xie. Numerical study on the combustion and emission characteristics of a methanol/diesel reactivity controlled compression ignition (RCCI) engine. Applied Energy.106(2013)184–197. [3] M. Brusstar, M. Stuhldreher, D. Swain, W. Pidgeon. High efficiency and low emissions from a port-injected engine with neat alcohol fuels. SAE technical paper No. 2002-01-2743; 2002. [4] K. Naganuma, J. Vancoillie, L. Sileghem, S. Verhelst, James Turner and Richard Pearson. Drive Cycle Analysis of Load Control Strategies for Methanol Fuelled ICE Vehicle. SAE technical paper No. 2012-01-1606; 2012. [5] M.Brusstar, M. Stuhldreher, D. Swain and W. Pidgeon. High Efficiency and Low Emissions from a Port-Injected Engine with Neat Alcohol Fuels. SAE technical paper No. 2002-01-2743; 2002. [6] S.H Liu, R. Eddy.C. Cuty, T.G. Hu , Y.J. Wei . Study of spark ignition engine fueled with methanol/gasoline fuel blends. Applied Thermal Engineering.27(2007) 1904–1910. [7] M. Eyidogan, A. N Ozsezen, M. Canakci, A. Turkcan. Impact of alcohol–gasoline fuel blends on the performance and combustion characteristics of an SI engine. Fuel. 89(2010)2713–2720. [8] C. Sayin, M. Ilhan, M. Canakci, M. Gumus. Effect of injection timing on the exhaust emissions of a diesel engine using diesel– methanol blends. Renew Energy.34(2009)1261–9. [9] C. Mishra, N. Kumar, B.S. Chauhan, H.C Lim, M. Padhy. Some Experimental Investigation on use of Methanol and Diesel Blends in a Single Cylinder Diesel Engine. International Journal of Renewable Energy Technology Research (IJRETR). 2(2013) 01 -16. [10] M. S Kumar, A. Ramesh B. Nagalingam. An experimental comparison of methods to use methanol and Jatropha oil in a compression ignition engine. Biomass and Bioenergy.3(2003)309-318. [11] M. Tuner. Review and Benchmarking of Alternative Fuels in Conventional and Advanced Engine Concepts with Emphasis on Efficiency, CO2, and Regulated Emissions. SAE technical paper No. 2016-01-0882; 2016. [12] L.Haraldson,“Methanol as fuel”, Methanol as Fuel & Energy Storage Workshop. Lund, Sweden, 2015, http://www.lth.se/ fileadmin/mot2030/filer/12._Haraldsson_-_Methanol_as_fuel. Pdf [13] A.Yousefi, M. Birouk, B. Lawler, A. Gharehghani. Performance and emissions of a dual-fuel pilot diesel ignition engine operating on various premixed fuels. Energy Conversion and Management.106(2015)322–336. [14] The OpenFOAM Foundation, 2015 [15] Cantera: chemical kinetics, thermodynamics, transport processes. 2015. Available at. [16] T.F Lu, C. K. Law, C.S Yoo, J. H. Chen. Dynamic stiffness removal for direct numerical simulations. Combustion and Flame.156(2009) 1542-1551. [17] J. Li, Z. W Zhao, A. Kazakov, M. Chaos, F. L.Dryer, J. I.Scire.JR. A Comprehensive Kinetic Mechanism for CO, CH2O, and CH3OH Combustion. Jr. Int. J. Chem. Kinet.39(2007)109-136.
Biography Prof. Xue-Song Bai leads a research group working on CFD modeling of turbulent reacting flows. Current research of the group aims at developing models for simulation of combustion processes of gas, liquid and solid fuels with industrial application to internal combustion engines, gas turbines and furnaces.
ScienceDirect Energy Procedia 105 (2017) 4943 – 4948
The 8th International Conference on Applied Energy – ICAE2016
Dual Fuel Combustion of N-heptane/methanol-air-EGR Mixtures Siyuan Hu, Cheng Gong, Xue-Song Bai* Division of Fluid Mechanics, Lund University, 221 00 Lund,Sweden
Abstract
Numerical simulations are performed to study the combustion processes of n-heptane and methanol/air/EGR under hightemperature and high-pressure conditions relevant to a dual-fuel compression-ignition engine. Detailed chemical kinetic mechanism and transport properties are considered in the simulation. The simulations are carried out by performing threedimensional (3D) direct numerical simulation (DNS) in a cuboid constant volume enclosure and two-dimensional (2D) DNS in a constant volume domain corresponding to the 3D domain. The results reveal three combustion modes involved in dual fuel engines, the ignition of the n-heptane jet, the propagation of thin reaction fronts in the methanol/air/EGR mixture, and finally the onset of ignition of the entire mixture. In dual-fuel combustion under high initial temperature conditions (1000K or higher) the three different combustion modes occur rather quickly with two distinct peaks of heat release corresponding to the two ignition modes. At low temperatures (800K or lower) the ignition delay time of the mixture is longer and more complete mixing is achieved before the onset of a single ignition of the n-heptane/methanol mixture. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 S.Y. Hu, C. Gong, X.S. Bai. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. Keywords: methanol/n-heptane combustion; openFoam; mixture-averaged; EGR; ignite-improving.
1.Introduction Petroleum-based fuels are widely used for internal combustion engines. The usage of petroleum-based fuels can however cause air pollution due to their inherent soot and NO x emissions. Thus, there is a great interest in finding alternative fuel sources and clean combustion solutions. To this end, methanol is an attractive fuel. It is a colorless, neutral and flammable liquid, which can be produced from not only the widely available fossil raw materials, such as coal, natural gas, coke-oven gas, but also from renewable fuel sources such as biomass etc [1]. Methanol has several physical and combustion properties similar to gasoline and diesel fuel. Compared with gasoline and diesel, the use of methanol can reduce the NOx formation owing to low flame temperature, and reduce the soot formation due to the higher stoichiometric fuel/air ratio [2]. Methanol can be used as a good quality fuel for spark ignition (SI) engines which exhibit better power performance due to its high anti-knock nature [3-5]. It can be also blended with gasoline, DME (dimethyl ether), water, and hydrogen etc., in SI engines to improve cold start and emissions [6-10]. However, it is difficult to use methanol directly to engines running at direct injection compression ignition (CI) conditions, since methanol has a high resistance to auto-ignition due to its low cetane number (CN<5). To address these challenges, the “dual-fuel” concept has been developed. In the duel-fuel approach, one fuel is port injected and another fuel is direct injected [11]. The port injected fuel and air are mixed and provided to the engine as in the SI engine, while instead of a spark ignition of the mixture, a high cetane number fuel (easily ignitable fuel, such as diesel or its surrogate n-heptane) is directly injected and ignited owing to the high pressure and temperature
* Corresponding author. Tel.: +46 46 222 48 60; fax: +46 46 222 47 17. E-mail address: [email protected].
1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.986
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Siyuan Hu et al. / Energy Procedia 105 (2017) 4943 – 4948
from the compression, which in turn ignites the methanol and air mixture. Previous studies have been focused on practical aspects of using dual-fuel in engines. Experimental results show that the dual-fuel strategy, whether both methanol and diesel are directly injected or mixing methanol with air in the chamber before the injection of diesel, can achieve clean combustion in CI engines [12]. Numerical studies have mostly been focusing on confirming the experimental findings [13]. There are several unclear fundamental questions on the dual-fuel combustion of methanol/diesel. For example, since the pressure is increased due to the combustion of diesel fuel, the combustion mode of the low CN methanol/air mixture is unclear. Does the mixture combust in the premixed flame propagation mode like a spark ignition engine, or in the auto-ignition mode like the HCCI engine? How changes in different operating conditions, such as the initial temperatures, ambient pressures and composition of surrounding mixture affect the combustion modes of methanol/air mixture? Detailed combustion model (e.g. DNS) is required to answer these questions. In this present study, DNS of dual-fuel combustion with methanol and n-heptane was carried out to gain deeper insights into the dual-fuel combustion process under CI engine conditions. More specifically, a homogeneous methanol/air/EGR mixture was assumed to exist in the computational domain at the start of the n-heptane injection. The simulation was performed at the start of the n-heptane injection. Combustion processes under various initial temperatures, ambient pressures and composition of surrounding mixture gas were investigated by performing three-dimensional (3D) and two-dimensional (2D) DNS. 2.Numerical setup and initial conditions A cuboid constant volume enclosure is chosen to mimic the condition of internal combustion engines with piston around the top-dead-center position. Figure 1 shows the schematic of the three dimensional (3D) computational domain. The domain has a dimension of 14mm, 12mm, and 2mm in the x, y, and z-directions, respectively. Wall boundary conditions are applied on all boundaries, except that in the boundaries of z=0 and z=h (h=2mm) where a periodic condition is used, and at y=0 boundary an inlet is placed at the center of the boundary, cf. Fig.1. The 3D simulation is performed to compare with a corresponding 2D simulation for the entire period of dual-fuel combustion, including the n-heptane injection, mixing, the first and second stage ignition, and until the appearance of different combustion modes of methanol/air/EGR mixture. During the early stage of dual-fuel combustion, the n-heptane mixing, ignition and combustion processes are found in the domain around the inlet as indicated by the dashed region in Fig.1. This inner domain has a dimension of 5mm, 10mm, and 2mm, in the x, y, and z-directions, respectively. In the 2D simulations six Cases are considered as listed in Table 1, with a range of initial temperature and pressure. The 2D simulations are performed in a constant volume domain with a dimension of 14mm and 12mm in x and y-directions, respectively.
Figure 1. A schematic illustration of the 3) domain. The injection flow is characterized by iso-surface of 2 ( 2 =2000).
A uniform grid is used for the 2D Cases with the cell size of 20 Pm and a total mesh cells of 420,000. For the 3D Case a locally refined grid system is employed. In the inner domain indicated by the dashed region a uniform grid is used with the same mesh as that in the 2D Cases; outside the inner domain a coarser grid is used since the main combustion events occur in the inner domain. The cell size in the coarse grid is 80 Pm . The total number of cells is about 13 million for the 3D Case. The conditions of the 3D Case, denoted as Case E, are identical to that of Case A. To compare with the 2D and 3D results one-dimensional (1D) simulations are also performed under the corresponding constant volume conditions. The initial distributions of the temperature and compositions of the 1D Cases are taken from the 3D Case across the n-heptane jet 0.5 mm downstream the jet inlet, at the instance of the end of injection. In the 2D and 3D Cases listed in Table 1, the initial mixture The simulations are performed using openFOAM [14] with an improved transport property. The original
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transport equations for the species and enthalpy in openFoam are based on two assumptions: a unity Schmidt number, and a unity Lewis number. The original solver was mainly developed for RANS (Reynolds averaged Navier-Stokes) simulations, where the molecular diffusion is negligible compared with the turbulent diffusion. In DNS, detailed transport properties are important since at the small scales (Kolmogorov scales) molecular transport is a dominant process. Thus, a moderate detailed account for the mass diffusion, namely the mixture-averaged approach, is implemented. The mixture-averaged transport properties are calculated using Cantera libraries [15]. The Soret and Dufour effects were neglected. A fourth-order Gaussian finite-volume scheme is used for the spatial discretization, together with a second-order implicit temporal integration scheme. A skeletal mechanism for n-heptane of Lu et al. [16], with 68 species and 283 reactions, is used. This mechanism is compared with a detailed methanol mechanism [17] in the 0D simulations. Table 1. Computational Cases and initial conditions. In all 2D and 3D Cases EGR to methanol/air ratio is 50:50 (volume basis), methanol/air equivalence ratio =0.6, n-heptane injection velocity v=45.2 m/s; n-heptane injection time tinj=0.1ms. EGR composition (on volume basis): CO2 (12.4%), H2O (15.6%), N2 (72%). N-heptane temperature in all Cases is 600K. Case No. A B C D E F G H
Initial T [K] 1000 900 800 1000 1000 1000 1000 1000
Initial P [bar] 42 42 42 24 42 42 42 42
phi 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
mixture n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air n-heptane/methanol/EGR/air methanol/EGR/air n-heptane/methanol/EGR/air methanol/EGR/air
Specification 2D 2D 2D 2D 3D 2D 1D 1D
3. Results and discussion 3.1 Ignition time First, ignition delay times of various mixtures under homogeneous adiabatic zero-dimensional constant volume conditions are computed. Figure 2a shows a comparison of the results from a detailed methanol mechanism of Li et al. [22] with that from the methanol sub-mechanism in the n-heptane mechanism [21]. The results are nearly identical, thus suggesting the adequateness of the n-heptane mechanism for the ignition simulation of not only the n-heptane/methanol mixture but also for the pure methanol mixture. It is seen that the ignition delay time of n-heptane is shorter than that of methanol when temperature is below 1000K, however, at higher temperatures pure methane/air mixture can ignite quicker. The impact of n-heptane on the ignition delay of methanol is evidenced in the figure: by mixing 50% volume n-heptane into methanol/air, the ignition delay time of methanol is prolonged when the temperature is above 1000K whereas at lower temperatures n-heptane can shorten the ignition delay time of methane significantly. The ignition behavior of n-heptane/methanol/air mixture is similar to that of n-heptane/air mixture, exhibiting a NTC (negative temperature coefficient) regime. To examine the effect of n-heptane on methanol/air combustion under low temperature, the temporal evolution of combustor pressure from combustion of methanol/EGR/air mixture with n-heptane injection (Case A) and without n-heptane injection (Case F) is plotted in Fig.2b. Compared with pure methanol/air combustion, the addition of n-heptane to methanol/air leads to earlier ignition of the mixture. The pressure evolution of the 2D case (Case A) and the 3D case (Case E) are similar, indicating that the 2D case replicates the 3D case reasonably well. The two-stage ignition behavior due to the NTC behavior of n-heptane leads to the earlier pressure rise around 0.8ms. The pressure evolution in 1D cases is also shown in Fig.2b. The 1D results show that the pure methanol/EGR/air mixture (Case H) has a similar combustion progress as that with n-heptane addition (Case G) under low temperature. This contrasts with the results of the 2D/3D cases (Case A/E vs F). The difference between results of Case F and Case H indicates the importance of flow transport in the overall combustion process. Figure 3a shows the total heat release rate (HRR) for the 2D Cases. Two HRR peaks can be seen in Case A. The first peak occurs at about 0.8ms after the injection of n-heptane. The second peak can be observed at 1.2ms. The HRR value of second peak is much higher than that of the first peak. Case B has a longer ignition
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delay time due to the lower initial methanol temperature (900K) as compared with Case A (1000K), and the duration of the first ignition spreads to a wider time interval, from 1 ms to 2 ms, and the second peak comes at rather later time (at 2.3 ms). There is only one HRR peak, at a rather later time, about 3 ms, can be observed in Case C due to the initial temperature of this Case is 800K, which is the lowest initial temperature in all Cases. The heat release rate profile of Case D is similar to that of Case A due to the same initial temperature and compositions; the initial pressure in Case D is lower than that in Case A, which leads to the heat release rate peaks at a relatively late time owing to the pressure effect on the ignition reactions. The same phenomenon is also found in the pressure evolution shown in Fig.3b.
(a) (b) Figure 2. (a) Comparison of ignition delay time in various mixtures of n-heptane, methanol, and air in a constant volume condition with initial pressure P=42 bar and equivalence ratio of 1. The dashed blue lines are results from the methanol mechanism of Li et al. [22], while the rest results are from the mechanism of Lu et al. [21]. In the n-heptane/methanol/air mixture the molar ratio between the two fuels is 1. (b) Comparison of temporal evolution of pressure for methanol/EGR/air mixture ignited by n-heptane (Case A [2D], Case E [3D], Case G [1D]) and pure methanol/EGR/air mixture (Case F [2D] and Case H [1D]) with initial pressure P=42 bar and equivalence ratio of 0.6. Detailed description of the Cases are given in Table 1.
(a) (b) Figure 3. Temporal evolution of heat release rate (a) and pressure (b) for four 2D Cases under different initial methanol/air/EGR mixture temperature and pressure (Case A [1000K, 42 bar], Case B [900K, 42 bar], C [800K, 42 bar] , D [1000K, 24 bar]).
3.2 Detailed ignition zone structure To gain insightful information about the heat release rate process, the temporal evolution of temperature and heat release rate field in the chamber of Case A is shown in Fig.4. At 0.4ms, the low temperature region is where n-heptane jet is penetrated through, whereas the high temperature region with uniform temperature is where the methanol/air/EGR mixture locates. At 0.9 ms the onset of ignition is shown to begin in the n-heptane jet. The entire n-heptane jet is ignited at 1.2ms while the methanol/air/EGR mixture is now starting to ignite, which yields the second peak of heat release rate shown in Fig. 3a. This behavior is essentially the same as the HCCI combustion process, which burns out the whole methanol fuel in a short duration. After 1.5 ms the mixture of pure methanol/air/EGR is nearly burned out, while certain mixture with n-heptane is still undergoing ignition. The entire combustion process can be divided into three stages: (1) ignition of the n-heptane/methanol mixing at proximity of the ne-heptane jet, (2) propagation of the reaction front into the premixed methanol/air/EGR mixture, and (3) ignition of the entire mixture of methanol/air/EGR. Stage 2 is rather short in Case A owing to the relative high initial temperature in the charge. Case D has a similar combustion process as that of Case A, except that the ignition is slightly delayed due to the lower chamber pressure .
Siyuan Hu et al. / Energy Procedia 105 (2017) 4943 – 4948
Fig.4 Temperature (K) and heat release rate (J/s) distributions for Case A with Tu=1000K at different time.
Fig.5 Temperature (K) and heat release rate (J/s) distributions for Case C with Tu=800K at different time.
The heat release rate profile has only one peak and longer duration for Case C (cf. Fig. 3a), which is due to the difference in the relative importance of above discussed three combustion stages. To examine this phenomenon, the temporal evolution of temperature of Case C is plotted in Fig.5. The ignition time of Case C is the longest since it has the lowest initial temperature in all Cases. This allows for a better mixing of n-heptane with methanol/air/EGR. The low temperature region shown in Fig.5 at 2.4 ms of Case C corresponds to higher concentration of n-heptane. The onset of ignition, at 2.4 ms, is located at the downstream region of the nheptane jet, rather different from that in Case A. At 3.0 ms, more ignition sites can be observed in the high nheptane region, including the center of the large recirculation zone. The heat release rate is still located in thin layers from 2.4 ms to 3.2 ms, indicating that the combustion of methanol is likely due to the reaction front propagation (stage 2). This process is similar to premixed flame propagation (although with the assistance of ignition in the entire unburned mixture), and it is a rather slow process, which explains the long heat release duration as shown in Fig. 3a. This combustion characteristic is mainly due to the low initial temperature, which has rather long ignition delay time. The initial temperature of Case B is moderate so that the heat release rate behavior (cf. Fig.3) is due to both the HCCI combustion of methanol/air/EGR mixture mode and the premixed reaction front propagation mode. The heat release rate profile has only one peak and longer duration for Case C (cf. Fig. 3a), which is due to the difference in the relative importance of above discussed three combustion stages. To examine this phenomenon, the temporal evolution of temperature of Case C is plotted in Fig.5. The ignition time of Case C is the longest since it has the lowest initial temperature in all Cases. This allows for a better mixing of n-heptane with methanol/air/EGR. The low temperature region shown in Fig.5 at 2.4 ms of Case C corresponds to higher concentration of n-heptane. The onset of ignition, at 2.4 ms, is located at the downstream region of the nheptane jet, rather different from that in Case A. At 3.0 ms, more ignition sites can be observed in the high nheptane region, including the center of the large recirculation zone. The heat release rate is still located in thin layers from 2.4 ms to 3.2 ms, indicating that the combustion of methanol is likely due to the reaction front propagation (stage 2). This process is similar to premixed flame propagation (although with the assistance of ignition in the entire unburned mixture), and it is a rather slow process, which explains the long heat release duration as shown in Fig. 3a. This combustion characteristic is mainly due to the low initial temperature, which
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has rather long ignition delay time. The initial temperature of Case B is moderate so that the heat release rate behavior (cf. Fig.3) is due to both the HCCI combustion of methanol/air/EGR mixture mode and the premixed reaction front propagation mode. 4. Conclusions The dual-fuel combustion process can be summarized below. First the onset of ignition is always at the proximity of the n-heptane jet. The n-heptane fuel is heated up by the hotter methanol/air/EGR mixture, thus the onset of ignition is first in a mixture of n-heptane and methanol. The ignition delay time of the nheptane/methanol mixture is governed by the n-heptane mixture. Second, a reaction front propagation is established to burn the methanol/air/EGR mixture, which gives rise to increase of the chamber temperature and pressure, and finally triggers the onset of ignition in the entire methanol/air/EGR mixture. A higher initial temperature in the methanol/air/EGR mixture can therefore give rise to faster ignition of the n-heptane jet as well as onset of ignition in the methanol/air/EGR mixture; a too low initial temperature delays the ignition of the n-heptane jet, which can allow for more complete mixing of n-heptane with methanol/air/EGR before the onset of ignition of n-heptane as well as the methanol/air/EGR mixture. The combustion process in this Case relays on the propagation of premixed flame, which can be slow or even difficult if the mixture is fuel lean or the EGR level is too high. Acknowledgements This work was partly sponsored by Swedish Research Council (VR), and the Swedish Energy Agency (STEM) through the national center for combustion science and technologies (CeCOST) and Lund University Competence Center for Combustion Process (KC-FP). S.Y. Hu was sponsored by China Scholarship Council (CSC). References [1] X.D Zhen, Y. Wang. An overview of methanol as an internal combustion engine fuel. Renewable and Sustainable Energy Reviews. 52(2015)477–493. [2] Y.P Li, M. Jia, Y.D Liu, M.Z Xie. Numerical study on the combustion and emission characteristics of a methanol/diesel reactivity controlled compression ignition (RCCI) engine. Applied Energy.106(2013)184–197. [3] M. Brusstar, M. Stuhldreher, D. Swain, W. Pidgeon. High efficiency and low emissions from a port-injected engine with neat alcohol fuels. SAE technical paper No. 2002-01-2743; 2002. [4] K. Naganuma, J. Vancoillie, L. Sileghem, S. Verhelst, James Turner and Richard Pearson. Drive Cycle Analysis of Load Control Strategies for Methanol Fuelled ICE Vehicle. SAE technical paper No. 2012-01-1606; 2012. [5] M.Brusstar, M. Stuhldreher, D. Swain and W. Pidgeon. High Efficiency and Low Emissions from a Port-Injected Engine with Neat Alcohol Fuels. SAE technical paper No. 2002-01-2743; 2002. [6] S.H Liu, R. Eddy.C. Cuty, T.G. Hu , Y.J. Wei . Study of spark ignition engine fueled with methanol/gasoline fuel blends. Applied Thermal Engineering.27(2007) 1904–1910. [7] M. Eyidogan, A. N Ozsezen, M. Canakci, A. Turkcan. Impact of alcohol–gasoline fuel blends on the performance and combustion characteristics of an SI engine. Fuel. 89(2010)2713–2720. [8] C. Sayin, M. Ilhan, M. Canakci, M. Gumus. Effect of injection timing on the exhaust emissions of a diesel engine using diesel– methanol blends. Renew Energy.34(2009)1261–9. [9] C. Mishra, N. Kumar, B.S. Chauhan, H.C Lim, M. Padhy. Some Experimental Investigation on use of Methanol and Diesel Blends in a Single Cylinder Diesel Engine. International Journal of Renewable Energy Technology Research (IJRETR). 2(2013) 01 -16. [10] M. S Kumar, A. Ramesh B. Nagalingam. An experimental comparison of methods to use methanol and Jatropha oil in a compression ignition engine. Biomass and Bioenergy.3(2003)309-318. [11] M. Tuner. Review and Benchmarking of Alternative Fuels in Conventional and Advanced Engine Concepts with Emphasis on Efficiency, CO2, and Regulated Emissions. SAE technical paper No. 2016-01-0882; 2016. [12] L.Haraldson,“Methanol as fuel”, Methanol as Fuel & Energy Storage Workshop. Lund, Sweden, 2015, http://www.lth.se/ fileadmin/mot2030/filer/12._Haraldsson_-_Methanol_as_fuel. Pdf [13] A.Yousefi, M. Birouk, B. Lawler, A. Gharehghani. Performance and emissions of a dual-fuel pilot diesel ignition engine operating on various premixed fuels. Energy Conversion and Management.106(2015)322–336. [14] The OpenFOAM Foundation, 2015 [15] Cantera: chemical kinetics, thermodynamics, transport processes. 2015. Available at
Biography Prof. Xue-Song Bai leads a research group working on CFD modeling of turbulent reacting flows. Current research of the group aims at developing models for simulation of combustion processes of gas, liquid and solid fuels with industrial application to internal combustion engines, gas turbines and furnaces.