Experimental and Modeling Studies of Heat Transfer, Fluid Dynamics, and Autoxidation Chemistry in the Jet Fuel Thermal Oxidation Tester (JFTOT)

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

Flow and chemical kinetics simulations of endothermic fuels

Advanced aircraft engines are reaching a practical heat transfer limit beyond which the convective heat transfer provided by hydrocarbon fuels is no longer adequate. One solution is to use an endothermic fuel that absorbs heat through chemical reactions. This paper describes the development of a two-dimensional computational model of the heat and mass transport associated with a flowing fuel using a unique global chemical kinetics model. Most past models do not account for changes in the chemical composition of a flowing fuel and also do not adequately predict flow properties in the supercritical regime. The two-dimensional computational model presented here calculates the changing flow properties of a supercritical reacting fuel by use of experimentally derived proportional product distributions. The present calculations are validated by measured experimental data obtained from a flow reactor of mildly cracked n-decane. It is believed that these simulations will assist the fundamental understanding of high temperature fuel flow experiments

Chemical Analysis of Jet Fuel Polar, Heteroatomic Species via High-Performance Liquid Chromatography with Electrospray Ionization-Mass Spectrometric Detection

High-performance liquid chromatography (HPLC) with electrospray ionization–mass spectrometry (ESI–MS) was used to identify several classes of heteroatomic, polar compounds containing oxygen, nitrogen, and sulfur in a variety of jet fuel samples. While nitrogen, oxygen, and sulfur compounds are present only at low concentrations in jet fuel, they contribute significantly to some important fuel properties. These trace, heteroatomic species can provide positive (e.g., improved lubricity) or negative (e.g., reduced thermal stability) impacts. Reversed-phase liquid chromatography with ESI–MS detection allows for the polar components to be selectively ionized and subsequently identified, despite the complex hydrocarbon fuel matrix. Phenols and carbazoles are detected in negative-ion [M – H]− mode, while anilines, pyridines, indoles, and quinolines are observed in positive-ion [M + H]+ mode. Accurate mass measurements allow for the molecular formula of the polar components to be determined, while different structural classes of isomeric compounds could be determined via HPLC separation and the formation of derivatives. Derivatization shifts the retention time, species masses, and potentially, the ion charge formed of specific compound classes, allowing them to be positively identified. The usefulness and limits of HPLC with ESI–MS for quantitation of these fuel polar, heteroatomic species are also explored