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Simplification and applicability studies of a hydrogen-air detailed reaction 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 8 ) 1 e5
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Simplification and applicability studies of a hydrogen-air detailed reaction mechanism Junfa Duan a, Paixia Wu a, Hairong Zhu b,*, Gaolin Qin a, Wei Wei a a
School of Mechanical Engineering, North China University of Water Resources and Power, Zhengzhou 450045, China b School of Mechanical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
article info
abstract
Article history:
The research of hydrogen fuel internal combustion engine (HICE) had great significance
Received 7 August 2018
facing the challenges of energy and environmental problems. Based on the detailed
Received in revised form
hydrogen-air reaction mechanism, the pre-mix model of CHEMKIN-Pro software was
28 August 2018
selected to simplify the detailed mechanism GRI-3.0. The most important elements and
Accepted 25 September 2018
reactions was chose to construct framework mechanism firstly based on the sensitivity
Available online xxx
coefficient for H2O and NO formation, and additional elements and reactions were added to framework mechanism for complementing the oxidation path of N2 and H2. A simplified
Keywords:
mechanism including 24-step elementary reaction was obtained and the laminar burning
Hydrogen fueled internal combus-
velocity calculated by this simplified mechanism matches well with the detailed mecha-
tion engine (HICE)
nism in a wide range. This simplified mechanism was also applied in a CFD model which
Simplified mechanism
predicted the cylinder instantaneous pressure and NOx emission accurately within a large
Sensitivity coefficient
range of fuel air equivalent ratio. Showing that this mechanism has good applicability.
Path analysis
© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Cylinder pressure NOx emissions
Introduction The utilization of power machinery and energy determines the range of human activities and creativity and at the same time puts great pressure on the living environment. The extensive application of internal combustion engine and fossil energy in the last hundred years has brought the development of human civilization to an unprecedented degree. Due to the challenge of energy shortage and pollution aggravation, it has become an important research direction to find new clean alternative fuels [1e4]. Hydrogen is the only carbon-free energy source, so it does not emit any carbon when used in internal combustion
engines. The combustion product of hydrogen is water, which can be easily regenerated by electrolysis to produce large amounts of hydrogen [5e7]. Hydrogen has a characteristic of low ignition energy and rapid combustion speed, which can ensure short combustion duration of the internal combustion engine so as to obtain a high efficiency. These characteristics give hydrogen an obvious advantage as a substitute fuel for internal combustion engine [8e12]. Great progress has been made in the study of combustion of hydrogen fuel internal combustion engines (HICE). Based on the test bench, the researchers can obtain the variation rule of cylinder pressure, temperature and thermal efficiency as well as the average discharge data under different operating conditions [13e17]. However, these data are not enough, based on
* Corresponding author. E-mail address: [email protected] (H. Zhu). https://doi.org/10.1016/j.ijhydene.2018.09.173 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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 8 ) 1 e5
chemical reaction kinetics method can obtain profound comprehensive hydrogen combustion and emission in cylinder forming rules, therefore, more and more attention has been paid in recent years [18e22]. Many researchers have studied the basic combustion characteristics of hydrogen based on shock tube and burner, and put forward a variety of detailed hydrogen-air reaction mechanisms based on the research of hydrogen and oxygen combustion mechanism. Detailed mechanism for internal combustion engine CFD simulation requires a lot of time, therefore, is of great significance to simplify it [23]. The detailed mechanism of extensive verification was selected in this study. Based on CHEMKIN software, sensitivity coefficient and path analysis were adopted to simplify the mechanism, which is used in CFD simulation model. Internal combustion engine test data to verify that adaptability of the simplify mechanism under load.
Method and model Software platform This article have selected the large-scale gas phase reaction dynamics simulation software CHEMKIN-Pro developed by the Sandia national laboratories as the simplified tool of detailed mechanism, selected PREMIX model, then simplified based on the sensitive coefficient and the reaction path method computational accuracy of the simplified mechanism of laminar burning velocity check was obtained by simulation. The applicability of the simplification mechanism was based on CFD simulation and actual test data of hydrogen fuel internal combustion engine. We selected CONVERGE as the simulation platform of software; CFD simulation model was established based on a spray hydrogen fuel engine inlet, compared to HICE and emissions of the simulation and the experiment data to validate the simplified mechanism to predict the applicability of hydrogen fuel engine simulation process. In this article, model coupled with chemical reaction mechanism was used to calculate the combustion exothermic process and emission generation process of HICE. The combustion model we adopted was the SAGE model combining chemical reaction mechanisms with 3D compute grids to calculate the combustion exothermic process and emission generation process of HICE. A three-dimensional solid model of HICE was established, when boundary conditions and encryption rules were determined, convergence software generated the computing grid autonomously in the calculation process. The maximum number of computing grids of 3d mesh model adopted in reached 425287. Data on in-cylinder combustion and emissions were obtained from the author's previous test results.
Detailed reaction mechanism Mr. Dryer's team at Princeton University [24e29] has been working on the hydrogen and oxygen reaction mechanism for 30 years since 1981. Their detailed reaction mechanism has been modified by Muller [28] and Li [29]several times. The
detailed mechanism can accurately predict the combustion speed and ignition delay of hydrogen and oxygen within the range of 298e3000k, 0.3e87atm, and the fuel air equivalent ratio between 0.25 and 5.0. The gas institute at the University of California, Berkeley, released the detailed mechanism of hydrogen and oxygen combustion, GRI1.0 [30], in 1995 after nearly a decade of research and later released the improved versions GRI2.11 and GRI3.0 [31,32]. This mechanism has been widely used and recognized in the prediction of small molecular hydrocarbon fuel combustion. Experience has shown that hydrogen and oxygen combustion can be predicted at high temperature pressure and concentration range. Williams at the University of California has also proposed a detailed mechanism for hydrogen burning in 1995, which can predict the ignition delay of hydrogen under 33 atm [33]. These detailed reaction mechanisms are more accurate in predicting laminar burning velocity and ignition delay and are widely used in one-dimensional simulation. Jochen compares several common detailed mechanisms, he found that the laminar burning velocity predicted by these mechanisms was relatively close and could accurately predict the laminar burning velocity of hydrogen and air mixture [34]. We use the GRI3.0 mechanism for simplification.
Simplified mechanism This study simplifies the mechanism based on the PREMIX model in CHEMKIN. The main initial condition parameters used in the simulation are shown in Table 1. According to the simulation parameters and detailed mechanism in Table 1, analyzing the oxidation path of H2 and N2 and the sensitivity of H2O and NO, to simplify the mechanism.
Sensitivity and path analysis of H2 oxidation Fig. 1 presents the sensitivity coefficients graph generated based on H2O. The figure shows that at 1850 k for the generation of H2O, HþO2〈 ¼ 〉OþOH is the most sensitive and important primitive reaction, and its sensitivity coefficient to H2O is close to 0.1, so it must be preserved in the simplified mechanism. The sensitivity coefficient of OHþH2〈 ¼ 〉HþH2O, HþO2þH2O〈 ¼ 〉HO2þH2O and OþH2〈 ¼ 〉HþOH also exceed 0.07, which is an important elementary reaction that determines the formation of H2O. Fig. 2 is the path analysis diagram of H2 oxidation. It can be seen from the figure:OHþH2〈 ¼ 〉HþH2O, H2/HO/H2O and H2/OH/H2O2/H2O are the three most important H2O generation paths. H2O2 is also an important substance in the formation path of H2O. To ensure the continuation of the
Table 1 e Simulation initial condition parameters. Para-meters units value
Fuel/air Initial Initial Mass flow equivalent temperature pressure 0.4e2.0
K 298
Atm 1
g/cm∧2-s 0.4
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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 8 ) 1 e5
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Fig. 1 e Sensitivity coefficients with generation of H2O.
Fig. 2 e The main paths of H2 oxidation.
reaction, the basic element H2O2, the reaction OHþ H2O2〈 ¼ 〉 H2OþHO2 were included in the simplification mechanism.
Sensitivity and path analysis of N2 oxidation Fig. 3 shows the sensitivity coefficient graph generated based on NO at the temperature of 2250K. It can be seen from the figure that HeO reaction has a great influence on the final generation of NO, and these reactions are essentially already simplifying mechanisms. In the direct generation path of NO,
Fig. 3 e Sensitivity coefficients with generation of NO.
NþNO〈 ¼ 〉N2þO,NNHþO〈 ¼ 〉NHþNO and N2OþH〈 ¼ 〉NHþNO are NeO reactions that generate sensitive coefficients for NO and play an important role in the generation of NO. At high temperature, the sensitivity of NþNO〈 ¼ 〉N2þO to NO generation reached 0.73, indicating that the thermal NO route plays a decisive role. This is consistent with the author's research results based on detailed mechanism analysis: the thermal NO, NNH and N2O route are the most important element reactions, which determine the formation of NO at high loads. Fig. 4 is the analysis diagram of the oxidation path of N2. At the elementary reaction rate, N2/NO, N2/N2O/NO, N2/NNH/NO are the most important three ways to generate NO. The NNHeNO pathway mainly includes the following three-step elementary reactions: N2þH 4 NNH, NNHþO 4 NHþNO, NHþO 4 NOþH; The generation path of N2OeNO includes the following five-step elementary reactions: N2þOþM 4 N2OþM, N2OþO 4 2NO, N2OþO 4 N2þO2, N2OþH 4 NHþNO, N2OþH 4 N2þOH. The simplified mechanism based on the sensitivity analysis and path analysis method above includes 24 reversible elementary reactions, among which NO mechanism contains 11 reactions. Compared with 69 reactions of the original detailed mechanism, the calculation amount is significantly reduced. The calculation results of the laminar burning velocity and detailed mechanism calculated based on simplified mechanism are compared, as shown in Fig. 5. As can be seen from the figure, the calculation results of the simplified mechanism
Fig. 4 e The main paths of N2 oxidation.
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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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 8 ) 1 e5
Fig. 5 e Comparison of simplification mechanism and detailed mechanism.
are very close to the detailed mechanism, and the maximum error is less than 5%. Meet the requirement of combustion prediction.
The applicability of the simplification mechanism The CFD model of the HICE based on the coupling simplified mechanism studied the combustion and emission process of the internal combustion engine of different fuel air equivalent ratio (0.6e1.0). Comparison between cylinder pressure curve predicted by simplification mechanism and test data under the same working condition is shown in Fig. 6. It can be seen from the figure that the cylinder pressure curve predicted by the simplified mechanism is basically the same. When the fuel air equivalent ratio is 0.6e1.0, the maximum pressure in the cylinder obtained by simulation remains the same, but the maximum pressure obtained by the simulation is slightly larger than the test data. The pressure increase rate calculated in the simulation of rapid combustion period is larger, and the maximum pressure error occurs when the fuel air equivalence ratio is 1.0, and the maximum error reaches 10%. The simulation model has high precision. The concentration comparison of the emission products obtained by simulation and test with different fuel air equivalence ratios is shown in Fig. 7. As can be seen from the figure,
Fig. 7 e Emission data for different fuel air equivalent ratio.
when the equivalence ratio is 0.6e1.0, the NOx emission data obtained by simulation are basically consistent with the test value, and only 0.8% error occurs at 0.8, indicating that the simplified mechanism for predicting emissions is acceptable. We can conclude from Figs. 3 and 4 that the CFD simulation method of simplified mechanism coupled unit grid can meet the calculation requirements of combustion and emission characteristics when different equivalent fuel to air ratio is used. The simplification mechanism has good applicability.
Conclusion (1) Based on sensitivity analysis and path analysis, the influence rules of different elementary reactions on products and reaction processes can be obtained, and the detailed mechanism can be simplified. Sensitivity analysis shows that, HþO2〈 ¼ 〉OþOH, OHþH2〈 ¼ 〉 HþH2O, HþO2þH2O〈 ¼ 〉HO2þH2O, OþH2〈 ¼ 〉HþOH are the most important reaction that affects the combustion of hydrogen. (2) The generation of NO depends not only on the combustion process of hydrogen, but also on the three most important paths: the thermal NO, NNH and N2O route. The sensitivity coefficient of thermal NO path reaches 73%, which almost determines the generation of NO. NNH and N2O route are also important paths. The addition of these two paths is very effective for improving the prediction accuracy in the cylinder environment. (3) The research of simplified mechanism for internal combustion engine cylinder combustion and emission simulation products, regardless of the forecast the change of the pressure in cylinder and the concentration of the pollutants has higher accuracy and can be applied to a wide range of fuel air equivalent ratio, this is very suitable for HICE with mass regulation mode.
Acknowledgements Fig. 6 e Cylinder pressure curves with different fuel air equivalent ratios.
This study is supported by National Natural Science Foundation of China (51276019)and (51706085).
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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 8 ) 1 e5
references
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Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Simplification and applicability studies of a hydrogen-air detailed reaction mechanism Junfa Duan a, Paixia Wu a, Hairong Zhu b,*, Gaolin Qin a, Wei Wei a a
School of Mechanical Engineering, North China University of Water Resources and Power, Zhengzhou 450045, China b School of Mechanical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China
article info
abstract
Article history:
The research of hydrogen fuel internal combustion engine (HICE) had great significance
Received 7 August 2018
facing the challenges of energy and environmental problems. Based on the detailed
Received in revised form
hydrogen-air reaction mechanism, the pre-mix model of CHEMKIN-Pro software was
28 August 2018
selected to simplify the detailed mechanism GRI-3.0. The most important elements and
Accepted 25 September 2018
reactions was chose to construct framework mechanism firstly based on the sensitivity
Available online xxx
coefficient for H2O and NO formation, and additional elements and reactions were added to framework mechanism for complementing the oxidation path of N2 and H2. A simplified
Keywords:
mechanism including 24-step elementary reaction was obtained and the laminar burning
Hydrogen fueled internal combus-
velocity calculated by this simplified mechanism matches well with the detailed mecha-
tion engine (HICE)
nism in a wide range. This simplified mechanism was also applied in a CFD model which
Simplified mechanism
predicted the cylinder instantaneous pressure and NOx emission accurately within a large
Sensitivity coefficient
range of fuel air equivalent ratio. Showing that this mechanism has good applicability.
Path analysis
© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Cylinder pressure NOx emissions
Introduction The utilization of power machinery and energy determines the range of human activities and creativity and at the same time puts great pressure on the living environment. The extensive application of internal combustion engine and fossil energy in the last hundred years has brought the development of human civilization to an unprecedented degree. Due to the challenge of energy shortage and pollution aggravation, it has become an important research direction to find new clean alternative fuels [1e4]. Hydrogen is the only carbon-free energy source, so it does not emit any carbon when used in internal combustion
engines. The combustion product of hydrogen is water, which can be easily regenerated by electrolysis to produce large amounts of hydrogen [5e7]. Hydrogen has a characteristic of low ignition energy and rapid combustion speed, which can ensure short combustion duration of the internal combustion engine so as to obtain a high efficiency. These characteristics give hydrogen an obvious advantage as a substitute fuel for internal combustion engine [8e12]. Great progress has been made in the study of combustion of hydrogen fuel internal combustion engines (HICE). Based on the test bench, the researchers can obtain the variation rule of cylinder pressure, temperature and thermal efficiency as well as the average discharge data under different operating conditions [13e17]. However, these data are not enough, based on
* Corresponding author. E-mail address: [email protected] (H. Zhu). https://doi.org/10.1016/j.ijhydene.2018.09.173 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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 8 ) 1 e5
chemical reaction kinetics method can obtain profound comprehensive hydrogen combustion and emission in cylinder forming rules, therefore, more and more attention has been paid in recent years [18e22]. Many researchers have studied the basic combustion characteristics of hydrogen based on shock tube and burner, and put forward a variety of detailed hydrogen-air reaction mechanisms based on the research of hydrogen and oxygen combustion mechanism. Detailed mechanism for internal combustion engine CFD simulation requires a lot of time, therefore, is of great significance to simplify it [23]. The detailed mechanism of extensive verification was selected in this study. Based on CHEMKIN software, sensitivity coefficient and path analysis were adopted to simplify the mechanism, which is used in CFD simulation model. Internal combustion engine test data to verify that adaptability of the simplify mechanism under load.
Method and model Software platform This article have selected the large-scale gas phase reaction dynamics simulation software CHEMKIN-Pro developed by the Sandia national laboratories as the simplified tool of detailed mechanism, selected PREMIX model, then simplified based on the sensitive coefficient and the reaction path method computational accuracy of the simplified mechanism of laminar burning velocity check was obtained by simulation. The applicability of the simplification mechanism was based on CFD simulation and actual test data of hydrogen fuel internal combustion engine. We selected CONVERGE as the simulation platform of software; CFD simulation model was established based on a spray hydrogen fuel engine inlet, compared to HICE and emissions of the simulation and the experiment data to validate the simplified mechanism to predict the applicability of hydrogen fuel engine simulation process. In this article, model coupled with chemical reaction mechanism was used to calculate the combustion exothermic process and emission generation process of HICE. The combustion model we adopted was the SAGE model combining chemical reaction mechanisms with 3D compute grids to calculate the combustion exothermic process and emission generation process of HICE. A three-dimensional solid model of HICE was established, when boundary conditions and encryption rules were determined, convergence software generated the computing grid autonomously in the calculation process. The maximum number of computing grids of 3d mesh model adopted in reached 425287. Data on in-cylinder combustion and emissions were obtained from the author's previous test results.
Detailed reaction mechanism Mr. Dryer's team at Princeton University [24e29] has been working on the hydrogen and oxygen reaction mechanism for 30 years since 1981. Their detailed reaction mechanism has been modified by Muller [28] and Li [29]several times. The
detailed mechanism can accurately predict the combustion speed and ignition delay of hydrogen and oxygen within the range of 298e3000k, 0.3e87atm, and the fuel air equivalent ratio between 0.25 and 5.0. The gas institute at the University of California, Berkeley, released the detailed mechanism of hydrogen and oxygen combustion, GRI1.0 [30], in 1995 after nearly a decade of research and later released the improved versions GRI2.11 and GRI3.0 [31,32]. This mechanism has been widely used and recognized in the prediction of small molecular hydrocarbon fuel combustion. Experience has shown that hydrogen and oxygen combustion can be predicted at high temperature pressure and concentration range. Williams at the University of California has also proposed a detailed mechanism for hydrogen burning in 1995, which can predict the ignition delay of hydrogen under 33 atm [33]. These detailed reaction mechanisms are more accurate in predicting laminar burning velocity and ignition delay and are widely used in one-dimensional simulation. Jochen compares several common detailed mechanisms, he found that the laminar burning velocity predicted by these mechanisms was relatively close and could accurately predict the laminar burning velocity of hydrogen and air mixture [34]. We use the GRI3.0 mechanism for simplification.
Simplified mechanism This study simplifies the mechanism based on the PREMIX model in CHEMKIN. The main initial condition parameters used in the simulation are shown in Table 1. According to the simulation parameters and detailed mechanism in Table 1, analyzing the oxidation path of H2 and N2 and the sensitivity of H2O and NO, to simplify the mechanism.
Sensitivity and path analysis of H2 oxidation Fig. 1 presents the sensitivity coefficients graph generated based on H2O. The figure shows that at 1850 k for the generation of H2O, HþO2〈 ¼ 〉OþOH is the most sensitive and important primitive reaction, and its sensitivity coefficient to H2O is close to 0.1, so it must be preserved in the simplified mechanism. The sensitivity coefficient of OHþH2〈 ¼ 〉HþH2O, HþO2þH2O〈 ¼ 〉HO2þH2O and OþH2〈 ¼ 〉HþOH also exceed 0.07, which is an important elementary reaction that determines the formation of H2O. Fig. 2 is the path analysis diagram of H2 oxidation. It can be seen from the figure:OHþH2〈 ¼ 〉HþH2O, H2/HO/H2O and H2/OH/H2O2/H2O are the three most important H2O generation paths. H2O2 is also an important substance in the formation path of H2O. To ensure the continuation of the
Table 1 e Simulation initial condition parameters. Para-meters units value
Fuel/air Initial Initial Mass flow equivalent temperature pressure 0.4e2.0
K 298
Atm 1
g/cm∧2-s 0.4
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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 8 ) 1 e5
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Fig. 1 e Sensitivity coefficients with generation of H2O.
Fig. 2 e The main paths of H2 oxidation.
reaction, the basic element H2O2, the reaction OHþ H2O2〈 ¼ 〉 H2OþHO2 were included in the simplification mechanism.
Sensitivity and path analysis of N2 oxidation Fig. 3 shows the sensitivity coefficient graph generated based on NO at the temperature of 2250K. It can be seen from the figure that HeO reaction has a great influence on the final generation of NO, and these reactions are essentially already simplifying mechanisms. In the direct generation path of NO,
Fig. 3 e Sensitivity coefficients with generation of NO.
NþNO〈 ¼ 〉N2þO,NNHþO〈 ¼ 〉NHþNO and N2OþH〈 ¼ 〉NHþNO are NeO reactions that generate sensitive coefficients for NO and play an important role in the generation of NO. At high temperature, the sensitivity of NþNO〈 ¼ 〉N2þO to NO generation reached 0.73, indicating that the thermal NO route plays a decisive role. This is consistent with the author's research results based on detailed mechanism analysis: the thermal NO, NNH and N2O route are the most important element reactions, which determine the formation of NO at high loads. Fig. 4 is the analysis diagram of the oxidation path of N2. At the elementary reaction rate, N2/NO, N2/N2O/NO, N2/NNH/NO are the most important three ways to generate NO. The NNHeNO pathway mainly includes the following three-step elementary reactions: N2þH 4 NNH, NNHþO 4 NHþNO, NHþO 4 NOþH; The generation path of N2OeNO includes the following five-step elementary reactions: N2þOþM 4 N2OþM, N2OþO 4 2NO, N2OþO 4 N2þO2, N2OþH 4 NHþNO, N2OþH 4 N2þOH. The simplified mechanism based on the sensitivity analysis and path analysis method above includes 24 reversible elementary reactions, among which NO mechanism contains 11 reactions. Compared with 69 reactions of the original detailed mechanism, the calculation amount is significantly reduced. The calculation results of the laminar burning velocity and detailed mechanism calculated based on simplified mechanism are compared, as shown in Fig. 5. As can be seen from the figure, the calculation results of the simplified mechanism
Fig. 4 e The main paths of N2 oxidation.
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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Fig. 5 e Comparison of simplification mechanism and detailed mechanism.
are very close to the detailed mechanism, and the maximum error is less than 5%. Meet the requirement of combustion prediction.
The applicability of the simplification mechanism The CFD model of the HICE based on the coupling simplified mechanism studied the combustion and emission process of the internal combustion engine of different fuel air equivalent ratio (0.6e1.0). Comparison between cylinder pressure curve predicted by simplification mechanism and test data under the same working condition is shown in Fig. 6. It can be seen from the figure that the cylinder pressure curve predicted by the simplified mechanism is basically the same. When the fuel air equivalent ratio is 0.6e1.0, the maximum pressure in the cylinder obtained by simulation remains the same, but the maximum pressure obtained by the simulation is slightly larger than the test data. The pressure increase rate calculated in the simulation of rapid combustion period is larger, and the maximum pressure error occurs when the fuel air equivalence ratio is 1.0, and the maximum error reaches 10%. The simulation model has high precision. The concentration comparison of the emission products obtained by simulation and test with different fuel air equivalence ratios is shown in Fig. 7. As can be seen from the figure,
Fig. 7 e Emission data for different fuel air equivalent ratio.
when the equivalence ratio is 0.6e1.0, the NOx emission data obtained by simulation are basically consistent with the test value, and only 0.8% error occurs at 0.8, indicating that the simplified mechanism for predicting emissions is acceptable. We can conclude from Figs. 3 and 4 that the CFD simulation method of simplified mechanism coupled unit grid can meet the calculation requirements of combustion and emission characteristics when different equivalent fuel to air ratio is used. The simplification mechanism has good applicability.
Conclusion (1) Based on sensitivity analysis and path analysis, the influence rules of different elementary reactions on products and reaction processes can be obtained, and the detailed mechanism can be simplified. Sensitivity analysis shows that, HþO2〈 ¼ 〉OþOH, OHþH2〈 ¼ 〉 HþH2O, HþO2þH2O〈 ¼ 〉HO2þH2O, OþH2〈 ¼ 〉HþOH are the most important reaction that affects the combustion of hydrogen. (2) The generation of NO depends not only on the combustion process of hydrogen, but also on the three most important paths: the thermal NO, NNH and N2O route. The sensitivity coefficient of thermal NO path reaches 73%, which almost determines the generation of NO. NNH and N2O route are also important paths. The addition of these two paths is very effective for improving the prediction accuracy in the cylinder environment. (3) The research of simplified mechanism for internal combustion engine cylinder combustion and emission simulation products, regardless of the forecast the change of the pressure in cylinder and the concentration of the pollutants has higher accuracy and can be applied to a wide range of fuel air equivalent ratio, this is very suitable for HICE with mass regulation mode.
Acknowledgements Fig. 6 e Cylinder pressure curves with different fuel air equivalent ratios.
This study is supported by National Natural Science Foundation of China (51276019)and (51706085).
Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173
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Please cite this article in press as: Duan J, et al., Simplification and applicability studies of a hydrogen-air detailed reaction mechanism, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.173