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High-Temperature Fuel Coking Mitigation Frangible Coatings for Fuel Nozzles and Screens


RT&L FOCUS AREA(S): General Warfighting Requirements (GWR);Hypersonics TECHNOLOGY AREA(S): Air Platforms;Materials / Processes OBJECTIVE: Develop a frangible coating that can slough off carbonaceous deposit precursors adhering and growing onto fuel-wetted surfaces, such as “last-chance screens”, to prevent dysfunction of critical aircraft fuel system components. DESCRIPTION: Increasing heat loads, projected today for advanced aircraft of the near future, will lead to higher average fuel system temperatures for both commercial and military aircraft [Ref 1]. However, fuel deposit issues currently prevent long-term fuel system operation at temperatures over ~300F [Refs 2, 3]. Fuel deposit issues are also projected to increase in current generation aircraft: for example, a recent analysis of F-24 fuel obtained from a military garrison showed extreme thermal stability problems – high coke deposition in fuel lines and hysteresis on critical valves – indicating a potential increase in thermal stability issues following the transition from JP-8 to F-24 [Ref 4]. Special consideration for Navy fuels, JP-5 with copper contamination should be made. While the formation of carbonaceous deposits can be problematic for several components of an aircraft fuel system, it is expected to have the highest negative impact in the vicinity of the fuel injectors, which are wetted by fuel with the highest time-at-temperature exposure. Of some concern are the “last-chance screens”, positioned immediately upstream of the fuel injectors, because the screen openings comprise some of the smallest fuel passages in the entire system and are exposed to fuel of temperature sufficient for coke formation. Blockage of these passages can have serious consequences in terms of aircraft propulsion control. A thin, conformal coating applied directly onto the screen would be a direct and cost-effective mitigation approach for deposit prevention; moreover, it would incur no weight penalty. However, the highly reactive radicals implicated in jet fuel deposition phenomena [Ref 5] are known to attach indiscriminately to essentially any organic or inorganic surface. For that reason, typical off-the-shelf “release coatings” are not a very effective mitigation approach even though some are better than others. The objective of the proposed SBIR topic is to create a frangible coating that can slough off carbonaceous deposit precursors adhering and growing onto fuel-wetted surfaces, thus preventing blockage on the most critical, most susceptible aircraft fuel system components. The thickness of the coating and its erosion rate should be adjusted such that it can remain operational for a time frame comparable to a typical fighter aircraft engine service interval, targeting a 5X increase in Mean-time Between Overhaul (MTBO) compared to the baseline at 400F fuel operation. KEY FRANGIBLE COATING ATTRIBUTES • “Frangibility”: Low cohesive strength of nano-layers prevents build-up by shedding adhered varnish precursors. • Nano-scale, conformal coating for complex geometries applied via vapor deposition. • Lubricity equal to or higher than underlying material, coefficient of friction equal to or lower than underlying metal, chemical inertness. • Temperature stability up to 600F. • Nano- to micro-meter coating thickness. • No off-gassing or other contamination. PHASE I: Focus on vapor deposition parameter optimization onto bill-of-materials last-chance screen samples. Confirm spectroscopically that a uniform, defect-free coating of thickness not-to-exceed 1 µm can be applied on a 200 to 120 µm mesh wire stainless steel screen such that it covers the entire surface (front and back) leaving no areas of exposed metal. PHASE II: During this phase, candidate samples resulting from the optimization efforts in Phase I will be tested with 400F flowing fuel under nominal cruise conditions and/or other conditions characterized by low fuel flow at high temperature. Evaluate within a long-duration test rig constructed with design parameters such that it simulates, as faithfully as possible, fuel system flow conditions and geometries expected in a real aircraft fuel system, with emphasis on time-at-temperature of the fuel entering the screen. The samples will be tested vis-à-vis a control (uncoated) wire mesh screen. Update vapor deposition application conditions and coating thickness based on evaluation of the flowing fuel test results in an iterative fashion until the coating application conditions which lead to the most successful coating validated under real aircraft fuel system flow conditions are identified. Demonstrate a coated screen exposed to flowing 400°F JP-5 fuel that exhibits a 5X increase in the run time to reach 80% blockage compared to a control (uncoated) screen. PHASE III DUAL USE APPLICATIONS: Focus on the development of manufacturing methods to improve component yield, production time, and component cost. Determine whether fuel system and components with the new screens require requalification or whether the screens can be qualified independently. Identify opportunities to use the technology in manufacturing areas, such as semiconductor fabrication and additive manufacturing, to prevent fouling of small and intricate tooling. REFERENCES: 1. Dahm, Werner J. A. et al. “United States Air Force Scientific Advisory Board Report on Thermal Management Technology Solutions, Vol. 2, SAB-TR-07-05-NP, August 2007. 2. Hazlett, R. N. “Thermal Oxidation Stability of Aviation Turbine Fuels.” ASTM, Philadelphia, January 1, 1991. 3. Zabarnick, S. “Studies of Jet Fuel Thermal Stability and Oxidation Using a Quartz Crystal Microbalance and Pressure Measurements,” Ind. Eng. Chem. Res. , 33, May 1, 1994, pp. 1348-1354. 4. Morris, Robert W., Jr.; Shardo, James R.; Marcum, Grady; Lewis, William K.; Wrzesinski, Paul J. and Bunker, Christopher E. “AFRL-RQ-WP-TR-2018-0019: Characterization of an On-spec, Commercial Grade, Jet A and a Near-off-spec Military F-24 (Final Report).” Air Force Research Laboratory, Aerospace Systems Directorate, January 2018. 5. Sander, Z. H. et al, “Experimental and Modeling Studies of Heat Transfer, Fluid Dynamics, and Autoxidation Chemistry in the Jet Fuel Thermal Oxidation Tester (JFTOT).” Energy Fuels 2015, 29, 11, pp. 7036-7047.
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