Energy & Fuels 1993, 7, 582-588
582
Monitoring Jet Fuel Thermal Stability Using a Quartz Crystal Microbalance E. A. Klavetter,' S. J. Martin, and K. 0. Wessendorf Process Research Department, Sandia National Laboratories, Albuquerque, New Mexico 87185 Received March 5, 1993. Revised Manuscript Received May 17, 1993
Thermal stability studies of aviation fuels are hindered by the inadequacy of current instrumentation in providing quantitative, in situ measurements of the rates of solid deposition on contacted surfaces. This paper reports the development of a test system that utilizes a quartz crystal microbalance to make in situ measurements of minute amounts of solids that deposit on the quartz crystal when a liquid fuel is thermally degraded. The quartz crystal microbalance jet fuel test system (QCM-JFTS) was developed in response to a need for quantitative information at temperature conditions representative of actual aviation fuel systems with which to develop mathematical models for predicting fuel thermal degradation and solids deposition. This bench-top system has a resolution of better than 0.3 pg/cm2with good reproducibility and accuracy. Data on deposition rates in the temperature range of 125-200 "C are obtained on three aviation fuels and simple global Arrhenius-type models formulated which demonstrate how the system can be used in model development. Additionally, the system can be used to obtain viscosity-density data on the fuels. Data obtained from the QCMJFTS on a model hydrocarbon compound and on a Jet A fuel compare favorably with literature values. The data demonstrate the potential for this system to be used in various aspects of aviation fuel thermal stability investigations.
Introduction Fuel thermal stability is an operational and performance concern in both civilian and military applications. The fuel in modern military aircraft is the primary coolant for on-board heat sources, cooling components such as the environmental control system, avionics, electrical systems, engine lubrication oil, and hydraulic fluid. The primary jet fuels in use can be reliably operated at a maximum temperature of 163 "C (325 "F);l fuels of subsonic commercial aircraft reach a maximum temperature, with the exception of fuel nozzles, of approximately 160 "C. Fuel temperatures will increase concurrently with the performance of future aircraft. Hydrocarbon-based fuels form deposita at elevated temperatures, reducing heat exchanger efficiencies, as well as obstructing valves, filters, and injection nozzles. Severe performance and operational problems can resulta2 These problems, and the trend to increasing heat loads, have prompted recent studies to understand thermal stability mechanisms and develop predictive mathematical models to describe fuel degradation and deposition of solids. These studies have shown the need to develop diagnostic instrumentation and apparatus to quantitatively monitor parameters related to these processes. In previous research activities, perhaps the most common parameter associated with fuel thermal stability has been the rate of solids deposition on a metal surface. Generally, this mass deposition rate has been determined posttest, resulting in only an average rate over the duration of a test. Additionally, because of the lack of sensitivity of previous test methods in measuring the quantity of solids (1)Gray, C.N.;Caiseon,M. W. 'Aircraft Fuel Heat Sink Utilization," Pt. No. AFAPL-TR- 73-51. General Electric Co.,July 1973. (2)Hazlett, R. N. Thermal Ozidation Stability of Aviation Turbine Fuels; ASTM PCM 81401092-12;ASTM: Philadelphia, 1991.
deposited, many methods have used accelerated test conditions. For example, the ASTM Standard Method D 3241 using the jet fuel thermal oxidation tester evaluates fuel at 260 "C, significantly above the maximum aircraft fuel system temperature. Other research test systems are generally run above 200 0C.2 This paper discusses the development of a system that uses a quartz crystal microbalance (QCM)to make in situ, real-time measurements of the mass deposited on a The resolution for this device is less than 1 pg/cm2, permitting mass deposition to be resolved at temperatures representative of those found in an aviation fuel system. The QCM sensor is integrated into a system for testing the thermal stability of hydrocarbon fuels at elevated temperatures and pressures. Experiments have been performed that demonstrate the capability of the device in monitoring fuel thermal stability. Kinetic model parameters in the form of global activation energies were determined for three fuels using mass deposition rates measured at temperatures from 125 to 200 "C. These parameters were determined under conditions of oxygen availability and oxygen deficiency. The mass deposition rates ranged from 0.3 to 13 pg/(cm2.h). The values determined from the QCM device were compared with data from independent tests, performed using ion beam sputtering combined with Auger spectroscopy, to directly determine the total thickness of solids deposited on the surface. Additionally, we show how data from the device are used to determine fuel viscosity values as a function of temperature. (3)Numura, T.; Mmemura, A. Nippon Kagaku Kaishi 1980,1621. (4)Sauerbrey, G.Z.Phys. 1959,155,206-222. (5)Shumacher, R. Angew. Chem., Znt. Ed. Engl. 1990,29, 329-343. (6)Ward,M. D.; Buttry, D. A. Science 1990,249, 1000-1007. (7)StockbridgeC.D.In VacuumMicrobaZanceTechniques;Behmdt., K. H., Ed.; Plenum: New York, 1966;Vol. 5, pp 193-205.
This article not subject to US.Copyright. Published 1993 by the American Chemical Society
Energy & Fuels, Vol. 7, No.5,1993 683
Jet Fuel Thermal Stability
mulation on the surface and liquid contact both lead to a decreme in the resonant frequency, given by11
upper and lower eloctrc&s
/
A t c u t qwrk cryrtrl
Figure 1. Schematic of quartz crystal microbalance (QCM), consisting of circular electrodes patterned on both sides of thinned AT-cut quartz disk. UWM P.'l
Y
t
7
X
Figure 2. Cross sectional view of a QCM simultaneously loaded on one side by a mass layer and a contacting liquid.
where fo is the unperturbed aeries resonant frequency, N is the harmonic number (N= 1,3,5, ...),h and pqare the quartz shear stiffness and mass density, p, is the surface mass density (mass/ area), p and 9 are the liquid density and viscosity, respectively, and n is the number of sides of the QCM in contact with liquid or coated by a mass layer. For AT-cut quartz crystals, values of h = 2.947 X 10" and pq = 2.651 g/cm3 have been determined.12 Equation 1 shows that, from frequency changes alone, we can not resolve changes in surface mass density (p,) from changes in the liquid density and viscosity ( p and 7). However, if liquid properties are constant, then changes in resonant frequency can be used to unambiguously measure mass accumulations. The surface mass density on each side of an immersed QCM arising from accumulated solid deposita is related to the change in resonant frequency as
for resonators operating in the fundamental resonance (N = 1). For the immersed device discussed in this paper, n = 2. If liquid properties are changing (e.g., due to temperature changes),then an independent measurement is required to resolve changes in surface mass accumulation from changes in liquid properties. The motional resistance, R, representing the impedance of the crystal a t resonance, is a convenient measure of crystal damping.ll The motional resistance arises primarily due to losses caused by viscous coupling to the adjacent liquid. Formation of a viscoelastic film on the device surface also contributes to motional resistance.lS R is related to liquid properties as (3)
Experimental Section Theory. The QCM is a bulk-wave resonator, consisting of a piezoelectric quartz wafer with metal electrodes on each face, that can be electrically excited into resonance. A schematic of the QCM used in these studies is shown in Figure 1. The metal electrodes, where the rate of surface mass accumulation is measured, can be formed from a variety of different metals by vapor deposition. The electrode geometry contains a grounded circular electrode on one side and, on the other side, a circular electrode a t rf potential. The active area for sensing mass accumulation is approximately the area of the smaller circular electrode where the electric field is confined.8 When the crystal is electrically excited a t ita resonant frequency, the crystal undergoes a shear deformation, with displacement maxima a t the crystal faces (see Figure 2). Numura and Minemura3 have shown that QCMs can be operated as microbalances in contact with liquids. Mass accumulated on the crystal surface moves synchronouslywiththeoscillatingsurfaces, resulting in adecreaae of the resonant frequency. In addition, a thin layer of liquid adjacent to the oscillating crystal surface is entrained by the surface (Figure 2), causing a decrease in the resonant frequency.' #lo An equivalent circuit model has been derived for the QCM contacted by a mass layer and a liquid that describesthe frequency response of the mam and liquid-loaded device." Mass accu(8) Hillier, A. C.; Ward,M.D.Anal. Chem. 1992,64,253+2554. (9) Kanmwa, K. K.; Gordon 11, J. G. Anal. Chem. 1985,57, 17701771. (10)Glassford, A. P. M. J. Vac. Sci. Technot. 1978, 15, 1836-1843. (11) Martin S.J.; Granataff, V. E.; Frye, G. C. Anal. Chem. 1991,63, 2272-2281.
where K2 is the quartz electromechanical coupling coefficient and COis the static capacitance of the device. For AT-cut quartz crystals, values of K2 = 7.74 X and CO= 4.25 pF have been determined.'* The relationship between the resonant frequency and the mass accumulated on the QCM surface allows the device to function as an extremely sensitive microbalance and monitor the accumulation of solids resulting from thermal degradation of aviation fuels. The sensitivity to liquid properties enable the densityviscosity product to be monitored. To instrument the QCM as a sensor,the QCM is typically incorporated in an oscillatorcircuit. A unique oscillator circuit has been developed to track the resonant frequency of the device and measure the motional resistance, R,a t this frequency. The oscillator has two outputa: (1) an ac voltage a t the QCM resonant frequency and (2) a dc voltage proportional to R. As solids deposit on the surface of the quartz crystal, the changing resonant frequency is measured and the corresponding surface mass density determined (from eq 2). The motional resistance is also monitored to indicate changes in liquid properties. System Description. A test fiaure was designed and constructed to allow the quartz microbalanceto monitor the solids deposited fromjet fuel thermal degradation. This fixture, shown in Figure 3, consists of a stainless steel vessel (ParrInstrument Co.) custom-modifiedto permit a hermetic, 3.5-mmrffeedthrough to be mounted in the lid. The QCM jet fuel sensor is suspended vertically by clamps a t the upper edge with the acousticallyactive area submersed in the jet fuel; vertical orientation was chosen (12)Ballato, A. IEEE Trans. Sonics Ultrason. 1978, SU-25,186-191. (13)Ristic, V. M.In Principles of Acoustic Devices; Wiley New York, 1983,p 127.
584 Energy & Fuels, Vol. 7, No. 5, 1993
Klauetter et al.
Jet Fuel
Figure 4. Schematic of the quartz crystal microbalance jet fuel test system.
Figure 3. Schematic of the QCM test fixture for monitoring jet fuel degradation and solids deposition. to eliminate gravitational effects on deposition. Electrical measurements are made on the quartz crystal through a 3.5-mm rf feedthrough that contacts the circular electrode pattern on the crystal. The quartz crystals used in this study, obtained from Maxtek (Torrance, CA), were 2.54-cm-diameter, synthetic AT-cut quartz wafers. The QCMs, nominally 0.33 mm thick, had planar faces that were lapped and polished to give a fundamental resonant frequency of 5.0 MHz. Gas inlet and outlet ports are provided on the lid of the fixture to allow exposure to an oxidative or inert atmosphere. A band heater is clamped around the periphery of the vessel to heat the chamber. Temperatures up to 300 "C can be achieved. A thermocouple embedded in the chamber indicates fuel temperature. A temperature controller uses the thermocouple to maintain fuel temperature a t the desired set point. The lid seals to the body of the vessel with a Teflon gasket and clamps,allowing a pressure of lo00 psig to be maintained. The unit is suspended by a clamp stand abovea magnetic stirrer, allowingthe fuel sample to be mixed during the test, if desired, to eliminate potential diffusional limitations during oxidative testing. The test fixture is integrated into the QCM jet fuel test system (QCM-JFTS);a schematic of the total system is shown in Figure 4. The system measures the in situ resonant frequency and voltage changes as the fuel thermally degrades and deposits on the QCM. Temperature, pressure, and time are also measured. A key component of the QCM-JFTS is an oscillator circuit that drives the quartz resonator suspended in the jet fuel. This circuit uses the impedance variations inherent in the QCM resonant response to track the resonant frequency of the sensor. Due to the severe operating environment, commercially-available oscillators proved inadequate and a circuit was custom-designed. This oscillator board is unique in providing both a frequency and amplitude output: mass accumulation causes a decrease in resonant frequency (eq 2); changes in liquid properties (densityviscosity)cause changesin both resonant frequency and motional resistance (eqs 1 and 3). The oscillator frequency output is read by a frequency counter (HP Model 5384A) and input to the personal computer (PC) as an indicator of mass accumulation. From eq 1,the mass density p, (mass/area) accumulated on each crystal face is proportional to the change in resonant frequency 4.Interpreting frequency changes in terms of mass accumulation according to eq 1will be
erroneous if the fuel and crystal temperatures are varying, because fuel viscosity and density will vary with temperature and contribute a frequency change also. In addition, the resonant frequency of the crystal itself has some slight temperature dependence. Consequently, the acquisition of frequency data is generally initiated only after the fuel sample has stabilized a t the test temperature. The dc voltage output of the oscillator is read by a digital voltmeter and input to the PC. This voltage increases with crystal damping and is proportional to the motional resistance (R) of the crystal. Monitoring the crystal damping is useful for several reasons. Crystal damping (asreflected by the motional resistance) is proportional to the square root of the liquid density-viscosity product1' (eq 3). First, while fuel density does not vary appreciably with temperature, fuel viscosity decreases significantly with temperature and can thus be monitored with the motional resistance output of the oscillator. Second, as fuel degradation byproducts accumulateon the crystal, the viscoelastic nature of these deposits leads to crystal damping in excess of that contributed by contact with the fuel. Thus, changes in the amplitude output serves as a secondary indicator of deposit thickness. Because the damping contributed by the surface film depends on the film's viscoelastic properties,ll the relationship between the damping output and deposit thickness is not as straightforward as between frequency output and depoait thickness. Third, the amplitude output is a useful diagnostic indicator: faulty electrical contact between the QCM and clamp, for example, is reflected in an intermittent or anomalously high R value. A personal computer acquires data during the test, displaying it in real time and storing it for later use. Software has been developed to set data acquisition rates and setpoint temperatures and to perform on-line data analysis to show mass deposition as a function of time. The computer can be any IBM AT-compatible computer capable of running Windows 3.1 and DOS 5.0. Experiment Description. For the experiments d i s d here, three different jet fuel samples were used. The fuels were either specificationJP-8or Jet A fuels. Because of the importance of oxygen in these fuel thermal degradation experiments, we attempted to control the initial oxygen concentration of each fuel sample. In a typical experiment, the fuels were sparged with oxygen to saturate each fuel sample prior to testing. A pressure of 50 psig of oxygen was then imposed to maintain liquid reaction conditions. In one experiment series, nitrogen was used as the overpressure gas to determine the effect of oxygen deficiency on mass deposition rates. In another series of experiments, the fuel was prepared by continuous nitrogen sparging to remove available oxygen and the experiments performed again usinga nitrogen overpressure. The experiments (14) Martin, S.J.; Frye, G.C. 2992 Ultwonics Symp. 1991,393-398.
Energy & Fuels, Vol. 7,No. 5, 1993 586
Jet Fuel Thermal Stability
Table 1. Comparison of Solid Deposit Thicknesses Determined from QCM Data and Ion Sputtering/Auger Saectroacoav
10
JP-8
f 7.5
s
Jet A (2)
170 180 170 200
1830 2820 680 11700
1600 3300 800 9900
13 16 16 17
different operators. This yields an areal mass density deposition rate of 1.17 pg/(cm2-h), with a standard deviation of 0.19. The areal mass density represent the mass accumulation averaged over the active surface area. I Three tests were performed at 170 "C; deposition rate values were 1.31,1.31,and 1.12 pg/(cm2.h). The average ii value was 1.22 pgl(cm2-h),with a standard deviation of 2.5 0.13. Accuracy. First, the accuracy of the QCM sensor in quantifying mass accumulation on the crystal surface was estimated by depositing a known thickness of gold on the I I I 0 QCM, measuring the resulting frequency shift, and de0 2ooo 4Ooo 8ooo 8Ooo termining the areal mass density and corresponding solid thickness. In previously reported data," a 124-nm gold Time (sec) layer was deposited on a quartz crystal to give a predicted Figure 6. Typical mass accumulation data from the QCM jet surface mass density of 225 pg/cm2. The QCM frequency fuel test system. shift data indicated a gold thickness of 117 nm, giving an were performed at temperatures between 125 and 200 "C. The error of 5.6% for this model system. experimentswere typically run for 2 h at constant temperature, Second, we attempted to determine the accuracy of the with a few testa run up to 24 h. The time to reach a constant mass accumulation data obtained from the thermal (h0.5O C ) temperaturewas generally 30-45 min. Mild mixing of degradation tests on aviation fuels by utilizing an indethe fuel to maintain uniform oxygen concentration in the vessel pendent technique to measure the solid carbonaceous was achieved using a magnetic stirrer and stir bar. deposit thickness. The deposits collected on four quartz crystal microbalances were depth-profiled using inert gas, Results and Discussion ion-beam sputtering, and Auger electron spectroscopy and deposit thicknesses determined. The crystals were from In situ accumulation of mass was measured on three QCM deposition-rate experiments on JP-8 fuel at 170and different aviation fuels to determine the affect of tem180 "C and on Jet A fuel at 170 and 200 "C. The sputter perature and atmosphere on mass deposition and to time for traversing the solid deposita on the quartz crystals demonstrate the capability of the quartz crystal microbalwas measured and converted to a thickness using asputter ance jet fuel test system (QCM-JFTS) to quantitatively rate calibrated on a carbon film of known thickness. Some evaluate fuel thermal stability. Deposition rates were difficulty was encountered because of the nonuniformity determined for each experiment as a measure of the fuel of the deposit film layer on the quartz crystals. A variation thermal stability. Figure 5 shows typical mass accumuup to a factor of 2 in thickness over the quartz crystal lation data on the QCM obtained in situ at isothermal surface was determined from the ion sputtering measureconditions as a function of time. A discussion of the ments on one sample; an average thickness was therefore resolution, precision, and accuracy of the mass accumuestimated. The average thickness values from the ionlation rate data is first presented. Deposition rate values beam sputtering tests for the four devices are shown in for the three fuels tested in the temperature range of 125Table I. The mass accumulation, as measured by the QCM 200 "C are then presented and global activation energies system, for each device was divided by the solid density calculated. The effect of oxygen on the deposition rates to provide an average thickness from the QCM data. If the is discussed. Finally, viscosity values are determined from nonuniformities in mass deposition on the surface are motional resistance measurements for a Jet A fuel and randomly distributed, the use of an average mass depotridecane as a function of temperature and compared with sition is appropriate in estimating an average thickness. literature values. The density of the deposited solids is not known, so a best Resolution. The temperature control a t the desired fit value of 0.32 g/cm3 (standard deviation of 0.076) was set point is the major factor in determining the resolution used. There exist no definitive density data for aviation for maas accumulation measurements on the metal surfaces fuel deposits, with densities values between 0.08 and 1.45 of the quartz crystal. Frequency fluctuations of 20 Hz at g/cm3determined from various researchem2 The density a resonant frequency of 5 MHz were measured as temvalue used here falls within this range. The thicknesses perature fluctuated f0.5 "C. This frequency fluctuation calculated from the QCM data are compared in Table I results in a limit of areal mass resolution estimated at 0.3 with the thicknesses determined from the ion sputtering/ pglcm2. Auger data. The percent differences between the two Precision. The precision of the deposition rate results determinations for the four devices range from 13 to 175%. was estimated by performing replicate tests at 160 and Because of the assumptions involved in the calculation of 170"C with samples taken from one fuel. At 160 "C,eight thicknesses from both techniques, we cannot quantitate testa were performed with resulting deposition rate values of0.94,1.05,1.O8,1.10,1.14,1.17,1.38,and1.52pg/(cm2~h) the accuracy of the QCM data but only state that the data appear to accurately represent the mass accumulation. using three different QCM jet fuel test systems with three
P-
i5
Klavetter et al.
586 Energy & Fuels, Vol. 7, No. 5, 1993 Table 11. QCM Measurements of Jet Fuel Solids DeDosition deposition activation sparge overpress. temp rate energy (OC) (pglcm2.h) (kcal/mol) fuelID gas gas JP-8 02 50psigof02 160 1.30 170 2.47 180 5.28 22 200 12.0 JetA(1) 02 50psigof 02 125 0.43 134 0.34 145 1.38 20 180 6.48 JetA(2) 0 2 50psigof02 160 1.18 170 1.22 180 2.92 26 200 13.1 02 50psigofN2 160 1.23 180 1.70 12 200 4.19 N2 50psigofN2 160 0.36 -0 180 0.36 200 0.26
~~
I
,
I
~~
(2);
'
I
2.1
'
'
22
'
I
2.3
'
24
2.5
2.8
lfJOo/T (1/K)
Figure 6. Arrhenius plot showing activation energies for the JP-8 and two Jet A aviation fuels tested at temperatures in the range 125-200 O C . I 3 -
The data discussed here indicate that the QCM-JFTS can provide quantitative evaluation of the mass accumulation of solid deposits resulting from the thermal degradation of aviation fuels. The system can measure extremely low deposition rates with good reproducibility. The accuracy of the technique appears to be good, with an error in predicted areal mass density of less than 6% for a model system. For tests performed with jet fuels, a quantitative determination of accuracy could not be made because of the lack of a known value for the density of the deposited solids and the variation in the solid thickness on the QCM surface, but the good correlation between the QCM data and the independent ion sputtering/ Auger data indicates that the QCM sensor system provides an accurate determination of the areal mass density. Deposition-Rate Experiments. Experiments were performed using the QCM-JFTS to demonstrate the system capability to provide quantitative data on the thermal stability characteristics of selected aviation fuels. Experiments were performed on a JP-8 fuel and two Jet A fuels, designated Jet A (1)and Jet A (2), to determine deposition rates at temperatures between 125and 200 "C. The data were used to calculate global deposition rate activation energies. Each fuel was tested at temperatures up to 200 "C with a 50 psig of 0 2 overpressure and pretest sparging with 0 2 for 20 min. The data are shown in Table 11. Deposition rates in the range 0.3-13 pg/(cm2.h)were measured. Figure 6 shows an Arrhenius plot and the calculated activation energies, E,, for the global deposition process for the three fuels. The activation energies of 24,20, and 26 kcal/mol are similar, indicating the similarity of the kinetics of the global deposition processes for the three fuels. Interestingly, the rates are very linear over the period of the test (2 h) with little to no induction period obvious prior to the observed deposition. Mass accumulation was measured from the point when the jet fuel temperature reached the set-point temperature. The calculated activation energies are well within the wide range of E, values (5-45 kcal/ mol) reported by various fuel researchers2 and are very similar to the activation energy values reported for hydroperoxide decomposition, considered to be an important fuel and hydrocarbon reaction. The values are also significantly higher than values associated with a heterogeneous reaction (catalyzed or wall-influenced)
.
0 -
fl
(2) (" 2.1
2.15
2.2
2.25
2.3
2.35
1 m / T (1/K)
Figure 7. Arrhenius plot showing activation energies for Jet A (2) fuel with (0) Ozsparging and 02overpressure, ( 0 )0 2 sparging and Nz overpressure, and (A)NZsparging and Nz overpressure.
reaction, which have E, values less than approximately 15 kcal/mol. This result supports the assumption that neither the walls of the test fixture (stainless steel) nor the QCM materials (quartz with gold electrodes) significantly catalyze the mass accumulation. Because of the importance of oxygen in the thermal decomposition and deposition processes at the studied temperatures, two additional series of testa were performed using the Jet A (2) fuel in which the availability of oxygen was limited. In the first series, the fuel was sparged with oxygen but the oxygen overpressure in the test fixture was replaced with a 50 psig of nitrogen overpressure. In the second series, the fuel was first sparged with nitrogen for 1h to remove oxygen and then thermally stressed with a 50 psi of nitrogen overpressure. These data are also shown in Table 11. As expected, the deposition rate decreases when oxygen is limited or removed. For the experiments where the fuel was sparged with nitrogen, extremely low deposition rates were measured at the three temperatures tested; the rates are near the lower limit of measurement of the system for the time tested and no activation energy was apparent. Figure 7 shows an Arrhenius plot of these data, comparing the activation energies of the fuel with oxygen and with limited and no oxygen. The effect of oxygen availability on fuel deposition rates was further demonstrated by two longer term testa (- 7 h) using Jet A (1)fuel at 135 OC. The fuel was sparged with oxygen in both tests. In one test, a continuous oxygen
Energy & Fuels, Vol. 7, No. 5,1993 587
Jet Fuel Thernuzl Stability
/I
Tridecane 0.025
A
z
n
0.02
QCM #1 0
QCM #I2
0
cn
0.015
m
v
.-3 v,
0
.-> V
0.01
v,
0
5,000
10,600
15,600 20,600 25,OOO 30,OOO
0.005
Time (sec)
Figure 8. Effect of 0 2 availability on mass accumulation of Jet A (1) thermally stressed at 135 "C.
supply was made available by flowing oxygen through the liquid fuel while maintaining an overpressure of oxygen of 50 psig. In the other test, a 50 psig overpressure of 0 2 was imposed and the system closed. The mass accumulation data are shown in Figure 8 (the data are intentionally offset for ease of viewing). As the data show, the deposition rates are both 0.6 pg/(cm2-h)up to about 3.5 h. After that time, the experimentwith the flowingoxygen maintained the deposition rate; however, the experiment with the limited oxygen showed a sharp decrease in deposition rate to 0.07 pg/(cm2=h),suggesting that the oxygen was depleted. These two experiments clearly show the importance of oxygen availability in solids deposition, as well as the importance of in situ measurement of deposition rates in realistic systems where oxygen is limited. TemperatureDependence of Viscosity. The QCMJFTS was also used to measure the temperature dependence of the viscosity of the Jet A (1)fuel, as well as for a model compound, tridecane, that has viscosity values similar to Jet A fuels. Due to the interaction of the oscillating devicesurface with a contacting fluid, electrical measurementscan be used to extract the product of density and viscosity of the fluid. These viscosity-density values were divided by tabulated values of the liquid density to determine the viscosity of the liquid at various temperatures. These viscosity values calculated from QCMderived measurements were compared with handbook viscosity values. Figure 9 shows the comparison between the viscosity values derived from data from two QCM tests on tridecane and viscosity values measured by Vargaftik15 from 25 to 200 OC. There is good agreement between the values, with an standard error between measurements of 0.001 g/(cm.s). Figure 10 shows a similar comparison for the Jet A (1) fuel, comparing the QCM-deriveddata with viscosity data on a typical Jet A fuel.16 The capability to determine in situ viscosity values is important because of the need for such informationin predictive modeling of fuel degradation mechanisms and also because of the need for viscosity values in analyzing data from other diagnostic systems ~
_
_
_
~
(15) Vargaftik, N. B. Tables on the Thernwphysical Roperties of Liquids and Cases; John Wiley & Sons, Inc.: New York, 1975. (16) Coordinating Research Council. Handbook of Aviation Fuel Properties; CRC Report No. 530; 1988,20-36. (17) OHern, T. J.; Trott, W. M.; Martin, S. J.; Klavetter, E. A. AIAA Paper, AIAA-93-0363; presentedat the Aermpace ScienceMeeting, Reno, NV, 1993.
,
n "
0
I
50
l
l
,
100
I
150
l
200
l
250
Temperature ( O C )
Figure9. CompariaonofviscosityvaluesdeterminedfromQCM data on tridecane between 30 and 200 "C with viscosity values from Vargaftik (1975) (ref 15).
Jet A (1)
\
Temperature ("C)
Figure 10. ComparisonofviscoeityvaluesdeterminedfromQCM data on Jet A (1) between 40 and 120 "C with viscosity values from CRC (1988) (ref 16).
being developed for monitoring other aspects of jet fuel degradation."
Conclusions We have developed a quartz crystal microbalance jet fuel test system that can provide quantitative values of in situ mass accumulation on a surface as a hydrocarbon fuel thermally degrades. The system utilizes a quartz crystal microbalancethat accurately and reproducibly determines the mass deposition resulting from thermal instabilities of aviation fuels, measuring rates as low as 0.3 pg/(cm2+h). Deposits were measured at temperatures as low as 125OC. Data obtained from experiments on multiple fuels have shown the capability to easily discriminate the thermal stability of the fuels, allowing quantitative evaluation of the fuels under various temperature and oxidative conditions. The data were used to obtain kinetic parameters required for use in developing mathematical models for the fuel deposition processes. The activation energies calculated are consistent with values for these type of
Klavetter et al.
588 Energy & Fuels, Vol. 7, No. 5, 1993
oxidative reactions reported in the literature. For the conditions tested, the mass accumulation values are linear with time when the availability of oxygen is not limited, suggesting no significant surface effects. The data clearly show the effect of oxygen availability on fuel thermal deposition rates and show the need to make in situ measurements of solid deposition rates for realistic environments where oxygen concentration varies. Additionally, the system provides information on the fluid viscosity values as a function of temperature. The QCM jet fuel test system shows good potential as a tool for evaluating the thermal stability of aviation fuels under a wide range of operating and pretreatment conditions at times and temperatures representative of those found in actual aviation fuel systems.
Acknowledgment. This work was jointly supported by the U.S. Air Force, Aero Propulsion and Power Directorate of Wright Laboratory, Wright-Patterson AFB, and the U S . Department of Energy, Pittsburgh Energy Technology Center (PETC), through Sandia National
Laboratories under Contract DE-AC04-76DP00789. We acknowledge the support of W. E. Harrison 111, Dr. W.M. Roquemore, and Major Donn Storch of Wright Laboratory and S. Rogers of the U.S.DOE PETC.
Glossary
co E,
f
fo
K2 n
N
R t 9
Ps P Ps PS
static capacitance, pF activation energy, kcal/mol frequency, Hz series resonant frequency, Hz quartz electromechanical coupling coefficient number of sides of QCM contact by liquid or coated with mass layer harmonic number impedance of the crystal at resonance, ohm thickness, A liquid viscosity, g/(cm.s) quartz shear stiffness, dyn/cm2 liquid density, g/cm3 quartz mass density, g/cm3 surface (areal) mass density, g/cmz
