- Email: [email protected]
methane blends using detailed kinetic mechanism
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism Kaveh Zaker a, Mohammad Hossein Askari b,*, Ali Jazayeri a, Reza Ebrahimi c, Behnam Zaker d, Mehdi Ashjaee b a
School of Mechanical Engineering, K.N Toosi University, Tehran, Iran Center of Excellence in Design and Optimization of Energy Systems (CEDOES), School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran c School of Aerospace Engineering, K.N Toosi University, Tehran, Iran d School of Mechanical and Aerospace Engineering, Politecnico di Torino University, Turin, Italy b
article info
abstract
Article history:
Hydrogen/methane mixtures have been considered as viable alternative fuels to gasoline,
Received 10 June 2015
due to overall lower emissions and much better lean burn capabilities. Nevertheless, due to
Received in revised form
different combustion characteristics of hydrogen compared to conventional fuels, its uti-
11 August 2015
lization in SI engines requires further investigations. The current study investigates the
Accepted 15 August 2015
effect of hydrogen addition to methane SI engines using an open cycle Computational Fluid
Available online xxx
Dynamics (CFD) simulation coupled with chemical kinetics. The Eddy dissipation concept (EDC) model is used in combination with a two-equation turbulence model (k-ε) and GRI-
Keywords:
Mech 3.0 kinetic mechanism. The effect of various volume fractions of hydrogen from
Hydrogen
0 to 0.5 on methane combustion is investigated and results are validated with experimental
Methane
data in literature. A good insight of combustion, turbulence flow and species consumption
Combustion engine
inside combustion chamber is obtained. A sensitivity analyses is also performed in order to
CFD
get profound understanding of combustion phenomena. The results showed that hydrogen
Emission
addition reduces combustion duration, improves flammability range, and reduces carbon
Blended fuel
monoxide and carbon dioxide amounts. Hydrogen also has a great effect on initiation of combustion process due to its lower ignition delay and higher flame propagation speed. Hydrogen addition does not affect NOx levels considerably, which is on the other hand much affected by combustion air and maximizes at 20e30 percent excessive air. The results also indicate that at Maximum Brake Torque (MBT) condition, all fuel is consumed up to 10 degrees after Top Dead Center (TDC), while carbon monoxide which is maximum at this point will transmute to carbon dioxide afterwards. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Present address: Optics and laser laboratory, Mechanical Engineering School, Tehran University, Tehran, Iran. Tel.: þ98 9112430179. E-mail address: [email protected] (M.H. Askari). http://dx.doi.org/10.1016/j.ijhydene.2015.08.040 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Nomenclatures ATDC After Top Dead Center BDC Bottom Dead Center BTDC Before Top Dead Center BSFC Break Specific Fuel Consumption CA Crank Angle CAD Crank Angle Degree CFD Computational Fluid Dynamics CNG Compressed Natural Gas COV imep Coefficient Of Variation of the Indicated Mean Effective Pressure EDC Eddy Dissipation Concept EGR Exhaust Gas Recirculation HC Hydro Carbon HCCI Homogeneous Charge Compression Ignition lambda air fuel ratio divided by stoichiometric air fuel ratio MBT Maximum Brake Torque NG Natural Gas REGR Reformed Exhaust Gas Recirculation rpm round per minute STP Standard Temperature and Pressure TDC Top Dead Center
Introduction Engine emissions and greenhouse gases have drawn government's attention towards stringent emission regulations. Finding practical solutions for reducing these emissions with concerns about energy deficiencies, has directed engine manufacturers to the development of alternative fueled engines. One of these solutions is using natural gas as a substitute fuel which is globally accepted and spread all around the world. Natural gas which mainly contains methane, offers significant economic and environmental advantages such as improved efficiency, availability, and less emissions. However, the low flame speed, poor lean-burn capability and ignitability of methane imposes great challenges for its utilization in internal combustion engines [1]. One of the most effective methods to deal with this problem is mixing natural gas with gases such as hydrogen which has considerable high burning velocity along with low ignition energy and well lean-burn capability. Combination of natural gas with hydrogen is expected to improve the lean-burn characteristics and decrease the engine emissions (mainly CO2, HC and CO), but the possibility of increased NOx emissions is of concern [2]. It improves combustion process with the possibility to develop engines with higher performance and lower environmental impact. Hydrogen itself has the potential to become an alternative to conventional fuels, since it is completely carbon-free and relatively easy to produce but costly. However, pure hydrogen has drawbacks such as too high adiabatic flame temperature, too low calorific value per unit volume, and preignition phenomena by contact with hot spots or residual gas due to lower ignition energy. The use of NG/hydrogen
mixtures containing H2 between 10 and 30 percent by volume offers good opportunity to achieve the positive aspects related to the hydrogen without substantial modification of already existing natural gas engines [3]. The use of hydrogen as a complement to natural gas also extends the lean burn limit due to the extended flammability range of hydrogen. Lean burn capability reduces the combustion temperature and NOx emission, increases thermal efficiency, and reduces knock incidence, which is a serious threat to spark ignition (SI) engines safe performance [4,5]. The anti-knock improvement feature of hydrogen makes it feasible to increase the compression ratio (CR) which can further improve the thermal efficiency [5]. Addition of hydrogen also can significantly reduce COV imep (coefficient of variation of the indicated mean effective pressure), decrease the combustion duration, achieve higher thermal efficiency and reduce fuel consumption [6,7]. A number of literatures have reported the development of hydrogen/methane internal combustion engines. Bauer and Forest [8] experimentally studied the effect of hydrogen addition on the performance of methane-fueled engine. They used a single cylinder research engine at compression ratio of 8.5. Four values of H2 ranging from 0 to 60 percent by volume and different values of equivalence ratios from low burn limit to equivalence ratio of 1.1 at different loads and speeds were tested. Their results revealed that when compared to pure methane, hydrogen addition up to 60% lowered the partial burn limit from equivalence ratio of 0.58 to 0.34. In addition, they reported a corresponding increase in brake power and decrease in BSFC in equivalence ratios ranging from 0.58 to 1.0. Their results also showed that decrease in CO, CO2 and HC and increase in NOx pollutant besides equivalence ratio, depends on hydrogen amount. Karim and Wong [9] examined an analytical method to find the effects of hydrogen addition to methane and propane fuelled HCCI engines on cyclic variation for a fuel-lean condition and low volumetric efficiency. It was shown that the addition of hydrogen, which has a relatively wide operating range, can reduce cyclic variations while extending the operating region of the engine. Mariani et al. [10] performed an experimental investigation on a passenger car engine fuelled with Compressed Natural Gas (CNG) and hydrogen-natural gas blends, with 15% and 30% of hydrogen by volume. Their results showed CO2 emission reduction while fuel consumption did not show significant differences for 15% hydrogen, although it reduced for 30% hydrogen. They also reported that the heat release rate increased with hydrogen content, the combustion duration shortened and cycle-by-cycle variability decreased due to the positive effect of hydrogen on combustion stability. Acikgoz and Celik [11] experimentally studied the performance and emission characteristics of a conventional twin cylinder, four stroke, sparkignition engine running with four premixed methanehydrogen blends with hydrogen ranging from 0 to 30 percent. They revealed that minor power loss occurs at low speed. Kahraman et al. [12] performed an experimental study on the performance and exhaust emissions of a spark ignition engine fuelled with methane with 0%, 10%, 20%, and 30% hydrogen enrichment at different engine speeds and excessive air ratios on a four-stroke engine with compression ratio of 10 at wide open throttle. Their results showed that increase in the speed
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
and excessive air ratio, reduces CO emission values, while addition of hydrogen into natural gas when operating on the lean mixture condition reduces HC, CO and CO2 emissions. They also reported that increasing the excessive air decreases the maximum peak cylinder pressure, and as the H2 fraction increases, maximum peak pressure gets close to the top dead center. Ghazal [13] evaluated the effect of blended hydrogenmethane fuel with two fuel induction systems of port injection and direct injection using a numerical simulation with Wiebe function for combustion modeling. Their results revealed that the addition of hydrogen to methane between 40% and 60% by volume with equivalence ratio near to stoichiometric condition and engine speed between 2000 and 3000 rpm improves engine performance and emissions. A general reduction in CO with the addition of hydrogen was also noticed. For port injection, hydrogen addition up to 60% increased the engine power while adding more hydrogen hampered it. For direct injection, hydrogen addition also increased the engine power. They declared that the direct injection system is more efficient than the port injection system due to turbulence intensity and combustion efficiency. Perini et al. [14] developed a predictive two-zone, quasidimensional combustion model for the simulation of the combustion process in spark ignition engines fueled with hydrogen, methane, or hydrogenemethane blends. Their model described the flame front development assuming a simplified spherical geometry, with an infinitesimal thickness. Iorio et al. [15] investigated methane/hydrogen blends combustion process in an optically accessible engine. They indicated an increase of thermal efficiency and a decrease of combustion duration with the increase of hydrogen fraction. Their optical results revealed an increased combustion reaction rate and a more uniform and rapid flame propagation by hydrogen addition. Zhou et al. [16] carried out an experimental study to investigate Combustion of a diesel engine fueled by addition of hydrogen or methane for dual-fuel operation, or mixtures of hydrogen-methane for tri-fuel operation. They showed that in-cylinder pressure and heat release rate change slightly at low to medium loads but increase dramatically at high loads. They indicated that tri-fuel operation with 30% hydrogen addition in methane is the best fuel in reducing particulate and NOx emissions at loads ranging from 70 to 90 percent of full load. Yadav et al. [17] performed an experiment to study the performance and combustion of a dual fuel hydrogen diesel engine. They reported lower exhaust emissions and improved performance using dual fuel mode with hydrogen as the supplementary fuel. It was also found that hydrogen enriched engine gives maximum efficiency at around 70% of full load whereas when operated with diesel this values come close to 80% of full load. At 70% of full load, the brake thermal efficiency was increased by 10.71%. Many other studies is available in literatures in which hydrogen was partly mixed with conventional hydrocarbon fuels such as natural gas [18e32], gasoline [33e35] or diesel oil [36e44]. All the mentioned works are done experimentally or numerically using simple thermodynamic models. Beside expensive experimental methods, trend to numerical methods alone or as a complement to experimental methods has been taken into considerations by researchers. With
3
progressions in computer technology, use of multidimensional computational fluid mechanics including numerical solution of Navier-Stokes equations and detail combustion mechanisms has become possible. However, there are not significant works done using computational fluid dynamics (CFD). Lilik et al. [45] investigated the hydrogen diesel blend fueled combustion engine by 0e15 percent of hydrogen using both experimental setup and CFD simulation. They carried out a closed loop sector mesh CFD simulation based on Reynolds averaged governing equations. Finite volume (FV) method with transported probability density function (PDF) to account for turbulenceechemistry interactions with a 71species n-heptane/NOx mechanism were used for combustion modeling. Azimovet et al. [46] performed a closed loop multidimensional CFD simulation of syngas combustion in a dual-fuel engine using a 9-step chemical kinetics mechanism. Their predicted and measured in-cylinder pressure, temperature, and rate of heat release data showed good agreement with experimental data for all types of syngas on various initial compositions. Kosmadakis et al. [47] investigated combustion of hydrogen-fueled spark-ignition engine operating with EGR using CFD method. A closed loop mesh with a simplified cylindrical geometry was used to investigate combustion and NOx emission. Yang et al. [48] performed a numerical study on a hydrogen assisted diesel engine. They revealed that in order to develop high-performance combustion engines, understanding of the combustion properties of hydrogen enriched methane is essential. Zheng et al. [49] using CFD coupled with chemical kinetics studied the effects of reformed exhaust gas recirculation (REGR) on combustion and emissions of dimethyl ether (DME) homogeneous charge compression ignition engines. Their results revealed that REGR can delay ignition time, and makes main combustion closer to top dead center, and as a result, reduces compression negative work and broadens load range of HCCI engines. They also reported that HC, CO and NOx emissions can be controlled simultaneously by REGR. Their simulation was performed using the AVL-FIRE CFD code coupled with CHEMKIN software package for a closed-cycle from intake valve closing (IVC) to exhaust valve opening (EVO). Duan et al. [50] conducted a numerical and experimental investigation for controlling backfire of a hydrogen engine. They reported that temperature and concentration of the gas mixture near the intake valves are the most effective factors that result in backfire. They optimized timing and pressure of hydrogen injection in order to reduce the concentration distribution of the intake mixture and the temperature of the high-concentration mixture through the inlet valve for controlling the backfire. The current study investigates the effect of hydrogen addition to methane fueled engine using an open cycle Computational Fluid Dynamics (CFD) simulation coupled with chemical kinetics. The Eddy Dissipation Concept (EDC) model is used in combination with a two-equation turbulence model (k-ε) and GRI-Mech 3.0 kinetic mechanisms which has been validated for CH4/H2 combustion to help rigorous understanding of combustion phenomena in engine. Effects of hydrogen addition at different air-fuel ratio and hydrogen content on emissions, efficiency and combustion duration is investigated using detailed three dimensional simulations. A
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
simplified geometry similar to Ma et al. [51] work is selected so it can be compared and verified with their experimental results. Hydrogen-Methane fuel is assumed to be inducted through the intake manifold as a premixed mixture with air including 0%, 30% and 50% of hydrogen by volume. A good insight of combustion phenomena, turbulence flow and species consumption inside combustion chamber is obtained. A sensitivity analyses is also performed in order to improve our understanding of chemical kinetic mechanism inside the combustion chamber.
Numerical approach In this work simulations are implemented in Fluent 6.3 Since a natural aspiration engine is considered in this investigation, inlet boundary condition is set to constant pressure at 105 kPa with regard to experimental reference work Ma et al. [51] and outlet boundary is set to constant atmospheric pressure. Piston and valves are moving walls and the other boundaries are set as stationary walls. Convergence criteria for continuity, energy and species are set respectively to 10e-4, 10e-7 and 10e-7. In our model we have taken the time steps equal to 0.1 CA during combustion process and 0.5 CA in the rest of the simulation.
Basic equations for mass, momentum, energy and species for compressible gas are illustrated by Bakul et al. [52]. Standard k ε model has been used in this research for modeling of turbulence. Two equations are needed for computation of turbulent kinetic energy k and dissipation rate ε which are presented as follows: For turbulent kinetic energy k m vk mþ t þ Gk rε þ Sk sk vxj
m vε ε ε2 mþ t þ C1ε ðGk Þ C2ε r þ Sε k sε vxj k
! ruej Yei
0 1 v B mt vYei C ui ¼ @ A þ r~ vxj sY vxj
i ¼ 1; : : : ; N
(3)
~ i expresses the net rate of where the chemical source term u production of species i due to chemical reaction. The eddy dissipation concept expresses the chemical reaction rate as: i h ~ε b p; Y b min þ Y b p gl b min ; Y ~ F ¼ Cu k min Y u k~
(4)
where: (5)
The mass fraction occupied by turbulent fine structure regions, gl , is expressed as g l ¼ Cg
1=4 n~ε 2 k~
(6)
And the correction factor for nonpremixedness, k, is written as
(1)
For dissipation ε v v v ðrεÞ þ ðrεui Þ ¼ vt vxi vxj
v vxj
i h b F; Y bO b min ¼ min Y Y
Basic governing equations
v v v ðrkÞ þ ðrkui Þ ¼ vt vxi vxj
because in those regions molecular diffusion is faster than turbulent transport. From this idea the concept of the Eddy Dissipation Concept (EDC) was developed. There is a range in which the reactions can be regarded as ideally mixed. Thus, chemical reaction kinetics determines the speed of the process, while outside this range the reactants are not mixed and do not react. So each computing cell is split into a reactive fraction (g3l ) which represents the part of the mixture inside the smallest turbulence structures and an inert fraction (1 g3l ) which is the surrounding fluid [53]. This combustion model (i.e. EDC) is used for combustion turbulent interaction consideration in the current research. The equation for conservation of individual species for a mixture of N species can be written as:
(2)
where Gk indicates turbulent energy production due to velocity gradient and C1ε ، C2ε and C3ε are adjustable constant coefficients. sk and sε are turbulence Prandtl number and Sk and Sε are source terms for k and ε respectively. Eddy viscosity 2 is represented as mt ¼ rCm kε where Cm , C1ε , C2ε , C3ε , sk and sε are constant coefficients which are considered to be 0.09, 1.44, 1.92, 0.09, 1.0 and 1.3 respectively.
Combustion modeling In simulating combustion systems, both physical and chemical processes are of great importance. The formation of turbulent structures is a physical process for which the concept of eddy cascade model is the basis. The smallest eddies happen in Kolmogorov Length Scales. At these scales, dissipation of turbulent kinetic energy k takes place. At a size smaller than Kolmogorov no turbulent structures exist,
2 b min þ Y bp Y k¼ bp Y bp bF þ Y bO þ Y Y
(7)
The terms Cu and Cg are constants with values 11.1 and 2.1 respectively [54]. Therefore in this model there are two regions: One which reaction rate is fast and mixing rate controls the combustion and the other which reactants are ideally mixed and the combustion is controlled by chemical kinetics.
Sensitivity analysis The sensitivity analysis method investigates the model output as a function of parameters. The local normalized sensitivity coefficient is defined as [55]: S¼
kj vci vln ci ¼ ci vkj vln kj
(8)
These coefficients form the normalized sensitivity matrix which represents the fractional change in concentration ci caused by a fractional change of parameter kj . The sensitivity matrix accounts for the change of a single variable as a result of the change of individual parameters.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Meshing The combustion chamber used to investigate the combustion of blended hydrogen methane is based on Ma et al. work [51]. In this research for straightforwardness, the geometry is simplified and cylinder head and piston are considered flat although some minor errors would be inevitable. In this study a tetrahedral mesh is used for dead volume and valve areas. Approximately 340,000 meshes are generated in top dead center (TDC) and 445,000 meshes in bottom dead center (BDC). This difference is because, the border between tetrahedral and hexahedral meshes also moves down while the piston moves down, leading to more mesh generations due to remeshing. According to Fluent a piston limit profile is chosen for this border as its movement profile. It means moving down with the piston to a limit and then staying and not accompanying the piston any more. Then waiting until the piston returns and then continuing moving up with it again. This is done due to the valve motions. If this border will be fixed, by valve motion the skewness of the dead volume tetrahedral cells will increase more than the acceptable limit and divergence would happen. The Hexahedral mesh is utilized for regions on the top of the valves in ports and near piston for faster dynamic mesh updating (approximately 5000 meshes in TDC and 135,000 in BDC) which in all will lead to a total of approximately 345,000 meshes in TDC and 580,000 meshes in BDC. The borders between two types of meshes are defined as mesh interfaces. The calculations are carried out using an Intel Corei7 e 2600 K CPU @ 3.4 GHz Processor and 8 GB RAM in a 64-bit Operating System. In order to study mesh independency the geometry at TDC was discretized with 175,000, 260,000, 345,000, and 455,000 cell volumes. Simulations using these grid sizes were compared for trapped air and fuel mixture mass and cylinder pressure, temperature and fuel mass fraction curves in the case of pure methane combustion and finally the grid size corresponding to 345,000 elements at TDC provided sufficiently accurate results without further change with increasing the number of cells. Therefore by considering the computational time this mesh is used for all the simulations. The solution domain and meshing at different positions of piston are shown in Fig. 1. As is shown in this Figure, at TDC unstructured mesh and only one row of structured mesh are used. As was mentioned this method will make a perfect dynamic mesh updating when piston and valves move.
Result and discussions Intake Turbulent kinetic energy and dissipation rate vs crank angle degree (CAD) is shown in Fig. 2. These parameters are very important for EDC combustion model which is used in this study. Turbulent kinetic energy generation source is usually the intake valve at the entrance to the combustion chamber. Turbulent kinetic energy increases up to 60 degrees ATDC and its generation is dominant over dissipation rate. From that degree due to decrease in intake of turbulent kinetic energy
5
and the fact that the turbulent kinetic energy dissipation proceeds the turbulent kinetic energy generation (which itself is decreasing due to valve opening), the average turbulent kinetic energy starts decreasing. Velocity vectors at different valve openings are shown in Fig. 3. Fig. 3A shows velocity vectors at the beginning of valve opening which is start of the vortex formation in combustion chamber. As can be seen from this figure, although close to TDC the intake valves are opened a little and because of it, it is expected that speed increases, but because the flow velocity is also dependent on piston velocity and piston velocity is approximately in its minimum value around TDC, therefore the flow velocity inside cylinder does not change that much. With the piston getting distance from TDC, flow velocity in combustion chamber gradually increases and at 25 degree ATDC velocity vortex is completely established and the flow tumble is apparently observable. These results reveal that intake valve opening and piston linear speed are two important factors for generation of velocity vortex in combustion chamber and therefore more monotone mixture formation, which leads to better combustion. In Fig. 4 temperature contours in combustion chamber at different crank angles are shown for better understanding of premixed cold air and fuel intake. The temperature contours declare how the cold air and fuel mixture enter the hot combustion chamber and how they decrease the average temperature in the cylinder.
Combustion The effect of hydrogen addition on the combustion of CNG engines is investigated using three different hydrogen ratios ranging from zero to fifty percent of hydrogen in fuel by volume. As mentioned, the engine performance information and geometry are set to be similar to their counterparts in the work of Ma et al. [51] so that the results can be validated. As can be seen from Figs. 5e10, there is a good agreement between the CFD and experimental results in trend but some quantity deviations are observable. These deviations are due to the nonconformities between the experimental conditions and CFD assumptions. In this study, exact geometry of intake and exhaust ports, intake and exhaust valve timing profiles, environmental conditions of the test (vs STP used in the CFD model), exact geometry of piston and cylinder head of the test engine were among the unknowns. In addition, some simplifications like pure CH4 as fuel vs NG in experimental work, ignoring radiation due to longer running time and neglectable effect on the performance, single cylinder instead of in-line 6 cylinder test engine would make the modeling results not to fit on the experimental results precisely. On the other hand, experimental set ups are not completely accurate themselves and have their own inherent uncertainties. For example the precision of the ALICAT flow meter used in experimental work of Ma et al. [51] is 1.5 percent [22], or the precision of Kistler 2613B used as the crank angel meter is 1 CA [51]which will impose an error when integrating Pdv for the calculation of work and power. In this section besides verifying the model with experimental results, most important in-cylinder combustion
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 1 e The solution domain and meshing at different crank angles A & B @ 10degrees ATDC, C @ 70degrees ATDC, D @ 55degrees ATDC.
phenomena are reported in order to complete understanding of what happens during the combustion process in the engine. Indicated thermal efficiency vs lambda for different hydrogen fractions is shown in Fig. 5. Regarding CFD results of this figure, hydrogen ability to extend lean limit is obvious. Also it is shown that when lambda was under 1.7, hydrogen addition did not have significant effect on efficiency improvement. After lambda ¼ 1.8, thermal efficiency exhibited an obvious drop with pure NG which is due to the lean burn limit of CH4 combustion being around this point. After this limit and for example around lambda ¼ 2 the indicated
Fig. 2 e Turbulence k-3 vs CAD for methane combustion.
thermal efficiency experiences a considerable drop from 37% to 20%. By adding 30% hydrogen and due to the effect of hydrogen on expanding the burning lean limit this indicated thermal efficiency goes back to 37% and even a bit higher. The CFD model shows similar trend with about 10% difference comparing to experimental results. Combustion duration vs lambda for different hydrogen fractions is shown in Fig. 6. The spark time instant to the moment that 99% of CH4 is consumed is defined as combustion duration. As is shown, at a given lambda, as hydrogen fraction increases, combustion duration is shortened, which is due to higher flame propagation speed of hydrogen. Fig. 7 shows the specific emissions of NOx, as a function of lambda for different hydrogen fractions. As shown in this figure, when engine works on lean mixture NOx emission first increases rapidly, reaches a peak, and then decreases gradually to a relatively small value, and this trend is independent of hydrogen fraction. Excess air have two opposing effects on the production of NOx. The more the excess air would be the more nitrogen would exist and the lower the combustion temperature would become. These two factors (more available N2 and lower combustion temperature) would lead to respectively higher and lower NOx production. That is why there would be a peak which is around 1.2e1.3. It is also noted that NOx emission can be very low after lambda exceeds 1.8 which indicates the greater weight of lowered temperature rather than higher available nitrogen amount. In other words after stoichiometric condition, with increasing the excess air, thermal NOx decreases while prompt NOx is increasing with a higher rate till 20e30% excess air. After this point thermal NOx decrease rate prevails prompt
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
7
Fig. 3 e Velocity Vectors in Combustion Chamber at different crank angles A) 10 degrees ATDC, B) 25 degrees ATDC, C) 70 degrees ATDC.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 4 e Temperature contours in Combustion Chamber at different crank angles A) 10 degrees ATDC, B) 25 degrees ATDC, C) 70 degrees ATDC. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 5 e Indicated thermal efficiency vs lambda for different hydrogen fractions.
Fig. 6 e Combustion duration vs lambda for different hydrogen fractions.
Fig. 7 e NOx emission vs lambda for different hydrogen fractions.
9
Fig. 8 e NOx mass fraction vs CAD for 30 percent hydrogen and different lambda.
NOx increase rate and the overall NOx reduces, and after a special CAD this NOx freezes which can be seen in Fig. 8. This figure indicates the mass fraction of NOx in different CADs and again in that figure it is obvious that NOx mass fraction after freezing is the highest in lambda between 1.2 and 1.3. Figs. 9 and 10 shows the specific emissions of CO and Unburned Hydro Carbons (UHC), as a function of lambda for different hydrogen fractions. As is shown in Fig. 9, after stoichiometric condition CO specific emission first drops gradually which is due to more oxygen and then starts climbing up rapidly due to misfire. Below 70 percent excess air, adding hydrogen showed no significant effect on CO, but once lambda exceeded 1.8, more hydrogen addition resulted in much less exhaust CO which again is due to extending the lean burn limit and preventing misfire.
Fig. 9 e CO emission vs lambda for different hydrogen fractions.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 10 e HC emission vs lambda for different hydrogen fractions.
HC specific emission shown in Fig. 10, is very low when lambda is smaller than 1.8, because at this point, on one hand there is extra air to ensure complete combustion and on the other hand, the fueleair mixture is not too lean, so the exhaust temperature can keep itself at a high level which is beneficial for further oxidation of HC remained in crevices and flame quenching zone. Reduction of HC emission by hydrogen enrichment is because of: first: lean burn capability which prevents combustion failure at higher lambdas, second: carbon-hydrogen molar ratio reduction, third: hydrogen
Fig. 12 e Cylinder temperature vs crank angle for spark ignition at MBT.
shorter quenching distance which will leave less UHC close to walls. Fig. 11 shows temperature contours at different crank angle degrees for equivalence ratio ¼ 1 and hydrogen volumetric ratio of zero. Because the spark is located at the center, flame starts propagation from the center. With initiation of combustion, the temperature around spark and the products of combustion start increasing and the amount of reactants in that same area start to decrease. With flame propagation the
Fig. 11 e Temperature contours at different crank angles for equivalence ratio of 1 for pure methane A) 25 degree BTDC B) 15 degree BTDC, C) TDC, D) 15 degree ATDC.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
hot zone develops which is clearly visible in the figure. The contours for reactants and products concentrations are a lot similar to the shown temperature contours in this figure. Cylinder temperature vs crank angle for spark ignition at MBT is shown in Fig. 12. As is shown in this figure with increasing the hydrogen amount, temperature increase rate after spark rises considerably. The combustion chamber maximum temperature of hydrogen may be thought to increase a lot due to its higher adiabatic flame temperature in comparison with methane. But this factor is balanced by the fact that hydrogen lower volumetric heating value is 0.3 of methane. Higher adiabatic flame temperature of hydrogen versus its lower volumetric heating value will lead to approximately the same maximum combustion chamber temperature of blended fuel in comparison with pure methane. As is declared in Fig. 12 maximum in-cylinder temperature increases slightly with increasing hydrogen amount in fuel composition. CO and CO2 mass fractions vs CAD are shown in Fig. 13 for various hydrogen concentrations in fuel in stoichiometric mixture. As shown in these figures, the maximum amount of CO happens slightly after 10ATDC in which methane is consumed completely. At the same time the CO2 amount is increased considerably up to this degree and after this point with slow transmutation of CO to CO2, CO2 amount increases gradually.
Sensitivity analysis Sensitivity analysis is performed using CHEMKIN PREMIXED code in isobaric condition of 30 bar and initial temperature of 1000 K which is the start of combustion of the engine under study using stoichiometric mixture using stoichiometric mixture. The chemical kinetic mechanism used in the present work is GRI-Mech 3.0 [56] which using sensitivity analysis is reduced to includes 50 species and 309 elementary chemical reactions to be adaptable to Fluent 6.3. As is depicted in Fig. 14 Methane is depleted by reaction OH þ CH4 ⇔CH3 þ H2 O in all cases. This shows that OH is the most important radical and combustion is mostly promoted by OH radicals. As is shown in Fig. 14 Chain-branching reaction H þ O2 ⇔O þ OH is the most important chain branching step in every combustion process where H atoms are present [55]. In the temperature regime
11
Fig. 14 e Normalized sensitivity coefficient of methane concentration caused by reactions rates.
studied in this article, fuel consumption is also controlled by the chain-propagating reaction HO2 þ CH3 ⇔OH þ CH3 O and chain integrating reactions H þ CH3 ðþMÞ⇔CH4 ðþMÞ andOH þ HO2 ⇔O2 þ H2 O. By increasing hydrogen fraction in the mixture the H radicals increase in the flame front and OH production is promoted as a result of reaction H þ O2 ⇔O þ OH. CH4 and H2 mass fraction vs CAD are shown in Fig. 15 for various hydrogen percentages in methane by volume in fuel for stoichiometric mixture. This figure is important because the combustion duration is calculated from it. As is shown in this figure methane is used up completely upto 12 degrees ATDC. After this degree some amount of hydrogen remains in the combustion chamber which reacts with the remaining radicals.
Conclusion In this study Eddy Dissipation Concept model is used in combination with a two-equation turbulence model (k-ε) and
Fig. 13 e A-CO and B-CO2 mass fractions vs CAD for different hydrogen contents. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
12
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 15 e Mass fractions vs CAD for different hydrogen contents A) CH4, B) H2.
GRI-Mech 3.0 kinetic mechanism as a valid CH4/H2 mechanism, for accurate understanding of chemical kinetics of methane and hydrogen combustion. Effects of hydrogen addition ranging from 0 to 50 percent in fuel by volume at different air-fuel ratios on emissions, efficiency and combustion duration is investigated. A great deal of combustion phenomena, turbulence flow and species consumption inside combustion chamber is obtained. A sensitivity analyses is also performed in order to great understanding of combustion phenomena. The most significance results of this investigation are summarized as follows: Hydrogen addition reduces combustion duration, extends lean burn range, and reduces carbon monoxide and carbon dioxide amounts. At MBT condition, fuel is used up till 10 degrees after TDC, while carbon monoxide has a maximum at this point and after that transmutes to carbon dioxide. Intake valve opening and piston motion are two important factors for generation of velocity vortex in combustion chamber. Maximum in-cylinder temperature increases slightly with increasing hydrogen amount in fuel composition. Based on sensitivity analysis methane is depleted by reaction OH þ CH4 ⇔CH3 þ H2 O in all cases. Chain-branching reaction H þ O2 ⇔O þ OH is the most important chain branching step in combustion. Methane is used up completely upto 12 degrees ATDC. After this degree some amount of hydrogen remains in the combustion chamber which reacts with the remaining radicals. This work reveals the high capability of contemporary CFD methods and the fact that they can be well trusted upon even for a complicated subject like an open cycle of an internal combustion engine with dynamic meshes besides complicated chemical mechanisms.
references
[1] Fotache TGKCG, Law CK. Ignition of hydrogen-enriched methane by heated air. Combust Flame 1997;110:429e40. [2] Hu E, Huang Z, Zheng J, Li Q, He J. Numerical study on laminar burning velocity and NO formation of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34:6545e57. [3] De Simio L, Gambino M, Iannaccone S. Experimental and numerical study of hydrogen addition in a natural gas heavy duty engine for a bus vehicle. Int J Hydrogen Energy 2013;38:6865e73. [4] Lim G, Lee S, Park C, Choi Y, Kim C. Effect of ignition timing retard strategy on NOx reduction in hydrogen-compressed natural gas blend engine with increased compression ratio. Int J Hydrogen Energy 2014;39:2399e408. [5] Lim G, Lee S, Park C, Choi Y, Kim C. Effects of compression ratio on performance and emission characteristics of heavyduty SI engine fuelled with HCNG. Int J Hydrogen Energy 2013;38:4831e8. [6] Akansu S. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int J Hydrogen Energy 2004;29:1527e39. [7] Deng J, Ma F, Li S, He Y, Wang M, Jiang L, et al. Experimental study on combustion and emission characteristics of a hydrogen-enriched compressed natural gas engine under idling condition. Int J Hydrogen Energy 2011;36:13150e7. [8] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. Int J Hydrogen Energy 2001;26(1):55e70. [9] Wong YK, Karim GA. An analytical examination of the effects of hydrogen addition on cyclic variations in homogeneously charged compressioneignition engines. Int J Hydrogen Energy 2000;25(12):1217e24. [10] Mariani A, Prati MV, Unich A, Morrone B. Combustion analysis of a spark ignition i. c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends. Int J Hydrogen Energy 2013;38:1616e23. [11] Acikgoz B, Celik C. An experimental study on performance and emission characteristics of a methaneehydrogen fuelled gasoline engine. Int J Hydrogen Energy 2012;37:18492e7.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
[12] Kahraman N, Ceper B, Akansu S, Aydin K. Investigation of combustion characteristics and emissions in a spark-ignition engine fuelled with natural gasehydrogen blends. Int J Hydrogen Energy 2009;34:1026e34. [13] Ghazal OH. A comparative evaluation of the performance of different fuel induction techniques for blends hydrogenemethane SI engine. Int J Hydrogen Energy 2013;38:6848e56. [14] Perini F, Paltrinieri F, Mattarelli E. A quasi-dimensional combustion model for performance and emissions of SI engines running on hydrogenemethane blends. Int J Hydrogen Energy 2010;35:4687e701. [15] Di Iorio S, Sementa P, Vaglieco BM. Experimental investigation on the combustion process in a spark ignition optically accessible engine fueled with methane/hydrogen blends. Int J Hydrogen Energy 2014;39:9809e23. [16] Zhou JH, Cheung CS, Leung CW. Combustion, performance and emissions of a diesel engine with H2, CH4 and H2eCH4 addition. Int J Hydrogen Energy 2014;39:4611e21. [17] Yadav VS, Sharma D, Soni SL. Performance and combustion analysis of hydrogen-fuelled C.I. engine with EGR. Int J Hydrogen Energy 2015;40:4382e91. [18] Gatts T, Liu S, Liew C, Ralston B, Bell C, Li H. An experimental investigation of incomplete combustion of gaseous fuels of a heavy-duty diesel engine supplemented with hydrogen and natural gas. Int J Hydrogen Energy 2012;37:7848e59. [19] Lee S-W, Choi W, Cho Y-S. Characterization of HCNG combustion and emission characteristics in a constant volume chamber with a single and a dual spark plug configuration. Int J Hydrogen Energy 2012;37:682e90. [20] Ma F. Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine. Int J Hydrogen Energy 2008;33:1416e24. [21] Ma F, Li S, Zhao J, Qi Z, Deng J, Naeve N, et al. Effect of compression ratio and spark timing on the power performance and combustion characteristics of an HCNG engine. Int J Hydrogen Energy 2012;37:18486e91. [22] Ma F, Wang M, Jiang L, Chen R, Deng J, Naeve N, et al. Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio. Int J Hydrogen Energy 2010;35:6438e47. [23] Ma F, Wang M, Jiang L, Deng J, Chen R, Naeve N, et al. Performance and emission characteristics of a turbocharged spark-ignition hydrogen-enriched compressed natural gas engine under wide open throttle operating conditions. Int J Hydrogen Energy 2010;35:12502e9. [24] Mariani A, Morrone B, Unich A. Numerical evaluation of internal combustion spark ignition engines performance fuelled with hydrogen e natural gas blends. Int J Hydrogen Energy 2012;37:2644e54. [25] Mathai R, Malhotra RK, Subramanian KA, Das LM. Comparative evaluation of performance, emission, lubricant and deposit characteristics of spark ignition engine fueled with CNG and 18% hydrogen-CNG. Int J Hydrogen Energy 2012;37:6893e900. [26] Mohammed SEL, Baharom MB, Aziz ARA. Analysis of engine characteristics and emissions fueled by in-situ mixing of small amount of hydrogen in CNG. Int J Hydrogen Energy 2011;36:4029e37. [27] Park C, Kim C, Choi Y. Power output characteristics of hydrogen-natural gas blend fuel engine at different compression ratios. Int J Hydrogen Energy 2012;37:8681e7. [28] Park C, Kim C, Choi Y, Won S, Moriyoshi Y. The influences of hydrogen on the performance and emission characteristics of a heavy duty natural gas engine. Int J Hydrogen Energy 2011;36:3739e45.
13
[29] Park C, Lee S, Lim G, Choi Y, Kim C. Effect of mixer type on cylinder-to-cylinder variation and performance in hydrogennatural gas blend fuel engine. Int J Hydrogen Energy 2013;38:4809e15. rez A, Gime nez B. Study of the cycle-to[30] Reyes M, Melgar A, Pe cycle variations of an internal combustion engine fuelled with natural gas/hydrogen blends from the diagnosis of combustion pressure. Int J Hydrogen Energy 2013;38:15477e87. nez B, Reyes M. Prediction of [31] Tinaut FV, Melgar A, Gime performance and emissions of an engine fuelled with natural gas/hydrogen blends. Int J Hydrogen Energy 2011;36:947e56. [32] Zheng J, Hu E, Huang Z, Ning D, Wang J. Combustion and emission characteristics of a spray guided direct-injection spark-ignition engine fueled with natural gas-hydrogen blends. Int J Hydrogen Energy 2011;36:11155e63. [33] Dandrea T. The addition of hydrogen to a gasoline-fuelled SI engine. Int J Hydrogen Energy 2004;29:1541e52. [34] Ji C, Wang S. Combustion and emissions performance of a hybrid hydrogenegasoline engine at idle and lean conditions. Int J Hydrogen Energy 2010;35:346e55. [35] Wang S, Ji C, Zhang B. Starting a spark-ignited engine with the gasolineehydrogen mixture. Int J Hydrogen Energy 2011;36:4461e8. [36] Chiriac R, Apostolescu N. Emissions of a diesel engine using B20 and effects of hydrogen addition. Int J Hydrogen Energy 2013;38:13453e62. [37] de Morais AM, Mendes Justino MA, Valente OS, Hanriot SDM, JR. Hydrogen impacts on performance and CO2 Sodre emissions from a diesel power generator. Int J Hydrogen Energy 2013;38:6857e64. [38] Ghazal OH. Performance and combustion characteristic of CI engine fueled with hydrogen enriched diesel. Int J Hydrogen Energy 2013;38:15469e76. [39] Ibrahim MM, Ramesh A. Experimental investigations on a hydrogen diesel homogeneous charge compression ignition engine with exhaust gas recirculation. Int J Hydrogen Energy 2013;38:10116e25. [40] Liu S, Li H, Liew C, Gatts T, Wayne S, Shade B, et al. An experimental investigation of NO2 emission characteristics of a heavy-duty H2-diesel dual fuel engine. Int J Hydrogen Energy 2011;36:12015e24. [41] Miyamoto T, Hasegawa H, Mikami M, Kojima N, Kabashima H, Urata Y. Effect of hydrogen addition to intake gas on combustion and exhaust emission characteristics of a diesel engine. Int J Hydrogen Energy 2011;36:13138e49. [42] Shin B, Cho Y, Han D, Song S, Min Chun K. Investigation of the effects of hydrogen on cylinder pressure in a splitinjection diesel engine at heavy EGR. Int J Hydrogen Energy 2011;36:13158e70. [43] Wang S, Ji C, Zhang J, Zhang B. Improving the performance of a gasoline engine with the addition of hydrogeneoxygen mixtures. Int J Hydrogen Energy 2011;36:11164e73. [44] Zhou JH, Cheung CS, Leung CW. Combustion, performance and emissions of ULSD, PME and B50 fueled multi-cylinder diesel engine with naturally aspirated hydrogen. Int J Hydrogen Energy 2013;38:14837e48. [45] Lilik GK, Zhang H, Herreros JM, Haworth DC, Boehman AL. Hydrogen assisted diesel combustion. Int J Hydrogen Energy 2010;35:4382e98. [46] Azimov U, Okuno M, Tsuboi K, Kawahara N, Tomita E. Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism. Int J Hydrogen Energy 2011;36:13793e807. [47] Kosmadakis GM, Rakopoulos CD, Demuynck J, De Paepe M, Verhelst S. CFD modeling and experimental study of combustion and nitric oxide emissions in hydrogen-fueled
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
14
[48]
[49]
[50]
[51]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
spark-ignition engine operating in a very wide range of EGR rates. Int J Hydrogen Energy 2012;37:10917e34. An H, Yang WM, Maghbouli A, Li J, Chou SK, Chua KJ. A numerical study on a hydrogen assisted diesel engine. Int J Hydrogen Energy 2013;38:2919e28. Zheng Z, Liu C, Zhang X. Numerical study of effects of reformed exhaust gas recirculation (REGR) on dimethyl ether HCCI combustion. Int J Hydrogen Energy 2014;39:8106e17. Duan J, Liu F, Sun B. Backfire control and power enhancement of a hydrogen internal combustion engine. Int J Hydrogen Energy 2014;39:4581e9. Ma F, Wang Y, Liu H, Li Y, Wang J, Zhao S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int J Hydrogen Energy 2007;32:5067e75.
[52] Bakul VYGCEJ, Xianming L. Computational fluid dynamics in industrial combustion. New York: CRC Press; 2001. [53] Rehm M, Seifert P, Meyer B. Theoretical and numerical investigation on the EDC-model for turbulenceechemistry interaction at gasification conditions. Comput Chem Eng 2009;33:402e7. [54] Gran MCMIR, Magnussen BF. Numerical simulation of local extinction effects in turbulent combustor flows of methane and air. Symp Combust 1994;25:1283e91. [55] Tunlnyi T. Applications of sensitivity analysis to combustion chemistry. Reliab Eng Syst Saf 1997;57:8. [56] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al. GRI-Mech 3.0. Available: http://www.me. berkeley.edu/gri_mech/.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism Kaveh Zaker a, Mohammad Hossein Askari b,*, Ali Jazayeri a, Reza Ebrahimi c, Behnam Zaker d, Mehdi Ashjaee b a
School of Mechanical Engineering, K.N Toosi University, Tehran, Iran Center of Excellence in Design and Optimization of Energy Systems (CEDOES), School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran c School of Aerospace Engineering, K.N Toosi University, Tehran, Iran d School of Mechanical and Aerospace Engineering, Politecnico di Torino University, Turin, Italy b
article info
abstract
Article history:
Hydrogen/methane mixtures have been considered as viable alternative fuels to gasoline,
Received 10 June 2015
due to overall lower emissions and much better lean burn capabilities. Nevertheless, due to
Received in revised form
different combustion characteristics of hydrogen compared to conventional fuels, its uti-
11 August 2015
lization in SI engines requires further investigations. The current study investigates the
Accepted 15 August 2015
effect of hydrogen addition to methane SI engines using an open cycle Computational Fluid
Available online xxx
Dynamics (CFD) simulation coupled with chemical kinetics. The Eddy dissipation concept (EDC) model is used in combination with a two-equation turbulence model (k-ε) and GRI-
Keywords:
Mech 3.0 kinetic mechanism. The effect of various volume fractions of hydrogen from
Hydrogen
0 to 0.5 on methane combustion is investigated and results are validated with experimental
Methane
data in literature. A good insight of combustion, turbulence flow and species consumption
Combustion engine
inside combustion chamber is obtained. A sensitivity analyses is also performed in order to
CFD
get profound understanding of combustion phenomena. The results showed that hydrogen
Emission
addition reduces combustion duration, improves flammability range, and reduces carbon
Blended fuel
monoxide and carbon dioxide amounts. Hydrogen also has a great effect on initiation of combustion process due to its lower ignition delay and higher flame propagation speed. Hydrogen addition does not affect NOx levels considerably, which is on the other hand much affected by combustion air and maximizes at 20e30 percent excessive air. The results also indicate that at Maximum Brake Torque (MBT) condition, all fuel is consumed up to 10 degrees after Top Dead Center (TDC), while carbon monoxide which is maximum at this point will transmute to carbon dioxide afterwards. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Present address: Optics and laser laboratory, Mechanical Engineering School, Tehran University, Tehran, Iran. Tel.: þ98 9112430179. E-mail address: [email protected] (M.H. Askari). http://dx.doi.org/10.1016/j.ijhydene.2015.08.040 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Nomenclatures ATDC After Top Dead Center BDC Bottom Dead Center BTDC Before Top Dead Center BSFC Break Specific Fuel Consumption CA Crank Angle CAD Crank Angle Degree CFD Computational Fluid Dynamics CNG Compressed Natural Gas COV imep Coefficient Of Variation of the Indicated Mean Effective Pressure EDC Eddy Dissipation Concept EGR Exhaust Gas Recirculation HC Hydro Carbon HCCI Homogeneous Charge Compression Ignition lambda air fuel ratio divided by stoichiometric air fuel ratio MBT Maximum Brake Torque NG Natural Gas REGR Reformed Exhaust Gas Recirculation rpm round per minute STP Standard Temperature and Pressure TDC Top Dead Center
Introduction Engine emissions and greenhouse gases have drawn government's attention towards stringent emission regulations. Finding practical solutions for reducing these emissions with concerns about energy deficiencies, has directed engine manufacturers to the development of alternative fueled engines. One of these solutions is using natural gas as a substitute fuel which is globally accepted and spread all around the world. Natural gas which mainly contains methane, offers significant economic and environmental advantages such as improved efficiency, availability, and less emissions. However, the low flame speed, poor lean-burn capability and ignitability of methane imposes great challenges for its utilization in internal combustion engines [1]. One of the most effective methods to deal with this problem is mixing natural gas with gases such as hydrogen which has considerable high burning velocity along with low ignition energy and well lean-burn capability. Combination of natural gas with hydrogen is expected to improve the lean-burn characteristics and decrease the engine emissions (mainly CO2, HC and CO), but the possibility of increased NOx emissions is of concern [2]. It improves combustion process with the possibility to develop engines with higher performance and lower environmental impact. Hydrogen itself has the potential to become an alternative to conventional fuels, since it is completely carbon-free and relatively easy to produce but costly. However, pure hydrogen has drawbacks such as too high adiabatic flame temperature, too low calorific value per unit volume, and preignition phenomena by contact with hot spots or residual gas due to lower ignition energy. The use of NG/hydrogen
mixtures containing H2 between 10 and 30 percent by volume offers good opportunity to achieve the positive aspects related to the hydrogen without substantial modification of already existing natural gas engines [3]. The use of hydrogen as a complement to natural gas also extends the lean burn limit due to the extended flammability range of hydrogen. Lean burn capability reduces the combustion temperature and NOx emission, increases thermal efficiency, and reduces knock incidence, which is a serious threat to spark ignition (SI) engines safe performance [4,5]. The anti-knock improvement feature of hydrogen makes it feasible to increase the compression ratio (CR) which can further improve the thermal efficiency [5]. Addition of hydrogen also can significantly reduce COV imep (coefficient of variation of the indicated mean effective pressure), decrease the combustion duration, achieve higher thermal efficiency and reduce fuel consumption [6,7]. A number of literatures have reported the development of hydrogen/methane internal combustion engines. Bauer and Forest [8] experimentally studied the effect of hydrogen addition on the performance of methane-fueled engine. They used a single cylinder research engine at compression ratio of 8.5. Four values of H2 ranging from 0 to 60 percent by volume and different values of equivalence ratios from low burn limit to equivalence ratio of 1.1 at different loads and speeds were tested. Their results revealed that when compared to pure methane, hydrogen addition up to 60% lowered the partial burn limit from equivalence ratio of 0.58 to 0.34. In addition, they reported a corresponding increase in brake power and decrease in BSFC in equivalence ratios ranging from 0.58 to 1.0. Their results also showed that decrease in CO, CO2 and HC and increase in NOx pollutant besides equivalence ratio, depends on hydrogen amount. Karim and Wong [9] examined an analytical method to find the effects of hydrogen addition to methane and propane fuelled HCCI engines on cyclic variation for a fuel-lean condition and low volumetric efficiency. It was shown that the addition of hydrogen, which has a relatively wide operating range, can reduce cyclic variations while extending the operating region of the engine. Mariani et al. [10] performed an experimental investigation on a passenger car engine fuelled with Compressed Natural Gas (CNG) and hydrogen-natural gas blends, with 15% and 30% of hydrogen by volume. Their results showed CO2 emission reduction while fuel consumption did not show significant differences for 15% hydrogen, although it reduced for 30% hydrogen. They also reported that the heat release rate increased with hydrogen content, the combustion duration shortened and cycle-by-cycle variability decreased due to the positive effect of hydrogen on combustion stability. Acikgoz and Celik [11] experimentally studied the performance and emission characteristics of a conventional twin cylinder, four stroke, sparkignition engine running with four premixed methanehydrogen blends with hydrogen ranging from 0 to 30 percent. They revealed that minor power loss occurs at low speed. Kahraman et al. [12] performed an experimental study on the performance and exhaust emissions of a spark ignition engine fuelled with methane with 0%, 10%, 20%, and 30% hydrogen enrichment at different engine speeds and excessive air ratios on a four-stroke engine with compression ratio of 10 at wide open throttle. Their results showed that increase in the speed
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
and excessive air ratio, reduces CO emission values, while addition of hydrogen into natural gas when operating on the lean mixture condition reduces HC, CO and CO2 emissions. They also reported that increasing the excessive air decreases the maximum peak cylinder pressure, and as the H2 fraction increases, maximum peak pressure gets close to the top dead center. Ghazal [13] evaluated the effect of blended hydrogenmethane fuel with two fuel induction systems of port injection and direct injection using a numerical simulation with Wiebe function for combustion modeling. Their results revealed that the addition of hydrogen to methane between 40% and 60% by volume with equivalence ratio near to stoichiometric condition and engine speed between 2000 and 3000 rpm improves engine performance and emissions. A general reduction in CO with the addition of hydrogen was also noticed. For port injection, hydrogen addition up to 60% increased the engine power while adding more hydrogen hampered it. For direct injection, hydrogen addition also increased the engine power. They declared that the direct injection system is more efficient than the port injection system due to turbulence intensity and combustion efficiency. Perini et al. [14] developed a predictive two-zone, quasidimensional combustion model for the simulation of the combustion process in spark ignition engines fueled with hydrogen, methane, or hydrogenemethane blends. Their model described the flame front development assuming a simplified spherical geometry, with an infinitesimal thickness. Iorio et al. [15] investigated methane/hydrogen blends combustion process in an optically accessible engine. They indicated an increase of thermal efficiency and a decrease of combustion duration with the increase of hydrogen fraction. Their optical results revealed an increased combustion reaction rate and a more uniform and rapid flame propagation by hydrogen addition. Zhou et al. [16] carried out an experimental study to investigate Combustion of a diesel engine fueled by addition of hydrogen or methane for dual-fuel operation, or mixtures of hydrogen-methane for tri-fuel operation. They showed that in-cylinder pressure and heat release rate change slightly at low to medium loads but increase dramatically at high loads. They indicated that tri-fuel operation with 30% hydrogen addition in methane is the best fuel in reducing particulate and NOx emissions at loads ranging from 70 to 90 percent of full load. Yadav et al. [17] performed an experiment to study the performance and combustion of a dual fuel hydrogen diesel engine. They reported lower exhaust emissions and improved performance using dual fuel mode with hydrogen as the supplementary fuel. It was also found that hydrogen enriched engine gives maximum efficiency at around 70% of full load whereas when operated with diesel this values come close to 80% of full load. At 70% of full load, the brake thermal efficiency was increased by 10.71%. Many other studies is available in literatures in which hydrogen was partly mixed with conventional hydrocarbon fuels such as natural gas [18e32], gasoline [33e35] or diesel oil [36e44]. All the mentioned works are done experimentally or numerically using simple thermodynamic models. Beside expensive experimental methods, trend to numerical methods alone or as a complement to experimental methods has been taken into considerations by researchers. With
3
progressions in computer technology, use of multidimensional computational fluid mechanics including numerical solution of Navier-Stokes equations and detail combustion mechanisms has become possible. However, there are not significant works done using computational fluid dynamics (CFD). Lilik et al. [45] investigated the hydrogen diesel blend fueled combustion engine by 0e15 percent of hydrogen using both experimental setup and CFD simulation. They carried out a closed loop sector mesh CFD simulation based on Reynolds averaged governing equations. Finite volume (FV) method with transported probability density function (PDF) to account for turbulenceechemistry interactions with a 71species n-heptane/NOx mechanism were used for combustion modeling. Azimovet et al. [46] performed a closed loop multidimensional CFD simulation of syngas combustion in a dual-fuel engine using a 9-step chemical kinetics mechanism. Their predicted and measured in-cylinder pressure, temperature, and rate of heat release data showed good agreement with experimental data for all types of syngas on various initial compositions. Kosmadakis et al. [47] investigated combustion of hydrogen-fueled spark-ignition engine operating with EGR using CFD method. A closed loop mesh with a simplified cylindrical geometry was used to investigate combustion and NOx emission. Yang et al. [48] performed a numerical study on a hydrogen assisted diesel engine. They revealed that in order to develop high-performance combustion engines, understanding of the combustion properties of hydrogen enriched methane is essential. Zheng et al. [49] using CFD coupled with chemical kinetics studied the effects of reformed exhaust gas recirculation (REGR) on combustion and emissions of dimethyl ether (DME) homogeneous charge compression ignition engines. Their results revealed that REGR can delay ignition time, and makes main combustion closer to top dead center, and as a result, reduces compression negative work and broadens load range of HCCI engines. They also reported that HC, CO and NOx emissions can be controlled simultaneously by REGR. Their simulation was performed using the AVL-FIRE CFD code coupled with CHEMKIN software package for a closed-cycle from intake valve closing (IVC) to exhaust valve opening (EVO). Duan et al. [50] conducted a numerical and experimental investigation for controlling backfire of a hydrogen engine. They reported that temperature and concentration of the gas mixture near the intake valves are the most effective factors that result in backfire. They optimized timing and pressure of hydrogen injection in order to reduce the concentration distribution of the intake mixture and the temperature of the high-concentration mixture through the inlet valve for controlling the backfire. The current study investigates the effect of hydrogen addition to methane fueled engine using an open cycle Computational Fluid Dynamics (CFD) simulation coupled with chemical kinetics. The Eddy Dissipation Concept (EDC) model is used in combination with a two-equation turbulence model (k-ε) and GRI-Mech 3.0 kinetic mechanisms which has been validated for CH4/H2 combustion to help rigorous understanding of combustion phenomena in engine. Effects of hydrogen addition at different air-fuel ratio and hydrogen content on emissions, efficiency and combustion duration is investigated using detailed three dimensional simulations. A
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
simplified geometry similar to Ma et al. [51] work is selected so it can be compared and verified with their experimental results. Hydrogen-Methane fuel is assumed to be inducted through the intake manifold as a premixed mixture with air including 0%, 30% and 50% of hydrogen by volume. A good insight of combustion phenomena, turbulence flow and species consumption inside combustion chamber is obtained. A sensitivity analyses is also performed in order to improve our understanding of chemical kinetic mechanism inside the combustion chamber.
Numerical approach In this work simulations are implemented in Fluent 6.3 Since a natural aspiration engine is considered in this investigation, inlet boundary condition is set to constant pressure at 105 kPa with regard to experimental reference work Ma et al. [51] and outlet boundary is set to constant atmospheric pressure. Piston and valves are moving walls and the other boundaries are set as stationary walls. Convergence criteria for continuity, energy and species are set respectively to 10e-4, 10e-7 and 10e-7. In our model we have taken the time steps equal to 0.1 CA during combustion process and 0.5 CA in the rest of the simulation.
Basic equations for mass, momentum, energy and species for compressible gas are illustrated by Bakul et al. [52]. Standard k ε model has been used in this research for modeling of turbulence. Two equations are needed for computation of turbulent kinetic energy k and dissipation rate ε which are presented as follows: For turbulent kinetic energy k m vk mþ t þ Gk rε þ Sk sk vxj
m vε ε ε2 mþ t þ C1ε ðGk Þ C2ε r þ Sε k sε vxj k
! ruej Yei
0 1 v B mt vYei C ui ¼ @ A þ r~ vxj sY vxj
i ¼ 1; : : : ; N
(3)
~ i expresses the net rate of where the chemical source term u production of species i due to chemical reaction. The eddy dissipation concept expresses the chemical reaction rate as: i h ~ε b p; Y b min þ Y b p gl b min ; Y ~ F ¼ Cu k min Y u k~
(4)
where: (5)
The mass fraction occupied by turbulent fine structure regions, gl , is expressed as g l ¼ Cg
1=4 n~ε 2 k~
(6)
And the correction factor for nonpremixedness, k, is written as
(1)
For dissipation ε v v v ðrεÞ þ ðrεui Þ ¼ vt vxi vxj
v vxj
i h b F; Y bO b min ¼ min Y Y
Basic governing equations
v v v ðrkÞ þ ðrkui Þ ¼ vt vxi vxj
because in those regions molecular diffusion is faster than turbulent transport. From this idea the concept of the Eddy Dissipation Concept (EDC) was developed. There is a range in which the reactions can be regarded as ideally mixed. Thus, chemical reaction kinetics determines the speed of the process, while outside this range the reactants are not mixed and do not react. So each computing cell is split into a reactive fraction (g3l ) which represents the part of the mixture inside the smallest turbulence structures and an inert fraction (1 g3l ) which is the surrounding fluid [53]. This combustion model (i.e. EDC) is used for combustion turbulent interaction consideration in the current research. The equation for conservation of individual species for a mixture of N species can be written as:
(2)
where Gk indicates turbulent energy production due to velocity gradient and C1ε ، C2ε and C3ε are adjustable constant coefficients. sk and sε are turbulence Prandtl number and Sk and Sε are source terms for k and ε respectively. Eddy viscosity 2 is represented as mt ¼ rCm kε where Cm , C1ε , C2ε , C3ε , sk and sε are constant coefficients which are considered to be 0.09, 1.44, 1.92, 0.09, 1.0 and 1.3 respectively.
Combustion modeling In simulating combustion systems, both physical and chemical processes are of great importance. The formation of turbulent structures is a physical process for which the concept of eddy cascade model is the basis. The smallest eddies happen in Kolmogorov Length Scales. At these scales, dissipation of turbulent kinetic energy k takes place. At a size smaller than Kolmogorov no turbulent structures exist,
2 b min þ Y bp Y k¼ bp Y bp bF þ Y bO þ Y Y
(7)
The terms Cu and Cg are constants with values 11.1 and 2.1 respectively [54]. Therefore in this model there are two regions: One which reaction rate is fast and mixing rate controls the combustion and the other which reactants are ideally mixed and the combustion is controlled by chemical kinetics.
Sensitivity analysis The sensitivity analysis method investigates the model output as a function of parameters. The local normalized sensitivity coefficient is defined as [55]: S¼
kj vci vln ci ¼ ci vkj vln kj
(8)
These coefficients form the normalized sensitivity matrix which represents the fractional change in concentration ci caused by a fractional change of parameter kj . The sensitivity matrix accounts for the change of a single variable as a result of the change of individual parameters.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Meshing The combustion chamber used to investigate the combustion of blended hydrogen methane is based on Ma et al. work [51]. In this research for straightforwardness, the geometry is simplified and cylinder head and piston are considered flat although some minor errors would be inevitable. In this study a tetrahedral mesh is used for dead volume and valve areas. Approximately 340,000 meshes are generated in top dead center (TDC) and 445,000 meshes in bottom dead center (BDC). This difference is because, the border between tetrahedral and hexahedral meshes also moves down while the piston moves down, leading to more mesh generations due to remeshing. According to Fluent a piston limit profile is chosen for this border as its movement profile. It means moving down with the piston to a limit and then staying and not accompanying the piston any more. Then waiting until the piston returns and then continuing moving up with it again. This is done due to the valve motions. If this border will be fixed, by valve motion the skewness of the dead volume tetrahedral cells will increase more than the acceptable limit and divergence would happen. The Hexahedral mesh is utilized for regions on the top of the valves in ports and near piston for faster dynamic mesh updating (approximately 5000 meshes in TDC and 135,000 in BDC) which in all will lead to a total of approximately 345,000 meshes in TDC and 580,000 meshes in BDC. The borders between two types of meshes are defined as mesh interfaces. The calculations are carried out using an Intel Corei7 e 2600 K CPU @ 3.4 GHz Processor and 8 GB RAM in a 64-bit Operating System. In order to study mesh independency the geometry at TDC was discretized with 175,000, 260,000, 345,000, and 455,000 cell volumes. Simulations using these grid sizes were compared for trapped air and fuel mixture mass and cylinder pressure, temperature and fuel mass fraction curves in the case of pure methane combustion and finally the grid size corresponding to 345,000 elements at TDC provided sufficiently accurate results without further change with increasing the number of cells. Therefore by considering the computational time this mesh is used for all the simulations. The solution domain and meshing at different positions of piston are shown in Fig. 1. As is shown in this Figure, at TDC unstructured mesh and only one row of structured mesh are used. As was mentioned this method will make a perfect dynamic mesh updating when piston and valves move.
Result and discussions Intake Turbulent kinetic energy and dissipation rate vs crank angle degree (CAD) is shown in Fig. 2. These parameters are very important for EDC combustion model which is used in this study. Turbulent kinetic energy generation source is usually the intake valve at the entrance to the combustion chamber. Turbulent kinetic energy increases up to 60 degrees ATDC and its generation is dominant over dissipation rate. From that degree due to decrease in intake of turbulent kinetic energy
5
and the fact that the turbulent kinetic energy dissipation proceeds the turbulent kinetic energy generation (which itself is decreasing due to valve opening), the average turbulent kinetic energy starts decreasing. Velocity vectors at different valve openings are shown in Fig. 3. Fig. 3A shows velocity vectors at the beginning of valve opening which is start of the vortex formation in combustion chamber. As can be seen from this figure, although close to TDC the intake valves are opened a little and because of it, it is expected that speed increases, but because the flow velocity is also dependent on piston velocity and piston velocity is approximately in its minimum value around TDC, therefore the flow velocity inside cylinder does not change that much. With the piston getting distance from TDC, flow velocity in combustion chamber gradually increases and at 25 degree ATDC velocity vortex is completely established and the flow tumble is apparently observable. These results reveal that intake valve opening and piston linear speed are two important factors for generation of velocity vortex in combustion chamber and therefore more monotone mixture formation, which leads to better combustion. In Fig. 4 temperature contours in combustion chamber at different crank angles are shown for better understanding of premixed cold air and fuel intake. The temperature contours declare how the cold air and fuel mixture enter the hot combustion chamber and how they decrease the average temperature in the cylinder.
Combustion The effect of hydrogen addition on the combustion of CNG engines is investigated using three different hydrogen ratios ranging from zero to fifty percent of hydrogen in fuel by volume. As mentioned, the engine performance information and geometry are set to be similar to their counterparts in the work of Ma et al. [51] so that the results can be validated. As can be seen from Figs. 5e10, there is a good agreement between the CFD and experimental results in trend but some quantity deviations are observable. These deviations are due to the nonconformities between the experimental conditions and CFD assumptions. In this study, exact geometry of intake and exhaust ports, intake and exhaust valve timing profiles, environmental conditions of the test (vs STP used in the CFD model), exact geometry of piston and cylinder head of the test engine were among the unknowns. In addition, some simplifications like pure CH4 as fuel vs NG in experimental work, ignoring radiation due to longer running time and neglectable effect on the performance, single cylinder instead of in-line 6 cylinder test engine would make the modeling results not to fit on the experimental results precisely. On the other hand, experimental set ups are not completely accurate themselves and have their own inherent uncertainties. For example the precision of the ALICAT flow meter used in experimental work of Ma et al. [51] is 1.5 percent [22], or the precision of Kistler 2613B used as the crank angel meter is 1 CA [51]which will impose an error when integrating Pdv for the calculation of work and power. In this section besides verifying the model with experimental results, most important in-cylinder combustion
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 1 e The solution domain and meshing at different crank angles A & B @ 10degrees ATDC, C @ 70degrees ATDC, D @ 55degrees ATDC.
phenomena are reported in order to complete understanding of what happens during the combustion process in the engine. Indicated thermal efficiency vs lambda for different hydrogen fractions is shown in Fig. 5. Regarding CFD results of this figure, hydrogen ability to extend lean limit is obvious. Also it is shown that when lambda was under 1.7, hydrogen addition did not have significant effect on efficiency improvement. After lambda ¼ 1.8, thermal efficiency exhibited an obvious drop with pure NG which is due to the lean burn limit of CH4 combustion being around this point. After this limit and for example around lambda ¼ 2 the indicated
Fig. 2 e Turbulence k-3 vs CAD for methane combustion.
thermal efficiency experiences a considerable drop from 37% to 20%. By adding 30% hydrogen and due to the effect of hydrogen on expanding the burning lean limit this indicated thermal efficiency goes back to 37% and even a bit higher. The CFD model shows similar trend with about 10% difference comparing to experimental results. Combustion duration vs lambda for different hydrogen fractions is shown in Fig. 6. The spark time instant to the moment that 99% of CH4 is consumed is defined as combustion duration. As is shown, at a given lambda, as hydrogen fraction increases, combustion duration is shortened, which is due to higher flame propagation speed of hydrogen. Fig. 7 shows the specific emissions of NOx, as a function of lambda for different hydrogen fractions. As shown in this figure, when engine works on lean mixture NOx emission first increases rapidly, reaches a peak, and then decreases gradually to a relatively small value, and this trend is independent of hydrogen fraction. Excess air have two opposing effects on the production of NOx. The more the excess air would be the more nitrogen would exist and the lower the combustion temperature would become. These two factors (more available N2 and lower combustion temperature) would lead to respectively higher and lower NOx production. That is why there would be a peak which is around 1.2e1.3. It is also noted that NOx emission can be very low after lambda exceeds 1.8 which indicates the greater weight of lowered temperature rather than higher available nitrogen amount. In other words after stoichiometric condition, with increasing the excess air, thermal NOx decreases while prompt NOx is increasing with a higher rate till 20e30% excess air. After this point thermal NOx decrease rate prevails prompt
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
7
Fig. 3 e Velocity Vectors in Combustion Chamber at different crank angles A) 10 degrees ATDC, B) 25 degrees ATDC, C) 70 degrees ATDC.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 4 e Temperature contours in Combustion Chamber at different crank angles A) 10 degrees ATDC, B) 25 degrees ATDC, C) 70 degrees ATDC. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 5 e Indicated thermal efficiency vs lambda for different hydrogen fractions.
Fig. 6 e Combustion duration vs lambda for different hydrogen fractions.
Fig. 7 e NOx emission vs lambda for different hydrogen fractions.
9
Fig. 8 e NOx mass fraction vs CAD for 30 percent hydrogen and different lambda.
NOx increase rate and the overall NOx reduces, and after a special CAD this NOx freezes which can be seen in Fig. 8. This figure indicates the mass fraction of NOx in different CADs and again in that figure it is obvious that NOx mass fraction after freezing is the highest in lambda between 1.2 and 1.3. Figs. 9 and 10 shows the specific emissions of CO and Unburned Hydro Carbons (UHC), as a function of lambda for different hydrogen fractions. As is shown in Fig. 9, after stoichiometric condition CO specific emission first drops gradually which is due to more oxygen and then starts climbing up rapidly due to misfire. Below 70 percent excess air, adding hydrogen showed no significant effect on CO, but once lambda exceeded 1.8, more hydrogen addition resulted in much less exhaust CO which again is due to extending the lean burn limit and preventing misfire.
Fig. 9 e CO emission vs lambda for different hydrogen fractions.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 10 e HC emission vs lambda for different hydrogen fractions.
HC specific emission shown in Fig. 10, is very low when lambda is smaller than 1.8, because at this point, on one hand there is extra air to ensure complete combustion and on the other hand, the fueleair mixture is not too lean, so the exhaust temperature can keep itself at a high level which is beneficial for further oxidation of HC remained in crevices and flame quenching zone. Reduction of HC emission by hydrogen enrichment is because of: first: lean burn capability which prevents combustion failure at higher lambdas, second: carbon-hydrogen molar ratio reduction, third: hydrogen
Fig. 12 e Cylinder temperature vs crank angle for spark ignition at MBT.
shorter quenching distance which will leave less UHC close to walls. Fig. 11 shows temperature contours at different crank angle degrees for equivalence ratio ¼ 1 and hydrogen volumetric ratio of zero. Because the spark is located at the center, flame starts propagation from the center. With initiation of combustion, the temperature around spark and the products of combustion start increasing and the amount of reactants in that same area start to decrease. With flame propagation the
Fig. 11 e Temperature contours at different crank angles for equivalence ratio of 1 for pure methane A) 25 degree BTDC B) 15 degree BTDC, C) TDC, D) 15 degree ATDC.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
hot zone develops which is clearly visible in the figure. The contours for reactants and products concentrations are a lot similar to the shown temperature contours in this figure. Cylinder temperature vs crank angle for spark ignition at MBT is shown in Fig. 12. As is shown in this figure with increasing the hydrogen amount, temperature increase rate after spark rises considerably. The combustion chamber maximum temperature of hydrogen may be thought to increase a lot due to its higher adiabatic flame temperature in comparison with methane. But this factor is balanced by the fact that hydrogen lower volumetric heating value is 0.3 of methane. Higher adiabatic flame temperature of hydrogen versus its lower volumetric heating value will lead to approximately the same maximum combustion chamber temperature of blended fuel in comparison with pure methane. As is declared in Fig. 12 maximum in-cylinder temperature increases slightly with increasing hydrogen amount in fuel composition. CO and CO2 mass fractions vs CAD are shown in Fig. 13 for various hydrogen concentrations in fuel in stoichiometric mixture. As shown in these figures, the maximum amount of CO happens slightly after 10ATDC in which methane is consumed completely. At the same time the CO2 amount is increased considerably up to this degree and after this point with slow transmutation of CO to CO2, CO2 amount increases gradually.
Sensitivity analysis Sensitivity analysis is performed using CHEMKIN PREMIXED code in isobaric condition of 30 bar and initial temperature of 1000 K which is the start of combustion of the engine under study using stoichiometric mixture using stoichiometric mixture. The chemical kinetic mechanism used in the present work is GRI-Mech 3.0 [56] which using sensitivity analysis is reduced to includes 50 species and 309 elementary chemical reactions to be adaptable to Fluent 6.3. As is depicted in Fig. 14 Methane is depleted by reaction OH þ CH4 ⇔CH3 þ H2 O in all cases. This shows that OH is the most important radical and combustion is mostly promoted by OH radicals. As is shown in Fig. 14 Chain-branching reaction H þ O2 ⇔O þ OH is the most important chain branching step in every combustion process where H atoms are present [55]. In the temperature regime
11
Fig. 14 e Normalized sensitivity coefficient of methane concentration caused by reactions rates.
studied in this article, fuel consumption is also controlled by the chain-propagating reaction HO2 þ CH3 ⇔OH þ CH3 O and chain integrating reactions H þ CH3 ðþMÞ⇔CH4 ðþMÞ andOH þ HO2 ⇔O2 þ H2 O. By increasing hydrogen fraction in the mixture the H radicals increase in the flame front and OH production is promoted as a result of reaction H þ O2 ⇔O þ OH. CH4 and H2 mass fraction vs CAD are shown in Fig. 15 for various hydrogen percentages in methane by volume in fuel for stoichiometric mixture. This figure is important because the combustion duration is calculated from it. As is shown in this figure methane is used up completely upto 12 degrees ATDC. After this degree some amount of hydrogen remains in the combustion chamber which reacts with the remaining radicals.
Conclusion In this study Eddy Dissipation Concept model is used in combination with a two-equation turbulence model (k-ε) and
Fig. 13 e A-CO and B-CO2 mass fractions vs CAD for different hydrogen contents. Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
12
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
Fig. 15 e Mass fractions vs CAD for different hydrogen contents A) CH4, B) H2.
GRI-Mech 3.0 kinetic mechanism as a valid CH4/H2 mechanism, for accurate understanding of chemical kinetics of methane and hydrogen combustion. Effects of hydrogen addition ranging from 0 to 50 percent in fuel by volume at different air-fuel ratios on emissions, efficiency and combustion duration is investigated. A great deal of combustion phenomena, turbulence flow and species consumption inside combustion chamber is obtained. A sensitivity analyses is also performed in order to great understanding of combustion phenomena. The most significance results of this investigation are summarized as follows: Hydrogen addition reduces combustion duration, extends lean burn range, and reduces carbon monoxide and carbon dioxide amounts. At MBT condition, fuel is used up till 10 degrees after TDC, while carbon monoxide has a maximum at this point and after that transmutes to carbon dioxide. Intake valve opening and piston motion are two important factors for generation of velocity vortex in combustion chamber. Maximum in-cylinder temperature increases slightly with increasing hydrogen amount in fuel composition. Based on sensitivity analysis methane is depleted by reaction OH þ CH4 ⇔CH3 þ H2 O in all cases. Chain-branching reaction H þ O2 ⇔O þ OH is the most important chain branching step in combustion. Methane is used up completely upto 12 degrees ATDC. After this degree some amount of hydrogen remains in the combustion chamber which reacts with the remaining radicals. This work reveals the high capability of contemporary CFD methods and the fact that they can be well trusted upon even for a complicated subject like an open cycle of an internal combustion engine with dynamic meshes besides complicated chemical mechanisms.
references
[1] Fotache TGKCG, Law CK. Ignition of hydrogen-enriched methane by heated air. Combust Flame 1997;110:429e40. [2] Hu E, Huang Z, Zheng J, Li Q, He J. Numerical study on laminar burning velocity and NO formation of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34:6545e57. [3] De Simio L, Gambino M, Iannaccone S. Experimental and numerical study of hydrogen addition in a natural gas heavy duty engine for a bus vehicle. Int J Hydrogen Energy 2013;38:6865e73. [4] Lim G, Lee S, Park C, Choi Y, Kim C. Effect of ignition timing retard strategy on NOx reduction in hydrogen-compressed natural gas blend engine with increased compression ratio. Int J Hydrogen Energy 2014;39:2399e408. [5] Lim G, Lee S, Park C, Choi Y, Kim C. Effects of compression ratio on performance and emission characteristics of heavyduty SI engine fuelled with HCNG. Int J Hydrogen Energy 2013;38:4831e8. [6] Akansu S. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int J Hydrogen Energy 2004;29:1527e39. [7] Deng J, Ma F, Li S, He Y, Wang M, Jiang L, et al. Experimental study on combustion and emission characteristics of a hydrogen-enriched compressed natural gas engine under idling condition. Int J Hydrogen Energy 2011;36:13150e7. [8] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. Int J Hydrogen Energy 2001;26(1):55e70. [9] Wong YK, Karim GA. An analytical examination of the effects of hydrogen addition on cyclic variations in homogeneously charged compressioneignition engines. Int J Hydrogen Energy 2000;25(12):1217e24. [10] Mariani A, Prati MV, Unich A, Morrone B. Combustion analysis of a spark ignition i. c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends. Int J Hydrogen Energy 2013;38:1616e23. [11] Acikgoz B, Celik C. An experimental study on performance and emission characteristics of a methaneehydrogen fuelled gasoline engine. Int J Hydrogen Energy 2012;37:18492e7.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
[12] Kahraman N, Ceper B, Akansu S, Aydin K. Investigation of combustion characteristics and emissions in a spark-ignition engine fuelled with natural gasehydrogen blends. Int J Hydrogen Energy 2009;34:1026e34. [13] Ghazal OH. A comparative evaluation of the performance of different fuel induction techniques for blends hydrogenemethane SI engine. Int J Hydrogen Energy 2013;38:6848e56. [14] Perini F, Paltrinieri F, Mattarelli E. A quasi-dimensional combustion model for performance and emissions of SI engines running on hydrogenemethane blends. Int J Hydrogen Energy 2010;35:4687e701. [15] Di Iorio S, Sementa P, Vaglieco BM. Experimental investigation on the combustion process in a spark ignition optically accessible engine fueled with methane/hydrogen blends. Int J Hydrogen Energy 2014;39:9809e23. [16] Zhou JH, Cheung CS, Leung CW. Combustion, performance and emissions of a diesel engine with H2, CH4 and H2eCH4 addition. Int J Hydrogen Energy 2014;39:4611e21. [17] Yadav VS, Sharma D, Soni SL. Performance and combustion analysis of hydrogen-fuelled C.I. engine with EGR. Int J Hydrogen Energy 2015;40:4382e91. [18] Gatts T, Liu S, Liew C, Ralston B, Bell C, Li H. An experimental investigation of incomplete combustion of gaseous fuels of a heavy-duty diesel engine supplemented with hydrogen and natural gas. Int J Hydrogen Energy 2012;37:7848e59. [19] Lee S-W, Choi W, Cho Y-S. Characterization of HCNG combustion and emission characteristics in a constant volume chamber with a single and a dual spark plug configuration. Int J Hydrogen Energy 2012;37:682e90. [20] Ma F. Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine. Int J Hydrogen Energy 2008;33:1416e24. [21] Ma F, Li S, Zhao J, Qi Z, Deng J, Naeve N, et al. Effect of compression ratio and spark timing on the power performance and combustion characteristics of an HCNG engine. Int J Hydrogen Energy 2012;37:18486e91. [22] Ma F, Wang M, Jiang L, Chen R, Deng J, Naeve N, et al. Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio. Int J Hydrogen Energy 2010;35:6438e47. [23] Ma F, Wang M, Jiang L, Deng J, Chen R, Naeve N, et al. Performance and emission characteristics of a turbocharged spark-ignition hydrogen-enriched compressed natural gas engine under wide open throttle operating conditions. Int J Hydrogen Energy 2010;35:12502e9. [24] Mariani A, Morrone B, Unich A. Numerical evaluation of internal combustion spark ignition engines performance fuelled with hydrogen e natural gas blends. Int J Hydrogen Energy 2012;37:2644e54. [25] Mathai R, Malhotra RK, Subramanian KA, Das LM. Comparative evaluation of performance, emission, lubricant and deposit characteristics of spark ignition engine fueled with CNG and 18% hydrogen-CNG. Int J Hydrogen Energy 2012;37:6893e900. [26] Mohammed SEL, Baharom MB, Aziz ARA. Analysis of engine characteristics and emissions fueled by in-situ mixing of small amount of hydrogen in CNG. Int J Hydrogen Energy 2011;36:4029e37. [27] Park C, Kim C, Choi Y. Power output characteristics of hydrogen-natural gas blend fuel engine at different compression ratios. Int J Hydrogen Energy 2012;37:8681e7. [28] Park C, Kim C, Choi Y, Won S, Moriyoshi Y. The influences of hydrogen on the performance and emission characteristics of a heavy duty natural gas engine. Int J Hydrogen Energy 2011;36:3739e45.
13
[29] Park C, Lee S, Lim G, Choi Y, Kim C. Effect of mixer type on cylinder-to-cylinder variation and performance in hydrogennatural gas blend fuel engine. Int J Hydrogen Energy 2013;38:4809e15. rez A, Gime nez B. Study of the cycle-to[30] Reyes M, Melgar A, Pe cycle variations of an internal combustion engine fuelled with natural gas/hydrogen blends from the diagnosis of combustion pressure. Int J Hydrogen Energy 2013;38:15477e87. nez B, Reyes M. Prediction of [31] Tinaut FV, Melgar A, Gime performance and emissions of an engine fuelled with natural gas/hydrogen blends. Int J Hydrogen Energy 2011;36:947e56. [32] Zheng J, Hu E, Huang Z, Ning D, Wang J. Combustion and emission characteristics of a spray guided direct-injection spark-ignition engine fueled with natural gas-hydrogen blends. Int J Hydrogen Energy 2011;36:11155e63. [33] Dandrea T. The addition of hydrogen to a gasoline-fuelled SI engine. Int J Hydrogen Energy 2004;29:1541e52. [34] Ji C, Wang S. Combustion and emissions performance of a hybrid hydrogenegasoline engine at idle and lean conditions. Int J Hydrogen Energy 2010;35:346e55. [35] Wang S, Ji C, Zhang B. Starting a spark-ignited engine with the gasolineehydrogen mixture. Int J Hydrogen Energy 2011;36:4461e8. [36] Chiriac R, Apostolescu N. Emissions of a diesel engine using B20 and effects of hydrogen addition. Int J Hydrogen Energy 2013;38:13453e62. [37] de Morais AM, Mendes Justino MA, Valente OS, Hanriot SDM, JR. Hydrogen impacts on performance and CO2 Sodre emissions from a diesel power generator. Int J Hydrogen Energy 2013;38:6857e64. [38] Ghazal OH. Performance and combustion characteristic of CI engine fueled with hydrogen enriched diesel. Int J Hydrogen Energy 2013;38:15469e76. [39] Ibrahim MM, Ramesh A. Experimental investigations on a hydrogen diesel homogeneous charge compression ignition engine with exhaust gas recirculation. Int J Hydrogen Energy 2013;38:10116e25. [40] Liu S, Li H, Liew C, Gatts T, Wayne S, Shade B, et al. An experimental investigation of NO2 emission characteristics of a heavy-duty H2-diesel dual fuel engine. Int J Hydrogen Energy 2011;36:12015e24. [41] Miyamoto T, Hasegawa H, Mikami M, Kojima N, Kabashima H, Urata Y. Effect of hydrogen addition to intake gas on combustion and exhaust emission characteristics of a diesel engine. Int J Hydrogen Energy 2011;36:13138e49. [42] Shin B, Cho Y, Han D, Song S, Min Chun K. Investigation of the effects of hydrogen on cylinder pressure in a splitinjection diesel engine at heavy EGR. Int J Hydrogen Energy 2011;36:13158e70. [43] Wang S, Ji C, Zhang J, Zhang B. Improving the performance of a gasoline engine with the addition of hydrogeneoxygen mixtures. Int J Hydrogen Energy 2011;36:11164e73. [44] Zhou JH, Cheung CS, Leung CW. Combustion, performance and emissions of ULSD, PME and B50 fueled multi-cylinder diesel engine with naturally aspirated hydrogen. Int J Hydrogen Energy 2013;38:14837e48. [45] Lilik GK, Zhang H, Herreros JM, Haworth DC, Boehman AL. Hydrogen assisted diesel combustion. Int J Hydrogen Energy 2010;35:4382e98. [46] Azimov U, Okuno M, Tsuboi K, Kawahara N, Tomita E. Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism. Int J Hydrogen Energy 2011;36:13793e807. [47] Kosmadakis GM, Rakopoulos CD, Demuynck J, De Paepe M, Verhelst S. CFD modeling and experimental study of combustion and nitric oxide emissions in hydrogen-fueled
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040
14
[48]
[49]
[50]
[51]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 4
spark-ignition engine operating in a very wide range of EGR rates. Int J Hydrogen Energy 2012;37:10917e34. An H, Yang WM, Maghbouli A, Li J, Chou SK, Chua KJ. A numerical study on a hydrogen assisted diesel engine. Int J Hydrogen Energy 2013;38:2919e28. Zheng Z, Liu C, Zhang X. Numerical study of effects of reformed exhaust gas recirculation (REGR) on dimethyl ether HCCI combustion. Int J Hydrogen Energy 2014;39:8106e17. Duan J, Liu F, Sun B. Backfire control and power enhancement of a hydrogen internal combustion engine. Int J Hydrogen Energy 2014;39:4581e9. Ma F, Wang Y, Liu H, Li Y, Wang J, Zhao S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int J Hydrogen Energy 2007;32:5067e75.
[52] Bakul VYGCEJ, Xianming L. Computational fluid dynamics in industrial combustion. New York: CRC Press; 2001. [53] Rehm M, Seifert P, Meyer B. Theoretical and numerical investigation on the EDC-model for turbulenceechemistry interaction at gasification conditions. Comput Chem Eng 2009;33:402e7. [54] Gran MCMIR, Magnussen BF. Numerical simulation of local extinction effects in turbulent combustor flows of methane and air. Symp Combust 1994;25:1283e91. [55] Tunlnyi T. Applications of sensitivity analysis to combustion chemistry. Reliab Eng Syst Saf 1997;57:8. [56] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al. GRI-Mech 3.0. Available: http://www.me. berkeley.edu/gri_mech/.
Please cite this article in press as: Zaker K, et al., Open cycle CFD investigation of SI engine fueled with hydrogen/methane blends using detailed kinetic mechanism, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.040