Article Cite This: Energy Fuels 2018, 32, 4732−4746
pubs.acs.org/EF
Investigation into the Influence of the Ethanol Concentration on the Flame Structure and Soot Precursor Formation of the n‑Heptane/ Ethanol Premixed Laminar Flame Runzhao Li, Zhongchang Liu, Yongqiang Han,* Manzhi Tan, Yun Xu, Jing Tian, Jiayao Yan, Xiangfeng Yu, Jiahui Liu, and Jiahong Chai State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, Jilin 130025, People’s Republic of China ABSTRACT: The purpose of this work is to investigate the effect of the ethanol blending ratio on the flame structure (species concentrations, heat release rate, and flame temperature profile) and soot precursor (CH3, C2H2, aC3H5, and C3H3) formation of the n-heptane/ethanol premixed laminar flame at the equivalence ratio of 2 and pressure of 3.861 MPa. The results indicate the following: First, n-heptane consumes more rapidly than ethanol for its high dehydrogenation reaction rate under the research temperature range. n-Heptane decomposes completely (over 99% n-heptane is consumed) at about 0.723−0.839 mm above the burner surface, while that of ethanol (over 99% ethanol is consumed) ranges from 1.657 to 8.021 mm. Second, the ethanol addition facilitates the reaction sequences of CH2O → HCO → CO → CO2 by providing reactive radicals, such as O, OH, and H. Third, the thickness of the reaction zone increases by 2.34% when the ethanol blending ratio grows from 0 to 25% because the dehydrogenation reaction rates of ethanol by H/OH radicals are slower than that from n-heptane at the studied flame temperature. However, it decreases by 0.585, 7.018, and 23.0021% at the ethanol blending ratios of 50, 75, and 100% compared to pure n-heptane as a result of the low-temperature heat release inhibition by ethanol. Fourth, even though the ethanol addition reduces the aC3H5/C3H3 and C2H2 formation, their principles are different. The ethanol addition has no direct impact on the reaction flux of the aC3H5 and C3H3 formation, and their reductions are caused by replacing n-heptane with ethanol. On the contrary, the C2H2 reduction with ethanol addition is attributed to the increasing active radical (such as CH2O, OH, and H) concentration. However, the existence of a high concentration of CH3 and sequential reactions of CH3 → sC3H5CO → sC3H5 → C2H2 also facilitate the acetylene formation. Therefore, the decline of acetylene is about 12−43% lower than aC3H5 and C3H3 at the same ethanol blending ratio. The CH3 concentration increases through C2H5OH → CH4 → CH3 with ethanol addition. release (LTHR),8−10 second, the autoignition characteristic and combustion process,11−15 and third, the reduction of polycyclic aromatic hydrocarbons (PAHs) and soot resulting from ethanol addition.12,16 However, the numerical investigation of nheptane/ethanol oxidation lags behind the experimental research as a result of the lack of the detailed chemical kinetic mechanism. Dagaut et al.14,15 construct the n-heptane/ethanol oxidation detailed mechanism for the jet-stirred reactor (JSR) and HCCI combustion study. They merge the n-heptane oxidation mechanism proposed by Curran et al.,17 and the ethanol sub-mechanism is raised by their own.18 The detailed mechanism suggested by Dagaut et al.14,15 omits the NO/NO2 formation sub-mechanism. Vuilleumier et al.11 also model the heat release rate in a HCCI engine fueled by a n-heptane/ ethanol mixture. The n-heptane, ethanol, and C 0 −C 4 mechanisms are derived from Curran et al.,17,19,20 Marinov,21 and the National University of Ireland (NUI) Galway,22,23 respectively; however, Vuilleumier et al.11 provide little information about their mechanism. Generally, the detailed mechanism should reproduce the macroscopic phenomenon of the experimental results, and the forecast capability is assessed by evaluating the agreement against the observed data. The frequently used techniques for elucidating chemical kinetics and
1. INTRODUCTION Homogeneous charge compression ignition (HCCI) has great potential to improve thermal efficiency and reduce NOx/soot emissions simultaneously.1,2 However, there are still some “bottlenecks” restricting the practical application of HCCI combustion mode.3−5 First, the autoignition timing depends upon the chemical kinetics and lacks physical parameters for accurate control. Second, the unacceptable high in-cylinder pressure rise rate caused by the high heat release rate confines the engine high load operation. Third, the chemical-kineticscontrolled ignition readily results in misfire during cold start and idle operation. Fourth, the extremely high CO and hydrocarbon emissions result from the low-temperature combustion. The effective control of the heat release rate is the prerequisite of load extension and the combustion phase control. The combustion phase can be shifted by altering incylinder charge reactivity or adjusting the in-cylinder temperature−time profile.6 The in-cylinder charge reactivity can be modified by blending two or more fuels with a remarkable reactivity difference, fuel reforming, introducing fuel additives, and exhaust gas recirculation (EGR). For example, the blended fuel (such as n-heptane/ethanol) HCCI combustion can readily adjust the combustion phasing by changing the fuel blending ratio.7 The experimental investigation of n-heptane/ethanol blended fuel combustion mainly concentrates on three aspects: first, the ethanol inhibition effect on low-temperature heat © 2018 American Chemical Society
Received: December 23, 2017 Revised: February 25, 2018 Published: March 21, 2018 4732
DOI: 10.1021/acs.energyfuels.7b04076 Energy Fuels 2018, 32, 4732−4746
closed homogeneous reactor closed partially stirred reactor HCCI engine rapid compression machine (RCM) jet-stirred reactor (JSR) plug-flow reactor (PFR) premixed burner-stabilized flame premixed stagnation flame freely propagating flame opposed-flow flame shock tube
technique
10−1−106
10−3−1
transient or steady-state well-mixed plasma reactor 10 −10 10−107 10−107 10−107 10−107 105−107
10 −1 10−4−10−2 10−4−10−2 10−4−10−2 10−4−10−2 10−6−10−3
plug-flow reactor, where convection dominates transport
one-dimensional, premixed, laminar, burner-stabilized flame
one-dimensional, premixed, laminar, burner-stabilized stagnation−flow flame, where the flow stagnates against a planar surface perpendicular to the flow from the burner one-dimensional freely propagating flame for determining premixed, laminar flame speeds
diffusion or premixed opposed-flow flame normal incident or reflected shock
−1
6
106−107 106−107
10−3−10−1 10−3−10−1
−3
103−109
10−106
transient, closed, partially stirred, or unmixed reactor, which determines rate limitations between mixing and kinetics closed, homogeneous charge compression ignition studies to simulate a single engine cycle of an internal combustion (IC) engine
800−2500 >1300
800−2500
800−2500
800−2500
800−1400
800−1400
1400−3000 1400−3000
