JOURNAL OF PROPULSION AND POWER

Vol. 16, No. 4, July–August 2000

Utilization of High-Density StrainedHydrocarbon Fuels for Propulsion

Gabriel D. Roy¤

Of� ce of Naval Research, Arlington, Virginia 22217

To increase the range of liquid-hydrocarbon-fueled ramjets and cruise missiles, a new class of high-densitystrained hydrocarbon fuels has been synthesized and characterized. By virtue of the increased density and addi-tional strain energy introduced during synthesis, these fuels have increased the volumetric heat of combustion.Theoxidation chemistry and short-term and long-term stability for storage of these fuels have been studied. Becauseof the high carbon-content-to-hydrogen ratio of the molecules, there is an increased tendency for sooting. Severalmethods have been identi� ed to eliminate this problem. These include sequential timed fuel injection, retardedfuel-stream velocities, and the use of additives. It has been shown that soot generation and transport can be con-trolled and that these fuels microexplode rapidly, thus offering potential for utilization in propulsion systems toobtain increased range.

Introduction

I NCREASED range, speed, stealth, reliability, and reduced vul-nerability are required of Navy propulsion systems. For a given

volume of fuel used, increasing the combustion ef� ciency of thepropulsion system can increase the range. Research has been pur-sued over the past couple of decades to achieve this. The increasein ef� ciency obtained by further improving mixing and combustionand combustion control will, however, be marginal. Navy propul-sion systems are volume limited, and a more attractive and logicalway to increase the range for a given propulsion system would beto increase the energy per unit volume of the fuel taken aboard. Forliquid-hydrocarbon-fueled ramjets and cruise missiles, a new classof high-density strained hydrocarbon fuels has been synthesized.1

These fuels, by virtue of their increased density, and the additionalstrain energy stored during synthesis, when properly burned in thecombustion chamber, can substantially increase the range.

In conventional hydrocarbon fuels, the angle between the bondsis 109.5 deg and, by the open nature of the molecules, they are notpacked densely. The strained hydrocarbon molecules studied arestrained to form triangles,squares, and polygons,or strained furtherto form tetrahedronsand prisms (Fig. 1), thereby increasingthe den-sity and adding strained energy.1 In Fig. 2, the gravimetric heat ofcombustion is plotted against the density of common hydrocarbonfuels, including other fuels synthesized elsewhere.1 As the densityof a conventionalhydrocarbonincreases,a decline in thegravimetricheat of combustion is noted. To take full advantage of this increasein density, the gravimetric heat of combustion should remain con-stant. However, the present fuels exhibit an increase in the heat ofcombustion with an increase in density, resulting in improved volu-metric and gravimetric heat release. Consequently, improvement inrange of volume-limited (greater extent) and weight-limited (lesserextent) propulsion systems is achieved. The synthesis aspects ofthese fuels have been reviewed by Roy.1 Because of the strainednature of the fuels, storage and long-term shelf life are to be con-sidered. The combustion of these fuels poses several problems, themost signi� cant being a high propensity to sooting that is due tothe high carbon content (carbon-to-hydrogenratio) of the fuels.Theother issues in the combustion of these fuels are the in� uence of thestrainenergyon thedecomposition2 processand any differencein itsinjection/atomization characteristics and spray and vortex dynam-ics compared with those of conventional liquid hydrocarbon fuels,

Received 23 February 1999; revision received 16 March 2000; acceptedfor publication 16 March 2000. This material is declared a work of the U.S.Government and is not subject to copyright protection in the United States.

¤ ProgramOf� cer, Energy ConversionPropulsionProgram. Associate Fel-low AIAA.

liquid-phasereaction3 vaporization,and microexplosion4 character-istics. These issues are illustrated in Fig. 3.5

Early fuel-synthesis efforts focused on strained hydrocarbon fu-els containing azido groups.6 It has been shown that certain di-azido compounds are effective in controlling the droplet burningrate and microexplosioncharacteristicsand thus moderate combus-tion instability.6 The idea behind this approach is that the combina-tion of the inherent strained energy in the substrate fuel moleculecoupled with the attachment of azido functionality could yield asuperior fuel. The concept is called synergetic microexplosion, inthe sense that microexplosionis coupled with exothermic strain re-lease in the fragmentation of the strained hydrocarbon.7 The moststrained hydrocarbon is cubane (C8H8, Fig. 1), in which the strainenergy is the result of cyclobutanerings fused together in the cubanemolecule.The strain energyof cubane is 161 kcal/mole (Ref. 8). Theother molecules considered, namely, dihydrobenzvalene (DHBV)(C6H8 ) and quadricycline (C7H8 ) have strain energies of 72.8 and78.7 kcal/mole, respectively (Santoro, R., Private Communication,1999).9

Storage StabilityAlthough these fuels were chosen for their higher density and

strain energy, other chemical and physical properties, as well asshort-termand long-termstabilityfor storage,shouldalsobe consid-ered for propulsionapplications.The DHBV preparedby Moriarty7

contained20% benzeneas the major impurity and other trace impu-rities. The results of a study on the effect of long-term storage10 ofDHBV are shown in Fig. 4. Here the lowest trace (original mixture)is a portionof the in infrared(IR) spectrumof a typicalDHBV/argonmatrix sample prepared immediately after the sample was received.The main peaks that are due to DHBV are labeled D, peaks that aredue to benzene are labeled B, and the unlabeled peaks are eithercontaminants or, in some cases, a combination band from DHBVand benzene.The second trace (old mixture) shows the IR spectrumof a matrix grown from the same DHBV/argonmixture after storagefor 3 months at room temperature in the dark. It can be seen that theDHBV has almost decomposedduring storage.A mass spectrumofthis decomposed mixture shows a signi� cant intensity of mass 80,79, and 77 as well as lower mass cracks, therefore suggesting thatthe decomposition was simply an isomerization, as this crackingpattern is very similar to that of DHBV. Another sample preparedby Moriarty was stored for a year and used for Fourier transform IR(FTIR) experiments.Although this sample also contained the sameamount of benzene as impurity, no decompositionwas noted.11 Theonly observable difference between these samples is that the � rstwas slightly colored, and the second was clear. It appears that thetrace contaminants could possibly accelerate or even catalyze the

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decompositionof DHBV. It becomesnecessaryto identifysuch con-taminants that can cause rapiddecompositionof the fuels, and atten-tion needs to be focused on synthesizingthese fuels in a pure form.

Because these fuels are expensive to synthesize, it is necessaryto characterize these fuels by use of very small quantities, andthe not-so-promising candidate fuels can be eliminated. A uniqueinstrument, a micro-� ow-tube reactor/mass spectrometer (micro-FTRMS) has been designed and built to meet the requirement ofanalyzing small samples.12 This instrument will allow the determi-nation of those fuels that indicate promise for follow-up.

Droplet Combustion StudiesBecause of the small quantities of fuels synthesized, spray com-

bustion studies were initially carried out with surrogate fuels thathave similar properties, whereas droplet combustion studies werecarriedoutwith fuelssynthesizedatdifferentuniversitylaboratories.

To study the dropletburningcharacteristicsof these fuels,a singlemonodispersestreamof dropletswas axiallyinjecteddownwardintothe postcombustion region of a premixed � at � ame burner.13 Thedroplet size couldbe varied from approximately150 to 250 l m, andthe chamber could be pressurized. The composition of the gaseous

Fig. 1 Strained molecular hydrocarbons.

Fig. 2 Comparison of heat of combustion of various fuels.

Fig. 3 Combustion issues.

mixturefed to theburnerwas controlledto yield combustiontemper-atures of approximately 1000–1200 K. The amount of soot emittedper drop is shown in Fig. 5 as a function of a normalized time, de-� ned as (1 ¡ d2 /d2

0 ), where d is the diameter of the droplet and d0 isthe initial diameter, for JP-10, benzene, and a mixture of 20% ben-zene and 80% DHBV.4 As the dropletburns, the amount of soot � rstincreasesand then decreases.As can be seen, the sootingpropensityof DHBV mixture is approximately the same as that of benzeneandapproximately � ve times higher than that of JP-10.

Only very small quantities of methyl cubane were synthesized,and hence single suspended droplet experiments were conducted.The � ame was characterized by intense sooting, and imaging ofthe droplet regression was not possible. Therefore additional ex-periments were performed14,15 with mixtures of methyl cubane andn-nonane under reduced pressures (the boiling points of these twocomponents are quite close so that preferential gasi� cation effectscan be eliminated). The burning rate constant K (the time rate ofchange of the square of the dropletdiameter) for combustionat 0.25and 0.13 atm is shown in Fig. 6. When the values of K for purenonane and methyl cubane at 0.13 atm are compared, it is seen thatmethyl cubane burns at least twice as fast as nonane, which is adecisive advantage.

In these fuels, the liquid-phasereaction is most likely to occur inthe relatively warmer portion of the droplet near the surface rather

Fig. 4 Decomposition of DHBV during storage.

Fig. 5 Soot emission from high-energy fuels.

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Fig. 6 Droplet burning rate constant K for mixtures of methyl cubaneand n-nonane at reduced pressures.

than the center of the droplet. It is not likely that the liquid-phasereaction occurs in a very thin spherical shell around the edge of thedroplet, which would require very fast reactions at relatively lowliquid temperatures.Hence it is likely that this reaction occurs overa � nite domain where liquid and gas exist. With a large content ofgas, all of the gas cannot remain in solution,but ratherbubbles couldexist, and a bubbly region forms.16 With this hypothesis, numeri-cal analysis was performed. The analysis indicated that the bubbleradius grows with radial position and time, causing the droplet toswell initially. A model incorporatingdecompositionof the dropletsurface predictedthat the droplet radius continuouslyincreaseswithtime until surface decompositionbecomes appreciable. Afterward,the droplet mass and radius decrease with time.16 However, exper-iments with certain diazidoalkanesdid not show any bubbly foamformation at the droplet surface.13 Although foaming combustionis an essential feature of solid propellant combustion, it is not clearthat foaming occurs in liquid propellant combustion because of themuch lower surface temperatures and smaller molecules comparedwith thoseof solidpropellants.However, it couldbedue to thepossi-bility that the resolutionobtained in the experiment is not suf� cientto identify this very thin region.

Soot Studies in Diffusion FlamesSeveral investigationswere carried out with differentmethodolo-

gies to minimize or eliminate soot that forms during combustionof these energetic fuels. A variety of methods have been tried inthe past to achieve rapid mixing between the fuel and oxidant-richstreams,which is known to be effectiveto controlsoot.However, theway in which the mixing is carried out is also very crucial. Existingevidence from both turbulent and laminar jet � ames and from em-pirical industrial practice indicates that soot reduction in diffusion� ames can be achieved if the fuel-stream velocity is retarded. Thisis schematically shown in Fig. 7, in which the hydrodynamics ofnonpremixed � ames is sketched.17 The soot-containingregions aresketchedfor conditionsunderwhich fuel-streamvelocitiesare larger(enhanced) and smaller (retarded) compared with those of the airstream.Becauseof their size, the sootparticlesdo notdiffuse likegasmolecules, but are convected at the local velocity of the � ow � eld.In the case of enhanced fuel-stream velocities, entrainment causedby the higher-velocity fuel stream causes streamlines to cross the� ame toward the fuel side. Consequently, soot particles nucleatemainly near the fuel-rich side of the � ame sheet and are convectedaway from the � ame surface. This results in long residence timesat fuel-rich conditions, and soot particle growth is enhanced.Theseparticlesaredif� cult to oxidizecompletelywhen they reachthe fuel-lean portions of the � ame. Thus the potential for soot emissions isincreased. On the other hand, with retarded fuel-stream velocity,entrainment from the higher-speedair stream causes streamlines tocross the � ame toward the side of air.The sootparticlesthen begin tonucleate in the cool core of the � ow and are drawn immediately to-ward the oxidationzone. Because they do not have suf� cient time togrow in the fuel-rich region, they are readily oxidized. Thus retard-ing the fuel velocity offers the potential of reducing soot emissionfrom the high-sooting fuels under consideration.18

In diffusion � ames, the mixing occurs in two stages. In the � rststage, large amounts of the reactants are brought together throughlarge-scale mixing, dominated by vortex dynamics, and in the sec-ond stage, molecular contact between the reactants is promoted by

Fig. 7 Effect of reactant stream velocities on soot processes withinlaminar mixing layers for nonpremixed hydrocarbon-fueled � ames. 1,� ame sheet (typical); 2, soot layer (typical); 3, streamline (typical).

small-scaleturbulence.Minimizing locationsof unmixednessis im-portant to reduce soot formation that is due to localized high fuel-to-air concentrations.Hence synchronizedperiodic injectionof fuelinto air vortices at a timing that ensures energizing of the vorticesand homogeneous mixing was investigated in another effort.19 Fora certain time delay, islands of unmixed fuels were generated insidethe core of the air vortices, leading to high soot formation, whereasfor other time delays, a homogeneous mixture of air and fuel waspromoted, reducing the soot level nearly to zero. Simultaneous pla-nar laser-induced� uorescence(PLIF) of OH and Mie scattering im-ages of soot was used to identify the relationshipbetween the initialsoot formation, the vortical structures, and the fuel-injection loca-tions (Fig. 8). It was shown that dual fuel injection during a singleair-vortexperiod can reduce the soot formationby several orders ofmagnitude relative to an unforced� ame (Fig. 9). Further, signi� cantimprovement in the combustion energy release was also obtained.The � rst fuel injection, which is synchronized with the formationof the incipient vortex, mixes homogeneously and initiates intensecombustion in the vortex as it is convecteddownstream.The secondinjection penetrates the jet and brings the fuel into the lean combus-tion products of the prior vortex. The process therefore simulatesstaged combustion without mechanical stages. This methodologyhas been applied to practical systems, resulting in ef� cient soot-freecombustion,higher temperatures,and an increase in energy release.

The control mechanisms developed in gaseous-fueled � ameswere applied to liquid-fueled � ames.20 It was found that properatomization was critical to the success of this technique. Althoughthe � ne componentof the spray follows the vortical � ow and partic-ipates in the control, larger droplets, because of their inertia, travelthrough the vortices and burn in a sooty � ame above the region ofvortex combustion. To guide the droplets into the desired spatiallocation, the interaction between fuel droplets and large-scale tem-poral features in the � ow (vortices) was utilized. By synchronizingthe fuel-injection process with a phase-delayed pressure signal, itwas possible to control the timing of fuel injection precisely withrespect to the air-vortex shedding process. The droplet trajectorieswere signi� cantlyaffectedby this timing.Dropletdispersioncharac-teristics of pulsed sprays at different timings are shown in Fig. 10.21

The improvement of spray characteristics was attempted throughcustom-built atomizers.

The fuel-additive approach has long been used to control sootemissions and was investigated in the control of soot of the strainedhigh-energy fuels.22 The physical considerations in the choice ofadditive are temperature effects that are due to the increased heatcapacity of the additive, dilution by a less energetic additive, dif-ferential diffusion that reduces the concentration of soot precursorspecies, and reduced thermal diffusivity that results in smaller resi-dence time in the soot growth region. Experiments were conductedwith benzene (C6H6 ) and quadricyclane (C7H8 ). Because of theheavy sooting characteristicsof these fuels, a fuel mixture approach

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Fig. 8 Soot generation with and without controlled fuel injection.

Fig. 9 Comparison of soot emission in uncontrolled and controlledethylene and propane � ames for different forcing levels.

was adoptedwith methaneas the base � ame.Figure 11 indicatesthatquadricyclanehas a higher propensity to sooting than benzene. Af-ter methanol addition, it was observed that the soot volume fraction( fv ) is reduced signi� cantly for heights at or below 20 mm. How-ever, higher in the � ame, soot reduction is comparatively lower.CS2 was found to be very effective in reducing the sootingnature ofthese fuels. A 60% soot reduction was observed with the additionof 25% CS2 (Fig. 12). Here, it is expected that most of the sulfuratoms in CS2 are converted to SO2 , which subsequently form theintermediate HSO2 . This catalyzes H, OH, and O radical combi-nation reactions, thereby reducing their concentrationclose to theirequilibriumvalues. As polycyclicaromatic hydrocarbon(PAH) for-mation and growth are sensitive to H radical concentrations, sucheffects can signi� cantly affect net soot formation.

The advances in supercomputing and parallel processing havemade it possible now to perform detailed calculations of complexmultiphase reacting � ows.23 As a complement to laboratory exper-iments, numerical simulations were performed24,25 both to obtain

further understanding of the mechanisms involved and to performdetailed parametric evaluation. In contrast to physical experiments,numerical experiments have the advantage of changing any desiredparameter at a time. Simulations were made with timing the particle(which simulates fuel) injection in or out of phase with the forcingat the sheddingfrequency.Dispersion increasedwhen injection wasin phase with the forcing because the particles traveled downstreamwith the � ow structures. Decreased dispersion was observed wheninjected out of phase. These simulations also demonstrated that, bychoosing suitable forcing frequency and timing the fuel injection,one could achieve enhanced dispersion of fuel droplets at differentlocations in the combustor, depending on the application. Simula-tions were also performed to determine the effect of particle size onits distribution in the vortical structure (Fig. 13). These results canbe utilized as the basis for the design of atomizers with optimumperformance.

A study on electrostaticdispersion and evaporationof dense anddilute clusters of drops was undertaken to determine the effective-ness of this technique in soot control.26 The effects of mechanicaldispersion on electrostatically charged as well as uncharged poly-disperseclusters of dropswere numericallyinvestigated.It has beenshown that drop-inducedmechanical dispersion cannot achieve thesame bene� cial effects as electrostatic dispersion in terms of re-ducing the mass fraction of the evaporated fuel and gas density (toreduce soot nucleation while promoting evaporation).

From the comparison of the above methodologies, it is evidentthat synchronized fuel injection seems to be the most promising.The rapiddevelopmentin microelectromechanicalsystems,sensors,and actuators would make active control of fuel modulation in realsystemspossible.Researchis underwayto achievethisgoal.Perhapsa hybrid technique involving more than one control methodologymay be the solution for utilizationof these fuels, in compliancewithexternal constraints as well.

High-Energy–Density Solid HydrocarbonsPentacyclic alkene dimers are high-energy–density (HED) hy-

drocarbons that exhibit attractive features for combustion systemsbecause of their high density (to q = 1.2–1.3 g/cm3 ). They contain

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Fig. 10 Spatial distribution of ethanol droplets (points) as a result of interaction with a large air vortex (continuous lines).

Fig. 11 Soot volume-fraction pro� les in methanol-added quadricy-clane � ames for measurement heights h of 20 and 60 mm.

a moderate amount of strain energy that contributes to the energyoutput during combustion. In addition, they dissolve in large por-tions in existing liquid-hydrocarbon formulations, forming stablesolutions to 18% w/w (Ref. 27). To assess their application toair-breathing propulsive devices, investigations have been focusedin two areas: 1) evaluation of combustion characteristics and, 2)modeling of the solid–gas interface of HED fuels in a turbulentreacting � ow. Droplet combustion experiments with binary solu-tions of methylated pentacycloundane(MPCU) dimer in JP-10 in-dicated high heat output of the binary system, doubling the pureJP-10 burning rate. Evaluations of liquid HED formulations basedon pentacyclic cages, as individual compounds, exhibited microex-plosive combustion and burning rates comparable with those ofazido-organiccompounds.27 Althoughthe pentacycloundane(PCU)dimer and spirocyclic alkene dimers dissolve in only a small quan-tity in liquid hydrocarbons, the MPCU alkene dimer dissolved inthe proportion of 22% w/w in a realistic additive to existing fuelsystems.28 Products resulting from the chemical decomposition ofHED compoundswere analyzedwith a quantitativemeasurementofthe combustionproductsofMPCU by FTIR spectroscopy.Figure 14shows the increase in the burning rate 2 > 2 of an 18% solution ofa MPCU alkene dimer [C22H22(CH3)2] in JP-10 (Ref. 29). Further,a new synthesis path produced a mixture of MPCU isomers withincreased solubility in JP-10. Solutions of 50% w/w have been ob-tained with stability at low temperatures veri� ed down to 70±C.The 50% w/w mixture increased the JP-10 burning rate by a factorof 5 (Ref. 30).

Fig. 12 Soot volume-fraction pro� les in CS2-added quadricyclanemixture � ames for measurement heights h of 20 and 60 mm.

Fig. 13 Vorticity and particle distribution.

Fig. 14 18% solution ofMPCU alkene dimer,C22H22(CH3 )2, in JP-10 in-dicates a burning rate in-crease by a factor of >2.

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ConclusionsA new class of high-densityhydrocarbonfuels with strained mo-

lecules has been synthesized and characterized. Their combustioncharacteristics have been investigated to assess their suitability forutilizationin the propulsionsystems for ramjets, cruise missiles andpossibly for aircraft. Systems studies indicate a substantial increasein the range of vehicles using these fuels for propulsion. However,there are several scienti� c issues that have to be resolved beforethese fuels can be utilized. Small quantitiesof various candidate fu-els have been synthesized,and their properties,oxidationchemistry,and reactionpathwayshavebeen characterized.Droplet combustionstudies indicate a propensity in soot formation;however, these fuelspossess good microexplosionand fast burning properties.An inno-vativeapproachof sequentialfuel injectionwith respectto thevortexgeneration in the mixing layer has resulted in orders-of-magnitudereduction in soot. Other methods investigated include control ofvelocities of fuel and oxidant and the addition of fuel modi� ers.These fundamental studies indicate that this class of fuels showspromise for longer range and speed of propulsion systems for thegiven tank volume. Efforts have been undertaken to produce largequantities of these fuels, as well as to demonstrate the utilizationofthese fuels in real systems.Althoughthe numberof steps involvedinthe synthesis of these fuels has been reduced through research, thecost would be still far above what commercial applications woulddictate because of the higher price of starting materials. However,as with most new products, continued research, alternative startingmaterials, improvements in the processes involved, and large-scaleproductionwill bring the fuel price down. Studies should also focuson the environmental impact in using these fuels. Each applicationcould have speci� c issues to be addressed. Experience has shownthat the development of new fuels takes several years. In the nearterm, these fuelsmay be added to conventionalfuels to enhanceper-formance and range. However, in the long range, the payoff fromthese fuels will be substantial.

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