Energy & Fuels 2005, 19, 1935-1947
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Experimental Studies of Jet Fuel Viscosity at Low Temperatures, Using a Rotational Viscometer and an Optical Cell Daniel 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 to evaluate the viscosity behavior of kerosene-based jet fuels from -20 °C to sample cloud point temperatures. The uncertainty of using the rotational viscometer technique is evaluated and determined to be acceptable. The rotational viscometer is used to perform shear rate studies on JP-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 crosspolarized 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 free droplets of a water/di-ethylene glycol monomethyl ether (water/DIEGE) mixture, while clouding of the jet fuel/additive blend was caused by crystallization. Viscosity results are presented for all jet fuel samples. Selected results are compared with viscosity measurements obtained from other methods 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. Viscosity results and the effects of shear rate changes matched observed solidification behavior.
Introduction Operators and designers of aircraft, both manned and unmanned, that fly at altitudes above 12 000 m for extended periods require high-fidelity fuel freezing prediction tools. An impediment to the understanding and simulation of jet fuel freezing has been the absence of published thermophysical properties of jet fuel at low temperatures. Common jet fuels, such as Jet A, Jet A-1, and JP-8, have little published property measurements at temperatures near their freeze point temperatures. The freeze point for a hydrocarbon liquid fuel (ASTM D2386-03) represents the temperature at which visible wax crystals melt upon warming. Therefore, jet fuel consists mostly of liquid several degrees (∼3-4 °C) below its freeze point. The absence of published property data is likely due to the variation of jet fuels, based on the composition of the original crude oil and the refinery methods used. Also, property measurements at temperatures approaching the onset of solidification are difficult to obtain, requiring specialized equipment and techniques. Commercial and military jet fuel specifications are not sufficiently rigorous to ensure consistent thermophysical properties. Moreover, the introduction of non-petroleum-derived fuels, such as Fischer-Tropsch fuels, are on the horizon. Therefore, jet fuel properties must be assessed regularly and new techniques * Author to whom correspondence should be addressed. Telephone: (937) 252-8878, ext. 114. Fax: (937) 252-9917. E-mail address:
[email protected].
developed to determine their low-temperature properties accurately. The Coordinating Research Council (CRC) has provided a compilation of aviation fuel properties that are widely used in the aerospace industry.1 Unfortunately, these properties are incomplete for temperatures near jet fuel freeze point temperatures. The low-temperature properties of jet fuels must be known to model and design aircraft fuel systems adequately. At present, the specified freeze point temperature and viscosity at a single temperature are used by aircraft designers and operators to determine a fuel’s ability to be pumped at lower temperatures. The usefulness of the specified viscosity requirement and freeze point temperature is limited because jet fuel is a liquid below its freeze point temperature. In addition, fuel viscosity greatly impacts the ability to re-light an engine at low temperature. More-complete data are needed for improved combustor design. The present specification method of determining the viscosity of jet fuel is the capillary method (ASTM D445). Continuous viscosity measurements of jet fuel, as a function of temperature, would provide important information on actual fuel flow characteristics at temperatures near the onset of crystallization. Viscosity measurements at varying shear rates could reveal possible non-Newtonian behavior caused by forming wax or water crystals. Fuel clouding behavior (1) Handbook of Aviation Fuel Properties, Coordinating Research Council Technical Report 635, Atlanta, GA, 2004.
10.1021/ef049683k CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005
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upon cooling, which is caused by precipitating water or other phenomena (e.g., large n-alkane gelation), may introduce non-Newtonian behavior. Two-phase slurries (wax particles suspended in liquid fuel), emulsions, or dispersions may change the flow properties from Newtonian to non-Newtonian behavior.2 The effects of fuel additives on flow characteristics could also be determined. The proposed method utilizes a temperature scanning rotational viscometer (ASTM D5133-01), which has been applied to low-temperature automotive lubricants, to investigate low-temperature jet fuel fluidity.3 The use of a rotational viscometer to determine the low-temperature viscosity of jet fuels has been limited but is emerging as a new technique.4 The viscosity of Jet A has been measured with a rotational viscometer at temperatures as low as -50 °C; however, these measurements did not include shear rate studies or the effects of additives. Moreover, variations in shear rate were required to maintain instrument scale, concealing potential non-Newtonian behavior.5 Other researchers investigated the viscosity of Russian kerosene-based jet fuels.6-11 For example, Mitin et al.6 used a kerosenebased thermally stable fuel (T-6) that was designed for high-performance aircraft in a rotational viscometer. They concluded that the fuel is non-Newtonian below 0 °C, because of the formation of “rheological structures.” T-6 has greater density than JP-8, because it consists mainly of cycloparaffins, reducing the lowerdensity n-alkane content.12 Gorenkov et al.7 measured the viscosity of Russian jet fuels using the capillary method (GOST 33-82) and identified the lack of continuous viscosity measurements at low temperatures. Likhterova et al.8 also identifies the lack of viscosity measurements at low temperatures and suggests that jet fuels may be non-Newtonian near the initial crystallization temperature. In this study, a rotational viscometer is used to study flow characteristics of Russian kerosene-based jet fuels at low temperatures.8 Unfortunately, this research provides data on only Russian fuels, lacks experimental detail, has translation issues (2) Pedersen, K. S.; Ronningsen, H. P. Energy Fuels 2000, 14, 4351. (3) Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique, ASTM D5133-01; American Society of Testing Materials: West Conshohocken, PA. Available via the Internet at URL: http://www.astm.org [cited June 2004]. (4) Selby, T. W. The Scanning Brookfield Technique of LowTemperature, Low-Shear RheologysIts Inception, Development, and Applications, ASTM STP 1143; American Society for Testing and Materials: Philadelphia, PA, 1992; pp 33-64. (5) Stockemer, F. J. NASA Technical Report CR-159615, National Aeronautics 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 of Khim. 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, 543547. (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 Development Center Technical Report No. WRDC-TR-89-2097, 1989.
Atkins et al. Table 1. Volume Percentages of Species Classes within Four Fuel Samples, as Determined by ASTM 2425 Composition of Jet A Fuel (vol %) species classes
F3686
F3688
F3773
F3804
paraffins cycloparaffins dicycloparaffins tricycloparaffins alkylbenzenes indan and tetralins indenes, CnH2n-10 naphthalene naphthalenes acenaphthenes acenaphthylenes tricyclic aromatics
43.6 20.0 11.7 0.9 12.7 6.6 0.3 0.4 3.5 0.3
