Experimental and Kinetic Modeling Study of Autoignition

Article Cite This: Energy Fuels 2017, 31, 13610−13626

pubs.acs.org/EF

Experimental and Kinetic Modeling Study of Autoignition Characteristics of n‑Heptane/Ethanol by Constant Volume Bomb and Detail Reaction Mechanism Runzhao Li, Zhongchang Liu, Yongqiang Han,* Manzhi Tan, Yun Xu, Jing Tian, Jiayao Yan, Jiahong Chai, Jiahui Liu, and Xiangfeng Yu State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China ABSTRACT: The purpose of this work is to investigate the autoignition characteristics of n-heptane/ethanol including the autoignition temperature (AIT), ignition delay time (τig) and the low/intermediate/high-temperature ignition. The AIT is measured by the combustion bomb under wide range conditions (φ = 0.2−2.0, Pinit = 0.772−3.861 MPa) while the influencing factors of τig and the dominant reactions of the high/intermediate/low-temperature ignition are investigated by detail chemical kinetic model. The results indicate that first, the maximum AITs difference is 46, 56, 62, 84, and 124 °C when the ethanol blending ratio is 0%, 25%, 50%, 75%, and 100% while the minimum AITs usually locate at the region of φ = 1.0−2.0, Pinit = 3.282−3.861 MPa. Second, the AITs become more vulnerable to the pressure and equivalence ratio at 3.809−3.861 MPa and the maximum AIT difference increases from 6 to 44 °C as the ethanol blending ratio increasing from 0% to 100%. Third, the sensitive factors for τig rank as initial mixture temperature ≫ ethanol blending ratio ≥ equivalence ratio ≥ initial mixture pressure. Fourth, the upper temperature limit of the NTC region can be determined by the mole fraction-balanced coefficient (MBC) equal to 2 which is the ratio of the maximum mole fraction of OH radicals to the maximum mole fraction of H2O2 molecule. The NTC region can be quantified by combining the criteria of MBC = 2 and the ceiling temperature. Fifth, at low-temperature regime, the H atom abstraction from n-heptane is the rate limiting step; at the NTC region, the reaction type of QOOH = olefin + HO2 displaces QOOH + O2= O2QOOH as the dominant reaction; the unimolecular fuel decomposition acts as the major initiation reactions and fuel consumption reactions at the high-temperature regime.

1. INTRODUCTION The increasingly stringent emission regulation and fuel consumption regulation are the two major impetuses to manufacture a cleaner and more fuel-efficient engine. Several new combustion processes such as homogeneous charge compression ignition (HCCI) or low-temperature combustion (LTC) are developed to simultaneously reduce the NOx/soot emission and elevate the indicated thermal efficiency.1−6 However, there are some technical bottlenecks concerning the HCCI combustion process. First is the difficulty in controlling the combustion phasing which depends on the charge chemical kinetics. The HCCI engine lacks the physical parameter to control the ignition timing such as the spark timing for the gasoline engine or injection timing for the diesel engine. Therefore, the ignition characteristics of the fuel−air mixture are extremely important to control the apparent heat release rate of the combustion process. Second, the unacceptable pressure rise rate (>1 MPa/°CA) caused by the rapid heat release rate confines the operating region of the HCCI engine. Generally, the new combustion mode has two essential features including a premixed fuel-lean mixture and autoignition, so that the ignition characteristics are always the most important issues for the HCCI combustion process.7−9 The dual fuel HCCI combustion mode is developed to adjust the ignition timing by varying the blending ratio of the two fuels which brings the favorable inspirations to control the heat release rate through the fuel reactivity.10 The contents of the ignition characteristics include three aspects: (1) autoignition temperature (AIT); (2) ignition delay times (τig); and (3) high/intermediate/low-temperature ignition © 2017 American Chemical Society

mechanics. According to the ASTM E659-15, the AIT is the minimum temperature at which a flammable mixture reaches hot-flame ignition under the specified condition.11 In other words, the AIT is the temperature corresponding to the maximum ignition delay time as shown in Figure 1. Since the major application and the comparison between different standard test methods for AIT determination have been elucidated in depth by the authors in ref 12, they will not be emphasized in this work. Ignition delay describes a period of time between start of injection (SOI) and start of combustion (SOC).13,14 The SOI generally represents the initial lift of the injector nozzle needle while the determination of SOC mainly depends on the certain experimental object and condition. In general, the methods of SOC determination fall into two categories: (1) configure certain thresholds for the measured combustion pressure. It is noteworthy that the first, second, and third derivatives of the pressure history and their integrals such as apparent heat release rate, accumulated heat release rate versus crank angle, and mass fraction burn (MFB), also belong to this field;15 (2) treat certain chemicals as the ignition indicator and the time of its occurrence as the SOC. Since the fuel droplet injected into the vessel undergoes not only the gas phase reaction process, but also the spray breakup, atomization, vaporization, and mixing process, the ignition delay time consists of the physical delay Received: October 23, 2017 Revised: November 28, 2017 Published: November 28, 2017 13610

DOI: 10.1021/acs.energyfuels.7b03247 Energy Fuels 2017, 31, 13610−13626

Article

Energy & Fuels

Figure 1. Framework of this article.

time and chemical delay time.16 In the ASTM D6890-16e1, ASTM D7170-16, and ASTM D7668-14a, the period of time between the SOI and the point when a significant and sustained increase in rate-of-change in the pressure is regarded as the ignition delay times. Ryan et al.17 further refine that it is the interval between the SOI and the rise in combustion pressure to the “pressure recovery point” (138 kPa above the initial chamber pressure prior to injection) which is followed by Borgin et al.18−20 and Allard et al.21−24 Meijer et al.25 propose that the ignition timing is bound up with the accumulation of OH radical even if only at the parts per billion level. Therefore, the presence of the OH radical can be considered as the indicator of ignition.26,27 In the combustion analysis of the internal combustion (IC) engine, the ignition delay is usually defined as the time interval between SOI and the certain threshold of mass fraction burn.28 In the field of the IC engine combustion analysis, the τig is an important indicator of the combustion phasing. However, the τig is also used to deduce the cetane number (CN), also known as derived cetane number (DCN), through specific correlations as shown in Table 1. The ASTM D6890-16e, ASTM D7170-16, and ASTM D7668-14a all adopt a constant volume vessel to

rapidly measure the ignition delay time of low volatility fuel since it enables the easy control of the equivalence ratio, temperature, pressure, fuel injection timing, and amount.29−31 Furthermore, the low fuel volume requirement is also an important property. However, the combustion process in the chamber is heterogeneous since the ignition delay time is too short (usually 0.1−100 ms under engine condition) to have enough time to form a uniform mixture. Borgin et al.19 report that it takes 20−26 ms for the droplet break up and vaporization, and more than 100 ms to produce the pseudohomogeneous charge where the local equivalence ratio approaches the global one. Therefore, the global equivalence ratio has little practical significance since the fuel−air mixture is heterogeneous. In this work, 3 min would be set aside to form a homogeneous fuel−air mixture after fuel injection into the combustion chamber, then the mixture is heated by the electric heating wire. By doing so, the physical effects of droplet break up, atomization, vaporization, and mixing process can be neglected because the ignition process is entirely controlled by the chemical kinetics. The ignition delay times of the hydrocarbons usually demonstrate the negative temperature coefficient (NTC) 13611


13612

2 + 25

cylindrical block having a volume of 0.60 ± 0.03 L, with external heating elements, heat shield, and electrically actuated intake and exhaust valves

constant volume compression ignition combustion chamber of 0.211−0.215 L with external electrical heating elements, suitable insulation and pneumatically actuated intake and exhaust valves

N/A

15 + 32

N/A

DCN = 83.99(ID-1.512)‑0.658+3.547

initial lift of the injector nozzle needle the point when a significant (+0.02 MPa above chamber static pressure) and sustained increase in rate-of change in pressure N/A

N/A DCN = 150.4(1/ID) + 5.3

N/A DCN = 4.460 + 186.6/ID

initial lift of the injector nozzle needle the point when a significant and sustained increase in rate-of change in pressure

This test method measures the ignition delay and utilizes a constant volume combustion chamber with direct fuel injection into heated, compressed air. An equation converts an ignition delay determination to a derived cetane number (DCN). 2.87−4.89 ms (59.6−35.0 DCN)

conventional diesel fuel oils, diesel fuel oils containing cetane number improver additives

ASTM D7170-1647

diesel fuel oil, oil-sands based fuels, hydrocarbon oils, blends of fuel containing biodiesel material, diesel fuel oils containing cetane number improver additives This test method measures the ignition delay of a diesel fuel injected directly into a constant volume combustion chamber containing heated, compressed air. An equation correlates an ignition delay determination to cetane number, resulting in a derived cetane number (DCN). 3.1−6.5 ms (64−33 DCN)

ASTM D6890-16ea46

20

single cylinder engine which consists of a standard crankcase with fuel pump assembly, a cylinder with separate head assembly of the precombustion type, thermal siphon recirculating jacket coolant system, multiple fuel tank system with selector valving, injector assembly with specific injector nozzle, electrical controls, and a suitable exhaust pipe

cylindrical chamber having a volume of 0.473 ± 005 L, with external heating elements, heat shield, and electrically actuated intake and exhaust valves

N/A

initial motion/lift of the injector pintle N/A

N/A

30−65CN CNARV = vol % n-cetane + (0.15)vol %heptamethylnonaned

N/A

The cetane number of a diesel fuel oil is determined by comparing its combustion characteristics in a test engine with those for blends of reference fuels of known cetane number under standard operating conditions.

diesel oil fuel

ASTM D613-17bc49

15

rise in the electronic signal that opens the injector the part of the pressure curve generated during the combustion cycle when significant (+0.02 MPa above the chamber static pressure) and sustained increase in rate-ofchange in pressure the part of the pressure curve midway between the chamber static pressure and the maximum pressure generated during the combustion cycle

N/A

N/A DCN = 13.028 − 5.3378/ID + 300.18/CD − 1267.90/CD2 + 3415.32/CD3

conventional diesel fuel oils, diesel fuel oils containing cetane number improver additives, low and ultralow-sulfur diesel fuel oils, biodiesel, blends of diesel fuel oils containing biodiesel material and diesel fuel oil blending components This test method utilizes a constant volume combustion chamber with direct fuel injection into heated, compressed synthetic air. A dynamic pressure wave is produced from the combustion of the sample. An equation converts the ignition delay and the combustion delay determined from the dynamic pressure curve to a derived cetane number (DCN). 1.9−25 ms (37−39 DCN)

ASTM D7668-14ab48

⎛ HWs − HWLRF ⎞ CNs = CNLRF + ⎜ ⎟(CNHRF − CNLRF) ⎝ HWHRF − HWLRF ⎠

ID denotes ignition delay, the period time between SOI and SOC. bCD denotes combustion delay, the period time between SOI and midpoint of the combustion pressure curve. cARV denotes accepted reference value, n-cetane and heptamethylnonane have assigned cetane number accepted reference values of 100 and 15 based on engine test by the ASTM Diesel National Exchange Group. dFor the equation, CNs = cetane number of sample; CNLRF = cetane number of low reference fuel; CNHRF = cetane number of high reference fuel; HWs = handwheel reading of sample; HWLRF = handwheel reading of low reference fuel; HWHRF = handwheel reading of high reference fuel.

a

midpoint of the combustion pressure curve complete sequence (preliminary cycles + further cycles) test facility

ignition delay time range (DCN range) CN range DCN or CN result inside the ignition delay range DCN result outside the ignition delay range SOI definition SOC definition

test method

fuel type

standard

Table 1. Comparison between Different Standard Test Methods for (Derived) Cetane Number Determination

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Energy & Fuels phenomenon.32−36 At the low-temperature ignition region, the τig decreases as the mixture temperature increases. When the temperature further increases and exceeds the onset of the NTC region, the τig reverses course and becomes longer within the intermediate temperature ignition regime. When the mixture temperature continues to rise about 50−200 K higher than the onset the of NTC, the τig returns to the trend that decreases with higher temperature. The cause of the NTC behavior in hydrocarbons was analyzed systematically in refs 37−42; therefore, the mechanics of the NTC behavior is not the emphasis of this work. Therein, the lower and upper temperature limits (also known as the onset and ending) of the NTC region act as the boundaries of the low/intermediate/high-temperature regions. Therefore, well-defined and general rules are needed to determine the onset and ending of particular regions among the many types of hydrocarbons. However, the definitions of the borders of the NTC region are ambiguous or not clearly clarified because the emphasis is putting on the phenomenon (the impact of equivalence ratio, temperature and pressure on the ignition delay time) and the cause (the chemical equilibrium transition of the reaction type of R+O2=RO2 and O2+QOOH = O2QOOH) of the NTC behavior. Benson43,44 and Morgan et al.45 propose a method to quantify the lower temperature limit of the NTC region (also known as “ceiling temperature”) by the equilibrium constant of reaction type R+O2=RO2 but the upper temperature limits have not clearly state or clarification until now. The purpose of this paper is to investigate the ignition characteristics of the n-heptane/ethanol which including three aspects: (1) autoignition temperature; (2) the ignition delay times; (3) the low/intermediate/high-temperature ignition. First, the AITs of the homogeneous n-heptane/ethanol mixture are measured under wide range conditions (φ = 0.2−2.0, step = 0.2; Pinit = 0.772−3.861 MPa, step 0.3861 MPa; δ = 0−100%, step 25%); second, an orthogonal experimental design is conducted to rank the sensitive factor of ignition delay time among equivalence ratio, mixture initial temperature, mixture initial pressure, and ethanol blending ratio; third, the upper temperature limits of the NTC region is quantified by the ratio of the maximum concentration of OH radical to the maximum concentration of H2O2 molecule and the dominant reactions of

the low/intermediate/high-temperature ignition are identified by sensitivity analysis.

2. EXPERIMENTAL SETUP AND COMPUTATIONAL MODEL In this section, the devices of autoignition temperature measurement, ignition criterion, experimental programs, and computational model development and validation are illustrated in detail. 2.1. Apparatus and Experimental Procedures. The experimental investigation is conducted using a constant volume combustion bomb system, as shown in Figure 2. The combustion bomb system consists of intake system, fuel supply system, bomb vessel, injection system, exhaust system, temperature control system, and data acquisition system. The test vessel is a vertical cylinder 83 mm high by 132 mm diameter with 1.014 L in volume. The required amount of air is supplied by adding appropriate pressure into the closed vessel. After reaching the desired air pressure, a specific amount of fuel is injected into the cylinder to attain a combustible mixture with specific equivalence ratio. The injection pressure is always 50 bar higher than the pressure inside the combustion chamber to facilitate homogeneous mixture formation. After the fuel injection into the combustion chamber, 3 min would be set aside to allow fuel−air mixing uniformity before heating begins. The formation of the homogeneous fuel−air mixture is a prerequisite of chemical kinetic controlled spontaneous ignition, otherwise, the global equivalence ratio would be meaningless. The electric heating wire is mounted inside the cylinder and the located range is from 106 mm to 116 mm in the radial direction. In addition, an external electric heating wire is available to promote temperature uniformity. The mixture temperature can be adjusted readily by the continuously adjustable transformer. The dynamic pressure and temperature histories are measured by a pressure transducer and temperature transducers. The pressure signal is amplified by a charge amplifier Kistler 5015A. There are three temperature sensors which are located at 0 mm, 66 mm, and 88 mm in diameter and they are all maintained at the same elevation (42 mm above the bottom). The thermocouples used in this study are nickel chromium type K thermocouples. The temperature measurement range, accuracy, and thermal response time constant of the sheathed thermocouples are 0−1300 °C, ±2.5 °C and 0.5 s, respectively. According to the temperature calibration result, the maximum temperature appears at the region that ranges from 50 mm to 88 mm in the radial direction, and the temperature difference in this region is within 50 °C. A high speed synchro-data acquisition device synchronizes the pressure/temperature signals and then transmits to the LabVIEW. Before each experiment,

Figure 2. Experimental setup. 13613


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Energy & Fuels compressed air is used to purge the product gases over 3 min to minimize the residual gas concentration. The main apparatus and the physical/chemical properties of the fuels used in this work are listed in Table 2 and Table 3, respectively.

temperature is measured by the thermocouple located at the diameter of 88 mm. 2.3. Experimental Programs. The AIT limits of specific fuel can be depicted as a contour plot with equivalence ratio on the horizontal and pressure on the vertical. Instead of varying the initial intake pressure, the equivalence ratio is varied at a constant pressure as shown in Table 4 which gives an example of the experimental program of 50%n-heptane50%ethanol at the pressure of 3.861 MPa. The intake air pressure ranges from 0.772−3.861 MPa with an interval of 0.3861 MPa, while the equivalence ratio ranges from 0.2−2.0 with an interval of 0.2. That is to say 90 test points are needed to describe the AIT limits for a specific fuel. Ethanol blending ratios of 0%, 25%, 50%, 75%, 100% are designed to investigate the effect of the blending ratio on the AIT limits, and there are total 450 test points for the experiment. 2.4. Chemical Kinetic Model Development and Model Validation. A detail chemical kinetic mechanism of n-heptane/ethanol can be systematically developed by means of hierarchical structure method which is advocated by Westbrook, Dryer,53,54 and Warnatz,55−57 respectively. As illustrated in Figure 4, a detailed mechanism initiates with the simplest species and reactions (such as CO, H2−O2 and CH2O) which are the subelements of more complex species (such as the C2, C3, and C4 species). Sequentially supplementing more elementary reactions describes the more complex species, and so on. For example, the hydrogen oxidation constitutes the foundation of most practical fuels. The resulting carbon monoxide oxidation is involved in the formaldehyde oxidation mechanism, and consecutively comes after methane oxidation. The C2, C3, C4, and larger hydrocarbons progress through the hierarchical process in such a manner. The simplest oxygenated fuels such as acetaldehyde, dimethyl ether, methanol, and ethanol are the subdivisions for the C2 species/formaldehyde oxidation.58,59 The Arrhenius rate coefficients of elementary reactions (pre-exponential factor, temperature exponent, and activation energy), thermodynamics parameters of species (standard state enthalpies/ entropies at 298 K, specific heats dependence on temperature etc.) and transport data (Lennard-Jones potential well depth, Lennard-Jones collision diameter, dipole moment, polarizability, rotational relaxation collision number, etc.) are integral physical and chemical parameters in mechanism construction. In fact, the comprehensive kinetic mechanism of n-heptane and ethanol have been built by Curran et al.37,60,61 and Marinov,62 respectively, which are constructed based on the hierarchical structure. Both of these mechanisms are chosen owing to their capability to emulate the fundamental combustion behaviors under wide range conditions. Comparison and mergence of both comprehensive kinetic mechanisms are performed. The n-heptane chemistry is set for a master mechanism while the ethanol chemistry acts as a donor mechanism because the kinetic parameters and thermodynamics data of n-heptane are compiled and revised according to the relatively new data source compared to ethanol. The master mechanism is defined as the mechanism that establishes defaults whenever conflicts arise during mechanism comparison, while the donor mechanism will be used as the secondary source of species and reactions that are not found in the master mechanisms. Besides, the GRI 3.0 mechanism proposed by the University of California, Berkeley,63,64 is incorporated in the n-heptane/ ethanol mechanism to describe the NO/NO2 formation through the thermal route, the prompt route, and the N2O route. The GRI 3.0 mechanism also complements and enriches the elementary reactions of the core C0−C2 submechanism. A comprehensive reaction mechanism should be tested and validated under wide range conditions to reproduce the macroscopic behavior such as ignition delay (measured by shock tube, constant volume vessel), premixed laminar flame velocity (measured by laminar flame burner), and diffusion combustion phenomenon (measured by coflow nonpremixed flame burner). The validation process must contain a transport-free condition (such as in static reactor and flow reactor) and a condition with transport effect (such as a premixed laminar flame). However, the experimental data about n-heptane/ethanol oxidation is scarce and mainly consists of constant volume vessel, JSR, and HCCI combustion.65−72 Thus, a detailed instead of comprehensive mechanism to mimic the macroscopic phenomenon under a specific

Table 2. Experimental Equipment apparatus injector pressure transducer temperature transducer charge amplifier temperature transmitter continuously adjustable transformer electrical control unit compressed air

manufacturer

type

Volkswagen Kistler Ming Yang Kistler PARAGON DELIXI

OEM:03C906036M 6125B WRNK-191 (K) 5015A PA-15-4-1 (K) TDGC2−5KVA

Freescale Ju Yang

MC9S12XEP100MAL O2/N2/Ar = 20.96/78.12/ 0.92 vol %

Table 3. Selected Physical and Chemical Properties of the Fuels Used in This Work component

n-heptane

ethanol

RON (ASTM D2699)50 MON (ASTM D2700)51 CN (ASTM D613)49 boing point (°C)52 triple point temperature (°C)52 critical temperature (°C)52 critical pressure (bar)52 enthalpy of vaporization (kJ/mol)52

0.0 0.0 56 98.35 ± 0.30 −90.59 ± 0.03 266.85 ± 2.00 27.4 ± 0.3 36.0 ± 3.0

109 90 8.0 78.35 ± 0.20 −123.15 ± 20.00 240.85 ± 7.00 63.0 ± 4.0 42.3 ± 0.4

2.2. Ignition Criterion. A combined temperature and pressure criterion is applied to determine autoignition. A pressure elevation more than 10% of the initial intake air pressure and a temperature increase more than 50 °C within 10 min (after heating initiation) is categorized as autoignition, while a pressure rise less than 10% of the initial intake air pressure or ae temperature elevation less than 50 °C is classified as autoignition not taking place. Since spatial temperature variation exists in the combustion chamber, it is difficult to measure the spontaneous ignition temperature accurately. To acquire the reliable autoignition temperature, the heterogeneous temperature distribution in the combustion chamber should be minimized. Therefore, the heating rate should be limited in order to keep the temperature field as uniform as possible. The maximum temperature of the three thermocouples corresponding to the moment the pressure undergoes a sharp increase is regarded as the autoignition temperature as shown in Figure 3. The term “Temperature D88” denotes that the observed

Figure 3. n-Heptane/air mixture ignition process: φ = 1, Tinit = 100 °C, Pinit = 1.544 MPa. 13614


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Energy & Fuels Table 4. Operating Condition of 50%n-Heptane-50%Ethanol Autoignition under Specific Intake Air Pressure air pressure (MPa)

air temperature (°C)

air mole (mol)

equivalence ratio

fuel mole (mol)

quantity of single injection (mg)

injection times

3.861 3.861 3.861 3.861 3.861 3.861 3.861 3.861 3.861 3.861

100 100 100 100 100 100 100 100 100 100

1.262 1.262 1.262 1.262 1.262 1.262 1.262 1.262 1.262 1.262

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

0.07554 0.06799 0.06043 0.05288 0.04533 0.03777 0.03022 0.02266 0.01511 0.00755

10.286 10.286 10.286 10.286 10.286 10.286 10.286 10.286 10.286 10.286

592 533 474 415 355 296 237 178 118 59

However, the p−T diagrams do not reflect the stoichiometry of the combustible. The AITs of n-heptane/ethanol mixtures under the pressure of 0.772−3.861 MPa and the equivalence ratio of 0.4−2.0 are demonstrated in the φ−p−T diagram in Figure 7. The AITs of pure n-heptane range from 258 to 304 °C, and its maximum temperature difference is within 46 °C, while the maximum temperature differences are 56, 62, 84, and 124 °C for the n-heptane blending ratios 75%, 50%, 25%, and 0%, respectively. The AITs of n-heptane are insensitive to the pressure and stoichiometry which is attributed to the multiple reaction pathways and low energy barriers for the formation and decomposition of ketohydroperoxides. The diverse reaction fluxes make the oxidation process less dependent on the boundary conditions such as stoichiometry, temperature, pressure, vessel shape, and size.12 On the opposite, the relatively simple chain-branching reaction pathways of ethanol make the oxidation processes rely more on the external factors as compared with the n-heptane. The tendency of AITs of n-heptane/ ethanol mixtures shown in Figure 7 further support the hypothesis proposed by the authors in ref 12. For the convenience of AITs analysis, the pressure range divides into three divisions including the low pressure regime (0.772−1.544 MPa), the intermediate pressure regime (1.544− 3.089 MPa), and the high pressure regime (3.089−3.861 MPa). The AITs of pure n-heptane exist at two distinct low AITs regions which are plotted as region A and B in Figure 7a. At the low pressure regime, the minimum AITs occur around stoichiometric conditions and modest fuel-rich conditions (φ = 1.0−1.5). At the intermediate pressure regime, the AITs endure modest pressure dependence within the equivalence ratio of 0.4−1.2. While the AITs decrease gradually when the equivalence ratio increases from 1.2−2.0. At the high pressure regime, the AITs further drop down as equivalence ratio increasing but the dependence on the equivalence ratio is not distinct. The authors have performed the extra autoignition experiment in which the pure n-heptane can still ignite around 250 °C even at φ = 3.0 and Pinit = 3.861 MPa. Generally, the autoignition limits of pure n-heptane in this research have obvious fuel-lean limits at around φ = 0.1−0.2 while the fuel-rich limits are not yet identified. In the gases phase reaction, the ignition process of fuel mixtures mainly depends on ignitibility of the reactive reactants with low octane number. The reactive fuel ignites first to bring about flame propagation and release a great amount of heat to elevate the mixture temperature and pressure. The less reactive fuel can be ignited by the flame front and begin thermal ignition due to increasing temperature and pressure. Therefore, the AITs of 75%n-heptane-25%ethanol are identical to those of pure n-heptane ranging from 250 to 310 °C. In the intermediate

Figure 4. Hierarchical structure of n-heptane/ethanol oxidation mechanism.53,54,73 regime would be more reasonable and realistic. The detailed mechanism contains all the necessary elementary reactions just like the comprehensive mechanism. Nevertheless, the detailed mechanism is validated for a specific condition only such as ignition delay times while the comprehensive mechanism needs to be verified under the wide range of operations mentioned above. Dagaut et al.65,66 constructed a detailed mechanism to reproduce the n-heptane/ethanol oxidation in JSR and constant volume vessel. The detailed mechanism proposed by this work is validated through the ignition delay times against that proposed by Dagaut et al.,65,66 Curran et al.,37,60,61 and Marinov62 as shown in Figure 5. The n-heptane blending ratio varies from 100% to 0% at 25% intervals (in mole fractions, similarly hereinafter). The parity plot shown in Figure 6 demonstrates that a very good agreement (R2 = 0.9853) is found between the predicted ignition delay times by Dagaut et al.,65,66 Curran et al.,37,60,61 Marinov,62 and by this work. On the basis of this result, the present detail chemical kinetic mechanism proposed by this work is appropriate for investigating the autoignition characteristics of the n-heptane/ethanol mixture including the ignition delay time and low/intermediate/high-temperature ignition.

3. RESULTS AND DISCUSSION 3.1. Autoignition Limits of n-Heptane/Ethanol Mixtures. As previously mentioned, the engine knock is the race between the velocity of flame front propagation and the ignition delay times of end gases autoignition. The ignitability under wide ranges of temperatures and pressures are of great interest which is usually displayed in the p−T ignition diagrams.74 13615


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Figure 5. Ignition delay time of (a) pure n-heptane, (b) 75%n-heptane−25%ethanol (c) 50%n-heptane−50%ethanol, (d) 25%n-heptane−75% ethanol and (e) pure ethanol at φ = 1.0, Pinit = 0.772−3.861 MPa, Tinit = 550−1250 K.

the elevated pressure and fuel-rich conditions. The AITs augment from 335 to 368 °C as equivalence ratio decreases from 2.0 to 0.4 at the low pressure regime of 25%n-heptane− 75%ethanol while the minimum AITs region shifts to the fuelrich condition (φ = 1.1−2.0) as shown in Figure 7d, region B. 3.2. Determinants of Ignition Delay Times. The n-heptane and the ethanol are typical two-stage ignition and single-stage ignition fuels, respectively. The ethanol addition would inhibit and retard the cool flame ignition caused by n-heptane.65,67−69,75 The calculated temperature and species histories of 50%n-heptane/50%ethanol fuel mixture oxidation in a closed homogeneous batch at φ = 1, Tinit = 280 °C, Pinit = 3.861 MPa are illustrated in Figure 8. The input parameters needed for defining a chemically reacting simulation in Chemkin are summarized in Table 5. The H2O2 molecules produce and accumulate by low and intermediate temperature reaction pathway when the mixture temperature is below 900 K.

pressure regime, the minimum AITs exist at around an equivalence ratio of 0.8−1.5 as shown in Figure 7b, region C. At the condition of 50%n-heptane−50%ethanol, the AITs increase from 320 to 334 °C as the decreasing equivalence ratio at the low pressure regime which is different from those of pure n-heptane and 75%n-heptane−25%ethanol. At the high pressure regime, the mixture ignites around 272−286 °C which is slightly above those of pure n-heptane and 75%n-heptane−25%ethanol. At the intermediate pressure regime, the general tendency of AITs is similar to that of 75%n-heptane−25%ethanol. Even when the ethanol blending ratio reaches 75%, the AITs are maintained around 284−293 °C at the high pressure regime which is about 30 °C higher than those of pure n-heptane. These phenomenon indicate that the autoignition process of fuel mixtures mainly depends on the ignition property of reactive reactants (those with low octane number) instead of the unreactive fuels (those with high octane number) especially at 13616


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blending ratio. The ranges R in Table 7 also support this result. The formulas for computing the ranges Rj are listed in eqs 9−eq 11. Figure 9 also reflects some issues of the mechanism validity especially the equivalence ratio and pressure. Generally, the decreasing equivalence ratio and pressure both result in longer ignition delay times. However, the ignition delay times of A factor (equivalence ratio) and C factor (pressure) in Figure 9 both appear to change suddenly in A4 and C4, respectively. These may attribute to the scope of the n-heptane37,60,61 and ethanol62 comprehensive mechanism. The n-heptane oxidation mechanism has been validated by comparison with experimental data over wide ranges of physical conditions: a equivalence ratio from 0.3 to 1.5, temperature from 550 to 1700 K, pressure from 1 to 42 atm. Similarly, the equivalence ratio of the ethanol mechanism ranges from 0.5 to 2.0, the temperature from 1000 to 1700 K, and the pressure from 1.0 to 4.5 atm. Five test conditions in this study are at the equivalence ratio of 0.2 where the improvement of the prediction ability of ignition delay times is necessary. But the object of this section is to distinguish the primary sensitive factor of ignition delay, the less important factors are under no consideration. Figure 6. Comparison between the predicted ignition delay time by Dagaut et al.,65,66 Curran et al.,37,60,61 Marinov,62 and by this work at φ = 1.0, Pinit = 0.772, 2.317, 3.861 MPa, Tinit = 550−1250 K.

5

K ij =

∑ τj i=1

The H2O2 decomposes rapidly to form OH radicals when the mixture temperature approaches 900−1000 K. The OH radicals consume the remaining fuels immediately which elevates the system temperature considerably, and the ignition is observed. As a result, the H2O2 decomposition temperature regime 900− 1000 K can be regarded as a threshold value for the ignition process.76 The time at which the combustible reaches the threshold temperature is responsible for the ignition delay times. Generally, any actions that accelerate attaining the critical temperature, either directly or indirectly, advance the ignition. While any actions that decelerate grasping the critical temperature retard the ignition. In Figure 8, the first stage ignition (cool flame) occurs at about 3179.40 ms which increases the mixture temperature from 952 to 1073 °C. The concentration of CH2O reaching the peak value up to 0.01275 mole fraction at about 3179.41 ms also proves the occurrence of cool flame heat release as shown in the partial enlargement of Figure 8. The high-temperature ignition occurs at 3179.43 ms afterward due to the early heat release, and the mixture temperature augments from 1154 to 2891 °C. The ignition process of the n-heptane/ethanol mixture in the constant volume vessel is affected by the equivalence ratio, mixture temperature, charge pressure, and n-heptane blending ratio. Their impacts on the ignition delay times are of great interest, therefore, an orthogonal experiment which includes four factors and five levels is designed to detect the most sensitive factor of ignition delay times. The four factors are equivalence ratio, mixture temperature, charge pressure, and n-heptane blending ratio, respectively, and each of them has five levels as shown in Table 6. The orthogonal experimental design test and corresponding ignition delay times are listed in Table 7. The results of the orthogonal experiment are processed by range analysis and variance analysis to detect the major and minor parameters of ignition delay times. The results of range analysis are demonstrated in Figure 9 and Table 6. Figure 9 indicates that the decrease of mixture temperature can effectively prolong the induction time of ignition compared to equivalence ratio, pressure, and ethanol

kij =

K ij max(i)

R j = max(kij) − min(kij)

(9)

(10) (11)

where i, j, e, and R denote levels, factors, error, and range, respectively. By the range analysis, little amount of computation is needed to identify the condition to acquire longer ignition delay times. However, the range analysis cannot estimate the errors caused by experimental processes and measurement. In other words, the range analysis cannot distinguish the difference of the orthogonal experiment resulting from the factors at different levels or experimental error (in this study is model inaccuracy). Therefore, the method of variance analysis is implemented to refine the analysis accuracy. The so-called variance analysis decomposes the sum of the squares of the total variation into the sum of the squares of the intra- (triggered by experimental error) and inter- (triggered by the factors at different levels) group variation. The F-test is adopted to perform the parameter significance test. A judgment about the significance can be made by comparing the F-distribution of factors with a certain threshold Fa( f j, fe) according to the rules below: a. If Fj > F0.01, factor j is highly significant on ignition delay times τig; b. If F0.01 > Fj > F0.05, factor j is significant on ignition delay times τig; c. If F0.05 ≥ Fj > F0.1, factor j has an impact on ignition delay times τig; d. If F0.1 ≥ Fj > F0.2, factor j has an impact on ignition delay times τig, to a lesser extent; e. If F0.2 ≥ Fj, factor j has a marginal impact on ignition delay times τig. The formulas for calculating the F-distribution of factors Fj are given in eq 12−eq 24 and the analysis of variance of the noninteracting orthogonal experiment is presented in Table 8. The results also support the conclusion obtained by range 13617


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Figure 7. AIT limits of (a) pure n-heptane, (b) 75%n-heptane−25%ethanol (c) 50%n-heptane−50%ethanol, (d) 25%n-heptane−75%ethanol, and (e) pure ethanol over a wide range of conditions, φ = 0.4−2.0, Pinit = 0.772−3.861 MPa.

Table 5. Inputs Used for Chemkin Simulation of n-Heptane/ Ethanol Oxidation items

value

problem type end time (sec) temp (°C) pressure (MPa) vol. (m3) heat loss (cal/sec) surface temperature (°C) equivalence ratio fuel mixture n-heptane blending ratio (mol %) oxidizer mixture complete combustion products

constrain volume and solve energy equation 10 280 3.861 0.001014 0 same as gas temperature 1 N−C7H16/C2H5OH 50 O2/N2 = 21/79 vol % CO2/H2O/N2

and ethanol blending ratio have minor effect compared to temperature.

Figure 8. Calculated species and temperature profiles for 50%nheptane/50%ethanol mixture, φ = 1, Tinit = 280 °C, Pinit = 3.861 MPa.

4

K=

analysis that the temperature is the main influencing factor on ignition delay times while the equivalence ratio, pressure,

5

∑ ∑ τi ,j j=1 i=1

13618

(12) DOI: 10.1021/acs.energyfuels.7b03247 Energy Fuels 2017, 31, 13610−13626

Article

Energy & Fuels P=

K2 no. of experiments

(13)

f j = levels − 1

(17)

Stotal = W − P

(18)

5

Qj =

∑ K ij

(14)

i=1

W = SUMSQ (τi , j)

(15)

Sj = Q j − P j

(16)

Table 6. Four Factors and Five Levels Orthogonal Table levelsfactors

A-equivalence ratio φ

B-temp Tinit (K)

C-pressure Pinit (MPa)

D-ethanol blending ratio δ

1 2 3 4 5

0.2 0.6 1.0 1.4 1.8

550 725 900 1075 1250

0.772 1.544 2.317 3.089 3.861

0 25 50 75 100

Figure 9. Indexes-factors diagram of an orthogonal experiment.

Table 7. Noninteracting Orthogonal Experimental Design and Corresponding Ignition Delay Times no. of experiment

equivalence ratio φ

temp (K)

pressure (MPa)

ethanol blending ratio δ

ignition delay τig (ms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 K1 K2 K3 K4 K5 k1 k2 k3 k4 k5 R Q W P S

0.2 0.2 0.2 0.2 0.2 0.6 0.6 0.6 0.6 0.6 1.0 1.0 1.0 1.0 1.0 1.4 1.4 1.4 1.4 1.4 1.8 1.8 1.8 1.8 1.8 28266.90157 5531.468248 1453.356041 23852.6887 1778.817041 5653.380314 1106.29365 290.6712082 4770.537741 355.7634082 5362.709106 280768411.5 1385171452 148270715.6 132497695.9

550.00 725.00 900.00 1075.00 1250.00 550.00 725.00 900.00 1075.00 1250.00 550.00 725.00 900.00 1075.00 1250.00 550.00 725.00 725.00 1075.00 1250.00 550.00 725.00 900.00 1075.00 1250.00 60283.493 563.376116 31.734303 4.1033185 0.52486805 12056.6986 93.89601933 7.93357575 0.8206637 0.10497361 12056.59363 726883589 1385171452 148270715.6 578612873.4

0.772 1.544 2.317 3.089 3.861 1.544 2.317 3.089 3.861 0.772 2.317 3.089 3.861 0.772 1.544 3.089 3.861 0.772 1.544 2.317 3.861 0.772 1.544 2.317 3.089 27966.61667 5817.593704 1220.037075 24102.16665 1776.817507 5593.323335 1163.518741 244.007415 4820.433329 355.3635013 5349.31592 280307210.5 1385171452 148270715.6 132036494.9

0 25 50 75 100 50 75 100 0 25 100 0 25 50 75 25 50 75 100 0 75 100 0 25 50 28219.50257 24130.84756 5552.700925 1781.645498 1198.535051 5643.900515 4826.169511 1110.540185 356.3290996 239.7070101 5404.193505 282816272.8 1385171452 148270715.6 134545557.2

27956 286.8169 22.21932 1.681074 0.1842771 5524.35 4.976208 1.628919 0.3595945 0.1535266 1192.522 257.3845 2.170229 1.177046 0.1022661 23841.43 4.912406 5.69495 0.6087026 0.04264543 1769.191 3.591152 5.715835 0.2769014 0.04215282 K 60883.23161 P 148270715.6 Stotal 1236900737 Se 259208115.4 f total 24 fe 8

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Energy & Fuels Table 8. An Analysis of Variance of the Noninteracting Orthogonal Experiment error source

S

f

S̅

F

Fa

statistical significance

A-φ B-Tinit C−Pinit D-δ e total

132497695.9 578612873.4 132036494.9 134545557.2 259208115.4 1236900737

4 4 4 4 8 24

33124423.98 144653218.3 33009123.73 33636389.29 32401014.43

1.022326756 4.464465724 1.018768218 1.03812766

F0.2(4,8) > F F0.01(4,8) > F > F0.05(4,8) F0.2(4,8) > F F0.2(4,8) > F

marginal impact significant marginal impact marginal impact

is defined by Benson43 and Morgan et al.45 in terms of the equilibrium constant for eq 25, which is demonstrated in eq 26. At the temperature lower than the ceiling temperature, the alkyl radicals larger than the C4 addition to molecular oxygen form the alkylperoxy species. When temperatures are above ceiling temperature, alkyl radicals undergo β-decomposition to produce olefin and hydroperoxides through eq 27.

4

Se = Stotal −

∑ Sj j=1

ftotal = no. of experiments − 1

(19) (20)

4

fe = ftotal −

∑fj j=1

Sj = Se = Fj =

Sj fj Se fe Sj Se

(21)

(22)

R + O2 = RO2

(25)

[RO2 ] [R]·[O2 ]

(26)

Kc =

R = olefin + HO2

(23)

(27)

At high-temperature regime (above 1000 K), the chain branching reactions such as eq 28−eq 29 proceed and dominate the fuel consumption rate. While at the NTC regime (about 800−1000 K), the chain propagation reactions such as eq 30−eq 31 are rate-limiting reactions.41,42,53,74,77 Therefore, the correlation between hydrogen peroxide molecules and hydroxyl radical has the potential to clarify the upper temperature limits of the NTC region. The calculated hydroxyl radical and hydrogen peroxide profiles at the temperature of 550, 725, 900, 1075, and 1250 K, respectively, cover the frequently used initial temperature of the combustion system. Since the ignition delay times vary by 4−6 orders of magnitude, they are all normalized for comparison purposes as shown in Figure 11. As the mixture temperature increases from 550 to 1250 K, the maximum mole fraction of OH radical also rises from 4.936 × 10−3 to 1.308 × 10−2 monotonously while the maximum mole fraction of H2O2 reduces from 1.016 × 10−2 to 6.322 × 10−3. Furthermore, the normalized time which corresponded to the maximum mole fraction of H2O2 advances from 0.99998 to 0.50492. Therefore, there may be a relation between the maximum mole fraction of OH radical and the maximum mole fraction of H2O2. The normalized time corresponding to the maximum mole fraction of H2O2 molecules is defined as the time-sequenced coefficient (TSC), and the ratio of the maximum mole fraction of OH radicals to the maximum mole fraction of H2O2 molecule is defined as the mole fraction-balanced coefficient (MBC). The TSC and MBC describe the phase of the maximum mole fraction of H2O2 molecule and the magnitudes contrast between OH radicals and H2O2 molecules. The temperature dependences of TSC and MBC at pressures of 0.772, 2.317, and 3.861 MPa are exhibited in Figure 12. As the mixture initial temperature increases, the relative phase of the maximum mole fraction of H2O2 molecules advances along with the reduction of ignition delay times. The combination of these two factors results in the reduction of the maximum mole fraction of H2O2 molecules. The MBC increases rapidly when the temperature rises above ∼850 K due to the OH explosion by eq 28 and eq 29.

(24)

where S, f, S̅, and F represent the sum of squares, the degree of freedom, the mean square value, and the F distribution, respectively. 3.3. High/Intermediate/Low Temperature Ignition Regimes Classification and the Corresponding Dominant Reactions. The object of this section is to determine the upper temperature limits of the NTC region. The calculated ignition delay times of 50%n-heptane−50%ethanol at the stoichiometric conditions, at the pressure of 0.772, 2.317, and 3.861 MPa are illustrated in Figure 10. The 50%n-heptane−50%ethanol

Figure 10. Calculated ignition delay times and ignition temperature partitions; φ = 1, Pinit = 0.772, 2.317, and 3.861 MPa, δ = 50%.

mixture still undergoes high/intermediate/low-temperature ignition. In the intermediate ignition region, the ignition delay times have negative temperature coefficients (NTC) behavior and the temperature regimes are shown in Table 9. The lower temperature limit of the NTC region, also known as “ceiling temperature”, 13620


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Energy & Fuels Table 9. High/Intermediate/Low-Temperature Ignition Temperature Classification δ (mol %)

Pinit (MPa)

low-temp ignition region (K)

NTC region (K)

high-temp ignition region (K)

upper temp limits of NTC region (K) [by Chemkin simulation]

upper temp limits of NTC region (K) [by MBC = 2]

relative deviationa

50%

0.772 2.317 3.861

550−750 550−820 550−875

750−850 820−905 875−955

850−1250 905−1250 955−1250

850 905 955

845 911 963

0.588% 0.663% 0.838%

a Relative deviation (%) =|((upper temp limits of NTC region calculated by MPC = 2) − (upper temp limits of NTC region calculated by Chemkin simulation))/(upper temp limits of NTC aregion calculated by Chemkin simulation)| × 100.

911 and 963 K, respectively. The results calculated by MBC = 2 are very similar to those calculated by Chemkin simulation as shown in Table 9, and the upper temperature limits of the NTC region calculated by MBC = 2 have a good agreement to those calculated by chemkin software (the relative deviations are all below 1%). Therefore, we can assume that the MBC = 2 may act as the indicator of the upper temperature limits of the NTC region. H + O2 = O + OH

(28)

H 2O2 ( +M) = 2OH( +M)

(29)

H + O2 ( + M) = HO2 ( +M)

(30)

H 2O2 + O2 = 2HO2

(31)

Since the OH radical is the ignition indicator, the sensitivity analyses of the OH radical are performed at the pressure of 3.861 MPa, and temperatures of 550, 725, 900, 1050, and 1250 K as shown in Figure 13 which cover the low-temperature ignition regime, NTC regime, and high-temperature ignition regime. This helps to reveal the dominant reactions for n-heptane/ethanol ignition under various temperature regimes. In the low-temperature ignition temperature regime, the dehydrogenation reaction from n-heptane (such as R1980−R1983) plays an important role to promote OH radical production by initiating the radical pool growth, while the H atom abstraction from the ethanol (such as R286−R288) consumes the OH radical. It is noteworthy that the alkyl peroxy radical isomerization (such as R2222, R2227, R2233, and R2237 in Figure 13) grows in importance as the initial mixture temperature increases from 550 to 900 K. As the temperature increases, the reverse reaction rate of alkyl radical addition to O2 exceeds the forward reaction rate resulting in the reduction of OH concentration because of the inhibition of keto-hydroperoxide decomposition. The reaction type of QOOH species undergoes β-scission to generate the conjugate olefin, and hydroperoxide becomes dominant within the NTC regime. Since it does not provide active radicals such as O, OH, and H but the relatively stable substance, the overall reactivity of the system decreases compared with the low-temperature ignition regime. This is the reason why the ignition delay time increases with initial temperature for alkanes. This phenomenon also reveals that the NTC behavior becomes less apparent as ethanol is added because the ethanol replaces the n-heptane. That is to say, the ethanol addition itself does not change the reaction pathway of the NTC behavior because the NTC is caused by the inhibition of the QOOH addition to the O2 of the n-heptane intermediate temperature oxidation. As the initial mixture temperature further increases up to 1050 K, the H atom abstraction from n-heptane by HO2 is the primary source of H2O2, It takes place due to a chain branching reaction decomposing into two OH radicals. On the contrary,

Figure 11. Calculated hydroxyl radical and hydrogen peroxide profiles; φ = 1, Pinit = 3.861 MPa, δ = 50%.

Figure 12. Time-sequenced coefficient (TSC) and mole factionbalanced coefficient (MBC) to describe the correlation between the hydroxyl radical and the hydrogen peroxide molecule; φ = 1, Pinit = 0.772, 2.317, 3.861 MPa, δ = 50%.

As mentioned above, the massive production of OH radicals is the indicator of the ignition process, and the chain branching reactions dominate the high-temperature ignition regime. Thus, a hypothesis is proposed that the temperature corresponding to MBC = 2 corresponds to the upper temperature limit of the NTC region. When the initial temperature exceeds this critical temperature, the net growth of free radicals especially the OH radical takes place, which is the characteristic of a high-temperature ignition region. At the initial temperature below the critical temperature, the free radicals consume more rapidly than their formation which is the characteristic of the NTC region. The upper temperature limits of the NTC region calculated by NBC = 2 at the pressure of 0.772, 2.317, and 3.861 MPa are 13621


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branching reactions R1, R16, and R109 act as the major source of the OH radical, while the R13, R110, R151, and R455 are the chain termination reactions that consume active radicals such as OH and HO2 to produce stable molecules. In addition, the unimolecular fuel decomposition reactions (R1970 and R1971 in Figure 13e) serve as the initiation reaction and fuel

those reactions consume the HO2 radical and H2O2 molecule without a net growth of active radical against the OH radical accumulation. For example, R110 in Figure 13d consumes the HO2 radical to produce the stable molecule CH4 and O2 which is a chain termination reaction. R14/R15 would reduce the system reactivity since the HO2 is a stable radical. The chain

Figure 13. continued 13622


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Figure 13. Sensitivity coefficients of OH species for constant volume bomb simulation of n-heptane/ethanol oxidation: φ = 1, Pinit = 3.861 MPa, δ = 50% (a) Tinit = 550 K, (b) Tinit = 725 K, (c) Tinit = 900 K, (d) Tinit = 1075 K, (e) Tinit = 1250 K.

consumption reaction only at the high-temperature ignition regime, because the high activation energy (5.839 × 1005− 5.977 × 1005 kJ/mol) as compared to that of H atom abstraction by OH radicals (about 3993 kJ/mol).

equivalence ratio of 0.4−2.0. While the maximum AIT difference is 56, 62, 84, and 124 °C as the ethanol blending ratio is 25%, 50%, 75%, and 100%, respectively. 2. Under the condition of n-heptane/ethanol autoignition, the AITs do not show a linear correlation with the initial pressure or equivalence ratio. But the minimum AITs generally occur at the pressure of 3.282−3.861 MPa, at the equivalence ratio of 1.0−2.0. 3. For n-heptane and 75%n-heptane/25%ethanol, the minimum AITs locate at stoichiometric and modest fuel-rich conditions (φ = 1.0−1.5) at the pressure of 0.772−1.544 MPa. As the ethanol blending ratio further increases, the AITs decrease with increasing equivalence ratio under the same pressure range. Even though the AITs do not exhibit distinct pressure or equivalence ratio dependence at 1.544−3.089 MPa, this still demonstrates that the minimum AITs always position at the equivalence ratio of 1.0−2.0. 4. At the high pressure regime (3.089−3.861 MPa), the AITs are more sensitive to both the pressure and equivalence ratio. The maximum AITs difference of pure n-heptane at this region is 6 °C while that of pure ethanol reaches 44 °C.

4. CONCLUSIONS We investigate the autoignition characteristics of the n-heptane/ ethanol mixture including the autoignition temperature (AIT), ignition delay times, and low/intermediate/high-temperature ignition. A combustion bomb experiment is performed to measure the AIT and an orthogonal experiment is conducted to identify the main influencing factors of ignition delay times. A hypothesis is proposed to give physical meaning of the upper temperature limit of the NTC region by reference to the lower temperature limit (ceiling temperature) which is proposed by Benson43 and Morgan et al.45 which help to quantify the boundaries of the low/intermediate/high-ignition region. The dominant reactions of the low/intermediate/high-temperature ignition are identified by sensitivity analysis. The main conclusions are summarized below: 1. The AIT range of the pure n-heptane is between 258 and 304 °C, in other words, the maximum AIT difference is 46 °C at the pressure of 0.772−3.861 MPa, at the 13623


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■

f = the degree of freedom HWHRF = handwheel reading of high reference fuel HWLRF = handwheel reading of low reference fuel HWs = handwheel reading of sample i = levels j = factors p = pressure (MPa) Pinit = initial intake air pressure (MPa) R = range S = the sum of squares S̅ = the mean square value T = temperature (°C) Tinit = initial intake air temperature (°C) V = volume (m3)

5. The sensitive factors affecting the ignition delay time in descending order is initial mixture temperature, ethanol blending ratio, equivalence ratio, and initial mixture pressure. The mixture initial temperature reduction increases the ignition delay times in an exponential rate according to the Arrhenius law. The fuel reactivity can be adjusted directly by changing the ethanol blending ratio which also plays an important role in the ignition delay time. 6. The mole fraction-balanced coefficient (MBC) is defined as the ratio of the maximum mole fraction of OH radicals to the maximum mole fraction of H2O2 molecules. The upper temperature limits of the NTC region calculated by MBC = 2 have a good agreement to those calculated by chemkin software (the relative deviation is less than 1%). The lower temperature limits of the NTC region (also known as “ceiling temperature”) is governed by the equilibrium constant of R + O2 = RO2, therefore, the low/intermediate/high-temperature regimes can be quantified. 7. The low-temperature (550−750 K) ignition of n-heptane/ ethanol mixture depends on the reaction rate of the H atom abstraction from n-heptane; while the reaction type of the QOOH + O2 = O2QOOH inhibiting within the temperature range of 750−955 K is the primary cause of the NTC region of n-heptane/ethanol oxidation. Therefore, the NTC inhibition behavior by adding ethanol is caused by the replacement of n-heptane by ethanol while the ethanol does not change the reaction pathway; the unimolecular fuel decomposition grows in importance at high-temperature regime (955−1250 K).

Greek Letters

φ = equivalence ratio δ = ethanol blending ratio τig = ignition delay time (ms) Abbreviations

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Runzhao Li: 0000-0001-5120-9849 Yongqiang Han: 0000-0002-7120-8218 Notes

The authors declare no competing financial interest.

■

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51576089), the National Key R&D Program of China (No. 2017YFB0103503, No. 2017YFB0306605) and the Graduate Innovation Fund of Jilin University (No. 2017060, No. 2016026). The authors wish to thank Prof. P. Dagaut and Prof. C. Mounaim-Rousselle for providing us with the details of the n-heptane/ethanol oxidation mechanism from their work to validate the proposed n-heptane/ethanol detailed mechanism in this work. Moreover, the field work is conducted in State Key Laboratory of Automotive Simulation and Control, Jilin University. The authors thank the laboratory managers and staff workers for their hospitability, time, and opinions. The authors are indebted to the reviewers of this article for their invaluable suggestions.

AIT = autoignition temperature ARV = accepted reference value ASTM = American Society for Testing and Materials CN = cetane number CD = combustion delay DCN = derived cetane number HCCI = homogeneous charge compression ignition IC = internal combustion ID = ignition delay LTC = low-temperature combustion MBC = mole fraction-balanced coefficient MFB = mass fraction burn MON = motor octane number N/A = not available or not applicable NTC = negative temperature coefficient RON = research octane number ROP = rate of production SOI = start of injection SOC = start of combustion TSC = time-sequenced coefficient vol = volume

REFERENCES

(1) Sharma, T. K.; Rao, G. A. P.; Murthy, K. M. Homogeneous Charge Compression Ignition (HCCI) Engines: A Review. Archives of Computational Methods in Engineering 2016, 23 (4), 623−657. (2) Bendu, H.; Murugan, S. Homogeneous charge compression ignition (HCCI) combustion: Mixture preparation and control strategies in diesel engines. Renewable Sustainable Energy Rev. 2014, 38, 732−746. (3) Saxena, S.; Bedoya, I. D. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog. Energy Combust. Sci. 2013, 39 (5), 457−488. (4) Gan, S.; Ng, H. K.; Pang, K. M. Homogeneous Charge Compression Ignition (HCCI) combustion: Implementation and effects on pollutants in direct injection diesel engines. Appl. Energy 2011, 88 (3), 559−567. (5) Yao, M.; Zheng, Z.; Liu, H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog. Energy Combust. Sci. 2009, 35 (5), 398−437. (6) Stanglmaier, R. H.; Roberts, C. E., Homogeneous Charge Compression Ignition (HCCI): Benefits, Compromises, and Future Engine Applications. SAE Tech. Pap. Ser..199910.4271/1999-01-3682.

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NOMENCLATURE CNHRF = cetane number of high reference fuel CNLRF = cetane number of low reference fuel CNs = cetane number of sample e = error F = F-distribution 13624


Article

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Experimental and Kinetic Modeling Study of Autoignition

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