Article pubs.acs.org/EF
Experimental and Modeling Studies of Heat Transfer, Fluid Dynamics, and Autoxidation Chemistry in the Jet Fuel Thermal Oxidation Tester (JFTOT) Zachary H. Sander, Zachary J. West, Jamie S. Ervin, and Steven Zabarnick* University of Dayton Research Institute, Dayton, Ohio 45469-0043, United States ABSTRACT: Modern military aircraft use jet fuel as a coolant before it is burned in the combustor. Prior to combustion, dissolved O2 and trace heteroatomic species react with the heated fuel to form insoluble particles and surface deposits that can impair engine performance. For safe aircraft operation, it is important to minimize jet fuel oxidation and resultant surface deposition in critical fuel system components. ASTM D3241, “Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels” (ASTM International: West Conshohocken, PA, 2014), defines the standard test method for evaluation of the thermal oxidation stability of aviation turbine fuels. The JFTOT is a thermal stability test that measures the tendency for fuel to form deposits via heated tube discoloration and/or an increased pressure drop across an outlet filter. It is used to discriminate between fuels of poor and acceptable thermal stability. However, the fluid dynamics, heat transfer characteristics, extent of oxidation, and corresponding deposition that occurs in the JFTOT are not fully understood. An improved understanding of these JFTOT characteristics should help in the interpretation of conventional and alternative fuel thermal stability measurements and provide important information for fuel thermal stability specification enhancements and revisions. In the current effort, the JFTOT was modified to include a bulk outlet thermocouple measurement and a downstream oxygen sensor to measure bulk oxygen consumption. Tube deposition profiles were measured via ellipsometry. External tube wall temperatures were measured via pyrometry and a computational fluid dynamic (CFD) with chemistry simulation was developed. The experimental temperature measurements show that the cooling of the outlet bus bar creates a wall hot zone near the center of the tube length. A direct relationship is found between the bulk outlet temperature and JFTOT set point temperature with the bulk outlet less than the set point temperature by 60−85 °C. Several fuels were tested at varying set point temperatures with complete oxygen consumption observed for all fuels by 320 °C; a wide oxygen consumption range from 10% to 85% was measured at a set point temperature of 260 °C. The CFD simulations demonstrated the importance of complex, three-dimensional fluid flows on the heat transfer, oxygen consumption and deposition. These three-dimensional simulations showed considerable flow recirculation due to buoyancy effects, which resulted in complex fuel residence time behavior. An optimized chemical kinetic model of autoxidation with a global deposition submechanism is able to reproduce the observed oxidation and depositions characteristics of the JFTOT. Simulations of deposition were of the right order of magnitude and matched the deposit profile of comparable experimental ellipsometric deposition data. This improved CFD with chemistry simulation provides the ability to predict the location and quantity of oxygen consumption and deposition over a wide range of temperatures and conditions relevant to jet fuel system operation.
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INTRODUCTION Jet fuel plays a vital role in modern aircraft systems beyond its primary use for propulsion via combustion in the gas turbine engine. With dramatically increasing heat loads in modern aircraft, jet fuel is used as a coolant during its passage through numerous aircraft fuels systems (e.g., avionic, hydraulic, lubrication, and environmental control) prior to combustion. Using jet fuel as a heat sink is an enabling technology for thermal control of such subsystems, but the resultant temperature rise in jet fuel can have detrimental effects. When the fuel bulk temperature approaches ∼140 °C, dissolved oxygen (∼70 ppm) reacts with fuel hydrocarbons via an autoxidation mechanism, resulting in the formation of surface deposits and bulk insolubles.1 ASTM D3241,2 “Standard Test Method for Thermal Oxidation Stability of Aviation Turbine Fuels”, defines the standard test method for thermal oxidation stability of aviation turbine fuels. The apparatus defined in the method is commonly referred to as the JFTOT, for Jet Fuel Thermal Oxidation Test. JFTOT is a trademark of the © 2015 American Chemical Society
Petroleum Analyzer Company, LLP, but specification instruments are also produced by the Falex Corporation. The JFTOT determines the tendency of an aviation turbine fuel to produce surface and bulk insolubles under such thermal-oxidative stresses. It is used to discriminate between fuels of poor and acceptable thermal stability. The test measures the formation of surface deposits on a heated aluminum rod (referred to as the JFTOT heater tube) and formation of insoluble deposits via an increase in pressure across a downstream filter. The ASTM D3241 specification rates thermal stability via a visual or metrological rating of tube surface deposition and the differential pressure increase during a test. ASTM D3241 does not attempt to closely reproduce jet fuel system deposit formation that might cause servo valve sticking, heat exchanger degradation, and/or filter plugging over the typical thousands Received: July 23, 2015 Revised: October 19, 2015 Published: October 26, 2015 7036
DOI: 10.1021/acs.energyfuels.5b01679 Energy Fuels 2015, 29, 7036−7047
Energy & Fuels
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of hours of aircraft operation and tens of thousands of gallons of fuel usage. But rather, this relatively rapid test (2.5 h) uses a small volume of fuel (500 mL) for rating the tendency of individual fuel samples to produce deposits within a fuel system. Most current petroleum-based jet fuel specifications require a set point test temperature of 260 °C, which is significantly higher than fuel temperatures in current aircraft which are typically
