- Email: [email protected]
air flames at elevated initial temperature and pressure
Journal of the Energy Institute 92 (2019) 1821e1830
Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Numerical investigation on combustion characteristics of laminar premixed n-heptane/air flames at elevated initial temperature and pressure Huaqiang Chu a, Fei Ren a, Longkai Xiang a, Shilin Dong a, Fen Qiao b, *, Guangju Xu c, ** a b c
School of Energy and Environment, Anhui University of Technology, Ma'anshan 243002, China School of Energy & Power Engineering, Jiangsu University, Zhenjiang 212013, China Department of Automobile Engineering, Changshu Institute of Technology, Changshu 215500, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2018 Received in revised form 26 November 2018 Accepted 26 November 2018 Available online 1 December 2018
Freely-propagating laminar premixed n-heptane/air flames were modeled using the Lawrence Livermore National Laboratory (LLNL) v3.1 n-heptane mechanism and the PREMIX code. Numerical calculations were conducted for unburned mixture temperature range of 298e423 K, at elevated pressures 1e10 atm and equivalent ratio 0.6e1.6, and the changes of laminar burning velocity (LBV), adiabatic flame temperature (AFT), heat release rate (HRR), and concentration profiles of important intermediate species were obtained. The results show that the overall results of LBVs of n-heptane at different elevated temperatures, pressures, and equivalence ratios are in good agreement with available experimental results. However, at the initial temperature 353 K, the calculated values of LBVs at pressure 1 atm and the 10 atm deviate significantly from the experimental results. The sensitivity analysis shows that, similar to many other hydrocarbon fuels, the most sensitive reaction in the oxidation of n-heptane responsible for the rise of flame temperature promoting heat release is R1 H þ O2<¼>O þ OH, and the reaction that has the greatest influence on heat release is R8 H2O þ M<¼>H þ OH þ M. In addition, when the initial temperature is 353, 398 and 423 K, the mole fractions of H, OH, and O increase rapidly around the flame front, while the mole fractions of C1eC3 dramatically decreases, reflecting the intense consumption of the intermediate products at the reaction zone. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: N-heptane LBV Adiabatic flame temperature Heat release rate
1. Introduction Currently, about 20% energy in the world is consumed for transportation, of which 95% liquid fuel comes from petroleum refining [1e4]. With the increase of prosperous population and environmental pressure, energy shortage is becoming increasingly prominent. While it is important to search for alternative fuels, it is also necessary to improve the design of combustion equipment to achieve high thermal efficiency and reduce pollutant emissions at the same time. Natural gas has been considered as the most potentially clean energy and engine alternative fuel because of its high octane number, large reserves, low price and low emission [5e10]. However, since its main component is methane, its spontaneous combustion temperature is high, the flame speed is low, and the combustion capacity is poor at low load, which lead to incomplete combustion and high misfire rate in lean burn [6e9]. In addition, compared with liquid fuels, vehicles fueled with natural gas, suffer a shorter mileage and high likelihood of corrosion of the combustion chamber components. Even if natural gas combustion is environmentally friendly, in many cases, it is not advisable to use natural gas as fuel, such as for long distance transportation, high-power output, and emergency fuel reserve. Consequently, it is important to continue to investigate the combustion characteristics of high energy-density liquid hydrocarbon gasoline alternative fuels. To understand the combustion characteristics of complex gasoline, which consists of tens of different hydrocarbon components, it is
* Corresponding author. Jiangsu University, Zhenjiang 212013, China. ** Corresponding author. Changshu Institute of Technology, Changshu 215500, China. E-mail addresses: [email protected] (F. Qiao), [email protected] (G. Xu). https://doi.org/10.1016/j.joei.2018.11.010 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
1822
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
necessary to develop gasoline surrogate fuel models. The earliest model is the primary reference fuel (PRF) containing n-heptane and isooctane, followed by more recently proposed toluene added to PRF to form a toluene reference fuel (TRF) and ethanol toluene reference fuel (ERF). In order to design clean and efficient gasoline engines, it is vital to gain insights into the basic combustion characteristics of major gasoline components. N-heptane was selected in this study as a single component alternative fuel to study the LBV and other flame properties of freely-propagating n-heptane/air mixtures under different conditions. The reasons to study n-heptane are mainly because it is a main component of gasoline surrogate to represent straight-chain paraffin with an assigned octane number of zero and it has a cetane number of 56, which is close to diesel, and has often been used to approximately simulate diesel combustion. Moreover, as a major component of gasoline surrogate fuels, n-heptane has received considerable research attention and extensive studies have been conducted to investigate its reaction mechanism and combustion characteristics experimentally and numerically. To save computing time, obtain fast and accurate results, and to facilitate multidimensional numerical simulation and mechanism validation, since Curran et al. [11] developed a detailed chemical reaction mechanism of n-heptane containing 544 components and 2446 elemental reactions, many related studies have been conducted on the laminar combustion characteristics of n-heptane, the simplification of the mechanism and the mechanism of soot formation during the combustion process. The laminar flame velocity of iso-octane/air and nheptane/air mixtures at room temperature and atmospheric pressure has been measured using the counterflow twin flame method by Davis and Law with both linear and nonlinear extrapolations [12]. Huang et al. [13] experimentally and numerically studied the LBV of PRF, nheptane, and iso-octane by means of the counterflow flame technique. It was found that the increase of iso-octane content in PRFs and reformer gas reduced the LBV, but the addition of reformer gas increased the LBVs of iso-octane/air mixtures. Kumar et al. [14] employed the Digital Particle Image Velocimetry (DPIV) technique in counterflow flames of premixed iso-octane/air and n-heptane/air mixtures to measure the LBVs over a wide range of equivalence ratios and at different unburned mixture temperatures. A 55 species and 51 global steps reduced mechanism for n-heptane oxidation has been obtained by Lu and Law [15] using the directed relation graph (DRG) and computational singular perturbation (CSP) methods. Jerzembeck et al. [16] experimentally determined the LBVs and Markstein numbers for nheptane, iso-octane, PRF87, and gasoline/air mixtures at engine-relevant conditions in constant volume bomb and validated a reduced kinetic mechanism derived from the LLNL detailed n-heptane and iso-octane mechanism in propagating flames. Using the counterflow twin-flame technique, Smallbone et al. [17] determined the LBVs for n-heptane/oxygen/nitrogen mixtures at various initial pressures and equivalence ratios and validated a 130 species and 955 reactions detailed kinetic model of n-heptane oxidation. Ji et al. [18] carried out experimental and numerical studies of the LBVs and extinction strain rates of premixed C5eC12 n-alkane flames in the counterflow configuration and conducted a comparison of the LBVs obtained by linear and non-linear extrapolations. Lipzig et al. [19] measured the LBVs of the adiabatic flames of n-heptane, iso-octane, ethanol, and their binary and tertiary mixtures on a perforated plate burner using the heat flux method. Kelley et al. [20] used the constant volume bomb to determine the LBVs of C5eC8 n-alkane and the Markstein lengths under different pressures and analyzed the flame structure similarities of alkane fuels and the influence of pressure on stretch and extrapolation. Sileghem et al. [21] conducted measurements of LBV for gasoline, iso-octane, n-heptane, toluene, and TRF on a flat flame adiabatic burner with the heat flux method. Dirrenberger et al. [22] conducted experiments through the heat flux method on a perforated plate burner to investigate the adiabatic LBVs of the four pure components of the model fuel, and a commercial gasoline and TRF at almost the same octane number, as well as compared LBV of TRF and TRF/ERF mixtures with that of the commercial gasoline by numerical simulation. In addition, the LBVs of iso-octane and n-heptane compared to three key biofuels n-butanol, iso-butanol and ethanol have been measured with the heat flux burner by Knorsch et al. [23] and the influence of exhaust gas recirculation (EGR) rates on the LBVs was investigated. Hakka et al. [24] performed an experiment on the ultra-rich oxidation of n-heptane with a jet-stirred reactor (JSR) and developed a detailed mechanism using computer-aided generation (EXGAS). A combustion chamber has been designed by Li et al. [25] to experimentally study the LBV and flame instability of methane/n-heptane/air mixture at different initial temperatures. Their results showed that when the methane content was over 0.75 in the fuel mixture, the LBV and Markstein length of methane/n-heptane/air mixture changed significantly. Using two different high-pressure shock tubes, Zhang et al. [26] measured the ignition delay times of stoichiometric n-heptane/air mixtures and verified the kinetic model of n-heptane oxidation under different temperature and pressure conditions. The numerical results are in good agreement with the experimental results. Hu et al. [27] measured the ignition delay times of dimethoxy methane, n-heptane, and their mixtures with the reflected shock waves. A chemical kinetic model for the blends was built and validated with the experimental results and the dimethoxy methane addition was found to decrease the ignition delay time of n-heptane. Shi et al. [28] numerically studied the stratified flames and homogeneous flames of methane, propane, and n-heptane. The LBV and propagation speed of the three fuels and the difference between them were compared and analyzed. Recently, Chen et al. [29] used DRGEP and TSA approaches to simplify the oxidation mechanisms of methane, propane, n-heptane and PRF. After comparison and validation, it was found that the newly proposed reduction methodology for developing accurate skeletal models that could take into account both ignition delay time and LBV. Although various simplified mechanisms for n-heptane oxidation have been proposed in the last two decades, only few can be considered reliable and accurate at specific conditions. It is worth pointing out that in the studies mentioned above the oxidation reaction mechanism of n-heptane developed by Curran et al. [11] is the most widely accepted reaction mechanism [26], which is often used for comparison, simplified mechanism, and new models. Based on the maturation mechanism, Mehl et al. [30,31] developed the Lawrence Livermore National Laboratory (LLNL) v3.0 and v3.1 nheptane mechanisms. The latter is modified and optimized on the basis of the previous v3.0 and has been verified in the experiments of shock tubes and rapid compression machines. LLNL v3.1 n-heptane mechanism contains 654 components and 2827 elementary reactions. In a series of experimental and numerical studies, the mechanism is found applicable to the initial pressure of 3e50 atm, the temperature of 650e1200 K, and the equivalence ratio of 0.3e1.0. The mechanism performs well at both low and high temperatures, as well as wide pressure ranges relevant to internal combustion engines. However, little attention has been paid to the prediction of combustion characteristics of n-heptane in premixed freely-propagating flame. In addition, to further understand the prediction accuracy of the LLNL v3.1 mechanism for n-heptane at fuel rich, low temperature, and near atmospheric pressure conditions, the detailed LLNL v.31 of n-heptane oxidation is used in this study to predict the combustion characteristics of premixed freely-propagating flames of n-heptane/air mixtures for different equivalence ratios, initial temperatures, and pressures. Therefore, by changing the initial conditions, the LBV, AFT, temperature sensitivity, HRR and the change of intermediate
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1823
components in combustion were studied. A comparison is carried out between simulation and available experimental results, and the sensitivity analysis is chosen to identify the most sensitive chemical reactions that affect the temperature rise in n-heptane/air mixtures combustion. 2. Computational model and condition The one-dimensional premixed laminar freely-propagating flames of n-heptane under different initial conditions were simulated using the PREMIX code [32]. Fig. 1 describes a schematic of the flame front structure of the premixed laminar flame and Table 1 lists the numerical simulation cases. In the calculation process, the n-heptane/air mixture flow rate is an eigenvalue of the problem and its initial value was assigned to be 0.04 g/(cm2.s). Using adaptive grid with GRAD ¼ 0.04 and CURV ¼ 0.04, the grid number upon convergence was in the range of 500e800, depending on the mixture properties (equivalence ratio, temperature, and pressure). The relative and absolute errors in the iterative process were set to 104 and 109, respectively. The left boundary of computational domain was at 0.2 cm and the right boundary location was set to between 6 and 10 cm to achieve zero gradients for all variables, which satisfied the calculation requirements. In addition, the fixed temperature was set at 500 K in the position of 0.5 mm. The windward difference was used to discretize the convection term and the average component method was used to calculate the diffusion coefficient of mixture. The Soret effect was considered in all the calculations. 3. Results and discussion 3.1. Laminar burning velocity LBV, an inherent characteristic of hydrocarbon fuels, influenced by the type of fuel and the initial combustion environment, is an important parameter for the propagation and stability of laminar flame, and it is also a target parameter for chemical reaction mechanism development and validation. Shown in Fig. 2 is the simulation results of LBVs compared with the experimental results in Refs. [12e14,17e23,25] at various equivalence ratios and initial temperatures, 298 K, 353 K, 398 K and 423 K, and 1 atm. From Fig. 2(a), it can be seen that at the initial temperature of 298 K, the simulation results are in good agreement with the experimental data of Huang et al. [13] obtained using counterflow flame and Dirrenberger et al. [22] and Sileghem et al. [21] by the heat flux method. In Fig. 2(b), at a temperature of 353 K, the simulation results are generally consistent with the experiments, especially the results of LBV obtained by the Li et al. [25] using a constant volume bomb. In addition, it is found that the overall agreement between the prediction and experiments is better in fuel-lean and -rich mixtures than those around the stoichiometry between about 0.9 and 1.3. In Fig. 2 (c), the simulation results are in good agreement with the experimental results of Dirrenberger et al. [22] and Li et al. [25], but are lower and higher than those Knorsch et al. and Kumar et al., respectively. With respect to 423 K, in Fig. 2 (d), the simulated values are in line with the trend of LBVs of both Knorsch et al. [23] and Li et al. [25], but in better overall agreement with those of Li et al. [25]. The overall observation from the results shown in Fig. 2 is that the peak value of LBV always occurs at a slightly fuel-rich side near the equivalence ratio of 1.1, regardless of the initial temperatures. By comparing the simulation and experimental results in Fig. 2, it can be found that the prediction of the LBVs of n-heptane appears to be inconsistent in experimental data over the range of initial temperature considered. In order to further and more intuitively analyze the numerical results of LBVs of n-heptane, the LBVs are compared with the experimental results of Kelley and Dirrenberger et al. [20,22] in terms of consistency of operating conditions for different pressures and temperatures in Fig. 3. From Fig. 3(a) and (b), it is seen that with increasing the pressure, the LBV gradually decreases, and as the pressure continues to increase, the effect of pressure on LBV is gradually weakened. The data in Fig. 3(b) demonstrate that at 353 K, compared with the results of Ref. [20], at 1 atm, the results on fuel-lean side are basically compatible with the experimental values, but the simulation values tend to be higher than the experimental values on fuel-rich side. Moreover, at 2 atm, the simulation agrees well with the experimental points, which is consistent with that of Zhang et al. [26]. At higher pressures of 5 and 10 atm, however, the simulated values are lower than the experiments, and the difference tends to increase with increasing the equivalence ratio. Furthermore, it is clear from Fig. 3(c) that the predicted LBV of n-heptane at temperature 298 and 398 K is greater than the experiments. When the temperature is at 353 K, the predicted value is slightly lower than the experimental points. According to Figs. 2 and 3, the LLNL v3.1 presents a good agreement with the experimental results. However, under the conditions of 353 K, 1 atm and 10 atm, further optimization for the mechanism should be carried out. And the conjecture requires further experimental and numerical investigations.
Fig. 1. Schematic of flame front structure of premixed laminar flame.
Table 1 Simulation cases. Simulated condition
Cases
Pressure
Temperature
Mole fraction of reactant
P (atm)
T (K)
O2 N2 n-C7H16
1e10
298e423
0.2037-0.2080 0.7666-0.7825 0.0095-0.0297
1824
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 2. LBVs of n-heptane at different initial temperatures and 1 atm.
Fig. 3. LBVs under different pressures and temperatures: solid lines:simulation, (a) data points: Ref. [22], (b) data points: Ref. [20], (c) data points: Ref. [22].
3.2. Adiabatic flame temperature, sensitivity analysis and net heat release rate The adiabatic flame temperatures at different initial pressures and temperatures are plotted in Fig. 4. From Fig. 4(a) and (b), it can be seen that the pressure effect on the AFT is concentrated near the equivalent ratio of 1, and the effect of pressure on the temperature is gradually decreased in the fuel-lean and -rich combustion. As the pressure increases, the AFT increases gradually. When the equivalent ratio is 1 and the pressure rises from 1 to 10 atm, the AFT rises from 2283 K to 2337 K at the initial temperature of 298 K, increased by 54 K. At the initial temperature 353 K, the AFT is changed from 2308 K to 2367 K, increased by 59 K. In Fig. 4(c), the AFT increases with increasing the initial temperature. Moreover, the initial temperature has a great influence on the AFT in the fuel-rich zone. At the equivalent ratio 1.0, the AFT rises
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1825
Fig. 4. Adiabatic flame temperatures at different pressures and temperatures.
from 2283 K to 2342 K with the initial temperature from 298 K to 423 K, increased by 59 K. While with increasing the initial temperature from 298 K to 423 K, the AFT increases from 2210 K to 2291 K at the equivalent ratio of 1.2, increased by 81 K. Fig. 5 shows the sensitivity coefficients of flame temperature under different initial temperatures and pressures at the maximum temperature gradient when the equivalence ratio is 1.0. From Fig. 5(a), (b) and (c), it can be concluded that the sensitivity of the key reactions to flame temperature is obviously influenced by the initial pressure. In the stoichiometric n-heptane/air flame with different initial pressures, the most sensitive reactions are those of C0eC2 chemistry, and the reaction R1 H þ O2<¼>H þ OH always has the largest absolute value of the sensitivity coefficient, which means that this reaction is the most sensitive to the flame temperature. The results obtained in the study of n-heptane propagation flame are consistent with Refs. [14,18,27], i.e., under the high temperature condition of the flame, the initial decomposition reaction rate of n-alkanes is too fast to play the role of rapid control step. Therefore, these reactions are so sensitive to the flame propagation. From Fig. 5(a), it can be observed that the sensitivity coefficients of R1 H þ O2<¼>O þ OH and R26 HCO þ M<¼>H þ CO þ M increase with the elevated pressure at the initial temperature of 298 K. The forward reactions R2 O þ H2<¼>H þ OH and R24 CO þ OH<¼>CO2 þ H promote temperature rise, but the pressure beyond 2 atm the two reactions suppress the temperature rise which is contrary to R9 H þ O2(þM)<¼>HO2(þM). The effects of reactions R11 HO2 þ H<¼>2OH and R268 C2H3 þ H<¼>C2H2 þ H2 on the temperature change from depression to promotion when the pressure are beyond 1 atm and 5 atm, respectively. Furthermore, it should be noted that, as shown Fig. 5(b), the absolute values of sensitivity coefficients for promoting reaction R1 H þ O2<¼>O þ OH and depressing reactions R8 H2O þ M<¼>H þ OH and R9 H þ O2(þM)<¼>HO2(þM) to temperature rise are increased with the elevated pressure 1e5 atm then decrease as the pressure continues to increase, at the initial temperature of 353 K. The forward reactions R24 CO þ OH<¼>CO2 þ H and R105 CH3 þ OH<¼>CH2(s) þ H2O promote temperature rise, but the pressure beyond 2 atm the two reactions suppress the temperature rise. However, The reactions R98 CH3þH(þM)<¼>CH4(þM) and R268 C2H3þH<¼>C2H2þH2 promote temperature rise with the pressure beyond 2 atm. In Fig. 5(c), R1 H þ O2<¼>O þ OH has the greatest impact on rising temperature and promoting heat release. The reaction that has the greatest influence on endothermic is R8 H2O þ M<¼>H þ OH þ M. With the increase of initial temperature, the sensitivity of the 10 reactions affecting the flame temperature first increases and then decreases. This could be attributed to that less decomposition of n-heptane at room temperature, with the increasing temperature, the heat absorption in the flame is strengthened, and the temperature of the transition point is between 298 K and 353 K. In addition, it was observed from the key reactions, the free radicals H, OH, O and CH3 are those substances promoting and inhibiting the heat release of flame [14,18,20,25,29] in the oxidation combustion of n-heptane. R1 H þ O2<¼>O þ OH, R2 O þ H2<¼>H þ OH and R24 CO þ OH<¼>CO2 þ H are important reactions [20,25,31] to affect LBV of n-heptane. CH3 promotes the increase of flame temperature when participating in the reaction R105 CH3þOH<¼>CH2(S) þH2O, while in the reaction R98 CH3þH (þM) <¼>CH4 (þM), the combination of the free radical H inhibits the propagation of the branched chain reaction [33]. Fig. 6 plots the net HRR under different pressures, temperatures and equivalence ratios. As we can see from Fig. 6(a), with increasing the temperature, the net HRR increases gradually, and the peak value of the net HRR moves upstream, which indicates that the position of the low temperature oxidation reaction occurs in advance as the initial temperature rises. Compared to the results at 398 K, the peak position at 423 K is actually slightly downstream away from the fresh mixture, which is not expected. It is interesting to observe that the net HRR at 353 K initial temperature is most advanced, i.e., it rises earliest compared to that at a higher temperature of 398 K and 423 K, though the peak HRR increases with increasing the initial temperature promoting the initial combustion reaction. As such, the peak value of the net HRR
1826
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 5. The sensitivity coefficients of flame temperature under the different initial pressure and temperature.
appears at a higher temperature, as shown in Fig. 6(b) and (c). With the increase of the equivalent ratio, the peak value of the net HRR increases first and then decreases, which is consistent with the trend of temperature profiles varying with the equivalence ratios, simultaneously, the peak position first moves upstream till about stoichiometry and then moves downstream for richer mixtures. And the net HRR is maximum at rich condition (F ¼ 1.2) for the exothermic recombination of radicals. Similarly, as the pressure increases, the temperature increases. Since the flame temperature varies little with pressure and the initial combustion reaction does not much strengthened, the peak position of HRR just moves slightly downstream. The net HRR can be increased evidently with the pressure increase. According to the reaction kinetics theory, the increase of pressure increases the collision rate of activated molecules and the chemical reaction rate, contributing to the increase of HRR. Moreover, the increase of pressure can increase the density of heat release, so the net heat release under different pressures is one or two orders of magnitude larger than that with varying initial temperatures and equivalent ratios. 3.3. Intermediate products From the analysis of Section 3.2, the free radicals H, OH, O, and CH3 are important species affecting the combustion characteristics of nheptane. Fig. 7 illustrates the H, OH, O, and CH3 mole fraction profiles at different temperatures. From Fig. 7(a), it is seen that the mole
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1827
Fig. 6. Net heat release rate: (a) equivalence ratio 1.0, 1 atm; (b) 353 K, 1 atm; (c) 353 K, equivalence ratio 1.0.
Fig. 7. Mole fraction of OH, H, O and CH3 at different temperatures.
fraction of H and CH3 gradually increases with increasing the equivalent ratio over the considered range, but the mole fraction of O gradually decreases because of the increase of the fuel content. Besides, the mole fraction of OH increases first and then decreases. At the same time, the peak position of the mole fraction of the four radicals moves downstream with the fuel from lean to rich at the initial temperature of 298 K. Under the same equivalence ratio, CH3 appears earlier than H, OH, and O. In addition, when the equivalent ratio is 1.0, the mole
1828
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 8. Mole fraction of C1eC3 at different temperatures.
fraction of H, OH, and O increases rapidly in the position of 0.8e0.9 mm, which reflects the intense consumption of the intermediate products, corresponding to the peak value of the net HRR in Fig. 6(a). In Fig. 7(b), (c), and (d), it is seen that the variation of the mole fraction with the equivalence ratio at three temperatures is consistent with that of 298 K. The difference is that the peak position of the mole fraction of the four radicals moves upstream. At the equivalent ratio 1.0, the mole fraction of H, OH, and O increases rapidly in the position of 0.55 mm, which is in accordance with the peak position of the net HRR
Fig. 9. Mole fraction of small molecule harmful substances at different temperatures.
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1829
shown in Fig. 6(a). In brief, Fig. 7 clearly shows the change trend of the effect of initial temperature on the four free radicals in the flame. At the same equivalent ratio, the peak values of the mole fraction of the four radicals increase with increasing the temperature. N-heptane and other macromolecules of alkanes usually first cleavage to produce a variety of small-molecule hydrocarbons in the process of oxidation. The mole fractions of C1eC3 at different temperatures are plotted in Fig. 8. From Fig. 8(a), it can be seen that the mole fraction of C2H4, C3H6, C2H6 and CH4 increases with the increase of the equivalent ratio at the initial temperature of 298 K, and the peak position moves gradually downstream further away from the fresh mixture inlet. Moreover, the peak position of C2H4, C3H6 and C2H6 appears earlier than that of CH4 under the same equivalence ratio, reflecting the hierarchical reactions of macromolecular oxidation cracking and dehydrogenation. The changes in the mole fraction of C1eC3 at three temperatures are consistent with those with the equivalent ratio at 298 K observed from Fig. 8(b), (c) and (d) that the mole fractions increase with the increasing equivalent ratios. But the peak position of the C1eC3 mole fraction at the three elevated temperatures occurs further upstream than that at 298 K. When the equivalent ratio is 1.0 and the initial temperature is 298 K, the mole fraction of C1eC3 decreases rapidly at the position of 0.8e0.9 mm, and the mole fraction of C1eC3 at the initial temperature of 353 K, 398 K and 423 K drops rapidly in the position of 0.55 mm, which is in accordance with the analysis of Figs. 6(a) and 7(a). The mole fraction profiles of C2H2, CH2O, CH2CO and CH2CHO are given in Fig. 9. Among the four species, C2H2 is an important precursor in the process of PAH production, and the other three are harmful products in the flame combustion process. They are very important for the oxidation combustion process of n-heptane [20,25], such as CH2O participating in the reaction CH2O þ H<¼>HCO þ H2, and the subsequent reactions HCO þ H2O<¼>CO þ H þ H2O and HCO þ H<¼>CO þ H2. From Fig. 9(a), (b), (c), (d), the mole fractions of these four species increase with the increase of temperature, which is consistent with the change of the equivalence ratio with the C1eC3 mentioned above that the mole fractions of the four harmful species increase with the increasing equivalent ratios. 4. Conclusions Freely-propagating laminar premixed n-heptane/air flames were simulated using the PREMIX Code and the LLNL n-heptane mechanism v3.1 under different initial conditions. The changing trends of LBV, AFT, HRR and intermediates of combustion are obtained and discussed. The main conclusions are as follows: (1) The prediction of detailed chemical reaction mechanism Version 3.1 is consistent with the experimental results of n-heptane premixed laminar combustion at different temperatures, pressures and equivalent ratios, but the experimental results are likely to be relatively poor under the conditions of temperature 353 K, pressure 1 atm and 10 atm, thus the mechanism needs to be further optimized. This conjecture requires further experimental and numerical investigations. (2) The effect of initial pressure on AFT is pronounced around the equivalence ratio between about 0.9 and 1.3 and the influence of pressure on temperature is relatively weak in leaner or richer mixtures outside the mentioned range of equivalence ratio. R1 H þ O2<¼>O þ OH is the most effective reaction to increase flame temperature and to promote heat release. R8 H2O þ M<¼>H þ OH þ M is the most sensitive endothermic reaction. With increasing the equivalent ratio, the peak value of the net HRR increases first and then decreases, consistent with the trend of temperature change. Besides, the peak position first moves downstream and then gradually moves pstream, and the peak value of HRR increases exponentially with the increase of pressure. (3) When the equivalent ratio is 1.0 and the initial temperature is 298 K, the mole fractions of H, OH, and O increase rapidly, while the mole fractions of C1eC3 decrease rapidly at the position of the peak HRR. When the initial temperature is elevated to 353 K, 398 K, and 423 K, the mole fractions of the radicals increase rapidly at a further upstream location and again around the peak HRR, reflecting the intense consumption of the intermediate products. Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Grant No. 51676002) and the Project of Support Program for Outstanding Young People in Colleges and Universities (Grant No. gxyqZD201830) for their financial support of this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
S.M. Sarathy, A. Farooq, G.T. Kalghatgi, Recent progress in gasoline surrogate fuels, Prog. Energy Combust. Sci. 65 (2018) 67e108. P. Jia, Y. Ying, M. Luo, et al., Effects of swirling combustion on soot characteristics in 2,5-dimethylfuran/n-heptane diffusion flames, Appl. Therm. Eng. 139 (2018) 11e24. A. Ambekar, A. Chowdhury, An experimental technique for determination of intrinsic burning rate constants of liquid fuels, Appl. Therm. Eng. 135 (2018) 238e245. F. Liu, Y. Hua, H. Wu, et al., Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames, J. Energy Inst. 92 (2019) 38e50. S.Y. Liao, D.M. Jiang, Q. Cheng, Determination of laminar burning velocities for natural gas, Fuel 83 (9) (2004) 1247e1250. J. Wang, Z. Huang, C. Tang, et al., Numerical study of the effect of hydrogen addition on methane-air mixtures combustion, Int. J. Hydrogen Energy 34 (2) (2009) 1084e1096. F. Ren, H. Chu, L. Xiang, W. Han, M. Gu, Effect of hydrogen addition on the laminar premixed combustion characteristics the main components of natural gas, J. Energy Inst. 92 (2019) 1178e1190. L. Xiang, H. Chu, F. Ren, M. Gu, Numerical analysis of the effect of CO2 on combustion characteristics of laminar premixed methane/air flame, J. Energy Inst. 92 (2019) 1487e1501. H. Chu, W. Han, W. Cao, et al., Effect of methane addition to ethylene on the morphology and size distribution of soot in a laminar co-flow diffusion flame, Energy 166 (2019) 392e400. X. Shen, G. Xiu, S. Wu, Experimental study on the explosion characteristics of methane/air mixtures with hydrogen addition, Appl. Therm. Eng. 120 (2017) 741e747. H.J. Curran, P. Gaffuri, W.G. Pitz, et al., A comprehensive modeling study of n-heptane oxidation, Combust. Flame 114 (1/2) (1998) 149e177. S.G. Davis, C.K. Law, Laminar flame speeds and oxidation kinetics of iso-octane-air and n-heptane-air flames, Symp. Combust. 27 (1) (1998) 521e527. Y. Huang, C.J. Sung, J.A. Eng, Laminar flame speeds of primary reference fuels and reformer gas mixtures, Combust. Flame 139 (3) (2004) 239e251. K. Kumar, J.E. Freeh, C.J. Sung, Y. Huang, Laminar flame speeds of preheated iso-octane/O2/N2 and n-heptane/O2/N2 mixtures, J. Propul. Power 23 (2) (2012) 428e436. T. Lu, C.K. Law, Strategies for mechanism reduction for large hydrocarbons: n-heptane, Combust. Flame 154 (1) (2008) 153e163. S. Jerzembeck, N. Peters, P. Pepiot-Desjardins, H. Pitsch, Laminar burning velocities at high pressure for primary reference fuels and gasoline: experimental and numerical investigation, Combust. Flame 156 (2) (2009) 292e301.
1830
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
[17] A.J. Smallbone, W. Liu, C.K. Law, et al., Experimental and modeling study of laminar flame speed and non-premixed counterflow ignition of n-heptane, Proc. Combust. Inst. 32 (1) (2009) 1245e1252. [18] C. Ji, E. Dames, Y.L. Wang, et al., Propagation and extinction of premixed C5-C12 n-alkane flames, Combust. Flame 157 (2) (2010) 277e287. [19] J.P.J.V. Lipzig, E.J.K. Nilsson, L.P.H.D. Goey, et al., Laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures, Fuel 90 (8) (2011) 2773e2781. [20] A.P. Kelley, A.J. Smallbone, D.L. Zhu, et al., Laminar flame speeds of C5 to C8 n-alkanes at elevated pressures: experimental determination, fuel similarity, and stretch sensitivity, Proc. Combust. Inst. 33 (1) (2010) 963e970. [21] L. Sileghem, V.A. Alekseev, J. Vancoillie, K.M. Van Geem, et al., Laminar burning velocity of gasoline and the gasoline surrogate components iso-octane, n-heptane and toluene, Fuel 112 (3) (2013) 355e365. [22] P. Dirrenberger, P.A. Glaude, R. Bounaceur, H. Le. Gall, et al., Laminar burning velocity of gasolines with addition of ethanol, Fuel 115 (1) (2014) 162e169. [23] T. Knorsch, A. Zackel, D. Mamaikin, et al., Comparison of different gasoline alternative fuels in terms of laminar burning velocity at increased gas temperatures and exhaust gas recirculation rates, Energy Fuels 28 (28) (2014) 1446e1452. [24] H.M. Hakka, R.F. Cracknell, A. Pekalski, et al., Experimental and modeling study of ultra-rich oxidation of n-heptane, Fuel 144 (2015) 358e368. [25] G. Li, J. Liang, Z. Zhang, et al., Experimental investigation on laminar burning velocities and markstein lengths of premixed methaneen-heptaneeair mixtures, Energy Fuels 29 (7) (2015) 4549e4556. [26] K. Zhang, C. Banyon, J. Bugler, et al., An updated experimental and kinetic modeling study of n-heptane oxidation, Combust. Flame 172 (2016) 116e135. [27] E. Hu, Z. Gao, Y. Liu, et al., Experimental and modeling study on ignition delay times of dimethoxy methane/n-heptane blends, Fuel 189 (2017) 350e357. [28] X. Shi, J.Y. Chen, Y. Chen, Laminar flame speeds of stratified methane, propane, and n-heptane flames, Combust. Flame 176 (2017) 38e47. [29] Y. Chen, J.Y. Chen, Towards improved automatic chemical kinetic model reduction regarding ignition delays and flame speeds, Combust. Flame 190 (2018) 293e301. [30] M. Mehl, W.J. Pitz, C.K. Westbrook, et al., Kinetic modeling of gasoline surrogate components and mixtures under engine conditions, Proc. Combust. Inst. 33 (1) (2011) 193e200. €berg, et al., Detailed Kinetic Modeling of Low-temperature Heat Release for PRF Fuels in an HCCI Engine//Powertrains, Fuels and Lubricants [31] M. Mehl, W.J. Pitz, M. Sjo Meeting, 2009, pp. 318e323. [32] R.J. Kee, J.F. Grcar, M.D. Smooke, PREMIX: a FORTRAN program for modeling steady laminar one-dimensional premixed flames, Sandia Report SAND85e8240, Sandia Natl. Labs 143 (5) (1985) 65. Modification of PREMIX version 2.5, 1991. [33] E. Ranzi, A. Frassoldati, R. Grana, A. Cuoci, et al., Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels, Prog. Energy Combust. Sci. 38 (4) (2012) 468e501.
Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Numerical investigation on combustion characteristics of laminar premixed n-heptane/air flames at elevated initial temperature and pressure Huaqiang Chu a, Fei Ren a, Longkai Xiang a, Shilin Dong a, Fen Qiao b, *, Guangju Xu c, ** a b c
School of Energy and Environment, Anhui University of Technology, Ma'anshan 243002, China School of Energy & Power Engineering, Jiangsu University, Zhenjiang 212013, China Department of Automobile Engineering, Changshu Institute of Technology, Changshu 215500, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2018 Received in revised form 26 November 2018 Accepted 26 November 2018 Available online 1 December 2018
Freely-propagating laminar premixed n-heptane/air flames were modeled using the Lawrence Livermore National Laboratory (LLNL) v3.1 n-heptane mechanism and the PREMIX code. Numerical calculations were conducted for unburned mixture temperature range of 298e423 K, at elevated pressures 1e10 atm and equivalent ratio 0.6e1.6, and the changes of laminar burning velocity (LBV), adiabatic flame temperature (AFT), heat release rate (HRR), and concentration profiles of important intermediate species were obtained. The results show that the overall results of LBVs of n-heptane at different elevated temperatures, pressures, and equivalence ratios are in good agreement with available experimental results. However, at the initial temperature 353 K, the calculated values of LBVs at pressure 1 atm and the 10 atm deviate significantly from the experimental results. The sensitivity analysis shows that, similar to many other hydrocarbon fuels, the most sensitive reaction in the oxidation of n-heptane responsible for the rise of flame temperature promoting heat release is R1 H þ O2<¼>O þ OH, and the reaction that has the greatest influence on heat release is R8 H2O þ M<¼>H þ OH þ M. In addition, when the initial temperature is 353, 398 and 423 K, the mole fractions of H, OH, and O increase rapidly around the flame front, while the mole fractions of C1eC3 dramatically decreases, reflecting the intense consumption of the intermediate products at the reaction zone. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: N-heptane LBV Adiabatic flame temperature Heat release rate
1. Introduction Currently, about 20% energy in the world is consumed for transportation, of which 95% liquid fuel comes from petroleum refining [1e4]. With the increase of prosperous population and environmental pressure, energy shortage is becoming increasingly prominent. While it is important to search for alternative fuels, it is also necessary to improve the design of combustion equipment to achieve high thermal efficiency and reduce pollutant emissions at the same time. Natural gas has been considered as the most potentially clean energy and engine alternative fuel because of its high octane number, large reserves, low price and low emission [5e10]. However, since its main component is methane, its spontaneous combustion temperature is high, the flame speed is low, and the combustion capacity is poor at low load, which lead to incomplete combustion and high misfire rate in lean burn [6e9]. In addition, compared with liquid fuels, vehicles fueled with natural gas, suffer a shorter mileage and high likelihood of corrosion of the combustion chamber components. Even if natural gas combustion is environmentally friendly, in many cases, it is not advisable to use natural gas as fuel, such as for long distance transportation, high-power output, and emergency fuel reserve. Consequently, it is important to continue to investigate the combustion characteristics of high energy-density liquid hydrocarbon gasoline alternative fuels. To understand the combustion characteristics of complex gasoline, which consists of tens of different hydrocarbon components, it is
* Corresponding author. Jiangsu University, Zhenjiang 212013, China. ** Corresponding author. Changshu Institute of Technology, Changshu 215500, China. E-mail addresses: [email protected] (F. Qiao), [email protected] (G. Xu). https://doi.org/10.1016/j.joei.2018.11.010 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
1822
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
necessary to develop gasoline surrogate fuel models. The earliest model is the primary reference fuel (PRF) containing n-heptane and isooctane, followed by more recently proposed toluene added to PRF to form a toluene reference fuel (TRF) and ethanol toluene reference fuel (ERF). In order to design clean and efficient gasoline engines, it is vital to gain insights into the basic combustion characteristics of major gasoline components. N-heptane was selected in this study as a single component alternative fuel to study the LBV and other flame properties of freely-propagating n-heptane/air mixtures under different conditions. The reasons to study n-heptane are mainly because it is a main component of gasoline surrogate to represent straight-chain paraffin with an assigned octane number of zero and it has a cetane number of 56, which is close to diesel, and has often been used to approximately simulate diesel combustion. Moreover, as a major component of gasoline surrogate fuels, n-heptane has received considerable research attention and extensive studies have been conducted to investigate its reaction mechanism and combustion characteristics experimentally and numerically. To save computing time, obtain fast and accurate results, and to facilitate multidimensional numerical simulation and mechanism validation, since Curran et al. [11] developed a detailed chemical reaction mechanism of n-heptane containing 544 components and 2446 elemental reactions, many related studies have been conducted on the laminar combustion characteristics of n-heptane, the simplification of the mechanism and the mechanism of soot formation during the combustion process. The laminar flame velocity of iso-octane/air and nheptane/air mixtures at room temperature and atmospheric pressure has been measured using the counterflow twin flame method by Davis and Law with both linear and nonlinear extrapolations [12]. Huang et al. [13] experimentally and numerically studied the LBV of PRF, nheptane, and iso-octane by means of the counterflow flame technique. It was found that the increase of iso-octane content in PRFs and reformer gas reduced the LBV, but the addition of reformer gas increased the LBVs of iso-octane/air mixtures. Kumar et al. [14] employed the Digital Particle Image Velocimetry (DPIV) technique in counterflow flames of premixed iso-octane/air and n-heptane/air mixtures to measure the LBVs over a wide range of equivalence ratios and at different unburned mixture temperatures. A 55 species and 51 global steps reduced mechanism for n-heptane oxidation has been obtained by Lu and Law [15] using the directed relation graph (DRG) and computational singular perturbation (CSP) methods. Jerzembeck et al. [16] experimentally determined the LBVs and Markstein numbers for nheptane, iso-octane, PRF87, and gasoline/air mixtures at engine-relevant conditions in constant volume bomb and validated a reduced kinetic mechanism derived from the LLNL detailed n-heptane and iso-octane mechanism in propagating flames. Using the counterflow twin-flame technique, Smallbone et al. [17] determined the LBVs for n-heptane/oxygen/nitrogen mixtures at various initial pressures and equivalence ratios and validated a 130 species and 955 reactions detailed kinetic model of n-heptane oxidation. Ji et al. [18] carried out experimental and numerical studies of the LBVs and extinction strain rates of premixed C5eC12 n-alkane flames in the counterflow configuration and conducted a comparison of the LBVs obtained by linear and non-linear extrapolations. Lipzig et al. [19] measured the LBVs of the adiabatic flames of n-heptane, iso-octane, ethanol, and their binary and tertiary mixtures on a perforated plate burner using the heat flux method. Kelley et al. [20] used the constant volume bomb to determine the LBVs of C5eC8 n-alkane and the Markstein lengths under different pressures and analyzed the flame structure similarities of alkane fuels and the influence of pressure on stretch and extrapolation. Sileghem et al. [21] conducted measurements of LBV for gasoline, iso-octane, n-heptane, toluene, and TRF on a flat flame adiabatic burner with the heat flux method. Dirrenberger et al. [22] conducted experiments through the heat flux method on a perforated plate burner to investigate the adiabatic LBVs of the four pure components of the model fuel, and a commercial gasoline and TRF at almost the same octane number, as well as compared LBV of TRF and TRF/ERF mixtures with that of the commercial gasoline by numerical simulation. In addition, the LBVs of iso-octane and n-heptane compared to three key biofuels n-butanol, iso-butanol and ethanol have been measured with the heat flux burner by Knorsch et al. [23] and the influence of exhaust gas recirculation (EGR) rates on the LBVs was investigated. Hakka et al. [24] performed an experiment on the ultra-rich oxidation of n-heptane with a jet-stirred reactor (JSR) and developed a detailed mechanism using computer-aided generation (EXGAS). A combustion chamber has been designed by Li et al. [25] to experimentally study the LBV and flame instability of methane/n-heptane/air mixture at different initial temperatures. Their results showed that when the methane content was over 0.75 in the fuel mixture, the LBV and Markstein length of methane/n-heptane/air mixture changed significantly. Using two different high-pressure shock tubes, Zhang et al. [26] measured the ignition delay times of stoichiometric n-heptane/air mixtures and verified the kinetic model of n-heptane oxidation under different temperature and pressure conditions. The numerical results are in good agreement with the experimental results. Hu et al. [27] measured the ignition delay times of dimethoxy methane, n-heptane, and their mixtures with the reflected shock waves. A chemical kinetic model for the blends was built and validated with the experimental results and the dimethoxy methane addition was found to decrease the ignition delay time of n-heptane. Shi et al. [28] numerically studied the stratified flames and homogeneous flames of methane, propane, and n-heptane. The LBV and propagation speed of the three fuels and the difference between them were compared and analyzed. Recently, Chen et al. [29] used DRGEP and TSA approaches to simplify the oxidation mechanisms of methane, propane, n-heptane and PRF. After comparison and validation, it was found that the newly proposed reduction methodology for developing accurate skeletal models that could take into account both ignition delay time and LBV. Although various simplified mechanisms for n-heptane oxidation have been proposed in the last two decades, only few can be considered reliable and accurate at specific conditions. It is worth pointing out that in the studies mentioned above the oxidation reaction mechanism of n-heptane developed by Curran et al. [11] is the most widely accepted reaction mechanism [26], which is often used for comparison, simplified mechanism, and new models. Based on the maturation mechanism, Mehl et al. [30,31] developed the Lawrence Livermore National Laboratory (LLNL) v3.0 and v3.1 nheptane mechanisms. The latter is modified and optimized on the basis of the previous v3.0 and has been verified in the experiments of shock tubes and rapid compression machines. LLNL v3.1 n-heptane mechanism contains 654 components and 2827 elementary reactions. In a series of experimental and numerical studies, the mechanism is found applicable to the initial pressure of 3e50 atm, the temperature of 650e1200 K, and the equivalence ratio of 0.3e1.0. The mechanism performs well at both low and high temperatures, as well as wide pressure ranges relevant to internal combustion engines. However, little attention has been paid to the prediction of combustion characteristics of n-heptane in premixed freely-propagating flame. In addition, to further understand the prediction accuracy of the LLNL v3.1 mechanism for n-heptane at fuel rich, low temperature, and near atmospheric pressure conditions, the detailed LLNL v.31 of n-heptane oxidation is used in this study to predict the combustion characteristics of premixed freely-propagating flames of n-heptane/air mixtures for different equivalence ratios, initial temperatures, and pressures. Therefore, by changing the initial conditions, the LBV, AFT, temperature sensitivity, HRR and the change of intermediate
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1823
components in combustion were studied. A comparison is carried out between simulation and available experimental results, and the sensitivity analysis is chosen to identify the most sensitive chemical reactions that affect the temperature rise in n-heptane/air mixtures combustion. 2. Computational model and condition The one-dimensional premixed laminar freely-propagating flames of n-heptane under different initial conditions were simulated using the PREMIX code [32]. Fig. 1 describes a schematic of the flame front structure of the premixed laminar flame and Table 1 lists the numerical simulation cases. In the calculation process, the n-heptane/air mixture flow rate is an eigenvalue of the problem and its initial value was assigned to be 0.04 g/(cm2.s). Using adaptive grid with GRAD ¼ 0.04 and CURV ¼ 0.04, the grid number upon convergence was in the range of 500e800, depending on the mixture properties (equivalence ratio, temperature, and pressure). The relative and absolute errors in the iterative process were set to 104 and 109, respectively. The left boundary of computational domain was at 0.2 cm and the right boundary location was set to between 6 and 10 cm to achieve zero gradients for all variables, which satisfied the calculation requirements. In addition, the fixed temperature was set at 500 K in the position of 0.5 mm. The windward difference was used to discretize the convection term and the average component method was used to calculate the diffusion coefficient of mixture. The Soret effect was considered in all the calculations. 3. Results and discussion 3.1. Laminar burning velocity LBV, an inherent characteristic of hydrocarbon fuels, influenced by the type of fuel and the initial combustion environment, is an important parameter for the propagation and stability of laminar flame, and it is also a target parameter for chemical reaction mechanism development and validation. Shown in Fig. 2 is the simulation results of LBVs compared with the experimental results in Refs. [12e14,17e23,25] at various equivalence ratios and initial temperatures, 298 K, 353 K, 398 K and 423 K, and 1 atm. From Fig. 2(a), it can be seen that at the initial temperature of 298 K, the simulation results are in good agreement with the experimental data of Huang et al. [13] obtained using counterflow flame and Dirrenberger et al. [22] and Sileghem et al. [21] by the heat flux method. In Fig. 2(b), at a temperature of 353 K, the simulation results are generally consistent with the experiments, especially the results of LBV obtained by the Li et al. [25] using a constant volume bomb. In addition, it is found that the overall agreement between the prediction and experiments is better in fuel-lean and -rich mixtures than those around the stoichiometry between about 0.9 and 1.3. In Fig. 2 (c), the simulation results are in good agreement with the experimental results of Dirrenberger et al. [22] and Li et al. [25], but are lower and higher than those Knorsch et al. and Kumar et al., respectively. With respect to 423 K, in Fig. 2 (d), the simulated values are in line with the trend of LBVs of both Knorsch et al. [23] and Li et al. [25], but in better overall agreement with those of Li et al. [25]. The overall observation from the results shown in Fig. 2 is that the peak value of LBV always occurs at a slightly fuel-rich side near the equivalence ratio of 1.1, regardless of the initial temperatures. By comparing the simulation and experimental results in Fig. 2, it can be found that the prediction of the LBVs of n-heptane appears to be inconsistent in experimental data over the range of initial temperature considered. In order to further and more intuitively analyze the numerical results of LBVs of n-heptane, the LBVs are compared with the experimental results of Kelley and Dirrenberger et al. [20,22] in terms of consistency of operating conditions for different pressures and temperatures in Fig. 3. From Fig. 3(a) and (b), it is seen that with increasing the pressure, the LBV gradually decreases, and as the pressure continues to increase, the effect of pressure on LBV is gradually weakened. The data in Fig. 3(b) demonstrate that at 353 K, compared with the results of Ref. [20], at 1 atm, the results on fuel-lean side are basically compatible with the experimental values, but the simulation values tend to be higher than the experimental values on fuel-rich side. Moreover, at 2 atm, the simulation agrees well with the experimental points, which is consistent with that of Zhang et al. [26]. At higher pressures of 5 and 10 atm, however, the simulated values are lower than the experiments, and the difference tends to increase with increasing the equivalence ratio. Furthermore, it is clear from Fig. 3(c) that the predicted LBV of n-heptane at temperature 298 and 398 K is greater than the experiments. When the temperature is at 353 K, the predicted value is slightly lower than the experimental points. According to Figs. 2 and 3, the LLNL v3.1 presents a good agreement with the experimental results. However, under the conditions of 353 K, 1 atm and 10 atm, further optimization for the mechanism should be carried out. And the conjecture requires further experimental and numerical investigations.
Fig. 1. Schematic of flame front structure of premixed laminar flame.
Table 1 Simulation cases. Simulated condition
Cases
Pressure
Temperature
Mole fraction of reactant
P (atm)
T (K)
O2 N2 n-C7H16
1e10
298e423
0.2037-0.2080 0.7666-0.7825 0.0095-0.0297
1824
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 2. LBVs of n-heptane at different initial temperatures and 1 atm.
Fig. 3. LBVs under different pressures and temperatures: solid lines:simulation, (a) data points: Ref. [22], (b) data points: Ref. [20], (c) data points: Ref. [22].
3.2. Adiabatic flame temperature, sensitivity analysis and net heat release rate The adiabatic flame temperatures at different initial pressures and temperatures are plotted in Fig. 4. From Fig. 4(a) and (b), it can be seen that the pressure effect on the AFT is concentrated near the equivalent ratio of 1, and the effect of pressure on the temperature is gradually decreased in the fuel-lean and -rich combustion. As the pressure increases, the AFT increases gradually. When the equivalent ratio is 1 and the pressure rises from 1 to 10 atm, the AFT rises from 2283 K to 2337 K at the initial temperature of 298 K, increased by 54 K. At the initial temperature 353 K, the AFT is changed from 2308 K to 2367 K, increased by 59 K. In Fig. 4(c), the AFT increases with increasing the initial temperature. Moreover, the initial temperature has a great influence on the AFT in the fuel-rich zone. At the equivalent ratio 1.0, the AFT rises
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1825
Fig. 4. Adiabatic flame temperatures at different pressures and temperatures.
from 2283 K to 2342 K with the initial temperature from 298 K to 423 K, increased by 59 K. While with increasing the initial temperature from 298 K to 423 K, the AFT increases from 2210 K to 2291 K at the equivalent ratio of 1.2, increased by 81 K. Fig. 5 shows the sensitivity coefficients of flame temperature under different initial temperatures and pressures at the maximum temperature gradient when the equivalence ratio is 1.0. From Fig. 5(a), (b) and (c), it can be concluded that the sensitivity of the key reactions to flame temperature is obviously influenced by the initial pressure. In the stoichiometric n-heptane/air flame with different initial pressures, the most sensitive reactions are those of C0eC2 chemistry, and the reaction R1 H þ O2<¼>H þ OH always has the largest absolute value of the sensitivity coefficient, which means that this reaction is the most sensitive to the flame temperature. The results obtained in the study of n-heptane propagation flame are consistent with Refs. [14,18,27], i.e., under the high temperature condition of the flame, the initial decomposition reaction rate of n-alkanes is too fast to play the role of rapid control step. Therefore, these reactions are so sensitive to the flame propagation. From Fig. 5(a), it can be observed that the sensitivity coefficients of R1 H þ O2<¼>O þ OH and R26 HCO þ M<¼>H þ CO þ M increase with the elevated pressure at the initial temperature of 298 K. The forward reactions R2 O þ H2<¼>H þ OH and R24 CO þ OH<¼>CO2 þ H promote temperature rise, but the pressure beyond 2 atm the two reactions suppress the temperature rise which is contrary to R9 H þ O2(þM)<¼>HO2(þM). The effects of reactions R11 HO2 þ H<¼>2OH and R268 C2H3 þ H<¼>C2H2 þ H2 on the temperature change from depression to promotion when the pressure are beyond 1 atm and 5 atm, respectively. Furthermore, it should be noted that, as shown Fig. 5(b), the absolute values of sensitivity coefficients for promoting reaction R1 H þ O2<¼>O þ OH and depressing reactions R8 H2O þ M<¼>H þ OH and R9 H þ O2(þM)<¼>HO2(þM) to temperature rise are increased with the elevated pressure 1e5 atm then decrease as the pressure continues to increase, at the initial temperature of 353 K. The forward reactions R24 CO þ OH<¼>CO2 þ H and R105 CH3 þ OH<¼>CH2(s) þ H2O promote temperature rise, but the pressure beyond 2 atm the two reactions suppress the temperature rise. However, The reactions R98 CH3þH(þM)<¼>CH4(þM) and R268 C2H3þH<¼>C2H2þH2 promote temperature rise with the pressure beyond 2 atm. In Fig. 5(c), R1 H þ O2<¼>O þ OH has the greatest impact on rising temperature and promoting heat release. The reaction that has the greatest influence on endothermic is R8 H2O þ M<¼>H þ OH þ M. With the increase of initial temperature, the sensitivity of the 10 reactions affecting the flame temperature first increases and then decreases. This could be attributed to that less decomposition of n-heptane at room temperature, with the increasing temperature, the heat absorption in the flame is strengthened, and the temperature of the transition point is between 298 K and 353 K. In addition, it was observed from the key reactions, the free radicals H, OH, O and CH3 are those substances promoting and inhibiting the heat release of flame [14,18,20,25,29] in the oxidation combustion of n-heptane. R1 H þ O2<¼>O þ OH, R2 O þ H2<¼>H þ OH and R24 CO þ OH<¼>CO2 þ H are important reactions [20,25,31] to affect LBV of n-heptane. CH3 promotes the increase of flame temperature when participating in the reaction R105 CH3þOH<¼>CH2(S) þH2O, while in the reaction R98 CH3þH (þM) <¼>CH4 (þM), the combination of the free radical H inhibits the propagation of the branched chain reaction [33]. Fig. 6 plots the net HRR under different pressures, temperatures and equivalence ratios. As we can see from Fig. 6(a), with increasing the temperature, the net HRR increases gradually, and the peak value of the net HRR moves upstream, which indicates that the position of the low temperature oxidation reaction occurs in advance as the initial temperature rises. Compared to the results at 398 K, the peak position at 423 K is actually slightly downstream away from the fresh mixture, which is not expected. It is interesting to observe that the net HRR at 353 K initial temperature is most advanced, i.e., it rises earliest compared to that at a higher temperature of 398 K and 423 K, though the peak HRR increases with increasing the initial temperature promoting the initial combustion reaction. As such, the peak value of the net HRR
1826
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 5. The sensitivity coefficients of flame temperature under the different initial pressure and temperature.
appears at a higher temperature, as shown in Fig. 6(b) and (c). With the increase of the equivalent ratio, the peak value of the net HRR increases first and then decreases, which is consistent with the trend of temperature profiles varying with the equivalence ratios, simultaneously, the peak position first moves upstream till about stoichiometry and then moves downstream for richer mixtures. And the net HRR is maximum at rich condition (F ¼ 1.2) for the exothermic recombination of radicals. Similarly, as the pressure increases, the temperature increases. Since the flame temperature varies little with pressure and the initial combustion reaction does not much strengthened, the peak position of HRR just moves slightly downstream. The net HRR can be increased evidently with the pressure increase. According to the reaction kinetics theory, the increase of pressure increases the collision rate of activated molecules and the chemical reaction rate, contributing to the increase of HRR. Moreover, the increase of pressure can increase the density of heat release, so the net heat release under different pressures is one or two orders of magnitude larger than that with varying initial temperatures and equivalent ratios. 3.3. Intermediate products From the analysis of Section 3.2, the free radicals H, OH, O, and CH3 are important species affecting the combustion characteristics of nheptane. Fig. 7 illustrates the H, OH, O, and CH3 mole fraction profiles at different temperatures. From Fig. 7(a), it is seen that the mole
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1827
Fig. 6. Net heat release rate: (a) equivalence ratio 1.0, 1 atm; (b) 353 K, 1 atm; (c) 353 K, equivalence ratio 1.0.
Fig. 7. Mole fraction of OH, H, O and CH3 at different temperatures.
fraction of H and CH3 gradually increases with increasing the equivalent ratio over the considered range, but the mole fraction of O gradually decreases because of the increase of the fuel content. Besides, the mole fraction of OH increases first and then decreases. At the same time, the peak position of the mole fraction of the four radicals moves downstream with the fuel from lean to rich at the initial temperature of 298 K. Under the same equivalence ratio, CH3 appears earlier than H, OH, and O. In addition, when the equivalent ratio is 1.0, the mole
1828
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
Fig. 8. Mole fraction of C1eC3 at different temperatures.
fraction of H, OH, and O increases rapidly in the position of 0.8e0.9 mm, which reflects the intense consumption of the intermediate products, corresponding to the peak value of the net HRR in Fig. 6(a). In Fig. 7(b), (c), and (d), it is seen that the variation of the mole fraction with the equivalence ratio at three temperatures is consistent with that of 298 K. The difference is that the peak position of the mole fraction of the four radicals moves upstream. At the equivalent ratio 1.0, the mole fraction of H, OH, and O increases rapidly in the position of 0.55 mm, which is in accordance with the peak position of the net HRR
Fig. 9. Mole fraction of small molecule harmful substances at different temperatures.
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
1829
shown in Fig. 6(a). In brief, Fig. 7 clearly shows the change trend of the effect of initial temperature on the four free radicals in the flame. At the same equivalent ratio, the peak values of the mole fraction of the four radicals increase with increasing the temperature. N-heptane and other macromolecules of alkanes usually first cleavage to produce a variety of small-molecule hydrocarbons in the process of oxidation. The mole fractions of C1eC3 at different temperatures are plotted in Fig. 8. From Fig. 8(a), it can be seen that the mole fraction of C2H4, C3H6, C2H6 and CH4 increases with the increase of the equivalent ratio at the initial temperature of 298 K, and the peak position moves gradually downstream further away from the fresh mixture inlet. Moreover, the peak position of C2H4, C3H6 and C2H6 appears earlier than that of CH4 under the same equivalence ratio, reflecting the hierarchical reactions of macromolecular oxidation cracking and dehydrogenation. The changes in the mole fraction of C1eC3 at three temperatures are consistent with those with the equivalent ratio at 298 K observed from Fig. 8(b), (c) and (d) that the mole fractions increase with the increasing equivalent ratios. But the peak position of the C1eC3 mole fraction at the three elevated temperatures occurs further upstream than that at 298 K. When the equivalent ratio is 1.0 and the initial temperature is 298 K, the mole fraction of C1eC3 decreases rapidly at the position of 0.8e0.9 mm, and the mole fraction of C1eC3 at the initial temperature of 353 K, 398 K and 423 K drops rapidly in the position of 0.55 mm, which is in accordance with the analysis of Figs. 6(a) and 7(a). The mole fraction profiles of C2H2, CH2O, CH2CO and CH2CHO are given in Fig. 9. Among the four species, C2H2 is an important precursor in the process of PAH production, and the other three are harmful products in the flame combustion process. They are very important for the oxidation combustion process of n-heptane [20,25], such as CH2O participating in the reaction CH2O þ H<¼>HCO þ H2, and the subsequent reactions HCO þ H2O<¼>CO þ H þ H2O and HCO þ H<¼>CO þ H2. From Fig. 9(a), (b), (c), (d), the mole fractions of these four species increase with the increase of temperature, which is consistent with the change of the equivalence ratio with the C1eC3 mentioned above that the mole fractions of the four harmful species increase with the increasing equivalent ratios. 4. Conclusions Freely-propagating laminar premixed n-heptane/air flames were simulated using the PREMIX Code and the LLNL n-heptane mechanism v3.1 under different initial conditions. The changing trends of LBV, AFT, HRR and intermediates of combustion are obtained and discussed. The main conclusions are as follows: (1) The prediction of detailed chemical reaction mechanism Version 3.1 is consistent with the experimental results of n-heptane premixed laminar combustion at different temperatures, pressures and equivalent ratios, but the experimental results are likely to be relatively poor under the conditions of temperature 353 K, pressure 1 atm and 10 atm, thus the mechanism needs to be further optimized. This conjecture requires further experimental and numerical investigations. (2) The effect of initial pressure on AFT is pronounced around the equivalence ratio between about 0.9 and 1.3 and the influence of pressure on temperature is relatively weak in leaner or richer mixtures outside the mentioned range of equivalence ratio. R1 H þ O2<¼>O þ OH is the most effective reaction to increase flame temperature and to promote heat release. R8 H2O þ M<¼>H þ OH þ M is the most sensitive endothermic reaction. With increasing the equivalent ratio, the peak value of the net HRR increases first and then decreases, consistent with the trend of temperature change. Besides, the peak position first moves downstream and then gradually moves pstream, and the peak value of HRR increases exponentially with the increase of pressure. (3) When the equivalent ratio is 1.0 and the initial temperature is 298 K, the mole fractions of H, OH, and O increase rapidly, while the mole fractions of C1eC3 decrease rapidly at the position of the peak HRR. When the initial temperature is elevated to 353 K, 398 K, and 423 K, the mole fractions of the radicals increase rapidly at a further upstream location and again around the peak HRR, reflecting the intense consumption of the intermediate products. Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Grant No. 51676002) and the Project of Support Program for Outstanding Young People in Colleges and Universities (Grant No. gxyqZD201830) for their financial support of this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
S.M. Sarathy, A. Farooq, G.T. Kalghatgi, Recent progress in gasoline surrogate fuels, Prog. Energy Combust. Sci. 65 (2018) 67e108. P. Jia, Y. Ying, M. Luo, et al., Effects of swirling combustion on soot characteristics in 2,5-dimethylfuran/n-heptane diffusion flames, Appl. Therm. Eng. 139 (2018) 11e24. A. Ambekar, A. Chowdhury, An experimental technique for determination of intrinsic burning rate constants of liquid fuels, Appl. Therm. Eng. 135 (2018) 238e245. F. Liu, Y. Hua, H. Wu, et al., Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames, J. Energy Inst. 92 (2019) 38e50. S.Y. Liao, D.M. Jiang, Q. Cheng, Determination of laminar burning velocities for natural gas, Fuel 83 (9) (2004) 1247e1250. J. Wang, Z. Huang, C. Tang, et al., Numerical study of the effect of hydrogen addition on methane-air mixtures combustion, Int. J. Hydrogen Energy 34 (2) (2009) 1084e1096. F. Ren, H. Chu, L. Xiang, W. Han, M. Gu, Effect of hydrogen addition on the laminar premixed combustion characteristics the main components of natural gas, J. Energy Inst. 92 (2019) 1178e1190. L. Xiang, H. Chu, F. Ren, M. Gu, Numerical analysis of the effect of CO2 on combustion characteristics of laminar premixed methane/air flame, J. Energy Inst. 92 (2019) 1487e1501. H. Chu, W. Han, W. Cao, et al., Effect of methane addition to ethylene on the morphology and size distribution of soot in a laminar co-flow diffusion flame, Energy 166 (2019) 392e400. X. Shen, G. Xiu, S. Wu, Experimental study on the explosion characteristics of methane/air mixtures with hydrogen addition, Appl. Therm. Eng. 120 (2017) 741e747. H.J. Curran, P. Gaffuri, W.G. Pitz, et al., A comprehensive modeling study of n-heptane oxidation, Combust. Flame 114 (1/2) (1998) 149e177. S.G. Davis, C.K. Law, Laminar flame speeds and oxidation kinetics of iso-octane-air and n-heptane-air flames, Symp. Combust. 27 (1) (1998) 521e527. Y. Huang, C.J. Sung, J.A. Eng, Laminar flame speeds of primary reference fuels and reformer gas mixtures, Combust. Flame 139 (3) (2004) 239e251. K. Kumar, J.E. Freeh, C.J. Sung, Y. Huang, Laminar flame speeds of preheated iso-octane/O2/N2 and n-heptane/O2/N2 mixtures, J. Propul. Power 23 (2) (2012) 428e436. T. Lu, C.K. Law, Strategies for mechanism reduction for large hydrocarbons: n-heptane, Combust. Flame 154 (1) (2008) 153e163. S. Jerzembeck, N. Peters, P. Pepiot-Desjardins, H. Pitsch, Laminar burning velocities at high pressure for primary reference fuels and gasoline: experimental and numerical investigation, Combust. Flame 156 (2) (2009) 292e301.
1830
H. Chu et al. / Journal of the Energy Institute 92 (2019) 1821e1830
[17] A.J. Smallbone, W. Liu, C.K. Law, et al., Experimental and modeling study of laminar flame speed and non-premixed counterflow ignition of n-heptane, Proc. Combust. Inst. 32 (1) (2009) 1245e1252. [18] C. Ji, E. Dames, Y.L. Wang, et al., Propagation and extinction of premixed C5-C12 n-alkane flames, Combust. Flame 157 (2) (2010) 277e287. [19] J.P.J.V. Lipzig, E.J.K. Nilsson, L.P.H.D. Goey, et al., Laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures, Fuel 90 (8) (2011) 2773e2781. [20] A.P. Kelley, A.J. Smallbone, D.L. Zhu, et al., Laminar flame speeds of C5 to C8 n-alkanes at elevated pressures: experimental determination, fuel similarity, and stretch sensitivity, Proc. Combust. Inst. 33 (1) (2010) 963e970. [21] L. Sileghem, V.A. Alekseev, J. Vancoillie, K.M. Van Geem, et al., Laminar burning velocity of gasoline and the gasoline surrogate components iso-octane, n-heptane and toluene, Fuel 112 (3) (2013) 355e365. [22] P. Dirrenberger, P.A. Glaude, R. Bounaceur, H. Le. Gall, et al., Laminar burning velocity of gasolines with addition of ethanol, Fuel 115 (1) (2014) 162e169. [23] T. Knorsch, A. Zackel, D. Mamaikin, et al., Comparison of different gasoline alternative fuels in terms of laminar burning velocity at increased gas temperatures and exhaust gas recirculation rates, Energy Fuels 28 (28) (2014) 1446e1452. [24] H.M. Hakka, R.F. Cracknell, A. Pekalski, et al., Experimental and modeling study of ultra-rich oxidation of n-heptane, Fuel 144 (2015) 358e368. [25] G. Li, J. Liang, Z. Zhang, et al., Experimental investigation on laminar burning velocities and markstein lengths of premixed methaneen-heptaneeair mixtures, Energy Fuels 29 (7) (2015) 4549e4556. [26] K. Zhang, C. Banyon, J. Bugler, et al., An updated experimental and kinetic modeling study of n-heptane oxidation, Combust. Flame 172 (2016) 116e135. [27] E. Hu, Z. Gao, Y. Liu, et al., Experimental and modeling study on ignition delay times of dimethoxy methane/n-heptane blends, Fuel 189 (2017) 350e357. [28] X. Shi, J.Y. Chen, Y. Chen, Laminar flame speeds of stratified methane, propane, and n-heptane flames, Combust. Flame 176 (2017) 38e47. [29] Y. Chen, J.Y. Chen, Towards improved automatic chemical kinetic model reduction regarding ignition delays and flame speeds, Combust. Flame 190 (2018) 293e301. [30] M. Mehl, W.J. Pitz, C.K. Westbrook, et al., Kinetic modeling of gasoline surrogate components and mixtures under engine conditions, Proc. Combust. Inst. 33 (1) (2011) 193e200. €berg, et al., Detailed Kinetic Modeling of Low-temperature Heat Release for PRF Fuels in an HCCI Engine//Powertrains, Fuels and Lubricants [31] M. Mehl, W.J. Pitz, M. Sjo Meeting, 2009, pp. 318e323. [32] R.J. Kee, J.F. Grcar, M.D. Smooke, PREMIX: a FORTRAN program for modeling steady laminar one-dimensional premixed flames, Sandia Report SAND85e8240, Sandia Natl. Labs 143 (5) (1985) 65. Modification of PREMIX version 2.5, 1991. [33] E. Ranzi, A. Frassoldati, R. Grana, A. Cuoci, et al., Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels, Prog. Energy Combust. Sci. 38 (4) (2012) 468e501.