Atkins 2005

Experimental Studies of Jet Fuel Viscosity at LowTemperatures, Using a Rotational Viscometer and an

Optical CellDaniel L. Atkins and Jamie S. Ervin*

University of Dayton, Mechanical and Aerospace Engineering Department, 300 College Park,Dayton, Ohio 45469-0210

Linda Shafer

University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0116

Received December 3, 2004. Revised Manuscript Received May 18, 2005

A temperature scanning rotational viscometer and visualization techniques are utilized toevaluate the viscosity behavior of kerosene-based jet fuels from -20 C to sample cloud pointtemperatures. The uncertainty of using the rotational viscometer technique is evaluated anddetermined to be acceptable. The rotational viscometer is used to perform shear rate studies onJP-8, Jet A, and JP-8 blended with a low-temperature additive designed to improve fluidity.Clouding of the neat fuel samples during cooling, visualized in a unique optical cell using cross-polarized light and a He-Ne laser, was determined to not cause non-Newtonian viscosity behavior.Polarized light is used to demonstrate that neat fuel clouding during cooling is likely due to freedroplets of a water/di-ethylene glycol monomethyl ether (water/DIEGE) mixture, while cloudingof the jet fuel/additive blend was caused by crystallization. Viscosity results are presented for alljet fuel samples. Selected results are compared with viscosity measurements obtained from othermethods and are determined to be in good agreement. Complex non-Newtonian viscosity behavior,including both pseudo-plasticity and dilatancy, was identified for the fuel/additive blend. Viscosityresults and the effects of shear rate changes matched observed solidification behavior.

Introduction

Operators and designers of aircraft, both manned andunmanned, that fly at altitudes above 12 000 m forextended periods require high-fidelity fuel freezingprediction tools. An impediment to the understandingand simulation of jet fuel freezing has been the absenceof published thermophysical properties of jet fuel at lowtemperatures. Common jet fuels, such as Jet A, Jet A-1,and JP-8, have little published property measurementsat temperatures near their freeze point temperatures.The freeze point for a hydrocarbon liquid fuel (ASTMD2386-03) represents the temperature at which visiblewax crystals melt upon warming. Therefore, jet fuelconsists mostly of liquid several degrees (3-4 C)below its freeze point. The absence of published propertydata is likely due to the variation of jet fuels, based onthe composition of the original crude oil and the refinerymethods used. Also, property measurements at temper-atures approaching the onset of solidification are dif-ficult to obtain, requiring specialized equipment andtechniques. Commercial and military jet fuel specifica-tions are not sufficiently rigorous to ensure consistentthermophysical properties. Moreover, the introductionof non-petroleum-derived fuels, such as Fischer-Trop-sch fuels, are on the horizon. Therefore, jet fuel proper-ties must be assessed regularly and new techniques

developed to determine their low-temperature proper-ties accurately.

The Coordinating Research Council (CRC) has pro-vided a compilation of aviation fuel properties that arewidely used in the aerospace industry.1 Unfortunately,these properties are incomplete for temperatures nearjet fuel freeze point temperatures. The low-temperatureproperties of jet fuels must be known to model anddesign aircraft fuel systems adequately. At present, thespecified freeze point temperature and viscosity at asingle temperature are used by aircraft designers andoperators to determine a fuels ability to be pumped atlower temperatures. The usefulness of the specifiedviscosity requirement and freeze point temperature islimited because jet fuel is a liquid below its freeze pointtemperature. In addition, fuel viscosity greatly impactsthe ability to re-light an engine at low temperature.More-complete data are needed for improved combustordesign. The present specification method of determiningthe viscosity of jet fuel is the capillary method (ASTMD445). Continuous viscosity measurements of jet fuel,as a function of temperature, would provide importantinformation on actual fuel flow characteristics at tem-peratures near the onset of crystallization.

Viscosity measurements at varying shear rates couldreveal possible non-Newtonian behavior caused byforming wax or water crystals. Fuel clouding behavior

* Author to whom correspondence should be addressed. Telephone:(937) 252-8878, ext. 114. Fax: (937) 252-9917. E-mail address:[email protected].

(1) Handbook of Aviation Fuel Properties, Coordinating ResearchCouncil Technical Report 635, Atlanta, GA, 2004.

1935Energy & Fuels 2005, 19, 1935-1947

10.1021/ef049683k CCC: $30.25 2005 American Chemical SocietyPublished on Web 06/28/2005

upon cooling, which is caused by precipitating water orother phenomena (e.g., large n-alkane gelation), mayintroduce non-Newtonian behavior. Two-phase slurries(wax particles suspended in liquid fuel), emulsions, ordispersions may change the flow properties from New-tonian to non-Newtonian behavior.2 The effects of fueladditives on flow characteristics could also be deter-mined. The proposed method utilizes a temperaturescanning rotational viscometer (ASTM D5133-01), whichhas been applied to low-temperature automotive lubri-cants, to investigate low-temperature jet fuel fluidity.3

The use of a rotational viscometer to determine thelow-temperature viscosity of jet fuels has been limitedbut is emerging as a new technique.4 The viscosity ofJet A has been measured with a rotational viscometerat temperatures as low as -50 C; however, thesemeasurements did not include shear rate studies or theeffects of additives. Moreover, variations in shear ratewere required to maintain instrument scale, concealingpotential non-Newtonian behavior.5 Other researchersinvestigated the viscosity of Russian kerosene-based jetfuels.6-11 For example, Mitin et al.6 used a kerosene-based thermally stable fuel (T-6) that was designed forhigh-performance aircraft in a rotational viscometer.They concluded that the fuel is non-Newtonian below0 C, because of the formation of rheological struc-tures. T-6 has greater density than JP-8, because itconsists mainly of cycloparaffins, reducing the lower-density n-alkane content.12 Gorenkov et al.7 measuredthe viscosity of Russian jet fuels using the capillarymethod (GOST 33-82) and identified the lack of continu-ous viscosity measurements at low temperatures.Likhterova et al.8 also identifies the lack of viscositymeasurements at low temperatures and suggests thatjet fuels may be non-Newtonian near the initial crystal-lization temperature. In this study, a rotational viscom-eter is used to study flow characteristics of Russiankerosene-based jet fuels at low temperatures.8 Unfor-tunately, this research provides data on only Russianfuels, lacks experimental detail, has translation issues

and conflicting non-Newtonian viscosity results, anddoes not consider the effect of additives.

In this paper, the rotational viscometer method isused with commonly employed commercial and militaryjet fuels (Jet A and JP-8) at low temperatures ap-proaching initial observable crystallization (the cloudpoint temperature). The objective is to determine theapplicability of the technique to jet fuel viscosity mea-surement and shear rate studies. We investigate theNewtonian and non-Newtonian viscosity behavior beforeinitial crystallization, because the previous research isincomplete and inconsistent. The viscosity behavior offuels mixed with additives that improve low-tempera-ture fluidity is also studied, and any resulting non-Newtonian behavior is characterized. Visualizationtechniques are used to investigate clouding behavior andto determine if the phenomena responsible for cloudingcause non-Newtonian behavior.

Experimental SectionJP-8 is the current primary jet fuel used by the United

States Air Force and is essentially Jet A-1 fuel that has beencombined with three additives. The first additive is a corrosioninhibitor that also improves lubricity. The second is a fuelsystem icing inhibitor, and the third is a static dissipater. JetA-1 is a commercial jet fuel that is used internationally,whereas Jet A is the primary U.S. commercial jet fuel.Therefore, JP-8 and Jet A are representative of extensivelyused jet fuels and were selected for analysis in the presentwork. Jet fuel samples for the present work were obtained fromthe Fuels Branch of the Turbine Engine Division of thePropulsion Directorate of the Air Force Research Laboratoryat Wright-Patterson Air Force Base. In addition, a proprietarylow-temperature additive was used in some experiments.Samples are identified by an alphabetic character, F or A,which designates fuel or additive, followed by four identifica-tion digits. The digits have no meaning other than to identifythe sample.

Table 1 lists the compositions (ASTM 2425) by speciesclasses of the four fuel samples studied in this work. Inaddition, Table 2 describes the fuel composition (on a volu-metric basis, ASTM 6379) in terms of the monoaromatics,

(2) Pedersen, K. S.; Ronningsen, H. P. Energy Fuels 2000, 14, 43-51.

(3) Standard Test Method for Low Temperature, Low Shear Rate,Viscosity/Temperature Dependence of Lubricating Oils Using a Tem-perature-Scanning Technique, ASTM D5133-01; American Society ofTesting Materials: West Conshohocken, PA. Available via the Internetat URL: http://www.astm.org [cited June 2004].

(4) Selby, T. W. The Scanning Brookfield Technique of Low-Temperature, Low-Shear RheologysIts Inception, Development, andApplications, ASTM STP 1143; American Society for Testing andMaterials: Philadelphia, PA, 1992; pp 33-64.

(5) Stockemer, F. J. NASA Technical Report CR-159615, NationalAeronautics and Space Administration, Washington, DC, 1979.

(6) Mitin, M. B.; Lyashenko, B. N.; Karpov, V. A.; Goncharov, L. A.;Kutuzova, O. V. Chem. Technol. Fuels Oils (English translation ofKhim. Tekhnol. Topl. Masel) 1989, 24, 219-221.

(7) Gorenkov, A. F.; Lifanova, T. A.; Klyuchko, I. G.; Saleev, V. A.Chem. Technol. Fuels Oils (English translation of Khim. Tekhnol. Topl.Masel) 1983, 19, 297-299.

(8) Likhterova, N. M.; Gorenkov, A. F.; Perepelkina, O. A. Chem.Technol. Fuels Oils (English translation of Khim. Tekhnol. Topl. Masel)1983, 19, 599-602.

(9) Belousov, A. I.; Bushueva, E. M.; Rudyakov, D. G. Chem. Technol.Fuels Oils (English translation of Khim. Tekhnol. Topl. Masel) 1984,20, 79-80.

(10) Dubovkin, N. F.; Malanicheva, V. G. Chem. Technol. Fuels Oils(English translation of Khim. Tekhnol. Topl. Masel) 1980, 16, 543-547.

(11) Gorenkov, A. F.; Stepanenko, V. S.; Saleev, V. A. Chem. Technol.Fuels Oils (English translation of Khim. Tekhnol. Topl. Masel) 1982,18, 94-98.

(12) Hanson, F. V. Air Force Wright Research and DevelopmentCenter Technical Report No. WRDC-TR-89-2097, 1989.

Table 1. Volume Percentages of Species Classes withinFour Fuel Samples, as Determined by ASTM 2425

Composition of Jet A Fuel (vol %)

species classes F3686 F3688 F3773 F3804

paraffins 43.6 60.5 57.2 58.5cycloparaffins 20.0 15.4 17.4 15.7dicycloparaffins 11.7 4.3 6.1 3.7tricycloparaffins 0.9

diaromatics, total aromatics, and total saturates. In low-temperature studies of jet fuel, the freeze point temperatureis a parameter commonly used to characterize the fluidity ofthe fuel. Table 3 lists both the measured freeze and cloud pointtemperatures for the fuel samples (using a Phase TechnologySeries 70V petroleum analyzer; ASTM D5972-96). For thetemperature regime in which fuel crystallization occurs, theincipient solid crystals consist of nonideal solid solutions ofthe larger n-alkanes (such as C16-C19) within the fuel.13 Theselarge n-alkanes represent a small fraction of the total n-alkaneconcentration. Thus, the precipitation of n-alkanes from thefuel (cloud point), and, therefore, the freeze point, are depend-ent on the n-alkane distribution of these species. Figure 1shows the n-alkane distribution for the fuel samples consid-ered. In addition, Figure 1 shows that fuels F3688 and F3686have n-alkane distributions that have a greater fraction of thelarger n-alkanes (C16-C19). Therefore, it may be anticipatedthat they would have the highest freeze point temperatures,as demonstrated by Table 3.

The n-alkane analyses of the fuels were performed usinggas chromatography (Agilent model 6890) combined with massspectrometry (GC/MS) and flame ionization detection (GC/FID). The same column model was used in both detectors(0.25 mm inside diameter (ID) 30 m DB-5MS with 0.25-mfilm). The GC temperature program involved an initial tem-perature of 40 C (2-min hold) followed by ramping (5 C/min)to 200 C and then an increase (30 C/min) to 280 C. Aconstant column flow rate of 1 mL/min and a 40:1 split ratiowere used. The injector temperature was 250 C. The massspectrometer, transfer line, and FID detector were held at a

temperature of 280 C. The GC/MS and GC/FID systems werecalibrated with standards containing n-alkanes (C8-C16) andan internal standard. Calibration curves were generated byobtaining response factors between the area responses for then-alkanes and the area response of the internal standard foreach level of calibration. The area responses for the GC/MScalibrations were extracted ion areas of the primary charac-teristic ions for each n-alkane. A minimum of four calibrationlevels was used to calibrate each compound. An averageresponse factor and the relative standard deviation (expressedas a percentage) were calculated for each of the n-alkanes. Fuelsamples were diluted so that the concentrations of the n-alkanes were in the linear range of the calibrations. Each sam-ple was diluted to at least two different concentration ranges.The higher concentration n-alkanes were quantified by GC/MS. The GC/MS quantitation involved the extracted ion areasof the primary characteristic ions, which provided baselineseparation of the n-alkanes from other fuel components. Thelower concentration n-alkanes were already baseline separatedfrom other fuel compounds and, thus, could be quantified byGC/FID. The estimated uncertainty in the n-alkane measure-ments is (10% for weight percentages of >1.0% (for example,the uncertainty in a stated value of 23% is (2.3%), (15% forweight percentages in the range of 0.11%-1.0%, and (25%for weight percentages in the range of 0.010%-0.10%.

ASTM D5133 describes the test method for the temperaturescanning rotational viscometer of Figure 2. This test methodprovides a means to determine the continuous rheologicalproperties of fluids over a desired temperature range. Itsapplication to petroleum products other than engine oils, suchas jet fuels, has not been established by ASTM. The temper-ature scanning rotational viscometer utilized in this study(13) Zabarnick, S.; Widmor, N. Energy Fuels 2001, 15, 1447-1453.

Figure 1. Measured distribution of n-alkanes for JP-8 and Jet A samples.

Table 3. Selected Low-Temperature Properties of Jet Fuel Samples

Density, F((0.001 g/mL)

Kinematic Viscosity, ((0.5 cSt)

Viscosity, ((0.5 cP)

fuelfreeze point((0.5 C)

cloud point((1.0 C)

pour point((2.0 C) @ -20 C @ -40 C @ -20 C @ -40 C @ -20 C @ -40 C

JP-8 (F3804) -48.3 -52.6 -58.0 0.822 0.834 3.9 8.4 3.2 7.0JP-8 (F3773) -49.4 -53.7 -58.0Jet A (F3686) -42.0 -46.2Jet A (F3688) -41.6 -46.5 0.824 0.833 4.9 11.9 4.0 9.3

Studies of Jet Fuel Viscosity at Low Temperatures Energy & Fuels, Vol. 19, No. 5, 2005 1937

(Tannas Company, model PlusTwo) is equipped with twoTAV-1 viscometer heads, each with an adapter that couplesthe rotor and glass stator cell (Figure 2) to the viscometer headwith a calibrated spring. The TAV-1 head contains a springthat has a full scale torque of 6.73 10-5 N m. A singleviscometer head was used to obtain all data to minimizeuncertainty. The unit is equipped with a programmable digitalcontroller and data acquisition system that controls andrecords the sample temperature, as well as records thepercentage of full-scale torque that is required to turn the rotor(hereafter referenced as relative torque). The desired sampletemperature is achieved by submersion of the rotor and statorcell into a stirred methanol bath. The maximum bath coolingrate is -15 C/h, and a sample cooling rate of -5 C/h wasselected to ensure that the sample was cooled at the same rateas the bath.

The recorded relative torque is that required to maintain aconstant angular velocity of the rotor. The sample fluid exertsa shear stress on the rotor, providing the opposing torque anddeflecting a torsional spring. The calibrated spring deflectionis measured with a rotary transducer and converted to relativetorque. For a Newtonian fluid, shear stress () is directlyproportional to the shear rate ( ). Viscosity () is defined asthe ratio of shear stress to shear rate:

For a rotational viscometer, eq 1 and the equipment dimen-sions can be used to derive a relationship between torque (M)and the rotor angular velocity (). Equation 2, which is calledthe Margules equation,14 shows the relationship between theviscosity, the dimensions of the viscometer, and the torque ina rotational viscometer. Equation 2 also shows that the torqueis directly proportional to the rotor angular velocity for aNewtonian fluid:

where h is the rotor height, RR the rotor radius, and RS thestator radius (Figure 2).

The instrument is preprogrammed for multiple rotor speeds,including 12, 24 and 60 rpm, which equate to shear rates of1.26, 2.51, and 6.28 s-1. Shear rate studies are performed byaltering the rotor rotational speed to determine whether the

jet fuel sample is a Newtonian fluid at low temperature.Equation 2 is valid for laminar flow only, and, because therotational viscometer has been applied to more-viscous low-temperature automotive lubricants, verification of laminar flowis required for jet fuel. Moreover, turbulent flow could resultin erroneously high viscosity measurements and mask non-Newtonian viscosity behavior. A Reynolds number of Re 5%) until the mea-sured relative torque is >6% of full scale. Figure 3shows that the measured relative torque is 6% at 60 rpm. Consequently, the figure shows thatthe relative torque measurements at 12 rpm and 24 rpm(above -35 C) have the greatest noise. Low relativetorque readings (

-20 C to -50 C, at 5 C increments, for the JP-8(F3804) sample. Figure 4 also shows the relative torqueat each shear rate just before solidification begins(-52.0 C). The curve fits of relative torque as a functionof shear rate (rotor speed) in Figure 4 have a high levelof linearity at all the temperatures, including -52 C.(The coefficient of determination or correlation coef-ficient, R2, is >0.9995 for all fits.) The fits have differentslopes at each temperature, which is expected as theviscosity varies with temperature. In addition, these fitlines show a nonzero relative torque at 0 rpm. Thisoffset varies with temperature and is likely due tomechanical friction and spring-constant variations withtemperature. The lines in Figure 4 show that the JP-8sample behaves as a Newtonian fluid at the low tem-peratures investigated, until the cloud point tempera-ture is reached. The relatively high level of uncertaintyin the measured relative torque and the averagingtechnique applied at discrete temperatures requirefurther consideration to verify Newtonian behavior.

Another method to analyze viscosity behavior requiresdata from only two rotor speeds and makes use of allthe data without averaging. Equation 2 indicates thatmeasured torque is directly proportional to rotor speedand that this relationship is independent of temperaturefor a Newtonian fluid in a rotational viscometer. Onlytwo rotor speeds are required to evaluate the directproportionality of eq 2. Moreover, continuous measuredrelative torque, as opposed to average values at discretetemperatures, may be utilized, enabling the use of allmeasurements. From eq 2, relative torque measure-ments obtained at 60 rpm divided by relative torqueobtained at 12 rpm should yield a torque ratio of 5.0 atall temperatures for Newtonian behavior. Similarly,torque ratios of 2.5 (60/24) and 2.0 (24/12) would beexpected for the other rotor speed combinations. Figure5 shows plots of the torque ratios. Table 6 shows thatthe propagated uncertainty in the calculated torqueratios is lowest for the 60/24 ratio. Consequently, Figure5 shows that the 60/24 torque ratios have the leastnoise, especially below -35 C. This is expected, becausethe measured relative torque exceeded 6% for both shearrates below -35 C (Figure 3). The calculated torque

ratio for the 60/24 rotor speeds for a Newtonian fluidshould be 2.5. From Figure 5, 60/24 torque ratios variedfrom 2.34 to 2.51 below -35 C. A statistical analysisof the 60/24 torque ratios yields a mean torque ratio of2.45 with a standard deviation of 0.029, which indicatesNewtonian behavior. Figure 5 also shows torque ratiosat selected temperatures (Figure 4) that have beencorrected for the offset derived from Figure 4. Thenonzero relative torque value at a rotor speed of 0 rpm(from Figure 4) has been subtracted from all relativetorque measurements. The corrected torque ratios havea mean value of 2.50 and a standard deviation of 0.018.Similarly, the corrected 24/12 and 60/12 torque ratiosof Figure 5 have mean torque ratios of 2.0 (standarddeviation of 0.032) and 5.0 (standard deviation of 0.046).Analysis of the torque ratios provides further evidencethat the JP-8 sample is Newtonian and that therotational viscometer can be used for shear rate analysisof jet fuel.

The compounds within a jet fuel each contribute tothe overall shear behavior and effective viscosity of thefuel mixture, and the contribution of each species alsois dependent on its concentration within the fuel.25Because jet fuel samples that originate from differentrefinery sources likely may have different flow behav-iors, a similar analysis was performed on the measure-ments obtained for a different JP-8 sample (F3773),which has a cloud point (-53.7 C) that is lower thanthat of F3804 (-52.6 C). Figures 3 and 6 together showthat the measured relative torque curves for F3804 havea tendency to be higher than those for fuel sampleF3773. The cloud point temperature of F3773 is slightlylower than that for F3804; therefore, it is reasonablethat the sudden increase in viscosity for F3773 (Figure6) occurs at a somewhat lower temperature than thatfor F3804 (Figure 3). Figure 7 shows the shear rateanalysis of the F3773 JP-8 sample. The linearity of therelative torque versus shear rate fit (R2 > 0.9995) atall temperatures, indicates that this JP-8 sample

(25) Boned, C.; Zeberg-Mikkelsen, C. K.; Baylaucq, A.; Dauge, P.Fluid Phase Equilib. 2003, 212, 143-164.

Figure 4. Relative torque versus rotor speed for JP-8 (F3804)cooled at a rate of -5 C/h.

Figure 5. JP-8 (F3804) torque ratios versus temperature forvarying shear rates at a cooling rate of -5 C/h. Open circlesrepresent corrected values, and solid data points representmeasured values.

1940 Energy & Fuels, Vol. 19, No. 5, 2005 Atkins et al.

exhibits Newtonian behavior from -20 C to the cloudpoint temperature.

Shear Rate Studies Involving Jet A. A shear rateanalysis was performed on the two Jet A samples F3686and F3688. The analysis was similar to that performedon the neat JP-8 samples. The Jet A samples chosenhave similar measured freeze point temperatures(Table 3). As in the case of the JP-8 fuels and asanticipated, the Jet A samples were determined tobehave as Newtonian fluids from -20 C to theirrespective cloud point temperatures. Fuel cloudingduring cooling does not cause non-Newtonian viscositybehavior in the JP-8 and Jet A fuel samples.

Effects of Low-Temperature Additive. The effecton viscosity of a proprietary additive (A4647) designedto improve the fluidity of kerosene-based jet fuel at lowtemperatures was analyzed. Relative torque measure-ments were obtained for JP-8 (F3804) that containedthe additive (Figure 8) for each shear rate over thetemperature range of -20 C to -70 C. For reference,Figure 8 shows relative torque measurements obtained

for the neat fuel. Several differences in the behavior ofthe neat fuel and the additive-containing fuel areobvious. The figure shows that the fuel with the additiveexerts more torque on the rotor, indicating that it ismore viscous. The 60-rpm shear rate curves show thisgreater viscosity beginning near -20 C. Unlike the neatfuel, the figure shows that the relative torque measure-ments of the fuel containing this low-temperatureadditive do not have a sudden increase in value at-52.7 C. Relative torque measurements for all threeshear rates show an abrupt increase near the temper-ature at which adhered solids form on the optical cellsurfaces (-55 C). To understand the differences in themeasured relative torque, the additive-containing fuelwas cooled in an optical cell and significant clouding ofthe fuel was observed, beginning near -40 C.

Flow Visualization in an Optical Cell withoutthe Low-Temperature Additive. Jet fuel cloudinghas been observed previously during cooling in fuels thatcontain additives designed to improve low-temperaturefluidity, as well as neat fuels.21,26 Figure 9 shows imagesof samples being cooled in a 114.3-mm-deep optical cell.Figure 9a shows a Jet A sample (F3219), obtained froma vented underground storage tank, cooled to -48.5 C.Adhered crystals are visible on the surface of the celland significant clouding is observed. Figure 9b showslaboratory-grade (bottled) kerosene cooled to -48.5 C,with adhered crystals, but no clouding behavior. Thecooling-induced clouding in jet fuels has not beenextensively reported; however, it is believed to be causedby dissolved water precipitating from solution as thewater solubility decreases with decreasing tempera-ture.27 GC was used to determine the dissolved watercontent of the samples.28 The Jet A (F3219) and JP-8(F3804) samples were found to contain as much as 100%more dissolved water than the kerosene. The handlingof the kerosene sample limits its exposure to water.Moreover, kerosene likely contains fewer polar com-pounds, resulting in lower water solubility. Conse-quently, the cooling of weathered jet fuel, such as theJP-8 and Jet A samples, results in a jet fuel/wateremulsion and/or ice crystal dispersion that causesclouding as the fuel is cooled.

To better understand the clouding phenomena, theJP-8 sample (F3804) was cooled from room temperature(18 C) in a 44.5-mm-deep optical cell.22 Unlike the114.3-mm-deep optical cell (Figure 9), visible cloudingwas difficult to observe in the 44.5-mm-deep cell. Crosspolarization has been shown to increase contrast be-tween wax crystals and liquid jet fuel in an optical cellgreatly, because of light depolarization by the crystals.22Figure 10 shows the results of using cross polarizationto improve the contrast of particles responsible for fuelclouding during cooling. Figure 10a shows the JP-8sample in the optical cell with cross polarization andwith the surfaces cooled to -53.0 C. Clouding was notobserved during cooling; however, wax crystals wereobserved forming on the cell surface near the cloud

(26) Leo, A.; von Meerwall, E.; Bruno, R.; Schruben, D.; Joshua, J.J. Aircr. 1989, 26, 465-469.

(27) Aviation Fuels Technical Review (FTR-3), Chevron ProductsCompany. Available via the Internet at URL: http://www.chevron.com/prodserv/fuels/bulletin/aviationfuel [cited March 2004].

(28) Anderson, S. D.; Rubey, W. A.; Striebich, R. Prepr. Pap.sAm.Chem. Soc., Div. Pet. Chem. 1998, 43, 471-475.

Figure 6. Relative torque measurements at rotor speeds of12, 24, and 60 rpm for JP-8 (F3773) cooled at a rate of-5 C/h.

Figure 7. Relative torque versus rotor speed for JP-8 (F3773)cooled at a rate of -5 C/h.


point temperature. Figure 10b-d shows that continuedcooling of the cell surfaces to the pour point temperature(-58 C) produced surface-adhered wax structures andvisible free-floating wax particles that have been de-scribed previously.22,24 The figure shows how waxcrystals are visible using cross polarization, but theparticles responsible for fuel clouding are not, indicating

that the particles responsible for clouding are notcrystalline.

Because the cross-polarization filtering did not showthe expected clouding of a weathered fuel, a He-Ne gaslaser was directed through the optical cell that con-tained the JP-8 sample (F3804). Figure 11 shows thatthe beam intensity was observed to increase as the fuelis cooled. Beam intensity continued to increase gradu-ally as the sample was cooled to its cloud point temper-ature. Figure 11a shows no detectable scattering at20 C, whereas Figures 11b-d shows increased scat-tering as the temperature is reduced. Light scatteringis a function of wavelength, as well as the number andsize of particles. The wavelength of the He-Ne laser is0.638 m. Increased scattering indicates that the num-ber and/or size of particles is increasing and is likelythe result of reduced water solubility as the fuel iscooled. It is believed that if the particles were wax orfree water crystals, they would have depolarized thelight and become visible, using cross-polarization filter-ing. If the particles are free water, they may not crystal-lize, because of their size and the presence of a fuelsystem icing inhibitor additive. Studies have shown thatsignificant supercooling is required to freeze emulsifiedwater droplets, depending on their size.29 Moreover, thepresence of DIEGME, which is intended to lower thefreeze point of free water in jet fuel and is present inthe fuel samples analyzed, inhibits crystallization. Theresulting water/DIEGME mixture freeze point can ap-proach the cloud point temperature of the fuel, inhibit-ing free water crystallization.30 Therefore, the particlesare likely free droplets of a water/DIEGME mixture.Shear rate studies described above for JP-8 show that

(29) Langer, G. Air Force Wright Air Development Division Techni-cal Report No. 60-826, 1960.

(30) Grosjean, D. F. Efforts to Advance Fuel-System Ice-Inhibitor(FSII) Technology; Prepared under contract for the Air Force ResearchLaboratory, Propulsion Directorate, Wright-Patterson Air Force Base,OH, September 18, 1998.

Figure 8. Relative torque measurements at rotor speeds of 12, 24, and 60 rpm for JP-8 (F3804) with 5000 mg/L of A4647 cooledat a rate of -5 C/h.

Figure 9. Observed clouding behavior in optical cell of (a)Jet A (F3219) and (b) kerosene at estimated wall surfacetemperatures. (From ref 21.)


the observed cloudiness of the fuel/water emulsion doesnot influence viscosity behavior and does not cause non-Newtonian viscosity behavior.

Flow Visualization Using the Low-TemperatureAdditive. Visualization is useful when analyzing fuelsthat have been blended with additives. Visualizationfrom the optical cell shows that the JP-8 (F3804) fuelblended with 5000 mg/L of A4647 additive begins tocloud near -40 C, which is 8 C above the neat fuelfreezing point (Figure 12). The A4647 low-temperatureadditive is believed to encourage nucleation sites to formin the cooled liquid fuel at temperatures well above thefreezing point of the neat fuel. The forming particlesdepolarize the light and clouding becomes visible,indicating that these particles are crystals. Figure 12ashows that the cloudiness increases in the coldest region

of liquid fuelsat the bottom of the cellsat -45 C.Figure 12b and c shows that this clouding behaviorexpands throughout the cell until solids begin to formon the sides of the optical cell at -55 C and extendinto the liquid, as shown in Figure 12d. Figure 12dshows that the clouding is more evenly distributed inthe cell. The relatively large, free-floating crystals thatformed in the JP-8 sample do not form in the fuel withthe additive. It was anticipated that the two-phasedispersion resulting from the use of the additive wouldcause non-Newtonian viscosity behavior.2

Rotational viscometer shear rate studies are per-formed to evaluate the viscosity behavior of the fuel/additive mixture. The increased relative torque mea-surements shown in Figure 8 for the additive-containingfuel is likely the result of the presence of a two-phasedispersion, which causes fuel clouding. Moreover, near-55 C, the measured relative torque briefly increasesrapidly but then returns to the previous rate of increase.It is believed that the additive alters the crystalmorphology by inhibiting the formation of large flatplate crystals that entrap liquid and cause an indefiniteincrease in relative torque measurements (Figure 3).The complex relative torque measurements of Figure 8suggest non-Newtonian behavior. Figure 13 shows thesmoothed relative torque plotted at specific tempera-tures, as a function of shear rate for the JP-8 withadditive. The curve fits have a high level of linearityfrom -20 C to -45 C with coefficients of determinationof >0.9995. However, at -50 C, the linearity dropsto 0.998, and at -55 C, drops to 0.986, indicating adegree of non-Newtonian behavior relative to the neatfuel (Figures 4 and 7). To further evaluate the non-Newtonian behavior, torque ratios are plotted withtemperature. Figure 14 shows that the torque ratios arestronger functions of temperature, when in comparisonto the neat fuel (Figure 5), further indicating non-Newtonian behavior as the mixture is cooled. Correc-tions were obtained, as done previously, from therelative torque plot versus rotor speed (Figure 13) forvalidation of non-Newtonian behavior. These correctionswere determined to be invalid due to the nonlinearbehavior and are not shown. The nonlinear plots ofrelative torque versus rotor speed (Figure 13) and the

Figure 10. JP-8 (F3804) in an optical cell with surfaces cooled to (a) the cloud point temperature (-53.0 C). With furthercooling, the surfaces reach -58.0 C and are held at that temperature for (b) 37.5 min, (c) 42.5 min, and (d) 47.5 min.

Figure 11. Laser scattering in JP-8 in an optical cell at (a)20 C, (b) -10 C, (c) -30 C, and (d) -40 C.


variation of torque ratio with temperature (Figure 14)demonstrate non-Newtonian behavior of the JP-8 andlow-temperature additive mixture. Optical cell imagery(Figure 12) shows that a sufficient number of evenlydistributed particles have formed that influence viscos-ity behavior.

Viscosity Determination and Analysis. From eq3 and Table 4, the correlation between measuredrelative torque and viscosity for the Newtonian calibra-tion fluid allows the absolute viscosity of jet fuel to bedetermined. The relative torque measurements obtainedat 60 rpm, which are least uncertain, are used todetermine the absolute viscosity of the jet fuel samples.Figure 15 shows the viscosity of the JP-8, JP-8 withadditive, and Jet A samples. For validation, the viscositywas determined by another method at -20 C and-40C and compared to the results in Figure 15 for theF3804 JP-8 and the F3688 Jet A samples. Kinematicviscosity () was measured using the capillary method

(ASTM D445), and the density (F) was measured bypycnometer to obtain absolute viscosity (, Table 3),where is equal to F. Figure 15 shows that the viscosityobtained from both methods agree reasonably well.

Figure 15 shows the viscosity of the neat fuel,compared to that of the fuel containing the low-temper-ature additive, derived from relative torque measure-ments at 60 rpm. The additive-containing fuel shows agrowing increase in viscosity, when compared to theneat fuel, as the temperature is reduced below -20 C.This difference is likely due to the presence of a two-phase dispersion caused by the action of the additiveand observed as clouding in Figure 12. The precipitatedwax/additive particles increase the viscosity, just asprecipitated wax particles increase the viscosity of oilin pipelines.2 Moreover, crude oils with suspended waxparticles exhibit non-Newtonian viscosity behavior.2 Aplot of viscosity as a function of shear rate, as shown inFigure 16, helps characterize the non-Newtonian be-havior caused by the action of the additive. Figure 16shows viscosity plotted for selected temperatures at eachof the shear rates. For Newtonian behavior, eq 1 dictatesno change in viscosity with shear rate. The curve fit ofviscosity at -30 C in Figure 16 shows no change inviscosity with shear rate. Visualization using the opticalcell showed no clouding at -30 C, validating theNewtonian viscosity behavior. The curve fits at -35 Cto -45 C show viscosity as an increasing function ofshear rate, decreasing with shear rate. Non-Newtonianpseudo-plastic liquids exhibit decreasing viscosity withincreasing shear rate (shear thinning). Dispersions, suchas wax crystals dispersed in liquid hydrocarbons, havebeen shown to exhibit pseudo-plastic viscosity behavior.2In the present work, it is believed that the turning rotorof the rotational viscometer has a tendency to align theadditive/wax particles parallel to the flow, reducing dragand decreasing the shear stress exerted on the rotor.Increasing the rotor speed (greater shear stress) wouldfurther align the particles and lower the viscosity.Visualization using the optical cell shows that theparticles become visible near -40 C surface tempera-tures in the coldest regions of the liquid (bottom of cell).Figure 12 shows that the clouding behavior increases

Figure 12. Clouding behavior of JP-8 (F3804) with A4647 additive at surface temperatures of (a) -45 C, (b) -50 C, (c) -55 C,and (d) -60 C.

Figure 13. Relative torque versus rotor speed for JP-8(F3804) with 5000 mg/L of A4647 cooled at a rate of -5 C/hat specific temperatures.


as the fuel is cooled, validating the increasing pseudo-plastic non-Newtonian behavior.

With decreasing temperatures, below -45 C, theviscosity behavior is more complex. Figure 16 showsthat, at -50 C, the viscosity remains constant at shearrates of 1.26 s-1 and 2.51 s-1 (12 and 24 rpm) butdecreases at a shear rate of 6.28 s-1 (60 rpm), indicatingviscosity behavior transition. The figure shows viscosity

increasing significantly from -50 C to -55 C. At-55 C, viscosity increases from 1.26 s-1 to 2.51 s-1,but decreases significantly at 6.28 s-1, more so than atall higher temperatures. Increasing viscosity with anincrease in shear rate (dilatancy or shear thickening)is observed in fluids that contain high levels of defloc-culated, or finely divided, solid particles colliding witheach other. From the visualization of the clouding of the

Figure 14. Torque ratios versus temperature for varying shear rates at a cooling rate of -5 C/h. JP-8 (F3804) with 5000 mg/Lof A4647 additive was used.

Figure 15. Absolute viscosity of JP-8, Jet A, and JP-8 with additive samples obtained using the rotational viscometer and capillarytube methods. The two symbols represent two capillary tube measurements, and the curves represent the viscometer measurements.


fuel with additive (Figure 12), it is believed that as thetemperature is reduced below -50 C, the particles haveincreased sufficiently in number to cause shear thicken-ing for the shear rates of 12 and 24 rpm. Near -55 C,Figure 12c shows adhered solids forming on the opticalcell surfaces, revealing solidification behavior and par-tially explaining the significant increase in viscosity at-55 C. This is in contrast to the solidification behaviorof the neat JP-8 (Figure 10). Adhered solids andrelatively large free-floating crystals begin to form inthe optical cell near -52 C for the neat JP-8. Figure15 shows that the neat JP-8 viscosity increases suddenlyand rapidly at this temperature, corresponding to thesolidification behavior observed in the optical cell(Figure 10).

Visualization of the solidifying JP-8 samples assistsin interrupting viscosity behavior. The magnified imageof freezing JP-8 in Figure 17 shows the adhered crystal-line structure, consisting of relatively large flat interlac-ing platelike crystals, that traps liquid fuel and with-stands the shear rate applied at rotor speeds of 12, 24,and 60 rpm. After wax crystals begin to form, whethersuspended or adhered, the crystalline structure is

sufficiently rigid to stop flow and behave as a solid.Modeling the solidification behavior of jet fuel, using amomentum exchange source term in the governingequations, yielded similar conclusions.24 This solidifica-tion behavior explains why viscosity increases rapidlynear -52 C for neat JP-8.

Figure 15 shows that the viscosity of JP-8 fuel withadditive experiences a rapid increase initially at-55 C, corresponding to solidification behavior ob-served in the optical cell (Figure 12), but then returnsto lesser rate of increase. Figure 18 shows markedlydifferent solidification behavior than the neat JP-8 fuel.Large, flat interlacing crystals do not form and thestructure may not be sufficiently rigid to withstandhigher shear rates, such as 60 rpm. A shearing of thestructures would result in a significant reduction inexerted rotor torque measurements and the resultingviscosity. Solidification and resulting shearing likelyexplain the complex viscosity behavior at -55 C inFigures 15 and 16. The phenomenon causing shearthickening, apparent at 12-24 rpm, is significantly lessthan the effects on viscosity of shearing the structures,hence, the large decrease in viscosity at 60 rpm.

Equation 1 has been modified to characterize non-Newtonian behavior. The effective viscosity of a pseudo-plastic fluid, such as the JP-8 fuel with additive above-50 C, could be represented by a simple two-parameterpower function (Ostwald-de Waele power law):31,32

The coefficients K and n are fit over a range of shearrates. Representing the non-Newtonian viscosity be-havior below -50 C for the JP-8 fuel blended with thelow-temperature additive is more complex, because ofparticle-induced shear thickening and structure-shear-ing behavior. Viscosity models that incorporate thepresence of particles and shear rates have been pro-posed. Krieger and Dougherty33 proposed eq 5, which

(31) Ostwald, W. Kolloid. Z. 1925, 36, 99.(32) De Waele, A. Oil Color Chem. Assoc. J. 1923, 6, 33.(33) Krieger, I. M.; Dougherty, T. J. Trans. Soc. Rheol. 1959, 3, 137-

152.

Figure 16. Viscosity of JP-8 (F3804) with 5000 mg/L of A4647additive, as a function of shear rate.

Figure 17. Magnified image of JP-8 (F3804) in an optical cell.

Figure 18. Magnified image of JP-8 (F3804) with A4647additive in an optical cell.

eff ) K(@u@y)n-1

(4)


shows an increase in liquid viscosity when particles areadded and the increase is dependent on particle con-centration, shape, and size.

Here, is the particle volume concentration, whereasA and B are constants that vary with particle shape andsize. Equation 5 does not include a shear rate term and,thus, could not reproduce the behavior observed inFigure 16 below -30 C. Pedersen and Ronningsen2proposed a more complex empirical representation forthe effects of dispersed wax particles in crude oil (eq 6),which includes the effects of particle volume concentra-tion and shear rate:

where D, E, and F are constants that are fit, and waxis the volume concentration of wax particles. Equations5 and 6 require knowledge of the particle volumeconcentration. Unfortunately, the particles observed inthe optical cell for the JP-8 blended with the low-temperature additive were too small to determine theparticle volume concentration. Estimates of volumeconcentration from phase-change energy obtained usinga differential scanning calorimeter proved inconclusive.Moreover, eq 6 does not include a shear thickening term.Based on the complex shear-thinning and shear-thickening behavior, as well as the adhered structureshearing, multiple viscosity models would be requiredto represent the viscosity of the fuel/additive mixtureat temperatures below -30 C.

Conclusions

The present work contains the first set of JP-8 jet fuelviscosity measurements obtained using a rotationalviscometer. The rotational viscometer can be used tomeasure the viscosity of kerosene-based jet fuels, suchas Jet A and JP-8, down to the cloud point temperature.Viscosity measurements from the rotational viscometerwere continuous from -20 C to the cloud point tem-perature, making the results useful for computationalfluid dynamics modeling. The viscosity of the neat fuelwas observed to increase exponentially with decreasingtemperature to the point of solidification, where viscos-ity values increased suddenly and rapidly. The rota-tional viscometer technique was determined to beapplicable to jet fuel; however, relative torque measure-ments must be >6% of full scale for this instrument tominimize uncertainty. The viscosity measurementsagree with results obtained from other techniques andpreviously published values for jet fuel. Moreover,visualization techniques were useful in interpreting theeffects of clouding and solidification behavior on viscos-ity.

The rotational viscometer may be used to performshear-rate studies on jet fuel to evaluate Newtonianbehavior, even with relatively high levels of uncertainty.JP-8, Jet A-1 (by inference), and Jet A fuels exhibitNewtonian viscosity behavior at low temperatures,down to temperatures approaching the onset of solidi-fication. The linearity of the fits of relative torque versusshear rate and the relationship between torque andshear rate, both as predicted by eq 2, demonstrate thatJP-8 and Jet A are Newtonian fluids near their cloudpoint temperature. Clouding of jet fuel during coolingwas determined to be likely caused by free water/di-ethyl glycol monomethyl ether (water/DIEGME) drop-lets that do not cause non-Newtonian behavior. Theadhered crystalline structure of the JP-8 fuel that wasforming at its cloud point temperature was sufficientlyrigid to withstand shear rates up to 6.28 s-1.

The rotational viscometer technique can also be usedto evaluate the viscosity of fuels containing additivesthat increase fluidity at low temperature. A class of low-temperature additives thought to encourage nucleationabove the fuel freeze point was observed to exhibit non-Newtonian viscosity behavior. This behavior was com-plex and seemed to be a function of solidificationbehavior and shear rates. Visualization techniques wereuseful in determining the relationship between clouding,solidification, and viscosity behavior. At temperaturesnear the freeze point, a dispersion of particles causedshear thinning non-Newtonian behavior. At tempera-tures near the cloud point, these particles are believedto be of sufficient size, number, and distribution to causeshear thickening. At higher shear rates (6.28 s-1), theforming adhered crystalline structures of the fuel withadditive were unable to withstand the shear rate.

NomenclatureDIEGME ) di-ethylene glycol monomethyl etherh ) rotor height (m)M ) spring torque (N m)RR ) rotor radius (m)RS ) stator (or cup) radius (m)T ) temperature (C)@u/@y ) velocity profile or shear rate (s-1) ) rotor rotational speed (rad/s) ) absolute or dynamic viscosity (Pa s) ) kinematic viscosity (cSt) ) shear stress (N/m2) ) shear rate (s-1)

Acknowledgment. This material is based on re-search sponsored by the Air Force Research Laboratory,under Agreement No. F33615-03-2-2347. The U.S.Government is authorized to reproduce and distributereprints for Governmental purposes notwithstandingany copyright notation thereon. The views and conclu-sions contained herein are those of the authors andshould not be interpreted as necessarily representingthe official policies or endorsements, either expressedor implied, of the Air Force Research Laboratory or theUnited States Government.

EF049683K

) liq(1 - A)-B

(5)

) liq[exp(Dwax) + Ewaxxdu/dy + Fwax4xdu/dy] (6)


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