Numerical simulations and experimental results of endothermic fuel reforming for scramjet cooling

application

Emeric DANIAU*, Marc BOUCHEZ†

MBDA-F, 8 rue Le Brix, 18020 Bourges CEDEX – France

Olivier HERBINET‡, and Paul Marie MARQUAIRE§, DCPR, UNR 7630 CNRS-INPL, 1 rue Grandville, 54000 Nancy – France.

The last years saw a renewal of interest for hypersonic research in general and regenerative cooling specifically, with a large increase of the number of dedicated facilities and technical studies. In order to quantify the heat transfer in the cooled structures and the composition of the cracked fuel entering the combustor, an accurate model of the thermal decomposition of the fuel is required. This model should be able to predict the fuel chemical composition and physical properties for a broad range of pressure, temperature and cooling geometry. For this purpose, an experimental and modeling study of the thermal decomposition of generic molecules (long-chain or polycyclic alkanes) that could be good surrogates of real fuels, has been started at the DCPR laboratory located in Nancy (France). This successful effort leads to several version of a complete kinetic model. The last version for n-dodecane will be described in this paper and now include a detailed mechanism for Poly-Aromatics Hydrocarbons synthesis. Validation have been done for a wide range of temperature and pressure (650-1500 K and 1-100 bar) and good agreement was generally found between the numerical model and available experimental results. In addition to this model, other molecules have been studied to investigate the differences between linear alkanes and cyclanes hydrocarbons. The unimolecular decomposition of cyclic hydrocarbons involves either the breaking of a C-H bond, which has a large bond energy, or the opening of the ring and the formation of a diradical. The behavior of the diradicals is very different from the one of linear radicals and should be investigated in order to gain a good knowledge of the thermal stability of such cyclanes.

* Aerospace engineer, AIAA member, † Aerospace engineer, AIAA member, ‡ Ph.D student, § Senior CNRS researcher,

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14th AIAA/AHI Space Planes and Hypersonic Systems and Technologies Conference AIAA 2006-7975

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

I. Introduction One of the main issues of the development of scramjet, an air breathing engine that could be used for

hypersonic flight, is the thermal management of the vehicle and more especially of the engine1 2. Studies are currently conducted in France by ONERA and MBDA-France in order to develop an active cooling system that consists in using the fuel heat sink capability due to the endothermicity of its thermal decomposition. Because of the large heat load found in a scramjet, engine temperature is expected to increase well above 800 K leading in the thermal decomposition of the fuel circulating at their walls before the injection in the combustion chamber. A consequence of the decomposition of the heavy hydrocarbon fuel is the production of lighter species and of poly-aromatic compounds. In order to quantify the heat transfer through the walls of the engine and the composition of the fuel entering the combustion chamber, a detailed kinetic model of the thermal decomposition of the fuel is required.

To answer this problem, ONERA developed a plug reactor using a relatively large flow of fuel3, and MBDA-F collaborated with the DCPR laboratory to study fuel kinetic in a small perfectly-stirred reactor (PSR).

A first study of the thermal decomposition of n-dodecane, a component of some jet fuels (high flash point fuel like JP5, JP7 and JP8), allowed us to propose a detailed kinetic model made of 1175 reactions involving 154 molecular and radical species4. This model was mainly validated from experimental results obtained in a plug flow reactor for temperatures between 950 and 1050 K and for residence times ranging from 0.05 to 0.2 s. In order to get experimental data in a different type of reactor and for a broader range of experimental conditions the new study presented here has been performed using a jet-stirred reactor, which is a well suited apparatus for gas phase kinetic study5 6 7 8. During this work 32 products including poly-aromatic hydrocarbons that were formed at traces level have been quantified. These new experimental results have led us to propose a more complete detailed kinetic model including a set of reactions explaining the formation and the consumption of poly-aromatic hydrocarbons.

II. Experimental investigation

A. Experimental apparatus Liquid n-dodecane (reactant) is contained in a

glass vessel pressurized with nitrogen. Before performing a series of experiments, oxygen traces are removed from the liquid hydrocarbon through nitrogen bubbling and vacuum pumping. Liquid hydrocarbon flow rate between 1 and 50 g.hr-1 is controlled, mixed to the carrier gas (helium) and evaporated in a Controlled Mixer and Evaporator (CEM) provided by “Bronkhorst”. This apparatus is composed of a liquid mass flow controller followed by a mixing chamber and a single pass heat exchanger. Carrier gas flow rate is controlled by a “3 nL.min-1 tylan – RDM280” gas mass flow controller set upstream the CEM. Molar composition of the gas at the outlet of the CEM is 2% hydrocarbon and 98% helium. Working with high dilution presents two advantages: the variation of molar flow rate due to the reaction is very weak and can be neglected and strong temperature gradients inside the reactor due to endothermic reactions are avoided.

The thermal decomposition of the fuel was performed in a quartz jet-stirred reactor that was developed by Matras and Villermaux9.

Figure 1. Global scheme of the experimental apparatus.

This type of reactor (Figure 2) which was

already used for numerous gas phase kinetic studies5 6 7 8 is a spherical reactor in which diluted reactant enters through an injection cross located in its center and that can be considered as well stirred for a residence time (τ) between 0.5 and 5 s.

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The diameter of the reactor is about 50 mm and its volume is about 90 cm3. The stirring of the whole reactor volume is achieved by the mean of four turbulent gas jets directed in different directions and produced by the four nozzles of the injection cross in the center of the reactor. Inside diameter of the nozzles is about 0.3 mm. The reactor is preceded by a quartz annular preheating zone in which the temperature of the vapor mixture is increased up to the reactor temperature before entering inside. The annular preheater is made of two concentric tubes, the inter-annular distance of which is about 0.5 mm. Gas mixture residence time inside the annular preheater is very short compared to its residence time inside the reactor.

inside diameter ofthe nozzles : 0.3 mm

0.5 mm inner distance

reactant &carrier gas

to analytical section

annularpreheating

zone

jet stirredreactor

V = 90 cm3

inside diameter ofthe nozzles : 0.3 mm

0.5 mm inner distance

reactant &carrier gas

to analytical section

annularpreheating

zone

jet stirredreactor

V = 90 cm3

Both spherical reactor and annular preheating zone are heated by the mean of “Thermocoax” heating resistances rolled up around their wall. Temperature control of each part is made by “Eurotherm 3216” controllers and type K thermocouples. Reaction temperature measurement is made by means of a thermocouple which is located inside the intra-annular space of the preheating zone and the extremity of which is on the level of the injection cross.

Figure 2. Scheme of the jet stirred reactor and the annular preheating.

Pressure inside the JSR is close to atmospheric

pressure (about 106 kPa). It is manually controlled thanks to a control valve placed after the liquid hydrocarbon trap.

32 products of n-dodecane thermal decomposition were identified and quantified by gas chromatography and

can be classified in two groups. First, light species (on-line analysis) which are hydrogen and C1-C5 hydrocarbons: methane, acetylene, ethylene, ethane, allene, propyne, propene, 1,3-butadiene, 1-butene and 1-pentene. Second, C5-C16 species: 1,3 cyclopentadiene, cyclopentene, benzene, 1-hexene, toluene, 1-heptene, phenyl ethylene, 1 octene, indene, 1-nonene, naphthalene, 1-decene, 1-undecene, 1- and 2-methyl-naphtalene, biphenyl, acenaphthalene, phenanthrene, anthracene, fluoranthene and pyrene.

B. The n-dodecane kinetic model Experimental results have been obtained at temperatures from 793 K to 1073 K, residence times ranging

from 1 to 5 s, and with a reactant mole fraction at the inlet of the reactor of 2%. On the following graphs (Figure 3 to Figure 5), symbols correspond to experiments and curves to modeling.

Figure 4 represents the evolution of the conversion of n-dodecane vs. residence time (Figure 4a) and vs. temperature for a residence time of 1 s (Figure 4b). It shows that the conversion strongly increases with temperature between 873 K and 1023 K.

Figure 3. Conversion of n-dodecane versus (a) residence time and (b) temperature (τ=1s).

Figure 4 and Figure 5 present the evolution of the mole fractions of the products of the reaction vs. residence

time (Figure 4) and vs. temperature (Figure 5). The major products are hydrogen and ethylene. For most of the pyrolysis products, an increase of the residence time or of the temperature leads to an increase of the mole fraction (e.g. hydrogen, ethylene and aromatic compounds), while the mole fractions of 1-alkenes from 1-butene

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to 1-undecene show an increase with residence time and temperature followed by a decrease. These 1-alkenes are probably primary products which decompose when temperature or residence time increase.

Figure 4. Evolution of products mole fraction with residence time (T=973 K).

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Figure 5. Evolution of products mole fractions with temperature (τ =1s).

Primary products of n-dodecane thermal decomposition were determined from a selectivity analysis

performed at 873K (selectivity is the ratio between the mole fraction of the considered product and the mole fractions of all products). In these conditions, the conversion of n-dodecane is less than 20% (maximum conversion corresponding to the residence time of 5s).

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If the extrapolation to origin of the selectivity of a species vs. residence time is different from zero then this product is probably a primary product. This is the case for hydrogen, methane, ethylene, ethane, propene and heavier 1-alkenes from 1-butene to 1 undecene from which extrapolated initial selectivities are summarized in Table 1. The sum of the initial selectivities of primary species is equal to 0.991 (theoretical value: 1). Species with the highest initial selectivities are ethylene, methane, propene and hydrogen. At 873 K, residence time has little effect on selectivities. Selectivities of hydrogen and 1-alkenes from 1 hexene to 1 undecene slightly decrease in a monotonous way with residence time whereas selectivity of ethane increases. Selectivities of 1-butene, methane and propene decrease first with residence time and then increase whereas selectivities of 1-pentene and ethylene present the opposite trend.

C. Modeling of n-dodecane thermal decomposition A first detailed kinetic model of the thermal decomposition of n-dodecane, made of 1175 reactions involving

154 species, was built using EXGAS software10 11 12 13 according to rules of construction previously described. Agreement between available experimental data and computed results was reasonable taking into account that no kinetic parameters fitting was attempted and that only few species were quantified. Some changes were brought to this previous model: the kinetic parameters of two types of very important reactions (recombination of large radicals and retroene reactions of 1-alkenes) have been modified according to data taken from literature and a sub-mechanism involving poly-aromatic compounds was added.

A set of 273 elementary reactions involving 117 new species (mainly aromatics, poly-aromatics and related radical species) has been added to the modified existing model in order to explain the formation and the consumption of species such as cyclopentene, 1,3-cyclopentadiene and aromatic compounds from benzene to pyrene. These reactions have been described in two papers about propane pyrolysis. It appears that cyclopentadiene, benzene and their related radicals (cyclopentadienyl, phenyl and benzyl radicals) play an important role in the formation of heavier aromatic and poly-aromatic species. The model explains the formation of cyclopentadienyl radical and cyclopentadiene from the addition of acetylene on propargyl radical (C3H3).

Benzene is formed through three

pathways (Figure 6): the combination of two propargyl radicals or of propargyl and allyl radicals (C3 pathway), the combination of cyclopentadienyl and methyl radicals through the formation of methyl-cyclopentadiene (C5 pathway), and the addition of acetylene to C4 radicals followed by a ring closure (C4 pathway).

Figure 6. The three pathways to benzene formation.

D. Cyclanes studies Given the large number of products found in a typical aviation-kerosene (more than 300 species), it was

interesting to expand our effort on kinetic investigations and to focus on less known compound like cyclanes and, still more, of polycyclanes, that raise several kinetic problems which have not yet been much investigated.

The unimolecular decomposition of cyclic hydrocarbons involves either the breaking of a C-H bond, which has a large bond energy (99 kcal.mol 1) or the opening of the ring and the formation of a diradical.

O'Neal and Benson studied diradical mechanism in the case the pyrolysis of some four and five members ring compounds and of some polycyclic molecules14 15. For most species, Arrhenius parameters estimated by these two authors were consistent with available experimental data. More recently Tsang16 17 has proposed reaction channels to account for the fate of these diradicals in the case of cyclohexane and cyclopentane and Billaud et al. 18 in the case of decaline.

Concerning propagation reactions, the rate constants of the decomposition by β-scission involving the opening of a ring or of the isomerization involving cyclic or polycyclic transition states are still very uncertain.

Three molecules were selected for this study (cyclopentane, cyclohexane and norbornane) but only results obtained with cyclohexane are included in this paper.

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1. Hydrocarbons thermal stability It was first anticipated that the strain of

cyclic molecules will decrease their thermal stability, so the pyrolysis conversion rate of cyclopentane, cyclohexane and n-dodecane was compared (Figure 7) in a PSR with a residence time of 1 sec.

Even if we take into account the difference in the number of C-C bond in the different molecules, (less C-C bond means less thermal reactivity because breaking of C-C bond is the first step of the pyrolysis mechanism), we could conclude that cyclanes generally exhibit a higher thermal stability than their linear counterparts.

This is linked with the fact that diradicals are prone to react, giving back the initial molecule, when the breaking of a linear molecule gives 2 separates radicals.

0

20

40

60

80

100

700 800 900 1000 1100Temperature (K)

Con

vers

ion

rate

(%)

Cyclohexane N-dodecane Cyclopentane

Figure 7. Experimental dodecane, cyclohexane and cyclopentane conversion as a function of the temperature, in a perfectly stirred reactor, residence time is 1 sec.

2. Decomposition products distribution For two conversion rates (~14 % and ~81 %) of cyclohexane and n-dodecane, the mass fraction of products

(excluding the initial compound) were compared (Figure 8, Figure 9, Figure 10 and Figure 11).

ethylene21%

1,3-butadiene47% propene

13%

methane3%

cyclohexene4%

ethane3%

hydrogene2%

benzène3%

cyclo-pentadiene

3%

hydrogene methane acetylene ethylene ethanepropene propadiene propyne 1,3-butadiene 1-butyne1,2-butadiene vinylacetylene 1,3-cyclopentadiene benzène cyclohexene1-hexene toluène styrène

Figure 8. Decomposition products of cyclohexane for a conversion rate of 13.6 %.

1-undecene3%1-decene

7%

1-octene7%

1-heptene8%

1-pentene8%

1-butene7%

ethylene25%

hydrogene<1%

propene11%1,3-butadiene

1%

methane5%1-nonene

7%

ethane3%

1-hexene9%

hydrogene methane ethylene ethane propene1,3-butadiene 1,3-cyclopentadiene benzène 1-hexene toluène1-butene cyclopentene 1-pentene 1-heptene 1-octene1-nonene 1-decene 1-undecene

Figure 9. Decomposition products of n-dodecane for a conversion rate of 14.3%.

At low conversion rate, the main products are very different, the cyclohexane giving a very high fraction of

butadiene (47 %), compared to only 1 % for n-dodecane. On the other hand, n-dodecane produces a large amount (55 %) of middleweight 1-alkenes not found with

cyclohexane. The mass fraction of other main products is very similar with a low fraction of hydrogen (2 % vs. 1 %),

methane (3% vs. 5 %), ethane (3 % vs. 3 %), a significant amount of propene (13 % vs. 11 %) and a large mass fraction of ethylene (21 % vs. 25 %).

It should be pointed out that in both cases, ethylene and propene account for more than one third of the mass of decomposition products and present some interesting properties from a combustion point of view.

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acetylene2%

cyclo-pentadiene

3%

naphtalene4%

styrène2%

toluène2%

ethane1%

propene4%

hydrogene2% methane

8%

benzène18%

1,3-butadiene13%

ethylene38%

hydrogene methane acetylene ethylene ethanepropene propadiene propyne 1,3-butadiene 1-butyne1,2-butadiene vinylacetylene 1,3-cyclopentadiene benzène cyclohexene1-hexene toluène styrène naphtalene indeneacenaphtalene phenanthrene anthracene methylnaphtalene biphenyl

Figure 10. Decomposition products of cyclohexane for a conversion rate of 81.1 %.

ethylene39%

ethane3%

methane9%

hydrogene1%

propene21%

1-butene9%

1-pentene2%

1-heptene1%

benzène3%

1,3-butadiene5%

cyclo-pentadiene

2%

hydrogene methane acetylene ethylene ethanepropene propadiene propyne 1,3-butadiene 1,3-cyclopentadienebenzène 1-hexene toluène styrène naphtaleneindene acenaphtalene methylnaphtalene biphenyl 1-butenecyclopentene 1-pentene 1-heptene 1-octene 1-nonene1-decene 1-undecene

Figure 11. Decomposition products of n-dodecane for a conversion rate of 80.3 %.

At high conversion rate, the mass fraction of butadiene in cyclohexane products decreases (from 47 % down

to 13 %) whiles the mass fraction of middleweight alkenes from n-dodecane decreases also (from 55 % down to 15 %).

The mass fraction of other main products remains very similar with a low fraction of hydrogen (2 % vs. 1 %), ethane (1 % vs. 3 %), an increase of methane fraction (8% and 9 %), and a large mass fraction of ethylene (38 % vs. 39 %).

Propene is very interesting because its mass fraction decreases in the case of cyclohexane (from 13 % down to 4 %), and increases in the case of n-dodecane (from 11 % up to 21 %).

Another difference is the fraction of aromatic products (benzene, toluene, styrene, naphtalene, indene...),

much higher in the case of cyclohexane (nearly 30 %) than in the case of n-dodecane (only 4.5 %), so at first it seems that, given the link between aromatic products and gums formation in the channels of a cooling system, cyclanes could lead to fooling problems for long duration engine operation. This issue should be investigated in details because of the high number of unknown factors related to coke formation (fuel) and deposition (walls and fuel – walls interactions), and the fact that aromatic compound are generally secondary products and that their production rate is a function of the reactor residence time.

III. Conclusion The thermal decomposition of n-dodecane has been studied in a jet-stirred reactor over a wide range of

temperatures from 793 K to 1073 K and for residence times between 1 and 5 s. 32 products of the reaction have been analyzed. Major products are ethylene, methane and hydrogen (low mass fraction but high mole fraction). Other products are mainly 1-alkenes from propene to 1-undecene and aromatic compounds. The formation of aromatic compounds intervenes rather quickly; it takes importance for conversions higher than 25 %.

A detailed kinetic model generated by the EXGAS software has been completed by a set of elementary reactions involving aromatic and poly-aromatic compounds. The new model obtained is made of 1449 reactions and contains 271 species.

Agreement between new experimental results presented here and computed results is satisfactory especially for primary products such 1-alkenes. As far aromatic compounds are concerned a light variation between experiments and modeling is observed for long residence times especially. This may be related to uncertainties on kinetic parameters of reactions involved in the formation of benzene (C5-pathway) and poly-aromatics compounds. The new model reproduces well experimental data presented in a previous paper and obtained with a different type of reactor working in different conditions.

To expand our knowledge on the pyrolysis of other hydrocarbons, some cyclanes (cyclopentane, cyclohexane and norbornane) were also studied in our jet-stirred reactor.

It was found that the thermal stability of cyclanes is generally better than its linear alkane counterpart and that some difference could be seen in decomposition products distribution.

Light species (hydrogen, methane, ethylene, ethane, propene) fractions are very similar but while middleweight 1-alkene (from butene to undecene) account for a large part of the n-dodecane decomposition products (at small conversion rate), 1-3 butadiene plays the same role for cyclohexane.

At higher conversion rate, ethylene becomes the major decomposition product (between 35 % and 40 %) and methane fraction increases but remains below 10 %. Aromatics like benzene, toluene, and styrene, increases also

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and in the case of cyclohexane account for nearly 30 % of decomposition products, compared to less than 5 % for n-dodecane in similar conditions.

Since jet fuels are generally a blend of linear and cyclic hydrocarbons and that cyclanes could be beneficial from some fuel point of view (density, viscosity...), the effect of this higher mass fraction of aromatic compounds should be investigated because of the link between aromatic compound and coke formation in fuel lines.

1 Dufour, E., and Bouchez, M., AIAA (2002), 5126. 2 Daniau, E., Bouchez, M., Herbinet, O., Marquaire, P.-M., Gascoin, N., and Gillard, P., AIAA (2005), 3403. 3 Sickard, M., Raepsaet, B., Ser, F., and Masson, C., AIAA (2006) 7974. 4 Dahm, K.D., Virk, P.S., Bounaceur, R., Battin-Leclerc, F., Marquaire, P.M., Fournet, R., Daniau, E., and Bouchez, M., J. Anal. Appl. Pyrol., 71 (2004), 865. 5 Chambon, M., Marquaire, P.M., and Come, G.M., C1 Mol. Chem., 2 (1987), 47. 6 Marquaire, P.M., Worner, R., Rambaud, P., and Baronnet, F., Organohalogen Compd., 40 (1999), 519. 7 Ziegler, I., Fournet, R., and Marquaire, P.M., J. Anal. Appl. Pyrol., 73 (2005), 107. 8 Fournet, R., Battin-Leclerc, F., Glaude, P.A., Judenherc, B., Warth, V., Come, G.M., Scacchi, G., Ristori, A., Pengloan, G., Dagaut, P., and Cathonnet, M., Int. J. Chem. Kinet., 33 (2001), 574. 9 Matras, D., and Villermaux, J., Chem. Eng. Sci., 28 (1973), 129. 10 Fournet, R., Battin-Leclerc, F., Glaude, P.A., Judenherc, B., Warth, V., Come, G.M., Scacchi, G., Ristori, A., Pengloan, G., Dagaut, P., and Cathonnet, M., Int. J. Chem. Kinet., 33 (2001), 574. 11 Warth, V., Stef, N., Glaude, P.A., Battin-Leclerc, F., Scacchi, G., and Come, G.M., Combust. Flame, 114 (1998), 81. 12 Battin-Leclerc, F., Glaude, P.A., Warth, V., Fournet, R., Scacchi, G., and Come, G.M., Chem. Eng. Sci., 55 (2000), 2883. 13 Warth, V., Battin-Leclerc, F., Fournet, R., Glaude, P.A., Come, G.M. and Scacchi, G., Comput. Chem., 24 (2000), 541. 14 O’Neal, H.E.; Benson, S.W., J. Phys. Chem., 1968, 72(6), 1866-1887. 15 O’Neal, H.E.; Benson, S.W., Int. J. Chem. Kinet., 1970, 2, 423-456. 16 Tsang, W. Int. J. Chem. Kinet., 1978, 10, 599-617. 17 Tsang, W. Int. J. Chem. Kinet., 1978, 10, 1119-1138. 18 Billaud, F., Chaverot, P., Freund, E., J. Anal. Appl. Pyrol., 1987, 11, 39-53.

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