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Dilution, Thermal and Chemical Effects of Carbon Dioxide on the Exergy Destruction in N-Heptane and IsoOctane Auto-ignition Processes: A Numerical Study Jiabo Zhang, Zhen Huang, Kyoungdoug Min, and Dong Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04018 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018
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Title: Dilution, Thermal and Chemical Effects of Carbon Dioxide on the Exergy Destruction in N-Heptane and Iso-Octane Auto-ignition Processes: A Numerical Study
Authors: Jiabo Zhang1, Zhen Huang1, Kyoungdoug Min2, Dong Han1
Affiliations: 1. Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China 2. Department of Mechanical & Aerospace Engineering, Seoul National University, Seoul 08826, South Korea
Corresponding author’s contact information: Name: Dong Han Mailing Address: Institute of Internal Combustion Engine, 800 Dongchuan Road, Shanghai Jiao Tong University, Shanghai 200240, China Fax: +86 21 34206860 Email:
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Dilution, Thermal and Chemical Effects of Carbon Dioxide on the Exergy Destruction in N-Heptane and Iso-Octane Auto-ignition Processes: A Numerical Study Jiabo Zhang, Zhen Huang, Kyoungdoug Min, Dong Han* 1. Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China 2. Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826, South Korea
Abstract A numerical analysis based on the second-law thermodynamics was conducted for the n-heptane and iso-octane auto-ignition processes in an adiabatic constant-volume system with CO2 addition. Detailed chemical mechanisms of n-heptane and iso-octane were used in the auto-ignition processes simulation at the initial temperatures located in the negative temperature coefficient (NTC) region and the high temperature region. The dilution, thermal and chemical effects of the CO2 addition were numerically isolated to evaluate each individual effect on the exergy destruction in different reaction stages of the auto-ignition processes, namely the fuel-series reaction stage, the fuel-fragment reaction stage, the H2O2 loop reaction stage and the H2-O2 reaction stage. It was observed that the exergy loss in the fuel-series reaction stage of iso-octane auto-ignition was lower than that of n-heptane, but the individual effect of CO2 played the same role in the exergy destruction for both fuels. The dilution effect of CO2 reduced the exergy loss in the H2O2 loop reaction stage and the reduction magnitude increased with the increased initial temperature or CO2 concentration. The thermal effect of CO2 slightly increased the exergy loss at both initial temperatures due to the increase in the overall heat capacity and the decrease in the bulk temperature. The chemical effect of CO2 reduced the exergy loss in the H2-O2 reaction stage at both initial temperatures. The exergy loss due to the incomplete combustion became apparent at the high initial temperature condition, and was increased by the dilution and chemical effects of CO2 while decreased by the thermal effect of CO2.
Keywords: Exhaust Gas Recirculation, Carbon Dioxide, Exergy, Auto-ignition, Chemical Kinetics
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Introduction The increasingly stringent emissions regulations and limited resources of fossil fuels behoove us to find an effective way for the emissions reduction and fuel economy improvement of internal combustion (IC) engines. Advanced engine combustion strategies, e.g. homogeneous charged compression ignition (HCCI), are as such proposed to solve this problem.1,2 Both nitrogen oxides and soot emissions could be reduced to the near-zero level in HCCI combustion due to the formation of homogenous lean mixture.3,4 However, the operational range of HCCI combustion strategy is narrow due to misfire at light loads and knocking at heavy loads.5,6 Exhaust gas recirculation (EGR) is an effective way to extend the HCCI operation to a broader window, as EGR could influence the fuel ignition timing and heat release processes in HCCI engines.7 The EGR effects on the energy conversion process in engines are also of great interests. While the analysis based on the first-law thermodynamics could evaluate the energy distribution in engine operation, this method only considered energy quantity rather than energy quality. The second-law thermodynamics was as such coupled with the first-law thermodynamics to provide more insights to the potential pathways towards high-efficiency engines.8-10However, there was still debates regarding to the EGR effects on the combustion-induced exergy loss in IC engines. Caton11 evaluated the combustion-induced exergy loss in a spark ignition (SI) engine as a function of the engine operation condition, engine design parameter and fuel type, and found cooled EGR resulted in increased exergy destruction due to the reduced combustion temperature. Mamalis et al. 12 established a one-dimension model for a boosted four-cylinder HCCI engine with negative valve overlap (NVO) strategy to generate internal EGR. They found the NVO strategy caused higher cylinder exergy destruction (~27%) than the conventional gasoline or diesel engine combustion (20%-22%), due to the higher dilution levels and lower cylinder temperatures. In contrast, in a numerical study on the exergy losses of the n-heptane constant-volume combustion process, the total combustion-induced exergy losses could be decreased as the oxygen concentration decreased13 . Similarly, Verma et al.14 conducted an exergy analysis for a CO2-diluted biogas-diesel dual-fuel engine and found the combustion irreversibility decreased with increased CO2 concentrations. Jafarmadar et al. 15 further analyzed the EGR effects on the exergy efficiency on a diesel/H2 dual-fuel engine. As the EGR rate varied from 0% to 30%, the accumulative irreversibility percentage accounting for the fuel chemical exergies decreased from 31.6% to 23.3%. Zheng et al.16 analyzed the exergy loss of a highly-diluted and late-injection diesel engine and found that the exergy destruction was lower than that in the conventional combustion mode by 50%. Moreover, Amjad et al. 17 experimentally analyzed the availability loss of the n-heptane and natural gas blends combustion in HCCI engines. The results revealed that the exergy loss decreased as the mass fraction of natural gas increased, and an optimum EGR rate existed for the exergy efficiency improvement.
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In this study, the chemical kinetic analysis and the thermochemistry analysis were combined to elucidate the EGR effects on the combustion-induced exergy loss in HCCI combustion. A numerical study based on detailed chemical mechanisms was first conducted to evaluate the exergy destruction process in the adiabatic constant-volume auto-ignition processes of n-heptane and iso-octane. EGR effects were simulated by replacing an amount of O2 in the oxidants with CO2. The dilution effect (intake oxygen concentration reduction by inert gases), the thermal effect (temperature change by increased gas heat capacity18), and the chemical effect (the participation of CO2 in chemical reactions) were numerically isolated as in the authors’ previous study.19 Further, the elementary reactions in the auto-ignition processes were divided into four stages, namely the fuel-series reaction stage, the fuel-fragment reaction stage, the H2O2 loop reaction stage and the H2-O2 reaction stage. The triple effects of CO2 addition on the exergy losses in each stage were analyzed under changed initial temperatures, equivalence ratios and CO2 concentrations. In this way, the contribution by the triple effects of CO2 addition, as well as by the key influential chemical reactions to the exergy losses of the auto-ignition processes were identified and compared.
Methodology The effects of CO2 addition on the n-heptane and iso-octane auto-ignition processes in an adiabatic constant-volume system were numerically studied, in the context of changes in the molar-basis oxidant composition from the baseline condition of 21% O2/79% N2 to 13% O2/8% CO2/79% N2 and 9% O2/12% CO2/79% N2, respectively. The auto-ignition processes were simulated using the detailed chemical kinetic mechanisms of n-heptane (654 species and 2827 reactions) and iso-octane (875 species and 3796 reactions) developed by Lawrence Livermore National Laboratories20,21. These mechanisms have been validated against the experimental data over a wide range of temperature, pressure and equivalence ratio.22-25 Figure 1 presents a comparison of the experimentally measured ignition delay times of n-heptane and iso-octane26, 27 with the simulated results by the chemical mechanisms used in this study. The oxidizers include air and buffer gases with CO2 addition. The satisfactory agreement between the experimental and simulation results certified that the chemical mechanism is suitable for the exergy loss calculation in the auto-ignition processes at conditions with and without CO2 addition.
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Figure 1. Comparison between the measured (by Hartmann et al. 26 and Di et al. 27) and simulated (LLNL mechanism) ignition delay times for n-heptane and iso-octane at high pressure conditions w and w/o CO2 dilution. In this study, two different initial temperatures (840 and 1500K) were selected for calculation, representing the negative temperature coefficient (NTC) region and the high temperature region, respectively. Moreover, the initial pressure was held at 50atm to simulate the high-pressure environment at the top dead center (TDC) in real engines. The baseline equivalence ratios were set as 0.3 and 0.5 considering the fuel-lean combustion in HCCI engines. To identify the three-fold effects of CO2 addition, a numerical method as proposed by Liu et al.28 was used in this study. Firstly, fictitious O2 (FO2) was introduced as it had the same thermodynamic and transport properties as real O2 but did not participate in any chemical reaction. By replacing an amount of O2 in the oxidant with FO2, the dilution effect of CO2 addition was isolated. Secondly, the same amount of O2 was replaced with the fictitious CO2 (FCO2), possessing the same thermodynamic and transport properties as real CO2 but did not participate in any chemical reaction. This represented the condition under the combined dilution and thermal effects of CO2. Thirdly, real CO2 was added to the mixture, and as such the combined triple effects could be obtained. The three conditions were denoted as D (only consider the dilution effect), D+T (consider both the dilution and thermal effects) and D+T+C (consider all the triple effects) in the following discussion, respectively. By comparing the D, D+T and D+T+C conditions as well as the baseline condition (oxidant: 21% O2 and 79% N2), the triple effects of CO2 addition can be isolated. The numerical computation was conducted using the Chemkin Pro29 software, while the VODE (variable-coefficient ordinary differential equation) solver was employed to solve the mass and energy conservation equations. The species mass equation can be written as
(1)
where is the mass fraction of the ith species, v is the specific volume, is the
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mole production rate of the ith species and is the molecular weight of the ith species. The energy conservation equation from the first-law thermodynamics is as
+ ∑ 0
(2)
where is the constant-volume specific heat capacity, is the specific internal energy of the ith species. The entropy conservation equation is as follows: ∆S (∆S) + (∆S) + (∆S) + (3) where ∆S, (∆S) , (∆S) , (∆S) and represent entropy change, thermal entropy flow, work entropy flow, mass entropy flow and entropy generation, respectively. In the adiabatic constant-volume model used in this study, Eq. (3) could be reduced to Eq. (4) as the heat transfer, external work and mass transfer are zero, ∆S (4) The irreversibility is calculated based on the Gouy-Stodla equation,30 I ! ! ∆ (5) where I is the combustion irreversibility and the subscript 0 indicates the dead state. In this study, the temperature and pressure of the dead state were defined as ! 298% , &! 1()* , and the molar-basis composition of the mixture is 75.60% N2, 20.34%O2, 3.12%H2O, 0.3%CO2 and 0.91%Ar based on the Kameyama-Yoshida system. The total entropy of the mixture at a state of (T, P) is
S(T, P) ∑ - . ∑ - (.̅ ( , &! ) − 123
4 5 56
)
(6)
where . is the specific entropy of the ith species at T and P, .̅ ( , &! ) is the specific entropy of the ith species at T and P0 calculated with the NASA Lewis polynomial coefficients,31 R is the universal gas constant and - is the molar-basis amount of the ith species, respectively. The system entropy change by each time step can be then calculated by Eq. (6). On the other hand, the entropy generation rate of each chemical reaction can be computed using the entropy generation equation13,32,33. As the fluid friction, heat transfer and mass transfer terms in the entropy generation equation were neglected in this adiabatic constant-volume system, the entropy generation due to chemical reactions was focused. Hence, the entropy generation equation could be reduced to
ρ
89
− ∑
:,9 ;,9
(7)
where ρ is the molar density (mole/m3),
