4 - surrogate mixtures to represent complex aviation and rocket fuels

8/12/2019 4 - Surrogate Mixtures to Represent Complex Aviation and Rocket Fuels

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462 EDWARDS AND MAURICE

Table 1 Typical classes of hydrocarbons in aviation fuels

Compound class Typical structure Name

n-parafn(n-alkane) n-dodecane

iso-parafn iso-octane

Naphthene butyl(cycloparafn) cyclohexane

Aromatic toluene

Alkene 1-pentene

Fig. 1 Total ion chromatogram of aviation gasoline and jet and rocket fuels. The labeled peaks are n-alkanes.

copper concentrations in fuels can be on the order of hundredsof parts per billion, a level that signicantly affects fuel thermalstability.

Surrogate Fuels

As discussed in the introduction,the type of surrogate fuel usedis dependent ideally upon the fuel properties that are being sim-ulated. For estimating heat transfer inside the fuel system, a sur-

rogate that gives reasonable physical properties(density, specicheat, viscosity, thermal conductivity)would be adequate. For ex-ample, as a rst approximation, dodecane has physical propertiessimilar to JP-7 and JP-8/Jet A over the 2001200F(100 650C)

temperature range,2;2528 as shown in Fig. 6. For simulating fu-els in situations where phase behavior is important, such as injec-tion modeling, then the fuel boiling range/phase behavior wouldhave to be simulated(Fig. 7).29 For example, Wood and cowork-ers performed experimental combustionstudies with JP-4 and JP-5surrogates.3;11 These surrogates, listed in Tables 4 and 5, were se-lected based upon ve criteria: 1 ) limit of 10 15 hydrocarbons,


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Table 4 JP-4 surrogate2

Compound Vol %

n-hexane 5.5Cyclohexane 8.0

n-heptane 8.0Methyl cyclohexane 8.0Toluene 8.0n-octane 8.0n-nonane 10.0

Cyclo-octane 8.0n-decane 10.0Decalin 5.0

Tetralin 1.0n-dodecane 10.01-methy l naphthalene 0.5n-tetradecane 10.0


Compound Blend 1, vol % Blend 2, vol %

n-decane 2.5 2.5Decalin 11.5 11.5

n-undecane 0.0 5.0

n-pentylcyclohexane 11.0 0.01,3-diisopropylbenzene 3.0 3.0

Tetralin 9.5 9.5n-dodecane 25.0 31.0

1-phenyl hexane 5.0 5.0n-tridecane 10.0 15.0n-heptylcyclohexane 11.0 0.0

1-methyl naphthalene 1.5 1.5n-tetradecane 5.0 11.0n-pentadecane 5.0 5.0

Fig. 6 Comparison of dodecaneproperties at 500 psia calculated withSupertrapp2;4 to published fuel property calculations.25 28

a simpler surrogate for fuel thermal oxidation studies in a askapparatus. The JP-8 surrogate model is listed in Table 6. The surro-gate reproducedthe generaloxidation behavior of JP-8, but did notreproducethe depositionlevels of distillatefuels.This is becauseofthe key role thattrace species,such as metals and heteroatoms,playin the depositionprocess.30 Distillate fuels can be groupedinto gen-eral classes by the method of processing(hydrotreatment, Merox,etc.)with the deposition generally being roughly similar for fuelsproduced by the same processing methods. A number of tests havebeen used to rank fuels by thermal stability,31 but there is no practi-cal method to assess the thermal stability of distillate fuels withouttesting the actual fuel sample under consideration.

The concept of surrogate fuels is also used for gasoline- anddiesel-fueledenginestudies(e.g., Ref. 32). Mixturesof heptane(oc-tane numberD 0)and iso-octane(octane numberD 100)are usedas primary reference fuels (PRF), with octane number adjustedby a linear combination of the two. However, it has been foundthat the simple two-componentsurrogatesdo not adequatelyrepro-duce the ignition behavior of the real multicomponent gasolinesand reference fuels in ow reactors and engines.33;34 The igni-tion of the PRFs occurred at a lower temperature than gasoline


Compound Wt %

iso-octane 5.0Methyl cyclohexane 5.0

m-xylene 5.0Cyclooctane 5.0Decane 15.0

Butyl benzene 5.01,2,4,5 tetramethyl benzene 5.0

Tetralin 5.0Dodecane 20.01-methyl naphthalene 5.0

Tetradecane 15.0Hexadecane 10.0

Table 7 RP-1 surrogate14

Compound Vol %

n-undecane 4.7n-dodecane 6.0n-tridecane 18.8n-tetradecane 12.5

n-hexylcyclopentane 2.7n-heptylcyclopentane 3.6n-octyl cyclopentane 11.2n-nonylcyclopentane 7.5Bicycloparafn 1 C11H20

a 11.3

Bicycloparafn 2 C12H22a 14.7

Pentamethyl benzene 1.3Hexamethylbenzene 1.7

Dimethylnaphthalene 4.0

a The identity of the bicycloparafns is not evident.

or standard multicomponent reference fuels. Addition of an alkene(1-pentene) andanaromatic (toluene) to theheptane/iso-octanemix-ture did reproducegasoline ignition behavior.33;34 The alkenes andaromatics acted as an ignition inhibitor. An effective gasoline sur-

rogate for ignition studies was determined to be 31.8 vol% toluene,4.6% 1-pentene,and 63.5% heptaneCiso-octane, with research oc-tane number adjusted by changing the ratio of the heptane andiso-octane.

AnRP-1 surrogatewas proposedbyFarmer et al.14 (Table7). Thissurrogate was developed for modeling purposes and was derivedfrom a detailed RP-1 analysis. The authors recognize that the aro-maticscontentis higher thanis really observed in practice (Table 2)

and probably resulted from a misidentication of some of the bi-cycloparafns as aromatics.14 The authors state that the surrogatecritical properties adequately match those of RP-1. An alternativeto surrogate fuels consisting of specic molecules is a surrogatemolecule that is just a representationof the overall stoichiometryofthe distillate fuel. This surrogate would be useful for performancecalculations, but not for experimental testing. A surrogate RP-1,C12H24, has been proposed.

13

JP-5, JP-7, JP-8, Jet A, and RP-1 all belong to the general classof fuel calledkerosene.Detailed kinetic modeling combustionstud-ies of kerosene aviation fuels are generally considered prohibitivebecause of their inherent chemical complexity. There have beenstudies of surrogates for kerosene, which can serve as general ex-amples. Aly and Salem35 have predicted the laminar burning ve-locitiesof lean to stoichiometriclaminar premixed kerosene amesusing a single global kinetic step. As a rst approximation,Gueretet al.16 modeled kerosene oxidation via quasi-global models forn-

decane, n-propylcyclohexane,tri-methyl benzene, xylene, toluene,and benzene. Concentration proles of molecular species in theow reactor were similar for the surrogate and kerosene; how-ever, the need for further renement of the aromatic models wasrecognized.16 Dagaut et al.17 later modeled kerosene combustionin low-temperature jet-stirred reactors using n-decane as a refer-ence hydrocarbon while neglecting the aromatic components andcaptured major species proles adequately. Ranzi et al.18 proposeda comprehensivereaction mechanism for higher-orderhydrocarbon


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EDWARDS AND MAURICE 465

Table 8 Recommendations on appropriate surrogate fuels

Physical situation Minimum complexity surrogate fuel recommendation

Single phase heat transfer without chemical reaction Single componentn-parafn with approximately correct critical temperatureHigher precision heat transfer, including Use appropriate surrogate with correct chemical compound classes and representative

enthalpy effects mixture critical temperature

Fuel vaporization/injection/mixing Use appropriate surrogate that matches distillation(boiling)curve(multiphase, no reaction)

F uel ign ition Use appropriate su rrogate that matches t he impor tant chemical classes. If ignition-improving

additives present(e.g., nitrates), then will need to include additive chemistry

Fuel heat release, ame speed Use appropriate surrogate that matches the important compound classesNOx emissions duringfuel combustion Use appropriate surrogate that matches the major important chemical classes

Aromatic and PAH emissions during combustion Use appropriate surrogate that matches the major and minor important chemical classesthat contribute to molecular growth

General fuel thermal-oxidation behavior Use surrogate that matches chemical class breakdown in fuel(inside fuel system)

Pyrolytic depositionfrom fuels Use appropriate surrogate that matches the major and minor important chemical classes

that contribute to molecular growthThermal-oxidative depositionfrom fuels No good surrogatedepositionlevels driven by trace fuel species

(heteroatomic species, dispersants)

Soo t f ormatio n in combu stio n No go od su rro gateso ot levels d riven b y trace fu el sp eci es(heteroatomic species, dispersants)

Fig. 7 Phase diagram for Jet A and diesel fuels.29 **, critical point; , bubble point; and, dew point.

fuels,which only considersbenzenein a semi-empiricalmanner. Bycontrast,Foelsche et al.36 modeled the efux composition of a richJP-7/toluene-fueled gas generator operating at intermediate tem-peratures (900 1300 K)using the toluene oxidation kinetic modelof Emdee et al.37 to describe the aromatic constituents; a surro-gate could not representthe soot formation process.Vovelle et al.38

studied low-pressure kerosene ames and modeled the aromaticcomponent with a simplied toluenemechanism. Discrepanciesbe-tween computations and experimental observations were attributedto uncertainties in the aromatic model. Leung39 studied kerosenediffusion ames and modeled the aliphatic component with a quasi-global undecane submechanismwhile assuming the aromatic com-ponent to be benzene. The preceding studies recognize the need toinclude an aromatic submechanism, preferably considering mono-substituted aromatic molecules, in detailed kerosene kinetic mod-eling. Maurice40 used a surrogate model of 89 mole% decane and11 mole% of various aromatic surrogates: benzene, toluene, ethyl-benzene, and ethylbenzene/naphthalene as an input to a detailedchemical kinetic model, which was compared to a number of ex-perimental data sets for species proles in kerosene ames. Then-decane/benzenesurrogatecapturedmajor species proles, but failedto predict benzene concentrations. By contrast, the major charac-teristics of the kerosene ame, including benzene proles, werepredictedaccuratelyusing n-decane/alkyl-substitutedaromatic sur-rogate blends. No differences were noted amongsthe various alkyl-benzene surrogates. Addition of naphthalene did not affect major

characteristics of the ame. However, if aromatics are used a s in-puts to soot formation model (e.g., Ref. 41) individual aromaticconcentrationsin the parent fuel must be considered.

An outgrowthof the work of Lindsted and Maurice15;40;41 was thecomparisonbetweennitrogenoxides(NOx) and CO emissionsfromcombustion of Jet A in a well-stirred reactorand model predictionsusing a surrogate model of 78% n-decane and 22% ethylbenzene.42

The comparisonwas generallygood. A generalconclusionfrom thiswork was thata detailedaromatic componentneededto be addedtothe two-component surrogate model to properly predict moleculargrowth.

Summary and Recommendations

This paper has briey reviewed several surrogate fuels. If careis taken, the use of these surrogates can dramatically simplify ex-perimental and modeling efforts. The authors take the bold stepof suggesting some guidelines for the various classes of surrogate

applications,as listed in Table 8.In our opinion, the onlyphysical situationthat can be represented

witha single-componentsurrogateis single-phaseheattransfer.Fuelvaporization,injection, and mixing without chemical reaction mayhave to be representedby a multicomponentsurrogate.The formersurrogateneedonlycapturethecorrectcriticaltemperature,whereasthe latter must match the distillation (boiling)curve.

Higher precision heat transfer,fuel heat release and ame speed,fuel ignition, and general fuel thermal-oxidation behavior inside


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the fuel system can be captured with surrogate fuels, which matchthe correct major chemical classes. However, if additives such asignition-enhancers additives are used it is critical to include theadditives in the surrogate model.

NOx emissions during fuel combustion can be captured with asurrogate that matches important chemical classes that drive tem-perature.However, capturing aromatic and polycyclic aromatic hy-drocarbon (PAH) emissions during combustionand pyrolyticdepo-sitionfrom fuels requiressurrogatesthatmatchthe major and minor

important chemical classes that contribute to molecular growth.Finally, capturingthermal deposition from fuels and soot forma-tion from combustion is in our opinion not viable with a surrogatefuel. The physical processes are driven by trace fuel species (e.g.,

heteroatomic species and dispersants), which cannot be accuratelycaptured by a surrogate.

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Paper 99-1038, Jan. 1999.

4 - surrogate mixtures to represent complex aviation and rocket fuels

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