A simplified mechanistic model of three-component surrogate fuels for

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Combustion

A simplified mechanistic model of three-component surrogate fuels for RP-3 aviation kerosene Yunpeng LIU, Yuchen LIU, Dengbing CHEN, Wen FANG, Jinghua Li, and Yingwen Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02094 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Energy & Fuels

1

A simplified mechanistic model of three-component surrogate fuels for

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RP-3 aviation kerosene

3

Yunpeng Liu, Yuchen Liu, Dengbing Chen, Wen Fang, Jinghua Li, Yingwen Yan

4

College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics,

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Nanjing, 210016, China

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ABSTRACT

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In this study, a detailed chemical reaction kinetic model of a three-component surrogate fuel (73%

8

n-dodecane, 14.7% 1,3,5–3-trimethylcyclohexane, and 12.3% n-propylbenzene) for RP-3 aviation

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kerosene was simplified, and a simplified mechanism for this fuel was obtained and validated. The

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detailed chemical reaction kinetic model included 257 components and 874 elementary reactions.

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In the first step, a mechanism consisting of 109 components and 423 elementary reactions was

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constructed from the detailed model using a directed relation graph (DRG). The second step,

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based on the first, was to construct an 84-component, 271-elementary-reaction mechanism using a

14

DRG based on error propagation (DRGEP) and computational singular perturbation (CSP). In the

15

third step, path analysis was applied to analyze the combustion paths under atmospheric pressure

16

and high-temperature conditions; the results were compared with those of the detailed mechanism

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and the simplified mechanism of the second step to remove unimportant reaction paths or to

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supplement important paths that were reduced in the first two steps. In the final step, a simplified

19

mechanism of the three-component surrogate fuel suitable for high temperature and atmospheric

20

pressure was obtained, which involved 59 components and 158 elementary reactions. Test data of

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the ignition delay time and the laminar flame velocity of RP-3 kerosene were used to verify the

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simplified mechanism for the three-component surrogate fuel. The results showed that the

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numerical results for the proposed simplified mechanism were consistent with the test data. Finally,

ACS Paragon Plus Environment

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to verify the engineering practicality of the simplified mechanism proposed in this study, taking a

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Bunsen burner as the physical model, a premixed pre-evaporation combustion flame was

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numerically simulated by using the simplified mechanism for the three-component surrogate fuel.

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The numerical calculation results of the simplified mechanism were consistent with the test data,

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and the computation time was within the acceptable range of the engineering application.

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Therefore, the simplified mechanism for the three-component surrogate fuel can be used for

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numerical simulation of engineering combustion with kerosene fuel.

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Keywords: Three-component surrogate fuels; RP3-Aviation kerosene; Ignition delay; Laminar

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flame speed; Simplified mechanism; Bunsen burner;

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1. Introduction

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Research on the combustion characteristics, combustion rate, flame structure, and stable

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combustion range of aviation kerosene has been key to the development of aeroengine combustors

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(1), so it is essential to study the combustion of aviation kerosene in depth. Aviation kerosene

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combustion in aeroengine combustors is a complex and turbulent combustion process, which is

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jointly determined by turbulent mixing and chemical kinetics. Therefore, to accurately simulate

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flame combustion in an aeroengine combustor, besides studying the turbulent combustion model,

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research on a chemical kinetic model of aviation kerosene is necessary. A combination of

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computational combustion and a chemical kinetic model can accurately predict the turbulent

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combustion characteristics of aeroengine combustor, and provide technical support for the design

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and optimization of aeroengine combustors. Since RP-3 aviation kerosene is the main fuel for

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combustion chambers of aeroengines in China, the multi-step chemical reaction kinetic model

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directly establishes the accuracy and reliability of numerical simulations of turbulent flow in


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Energy & Fuels

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combustion chambers. However, the combustion of macromolecular hydrocarbon in RP-3 aviation

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kerosene with oxygen is extremely complex. This involves two aspects (2): 1) aviation fuels are

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mixtures of hundreds of components and it is therefore impractical to provide a detailed chemical

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mechanism model to study and test for each component. 2) Generally, a detailed chemical kinetic

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mechanism for a surrogate fuel contains hundreds of components and thousands of elementary

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reactions. For such a complex reaction system, a detailed chemical kinetic model is too complex

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to use to calculate the three dimensional flow field at the current computer level. Therefore,

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numerical simulation of turbulent combustion in aeroengine combustors requires the detailed

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chemical kinetic mechanism to be simplified on the premise of ensuring the accuracy of the

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numerical simulation of the chemical reaction mechanism. The aim is to ensure the accuracy of

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the numerical simulation while reducing the computation time, which requires an increased

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calculation efficiency. Simplifying the kinetic model of the aviation kerosene combustion reaction

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is important for numerical simulation of combustion in aeroengine combustors at this stage. This

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can improve the computational efficiency and reduce the convergent stiffness for the CFD

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simulation of aviation kerosene combustion, to introduce the chemical reaction mechanism to the

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practical engineering problems of three-dimensional turbulent combustion and perform more

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accurate numerical simulations. While the detail reaction mechanisms could well reflect the

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complex reaction process, the mechanisms are hard to be applied to the engineering applications,

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for example, combustion numerical simulation of the aero engine combustor. Thus, appropriate

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methods and verifications should be employed to simplify the detail mechanisms. As conducted in

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the paper, the simplified mechanism could well be introduced to the engineering calculation.

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This study is mainly concerned with the engineering application of the chemical reaction


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mechanism of aviation kerosene. So it is different from the fundamental research about the

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surrogate fuel of aviation kerosene or the detail combustion reaction mechanism. The detail

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mechanism of three-component surrogate fuel of aviation kerosene is selected and simplified in

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the paper. The present study is devoted to investigating the simplification of the detail mechanism

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for engineering application, and the unique simplification scheme makes this paper especially

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concern the ignition delay time in the simplification process. A set of verification ensures the

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reasonability and accuracy of the simplified mechanism in this paper.

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2. State-of-the-art and development trends

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Because the composition of aviation kerosene is very complex, many researchers investigate

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surrogate fuel of aviation kerosene. For example, Yi W et al (3) investigated the laminar flame

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speed measurement of kerosene and its surrogate fuel (n-decane, n-propyl benzene, and propyl

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cyclohexane) and results confirm the reasonableness of surrogate fuel. To date, research on the

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reaction mechanisms of chemical combustion of aviation kerosene includes the following aspects.

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1) Surrogate fuel modes developed from monocomponent surrogate fuel modes to

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double-component surrogate fuel modes and subsequently to multi-component surrogate fuel

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modes. 2) Surrogate fuel modes developed from single chain hydrocarbon modes to

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multi-component surrogate fuel modes, which contain aliphatic and aromatic hydrocarbons. 3)

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The intermediate components increased from dozens to hundreds and elementary reactions

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increased from hundreds to thousands.

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Kundu et al. (4) studied the surrogate fuel C12H23, and a simplified mechanism involving 16

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components and 23 reactions was proposed to simulate the combustion of Jet A fuel. By

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comparing the temperature, the concentration of NOx, and other products calculated using the


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simplified mechanism with the experimental data, it was shown that the simplified mechanism can

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be applied to the numerical simulation of Jet A fuel combustion. Many other researchers (5-7) also

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investigated the skeletal mechanisms and mechanism reduction for aviation kerosene surrogate

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fuel. Moreover, using n-dodecane as a monocomponent surrogate fuel for RP-3 kerosene, a

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reaction model involving 11 components and 17 elementary reactions was proposed for

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n-dodecane by Wang et al.(8). The cracking of kerosene, the formation mechanism of carbon

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smoke, and the principle of oxidation were also explored. Patterson et al. (9) developed a

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surrogate for Jet A-1 kerosene comprising 89% n-decane and 11% methylbenzene to conduct

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experimental and mechanistic research. Honnet et al. (10) developed a surrogate for Jet A-1

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kerosene consisting of 80% n-decane and 20% 1,2,4–3-methylbenzene. The proposed mechanism

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involved 118 components and 914 elementary reactions. Vovelle et al. (11) developed a surrogate

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for Jet A-1 kerosene consisting of 90% n-decane and 10% methylbenzene. They proposed a

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simplified kinetic model consisting of 39 components and 207 reactions. Subsequently,

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researchers developed more complex multi-component dynamic models. Prior to this, the main

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principle involved adding naphthenic hydrocarbons to surrogate fuels consisting of saturated

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alkanes and aromatic hydrocarbons. For example, a detailed model was proposed involving a

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three-component surrogate fuel for Jet A-1 comprising 74 mol% n-decane, 11 mol%

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1,3,5-trimethylcyclohexane, and 15 mol% n-propylbenzene (12). This model consisted of 209

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components and 1,673 reactions. Chitral kumar V (13) introduced a detailed chemical kinetic

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mechanism for surrogate aviation fuel. First, using an available mechanism, the reaction

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mechanism was constructed by coupling the mechanisms of components of surrogate fuel, then

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adding the reaction mechanism for NOx. Second, a reaction mechanism for the three-component


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surrogate fuel was proposed. Finally, ideal numerical results were obtained after refining the main

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mechanism. Using 71% n-decane, 13% trimethylcyclohexane, and 16% ethylbenzene as a

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three-component surrogate fuel for kerosene, a detailed three-component surrogate fuel

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mechanism model including 109 components and 946 elementary reactions was proposed by Xiao

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(14). By using a quasi-steady-state hypothesis simplification method, a simplified reaction model

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of an overall reaction with 22 components and 18 elementary reactions was obtained. Additionally,

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the ignition delay time and the laminar flame speed were calculated by using the simplified

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mechanism. The calculation results were consistent with the experimental data for kerosene

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ignition. A mixture composed of 73 mol% n-dodecane, 14.7 mol% 1,3,5-trimethylcyclohexane,

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and 12.3 mol% n-propylbenzene was applied as a surrogate for RP-3 kerosene (15). The

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simplified mechanism involved 138 components and 530 reactions. A mixture consisting of 40

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mol% n-decane, 42 mol% n-dodecane, 13 mol% ethylcyclohexane, and 5 mol% p-xylene was

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proposed by Yu (2) as a four-component surrogate fuel for RP-3 kerosene. This was developed

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into a detailed mechanism containing 168 components and 1,089 elementary reactions. The results

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showed that the flame propagation speed obtained by this detailed mechanism was consistent with

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the measured RP-3 kerosene flame propagation speed. A four-component surrogate fuel for JP-8

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aviation kerosene proposed by Montgomery (16) consisted of 32.6 mol% decane, 34.7 mol%

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n-dodecane, and 16.7 mol% methylcyclohexane. The detailed chemical reaction mechanism of the

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four-component surrogate fuel involved 164 components and 1,162 elementary reactions. The

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mechanism of the detailed reaction was simplified to obtain two simplified reaction mechanisms

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composed of 15 and 20 components. The ignition delay times of the two simplified reaction

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mechanisms were consistent with the test data. From the above studies, it can be concluded that


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Energy & Fuels

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surrogate fuels for aviation kerosene consisted of three major categories: alkanes (straight-chain

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and branched paraffin), cycloalkanes, and aromatics. The main surrogate fuels for aviation

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kerosene were n-decane, n-dodecane, n-propylbenzene, n-butyl benzene, 1,2,4–3-methylbenzene,

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methylbenzene,

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hydrocarbons and other trace elements were not generally considered by researchers.

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3. Construction of simplified mechanism for three-component surrogate fuel

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In

this

1,3,5-trimethylcyclohexane,

study,

a

mixture

composed

and

of

methylcyclohexane,

73%

n-dodecane

while

unsaturated

(n-C12H26),

14.7%

141

1,3,5–3-methylcyclohexane (C9H18), and 12.3% n-propylbenzene (PHC3H7) was selected as a

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surrogate fuel for RP-3 aviation kerosene (15). The suitability of the proportion of each

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component was verified and a semi-detailed mechanism involving 257 components and 874

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elementary reactions was proposed (15). Additionally, by utilizing test data of aviation kerosene

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ignition delay times published by Tang (17) to verify the ignition delay time of this detailed

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mechanism, the results showed that the model can accurately describe the high-temperature

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ignition characteristics of RP-3 kerosene.

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Therefore, the detailed mechanism of the three-component surrogate fuel used in this study

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was the semi-detailed mechanism (257 components and 874 elementary reactions) of RP-3

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aviation kerosene that was previously verified (15, 18). However, as the number of components

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and elementary reactions remained high, this could not be applied to the numerical simulation for

152

combustion of RP-3 aviation kerosene in an aeroengine combustor. Further simplification was

153

therefore necessary. A three-step simplification scheme was adopted in this study. In the first step,

154

a directed relation graph model (DRG) (19) was used to simplify the semi-detailed chemical

155

kinetic model, and skeleton simplification mechanism Ⅰ was obtained. In the second step, a


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directed relation graph based on error propagation model (DRGEP) (20) was used for further

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simplification on the basis of simplification mechanism Ⅰ. Additionally, computational singular

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perturbation model (CSP) (21) was used to remove redundant reactions to obtain simplification

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mechanism. In the third step, a path analysis method in CHEMKIN-PRO software (22) was used

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to analyze the combustion paths at atmospheric pressure and high temperature. By comparing the

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detailed mechanism and simplification mechanism Ⅰ, the unimportant reaction paths (elementary

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reactions with low chemical reaction rates) were removed, and the important paths removed in the

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first and second steps were reinstated. Finally, a simplified mechanism for a three-component

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surrogate fuel suitable for combustion at atmospheric pressure and high temperature was obtained,

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which involved 59 components and 158 elementary reactions. Experimental data of RP-3 aviation

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kerosene ignition delay times published by Changhua Zhang (23) and test data of RP-3 kerosene

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laminar flame speeds published by Yu (2) were used to verify the simplified mechanism proposed

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in this study.

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The aim of the mechanistic study was to obtain a detailed understanding of each elementary

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reaction, to clarify the paths and the order of the reactions, and finally to clarify the rate and trend

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of the actual combustion reaction. Clarifying the reaction paths of the mechanism is essential for

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controlling the reaction and predicting the external macroscopic phenomena. To study the reaction

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mechanism of RP-3 kerosene combustion, it is necessary to understand how specific combustion

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paths of aviation kerosene develop. Some studies have been published on the analysis of paths.

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Generally, the reaction rate analysis method in CHEMKIN-PRO is used to analyze the main

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reaction paths of fuel molecules at high and low temperatures during combustion. At low

177

temperatures, fuel is consumed by dehydrogenation, oxygenation, and isomerization of large


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molecules and cracking reactions of ketones. At high temperatures, intermediate products

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generated at low temperatures are cracked into smaller molecules such as CH4. This may occur via

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cleavage of C-O, C-C, and β-bonds. The first and second steps of the simplification process have

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already been described (24). Based on the first two steps, the third step involving simplification

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and optimization using path analysis was performed in this study.

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In the third step, path analysis was adopted for quantitative and qualitative analysis of

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three-component surrogate fuel cracking using the simplified mechanism obtained in the second

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step and the detailed mechanism separately under the working conditions, including equivalence

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ratio Ø = 1, P = 1 atm, T = 1,150 K, 1,600 K, and 2,000 K. Additionally, paths that are not

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applicable at atmospheric pressure, those with low reaction rates, and unimportant paths were

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removed, while some important paths that were removed by the simplification program were

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reinstated. The simplified mechanism obtained involved 59 components and 158 elementary

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reactions (See Appendix A The simplified mechanism of three-component surrogate fuels for RP-3

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aviation kerosene for details).

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Figure 1 shows the probability of path analysis diagrams of fuel cracking during ignition

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delay times at three different initial temperatures for the simplified mechanism proposed in this

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study. As the ignition process of fuel plays an important role in mechanistic studies, and more than

195

half of the ignition delay time is occupied by macromolecular cracking, this study focuses on the

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path analysis of macromolecules of three-component surrogate fuel during the ignition delay time.

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In this study, the mechanism was simplified, while its cracking process was explored. In the first

198

step of Figure 1(a), s0C12H26 was cracked to nC3H7, s10C9H19, s12C5H11, and s13C7H15

199

through 109th and 110th reactions. The three intermediate components finally formed nC3H7


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through three paths via a series of reactions. Finally, nC3H7 was dehydrogenated to form C3H6

201

through the 97th reaction. Meanwhile, small molecules C2H4 and CH3 were formed through the

202

78th elementary reaction. As shown in Figure 1(b), 1,3,5–3-methylcyclohexane (s1C9H18) was

203

gradually cracked and oxidized via two paths into small elementary molecular groups. First,

204

s1C9H18 formed s58C9H18 and s59C9H18 via the 118th and 119th isomerization reactions,

205

respectively. The two components were then dehydrogenated with CH3 to form s164C9H17 and

206

s167C9H17 via the 121st and 122nd reactions. Next, nC3H7, iC3H7, and s482C6H10 were formed

207

via the 123rd and 124th cracking reactions. Then, s482C6H10 reacted with H, CH3, and OH to

208

crack into C4H6 and C2H3 via the 125th, 126th, and 127th reactions. Finally, nC3H7, iC3H7, and

209

C4H6 cracked into small molecules. Figure 1(c) shows that paths of n-propylbenzene PHC3H7

210

were significantly different from those of alkanes. The unique step of the high-temperature

211

combustion of aromatic hydrocarbons is a ring-opening reaction; the first mode of ring opening

212

was cleavage of a carbon-carbon bond on a single ring to generate a diradical, which cracked at

213

high temperature to form isoolefin or other small molecular groups. The second mode was the

214

reaction of alkyl on a single ring to form a mono-ring alkyl by hydrogen extraction, followed by

215

ring opening by cleavage of a β-bond. Reactions of the species after ring opening were consistent

216

with alkane cracking. As regards PHC3H7, in the first step, BPHC3H6 and CPHC3H6 were

217

formed by dehydrogenation via the 128th, 129th, 130th, 131st, 132nd, 133rd, 134th, 135th, and 136th

218

elementary reactions. Simultaneously, cracking was performed to form nC3H7, C6H6, PHCH2,

219

and APHC2H4 via the 137th, 138th, and 139th elementary reactions. C6H5 was then generated

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through a series of cracking and dehydrogenation reactions, after which a portion of C6H5

221

cracked into C5H5 via the 105th reaction, while the remainder was oxidized to C6H5O via the


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Energy & Fuels

222

104th and 106th reactions. Finally, small molecules such as C3H3 and C2H2 were formed by

223

cleavage of C-O, C-C, and β-bonds, cracking reactions, partial dehydrogenation, and

224

isomerization of C5H5.

225

In this study, The Bunsen burner employed in this paper works at atmosphere environment.

226

The Bunsen burner is feed with gaseous aero kerosene, and the fuel is premixed with the air. So

227

the factors affecting the combustion are relatively simply, for example, there is no atomization,

228

evaporation and mixing in the combustion process. For the simplification process, the premixed

229

model is also utilized in CHEMKIN to explore the important chemical reactions. Then the

230

numerical simulation is conducted under the premixed and pre-vaporized circumstance. Based on

231

this, the simplified mechanism is prone to be compared and verified by the experimental data. In

232

addition to the qualitative analysis of the simplified mechanism for three-component surrogate

233

fuel during the ignition delay time under conditions including equivalence ratio Ø = 1,

234

atmospheric pressure P = 1 atm, and temperature T = 1,150 K, 1,600 K, and 2,000 K, quantitative

235

analysis was also performed. The rates of all reactions at all micro-moments during the ignition

236

delay time at three different ignition temperatures were statistically analyzed to compare the

237

effects of different initial temperatures on the chemical reaction paths. The main principle was to

238

quantitatively calculate the proportion of each small path (each arrow) in the current consumption

239

process (the tail of the arrow represents the consumed material) through chemical reaction rates

240

during the ignition delay time. Combined with Figure 1(a, b, and c), it can be seen that the

241

proportion of dehydrogenation and isomerization paths of macromolecules and most intermediate

242

components decreased as the temperature increased, and the proportion of cracking paths

243

increased. The proportion of cracking paths of some intermediate components was reduced, but


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244

oxidation rather than dehydrogenation increased in proportion. Therefore, it can be concluded that

245

at relatively low temperatures, due to the energy being relatively low, C-H bonds were easily

246

broken, while C-C, C-O, and β-bonds were not, which increased the proportion of

247

dehydrogenation and isomerization paths at relatively low temperatures. At relatively high

248

temperatures, C-H, C-C, C-O, and β-bonds break, but C-C, C-O, and β-bonds react faster than

249

C-H. Thus, at high temperature, the proportion of cracking paths was larger than that of

250

dehydrogenation paths.

251 252

1(a) n-dodecane


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Energy & Fuels

253 254

1(b) 1,3,5-trimethylcyclohexane

255 256

1(c) n-propylbenzene

257

Figure 1. Path analysis of simplified mechanism of three-component surrogate fuel at three initial

258

temperatures during the ignition delay time

259

4. Sensitivity analysis

260

Sensitivity analysis is an ideal method available in CHEMKIN for judging and screening

261

important reactions in chemical reaction kinetic models. By studying the effects of small


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262

disturbances on the whole system, the elementary reactions in the model that play important roles

263

in the combustion process can be found. Temperature sensitivity analyses mainly study the effects

264

of temperature disturbance on reaction rate of the simplified mechanism. That is, sensitivity

265

analyses investigate the internal chemical reaction mechanisms of all species. From temperature

266

sensitivity analysis, primary elementary reaction (controlled step) could be explored. Currently,

267

the Jacobi matrix method is used to analyze the sensitivity of mechanistic models. In this study,

268

sensitivity analysis of the steady adiabatic flame temperature of the simplified mechanism was

269

performed at atmospheric pressure and different initial temperatures.

270

Figure 2 shows the sensitivity analysis of the simplified mechanism proposed in this study at

271

atmospheric pressure and different initial temperatures. Each part of Figure 2 lists the 10 reactions

272

with the highest temperature sensitivity coefficients. The simulation conditions included P = 1 atm,

273

Ø = 0.1, T = 1,400 K, 1,600 K, and 2,000 K. In Figure 2(a)~(d), it can be seen that as the initial

274

temperature increased, the overall temperature sensitivity coefficient decreased. This is because

275

most elementary reactions can reach the required activation energy at high temperatures, so they

276

become increasingly insensitive to temperature as the temperature increases. Furthermore, the

277

elementary reaction H + O2 OH + O can be seen as the main OH formation reaction, wherein

278

the OH group is considered to symbolize ignition and combustion; this elementary reaction was

279

sensitive to temperature at different initial temperatures. As the initial temperature increased,

280

compared to other elementary reactions with positive sensitivity coefficients, the temperature

281

sensitivity of the reaction increased (although not significantly), indicating that this reaction

282

promoted combustion and temperature increase. As regards reactions with negative sensitivity

283

coefficients, it can be seen that the sensitivity of reaction CHO + O2 = CO + HO2 was relatively


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284

larger, which indicated that this reaction inhibited ignition, combustion, and temperature increase.

285

Additionally, the cracking and dehydrogenation reactions of macromolecules such as s0C12H26

286

and PHC3H7 were among the first ten reactions at 1,200 K, but as the initial temperature

287

increased, the ten reactions with the highest temperature sensitivities were all violent reactions of

288

small molecular groups.

289 290

2(a) 1,200 K

2(b) 1,400 K

2(c) 1,600 K

2(d) 2,000 K

Figure 2. Sensitivity analysis of simplified mechanism of three-component surrogate fuel 5. Verification of multi-component simplification mechanistic model

291

A study on fuel combustion requires reliable and detailed kinetic analysis of the fuel ignition

292

process, flame propagation characteristics, flame stability, and concentration changes of

293

intermediate components. The ignition and flame propagation characteristics, adiabatic flame


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294

temperature, and concentrations of some intermediate groups were selected to verify the proposed

295

simplification mechanism for three-component surrogate fuel for RP-3 aviation kerosene. The

296

most important parameter of the ignition characteristics was the ignition delay time, and that of the

297

flame propagation characteristics was the laminar flame speed. In this paper, different mechanisms

298

schemes are listed in Table.1.

299

Table.1 Mechanisms schemes of different references

Case A B C D E 300

Author XU Jia-Qi XU Jia-Qi Kundu P K M. I. Strelkova Dai Chao

reference (15) (15) (4) (25) (26)

Species and reactions 257 species and 874 reactions 138 species and 530 reactions 16 species and 23 reactions 25 species and 38 reactions 36 species and 62 reactions

5.1 Ignition delay time

301

The ignition delay time occurs because, although the initial pressure, initial temperature, and

302

other parameters of the combustion system meet the critical ignition conditions at t = 0,

303

combustion does not begin until the temperature reaches Tc at t = t0, where t0 is the ignition delay

304

time. The ignition delay time is a crucial parameter for fuel ignition and combustion. It is also an

305

essential criterion for judging the rationality of a chemical kinetic model.

306

Firstly, the ignition delay time under different working conditions was calculated using the

307

closed homogeneous reaction module of CHEMKIN. It was then compared with the results of

308

other researchers. Figure 3 compares the simulated ignition delay times for the mechanism

309

proposed in this study, the results of other researchers, and the test data for aviation kerosene

310

published by Changhua Zhang et al. (23) under the following conditions. The mechanisms from

311

the literature is used for the model predictions of detail and simplified mechanisms. The pressure

312

(P) was 1 atm, the equivalence ratio (Φ) was 1, and the temperature was in the range 1,150–1,550


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K. The graph shows that the ignition delay time of the simplified mechanism proposed in this

314

study was consistent with the test data. The difference between them is less than 6.5%, which is

315

acceptable for engineering. While the ignition delay times of case A, B and C were all close to the

316

test data, the trends were different. The difference of ignition delay time between case D and the

317

test data is larger under these working conditions. In summary, Figure 3 shows that the proposed

318

simplified mechanism is closer to the test data when P = 1 atm and Φ = 1.

Figure 3. Ignition delay time for P = 1 atm and Φ = 1 319

5.2 Laminar flame speed

320

The laminar flame speed is an important parameter of the propagation characteristics of fuel

321

combustion. To verify the accuracy of the proposed simplified mechanism, its laminar flame speed

322

had to be verified. The laminar flame speed (S) is a function of the temperature coefficient (A) and

323

the reaction rate (W). It is a physical and chemical parameter of a combustible gas (and the most

324

affected by temperature).

325

The laminar flame speed without stretch was calculated with the premixed laminar

326

flame-speed calculation module in CHEMKIN-PRO. Figure 4 shows a comparison of the laminar

327

flame speeds of case A, B, C, E and the simplified mechanism proposed in this study, and the


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328

experimental data of the laminar flame speed is from the literature (2). The test data for RP-3

329

aviation kerosene published by Yu (2) worked under the same conditions where P = 1 atm, Φ =

330

0.7–1.4, and the initial unburned mixture temperature is 403 K. Figure 4 shows that while the

331

difference between the test data and the numerical simulation results of case E was 35%, they

332

follow the same trend. The difference between the results of case C and the test data was relatively

333

larger; furthermore, they followed different trends. As regards the simplified mechanism proposed

334

in this study, case A and case B, their results were consistent with test data. At Φ = 1.1, the laminar

335

flame speed calculated using the proposed mechanism was 67.2 cm/s, and the difference between

336

this and the test data (63.1 cm/s) was 6.5%. Additionally, they followed similar trends. Overall,

337

while the model predictions of the ignition delay time has a good agreement with the experimental

338

data, it has some different for the laminar flame speed, as shown in Figure 4. This is because the

339

numerical simulation is conducted under ideal conditions, in contrast, in the experimental

340

conditions, the heat transfer, measurement error, etc. will also have some effects on the results.

341

Even so, the simplified and detail mechanisms have good coincide characteristics with the

342

experimental data.

Figure 4. Laminar flame speed at P = 1 atm and T = 403 K


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6. Application of simplified mechanism for three-component surrogate fuel for RP-3 aviation

344

kerosene in numerical simulation of premixed pre-evaporation combustion

345

6.1 CFD numerical simulation and calculation conditions

346

To verify the engineering applicability of the proposed simplified mechanism for RP-3

347

aviation kerosene, in this study, the CFD calculation software FLUENT was used to simulate the

348

premixed pre-evaporation combustion flame of a Bunsen burner. The proposed simplified

349

mechanism was applied to the calculation to simulate the chemical reactions of RP-3 aviation

350

kerosene (24). The simulation results were compared with experimental data (24,27) to verify the

351

feasibility of the simplified mechanism for practical combustion problems. Figure 5 shows a

352

physical model of a Bunsen burner adopted in the numerical simulation. The outlet diameter of the

353

Bunsen burner was 12 mm and its height was 20 mm. It was surrounded by a lampshade that was

354

600 mm high and 350 mm wide. As regards the calculation model, the boundary conditions were

355

as follows. The exit of the Bunsen burner was set as a mass flow inlet, the air entrance of the

356

lampshade was set as pressure inlet boundary condition, the exit of the lampshade was set as

357

pressure outlet boundary condition, and the wall of the lampshade was set as an adiabatic wall.

358

Figure 6 shows the computational grids for the physical model of the Bunsen lamp and Figure 7

359

shows local enlarged detail. Because of the obvious velocity shear layer at the outlet of the nozzle,

360

the grid of fuel nozzle and a part of region close to nozzle outlet are refined. These were

361

unstructured and the mesh number was 2,430,000. Additionally, the grid independence was

362

performed before simulation. The velocity magnitude distributions along y axis on z=0.3m for

363

different grid number are presented in Figure 8. The number of grid is 1.46, 2.07, 2.43 and 3.18

364

million, respectively. The figure shows that the velocity distributions of 2.43 and 3.18 million


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365

grids are in good agreement, indicating that 2.43 million grids employed in this paper are

366

reasonable. The mathematical models used in the numerical calculation were the standard

367

K-epsilon turbulence model, the component transport model, and the eddy-dissipation-concept

368

model. The local mixture of fuel and air will be accelerated due to heat release of combustion. The

369

fluctuation in velocity will also be enhanced. In such, the flow under the combustion environment

370

is turbulent and the turbulent model should be employed, and the standard k-epsilon turbulence

371

model is employed in this paper. Discrete equations of pressure, momentum, energy, and all the

372

components were second order upwind scheme. Discrete equations of the turbulent kinetic energy

373

and turbulence dissipation were first order upwind scheme. In numerical simulation, the influence

374

of radiation is neglected. First of all, the radiation boundary conditions are not easy to determine.

375

Moreover, radiation models in FLUENT are not accurate enough. Additional equations will

376

increase the calculating time of numerical simulation at the current calculation level. According to

377

the numerical simulation convergence criteria standard of FLUENT, the numerical simulation was

378

judged to be convergent under conditions for which the relative errors of the inlet and outlet flows

379

were less than 1%, the residual of the continuity equations was below 1.0 × 10−6, and other

380

residuals were below 1.0 × 10−3.

381 382

Figure 5. Physical computation model of Bunsen burner

Figure 6. computational grid


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383 Figure 7. Local enlarged detail of the grid

Figure 8. Grid sensitive analysis (velocity magnitude distribution along y axis on z=0.3m)

384

Table 2 shows the conditions of the numerical simulation for premixed pre-evaporation

385

combustion, which were consistent with those in references (24) and (27). The oil gas ratio for

386

condition 1 was slightly higher than that for ideal conditions. Condition 2 represented oil-rich

387

conditions and condition 3 represented oil-lean conditions. The atmospheric temperature of the

388

test environment was 300 K, and its atmospheric pressure was 101,325 Pa.

389

Table 2. Numerical simulation conditions Pipe

Case

Fuel flow

Air flow

Inlet

Equivalence

ml/h

L/h

temperature/K

ratio

1

80

700

430

1.07

2

90

900

430

0.94

3

100

800

430

1.19

diameter/mm

12

390

6.2 Analysis of numerical results

391

Figure 9 shows the temperature contour maps of the center section of the premixed

392

pre-evaporation combustion flame of a Bunsen burner. As shown in the figure, the temperature

393

distribution of the Bunsen burner flame was the same under three working conditions, and the

394

premixed pre-evaporation gas of aviation kerosene and air was ejected from the exit of the Bunsen


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395

burner. This burned rapidly after ignition and formed a flame front, and the temperature rose to

396

form a red high-temperature zone. In front of the high-temperature zone, the inner flame exits

397

from a triangular thin layer region. At the back of the high-temperature zone, due to the decrease

398

in combustion intensity, the post-combustion airflow moved downstream, entraining surrounding

399

cold air, and the temperature gradually decreased and finally tended to ambient temperature.

400

However, there are some significant differences between the three conditions. Under condition 2

401

(oil-lean), the length of the high-temperature area (over 1,800 K) along the axis was 0.065 m,

402

which was shorter than those of condition 1 (0.069 m) and condition 3 (0.085 m). This was

403

because under oil-rich conditions, in addition to the premixed flame, there was diffusion flame,

404

which increased the length of the high-temperature area.

(a) Condition 1

(b) Condition 2

(c) Condition 3

405

Figure 9. Temperature contour maps of premixed pre-evaporation combustion flame in center

406

section

407

Figure 10 compares the temperatures on the central axis for the simulation results and test

408

data under the three conditions. The simplified mechanism is proposed in this paper and the

409

experimental data is resulted from the current work (from the Bunsen burner). In the experiment,

410

B-type thermocouple is used to measure the temperature of Bunsen burner, and its uncertainty is

411

±5K. In this figure, coordinate 0 represented the nozzle exit position. In the region of temperature

412

increase, the experimental data for each sampling point almost agreed with the calculated values,


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and they showed the same declining trend.

414

There were two explanations for the higher values of the calculation results in comparison

415

with the experimental data at the maximum value: 1) Errors originated from the measuring

416

equipment. 2) Numerical calculations were performed in an ideal environment, while the

417

experiment may have been subject to atmospheric interference in the environment, and the cooling

418

conditions for the experiment were different from those for the simulation.

(a) Condition 1

(b) Condition 2

(c) Condition 3

Figure 10. Temperature of premixed flame on central axis 419

Figure 11 shows the isoline nephogram of the OH mass fraction on the central section.

420

Hydroxyl, -OH, is regarded as a marker of the flame front and heat release rate. In this study, the

421

flame surface is judged using the OH diagram. In Figure 11(b), condition 2 corresponded to the

422

flame structure in oil-lean working conditions. It can be clearly seen that a high OH concentration

423

was present only at the outlet of the nozzle. This was because under this condition, premixed

424

flame existed without diffusion flame. Additionally, the cone of the premixed flame shrank to a

425

minimum under this condition. Figure 11(a) (condition 1) and Figure 11(c) (condition 3) show the

426

flame structures for oil-rich conditions. Condition 1 represents slightly oil-rich conditions, with

427

two layers having high OH concentrations. The diffusion flame front formed at the back of the

428

premixed flame. As condition 3 was more oil-rich than condition 1, the two layers can be seen

429

more clearly. Additionally, a crescent hollow area can be clearly seen between the two layers,


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430

which indicates separation between the premixed and diffusion flame. From the above simulation

431

results, it can be concluded that under oil-rich conditions, both premixed and diffusion flame

432

existed. However, only premixed flame existed under oil-lean conditions. By comparing the three

433

conditions, it was found that under condition 2 (oil-lean condition), the maximum OH

434

concentration was the highest, that under condition 1 (slightly oil-rich) was second, while the

435

maximum OH concentration under condition 3 (oil-rich condition) was the lowest.

(a) Condition 1

(b) Condition 2

(c) Condition 3

Figure 11. Isoline nephogram of OH mass fraction of the central section 436

Figure 12 shows the contours of the CO2 mass fraction of the central section under the three

437

conditions. In the experiment, the sampling gas of the Bunsen burner is transmitted to an infrared

438

continuous gas analyzer (SIEMENS U23, Germany) to measure the volume concentrations of CO,

439

NO, O2, and CO2 via a sampling pipe. The sampling pipe is electrically heated to assure the

440

measurement accuracy. The measuring range of CO2 volume concentration is 0 to 10%, and the

441

measuring lever is 1%. The large molecules cracked into small intermediate molecules at the exit

442

area of the Bunsen burner and eventually converted from CO to CO2 (19th reaction step CO + OH

443

↔ CO2 + H). The OH concentration directly determined the generation rate and concentration

444

distribution of CO2, and the OH concentration in the flame front was generally the highest.

445

Therefore, the highest concentration of CO2 appeared behind the flame front. Under condition 2

446

(oil-lean conditions, Figure 12(b)) the high concentration region of CO2 was further forward on


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447

the central axis. This was because the premixed flame was further forward under these conditions.

448

A high concentration region of CO2 (the region where the mass fraction of CO2 is higher than 0.14)

449

appeared at 0.018–0.056 m on the central axis. It can be clearly seen that high concentration

450

regions were relatively far from the nozzle exit under conditions 1 and 3 (oil-rich conditions). This

451

is because under oil-rich conditions, both premixed and diffusion flame existed; CO is generated

452

due to the oxygen deficiency of the premixed flame, and CO2 is formed by CO combustion in the

453

high OH concentration region of the diffusion flame. A high concentration of CO2 appeared at

454

0.03–0.061 m on the central axis under condition 1, and under condition 3, it appeared at

455

0.045–0.075 m. Figure 13 compares the CO2 along the central axis for the simulation results and

456

experimental data under the three conditions. Figure 13 shows that the CO2 concentration trend

457

was the same for the calculation results and experimental data. The highest concentration of CO2

458

was situated at Z = 0.032 m, which was closer to the nozzle exit under condition 2 than under

459

condition 1 (Z = 0.051 m) and condition 3 (Z = 0.66 m). This phenomenon was due to their

460

different flame structures; under oil-rich conditions, most of the CO2 was generated at the back of

461

the diffusion flame, which was further behind than for the premixed flame under oil-lean

462

conditions.

(a) Condition 1 (b) Condition 2 (c) Condition 3 Figure 12. Contours of CO2 mass fraction of the central section


Energy & Fuels 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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) condition 1

(b) Condition 2

Page 26 of 32

(c) Condition 3

Figure 13. Mass fraction of CO2 along the central axis 463

Figure 14 shows the temperature contours of premixed flame under condition 2 at three

464

positions, Z = 9 mm, Z = 25 mm, and Z = 47 mm. The three figures show that the calculation

465

results for Z = 25 mm and Z = 47 mm are similar, with high-temperature regions. The

466

high-temperature areas are situated at the center of the sections and diffuse continuously along the

467

radial direction, forming concentric circles of different temperatures. The temperature along the

468

radial direction decreases gradually. Because the plane of Z = 25 mm is located in the center of the

469

high-temperature region of Figure 9(b), while that of Z = 47 mm is located in the tail of the

470

high-temperature region, the radius of the high-temperature region at Z = 25 mm is larger than that

471

at Z = 47 mm. As regards Z = 9 mm, there is a low-temperature region within the

472

high-temperature region due to Z = 9 mm being located at the core of the premixed flame, in

473

which there is unburned kerosene. This proves that the flame of the Bunsen burner is hollow, and

474

that its surface is conical. Figure 15 compares the premixed flame temperatures of the

475

experimental data and simulation results under condition 2 at Z = 9 mm, Z = 25 mm, and Z = 47

476

mm. As shown in Figure 14, the temperature initially increased and then decreased along the

477

radius. This was because the inner flame was located at height Z = 9 mm and there was unburned

478

low-temperature mixed gas in the inner flame. The radial direction passed through the mixed

479

low-temperature region to the flame surface, which caused the gradual increase in temperature. As


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Energy & Fuels

480

the radius increased from the flame surface to the ambient atmosphere, the temperature decreased.

481

As regards heights Z = 25 mm and Z = 47 mm, these planes did not pass109th

482

Through the Bunsen inner flame region, so the temperature decreased as the radius

483

increased. By comparing experimental data and simulation results, it was found that, despite

484

minor differences between them, the trends of the experimental and calculated values were almost

485

identical.

(a) Z = 9 mm

(b) Z = 25 mm

(c) Z = 47 mm

Figure 14. Temperature contours of premixed flame for different heights under condition 2

(a) Z = 9 mm

(b) Z = 25 mm

(c) Z = 47 mm

Figure 15. Comparison of premixed flame temperatures at different heights 486

7. Conclusions

487

In this study, DRG, DRGEP, CSP, path analysis and sensibility analysis methods were

488

adopted to simplify the detailed mechanism for a three-component surrogate fuel, and a simplified

489

mechanism involving 59 components and 158 elementary reactions was obtained. The ignition

490

characteristics and flame propagation characteristics were then selected to verify the proposed

491

simplified mechanism using CHEMKIN. Finally, the simplified mechanism for three-component


Energy & Fuels 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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

492

surrogate fuel for RP-3 aviation kerosene was applied to numerical simulation of a premixed

493

pre-evaporation combustion flame, and a detailed comparison was made between the simulation

494

results and the experimental results. The conclusions are as follows.

495

(1) The path analysis showed that for fuel macromolecules and most intermediate

496

components, as the temperature increases, the proportion of dehydrogenation and isomerization

497

paths will decrease, while the proportion of decomposition paths will increase.

498

(2) The simplified mechanism for three-component surrogate fuel set up using DRG, DRGEP,

499

CSP, path analysis, and sensibility analysis was consistent with experimental data. The results

500

showed that the proposed simplified mechanism was consistent with the detailed mechanism.

501

(3) The simulation results for a premixed pre-evaporation combustion flame using the

502

proposed simplified mechanism showed that parameters such as combustion temperature

503

distribution and concentration of CO2 were consistent with the experimental data. The

504

computational load is acceptable.

505

In conclusion, by comparing the simulation calculated using the simplified mechanism for

506

the three-component surrogate fuel proposed in this study with experimental data, it was found

507

that the proposed mechanism can successfully simulate the combustion of kerosene and predict

508

the kinetic characteristics of combustion such as ignition delay time and flame propagation speed

509

within a large range of equivalence ratios. Additionally, the simplified mechanism was applied to

510

simulate a premixed pre-evaporation combustion flame using RP-3 aviation kerosene as fuel. The

511

simulation results were consistent with experimental data, and the computation time is acceptable

512

for engineering, which proved that the simplified mechanism for the three-component surrogate

513

fuel (59 components and 158 elementary reactions) proposed in this study can be used for


Page 28 of 32

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Energy & Fuels

514

numerical simulation of RP-3 aviation kerosene combustion engineering.

515 516

Acknowledgments This work received funding from National Natural Science Foundation of China

517 518

(No.51676097, No. 91741118).

519 520

Corresponding author:

521

Dr. Yingwen YAN, E-mail: [email protected], Tel: +86-13770507240.

522 523

References

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1. Yan C.; Fan W. Combustion. Xi'an: Northwestern Polytechnic University Press. 2005.

525

2. Yu W. Study on flame speed and chemical reaction mechanism for alternative fuels of aviation

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kerosene. Beijing, Tsinghua University 2014, 5–9.

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3. Wu Y.; Modica V.; Yu X.; Grisch F. Experimental investigation of laminar flame speed

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measurement for kerosene fuels: Jet A-1 surrogate fuel, and its pure components. Energy

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Fuels 2018, 32(2), 2332-2343, DOI: 10.1021/acs.energyfuels.7b02731

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4.

Kundu P.; Deur J. A simplified reaction mechanism for calculation of emissions in

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hydrocarbon (Jet-A) combustion. Joint Propulsion Conference and Exhibit, Monterey, CA,

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United States, AIAA paper 1993, 1993–2341, DOI: 10.2514/6.1993-2341

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5. Zhong F.; Ma S.; Zhang X.; Sung C.; Niemeyer K. Development of efficient andaccurate

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skeletal mechanisms for hydrocarbon fuels and kerosene surrogate. Acta Mechanica Sinica

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6. Zettervall N.; Fereby C.; Nilson E. J. K. Small skeletal kinetic mechanism for kerosene


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combustion. Energy Fuels 2016, 30(11), 9801-9813, DOI: 10.1021/acs.energyfuels.6b01664

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7. Wu K.; Yao W.; Fan X. Development and fidelity evaluation of a skeletal ethylene mechanism

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under scramjet-relevant conditions. Energy Fuels 2017, 31(12), 14296-14305, DOI:

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