Research Papers: Fuel Combustion

Ellipsometric Measurements of the Thermal Stability of Alternative Fuels

[+] Author and Article Information
Leigh Nash, Subith Vasu

Center for Advanced Turbomachinery and Energy
Research (CATER),
Mechanical and Aerospace Engineering,
University of Central Florida,
Orlando, FL 32816

Jennifer Klettlinger

NASA Glenn Research Center,
Cleveland, OH 44135

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received April 14, 2017; final manuscript received April 18, 2017; published online July 17, 2017. Editor: Hameed Metghalchi. This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.

J. Energy Resour. Technol 139(6), 062207 (Jul 17, 2017) (8 pages) Paper No: JERT-17-1165; doi: 10.1115/1.4036961 History: Received April 14, 2017; Revised April 18, 2017

Thermal stability is an important characteristic of alternative fuels that must be evaluated before they can be used in aviation engines. Thermal stability refers to the degree to which a fuel breaks down when it is heated prior to combustion. This characteristic is of great importance to the effectiveness of the fuel as a coolant and to the engine's combustion performance. The thermal stability of Sasol iso-paraffinic kerosene (IPK), a synthetic alternative to Jet-A, with varying levels of naphthalene has been studied on aluminum and stainless steel substrates at 300–400 °C. This was conducted using a spectroscopic ellipsometer to measure the thickness of deposits left on the heated substrates. Ellipsometry is an optical technique that measures the changes in a light beam's polarization and intensity after it reflects from a thin film to determine the film's physical and optical properties. It was observed that, as would be expected, increasing the temperature minimally increased the deposit thickness for a constant concentration of naphthalene on both substrates. The repeatability of these measurements was verified using multiple trials at identical test conditions. Finally, the effect of increasing the naphthalene concentration at a constant temperature was found to also minimally increase the deposit thickness.

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Fig. 1

JFTOT color standard adapted from Ref. [16]. There are ten designations including 0, <1, 1, <2, 2, <3, 3, <4, 4, and >4. The 0 rating corresponds to a clean tube, and the deposit gets darker as the rating number increases.

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Fig. 2

Naphthalene structure. Naphthalene is a two-ringed aromatic compound with the formula C10H8.

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Fig. 3

Hot liquid process simulator schematic. Fuel is pumped out of the reservoir, over the test tube, and then back into the reservoir at a rate controlled by the metering pump.

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Fig. 4

Primary components of the ellipsometer. Unpolarized light is generated by the light source and then linearly polarized. After the light reflects off of the sample, it is elliptically polarized and is collected by the detector. Finally, the signal from the detector is analyzed to determine the amplitude and phase of the light's components.

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Fig. 5

Horiba Scientific Auto SE. The expanded view shows the tube mount on the positioning stage. The boxes on either side of the tube house the optical components to perform the ellipsometric measurement.

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Fig. 6

Illustration of light reflection and refraction that occurs at an interface. A portion of the incoming light reflects at an angle equal to the angle of incidence, and the remainder is transmitted into the material at a refracted angle governed by Snell's law.

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Fig. 7

Light behavior at an interface. The overall reflected beam is made up of components that come from each interfacial interaction.

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Fig. 8

Example optical constant spectrum with a single peak. The important parameters for the new amorphous dispersion are indicated.

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Fig. 9

Tube discoloration and thickness profile. Thicker deposition leads to more discoloration (however there is deposit present on the section that is not discolored).

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Fig. 10

Tube 1304 thickness profile. This tube was exposed to Sasol IPK with 0% naphthalene at 300 °C, yielding a maximum deposit thickness of 35.77 nm and a deposit volume of 0.0112 mm3.

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Fig. 11

Tube 1321 thickness profile. This tube was exposed to Sasol IPK with 5% naphthalene, yielding a maximum deposit thickness of 21.56 nm and a deposit volume of 0.0087 mm3.

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Fig. 12

Tube 1311 thickness profile. This tube was exposed to Sasol IPK with 1% naphthalene at 385 °C, yielding a maximum deposit thickness of 134.51 nm and a deposit volume of 0.0438 mm3.

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Fig. 13

Tube 1328 thickness profile. This tube was exposed to Sasol IPK with 1% naphthalene at 400 °C, yielding a maximum deposit thickness of 200.24 nm and a deposit volume of 0.0649 mm3.




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