Effects of carbon dioxide on the combustion characteristics of the laminar premixed n-heptane/air flames at elevated pressures
Introduction
Exhaust gas recirculation (EGR) is an effective and widely used technology for emission reduction [[1], [2], [3]]. Carbon dioxide (CO2) is one of the main components of EGR gas which occupies a significant position in the combustion process. CO2 has high heat capacity, which leads to a lower adiabatic flame temperature [4] and its third body collision effect affects the intermolecular reaction [5]. Carbon dioxide can increase the intensity of flame radiation [6] and reduce minimum oxygen concentration needed to support combustion [7,8]. In addition, CO2 is not an inert gas, which can be directly involved in basic reactions and decomposed to release free radicals [9].
N-heptane (n-C7H16) is always used as an alternative fuel to diesel because of their similar cetane numbers [10]. There are few studies on physical and chemical effects of exhaust composition and combined effect of specific thermodynamic conditions on diesel engines. Therefore, systematic analysis is needed to further optimize combustion and emission performance of diesel engines.
In recent years, lots of scholars have carried out a series of numerical and experimental studies on the combustion of hydrocarbon fuels with CO2 addition. Xie et al. [11] and Mitu et al. [12] analyzed the effect of CO2 addition on laminar burning velocity (LBV). Hu and Yu [13] experimentally and numerically measured the propagation velocity of CH4/O2/CO2 laminar premixed flame by using the Bunsen burner. Halter et al. [14] reported that LBV decreased more promptly with iso-octane of CO2 addition than nitrogen. Miao et al. [15,16] experimentally investigated the effects of CO2 and N2 addition on laminar combustion characteristics of natural gas/hydrogen flame by using the constant volume combustion chamber. Han et al. [17], Burbano et al. [18], Prathap et al. [19], and Khan et al. [20] investigated the effect of CO2 on LBV of H2/CO at elevated pressures and temperatures and found that the CO2 dilution reduced LBV and adiabatic flame temperature (AFT). Fictitious gases (such as fictitious CO2, i.e., FCO2) have identical thermal and transport properties as real gases (such as CO2), but do not participate in chemical reactions. Therefore, fictitious gases addition in hydrocarbon fuels can be used to analyze their chemical effect. Chen et al. [21] studied thermal and chemical kinetics of various fuels with CO2, N2, Ar addition, and they analyzed the effect of CO2 on chemical kinetics. Li et al. [22] investigated chemical effect of CO2 on the oxidation of methyl butyrate (MB) under atmospheric pressure with laminar flow reactor. Liu [23] analyzed the chemical effect of hydrogen addition on dimethyl ether combustion characteristic. Liu et al. [24] firstly proposed the concept of fictitious gases and numerically considered the chemical effects of CO2 replacement of N2 in air on the LBVs of CH4 and H2 premixed flames. Hu et al. [25], Gascoin et al. [26], Ren et al. [27,28] and Xiang et al. [29] numerically studied the effect of CO2 addition for methane/air in LBV and AFT. They found that the dilution effect had the greatest inhibition for the LBV and AFT, the second was thermal effect and chemical effect was the minimal effect.
There are few studies on laminar premixed n-heptane/carbon dioxide/air flames, especially under high pressures. Li et al. [30] experimentally and numerically studied the influence of CO2 addition on C7H16/air laminar flame. Their results showed that LBV decreased significantly with the increase of CO2 doping ratios, where the physical effect and chemical effect of CO2 reduced LBV. For the hydrocarbon combustion, Chu et al. [[31], [32], [33], [34]] investigated the characteristics of hydrocarbon fuel and found that the peak value of NHRR increases exponentially with initial pressure increasing. Li et al. [35] studied the n-heptane pyrolytic characteristics in CO2/H2O. Under low residence times, the research results showed that the mass yield of ethylene and propylene under the CO2/H2O atmosphere were higher than H2O atmosphere. Dai et al. [36] analyzed the autoignition and detonation development induced by CO2 diluted n-heptane/air mixtures and found that the increase of CO2 dilution could reduce the heat release and chemical reaction became weaker.
In this paper, exhaust gas CO2 is brought into n-heptane combustion, which has certain enlightenment significance for emission reduction of diesel engines. The purposes of this research are to investigate the effects of carbon dioxide on the combustion characteristics of the laminar premixed n-heptane/air flames at elevated pressures and reveal physical and chemical effects of CO2. The research can help comprehend the combustion characteristics of n-heptane/carbon dioxide/air mixture at elevated pressure.
Section snippets
Calculation method
One-dimensional premixed laminar freely propagating flames in the Chemkin/Premix code is used to simulate n-heptane/carbon dioxide/air combustion under different conditions. Thermal and transport properties can be obtained from LLNL v3.1 mechanism [37]. The LLNL v3.1∗ mechanism is been modified to investigate the chemical and physical effects of CO2. The maximum initial pressure is set as 20 atm, which well makes up for the lack of high-pressure environment data and provides a theoretical basis
Effects of CO2 doping ratio and initial pressure on LBV and AFT
LBV and AFT are the most important parameters to characterize the combustion process and factors to control flame propagation. AFT has a great influence on the change of LBV. In general, the relationship between them is positively correlated.
Fig. 1 shows the comparison of LBVs calculated by using different mechanisms with the experimental results. As can be seen from Fig. 1 (a), under the condition of P = 1 atm, T = 298 K and α = 0.05, the simulation results of Cai et al.'s mechanism [38] at
Conclusions
The effect of carbon dioxide adding into the gasoline surrogate fuel (n-heptane) and carbon dioxide mixture is investigated by using Chemkin/Premix code. The combustion characteristics of LBV, ATF, NHRR, temperature sensitivity and ROP have been studied. The major conclusions are summarized as follows:
- (1)
With the increase of initial pressures, LBV decreases greatly, adiabatic temperature and net heat release rate increase continuously; with the increase of CO2 doping ratios, LBV, AFT and NHRR
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 52176095), Anhui Provincial Natural Science Foundation (No. 2008085J25), State Key Laboratory of Engines in Tianjin University (No. K2020-11), Open Project of State Key Laboratory of Clean Energy Utilization, Zhejiang University (No. ZJU-CEU2020001), Opening Project of Engineering Technology Research Center of Anhui Education Department for Energy Saving and Pollutant Control in Metallurgical Process (No. GKF20-5),
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