S. P. Heneghan

Oxidation of jet fuels and the formation of deposit

Abstract Jet fuels and jet fuel surrogate were thermally stressed to simulate the time-temperature history of aircraft fuel-handling systems. The resulting fuels and soluble and insoluble products were analysed. The results are shown to be incompatible with previous mechanisms concerning the source of deposit precursors. In general, two important dependences on oxygen have been found: 1. (1) in agreement with previous research, the amount of deposit formed decreases significantly if oxygen is removed from the fuel before thermal stressing; 2. (2) fuels that oxidize easily are likely to be more stable (as measured by deposits). New evidence is presented to support the free-radical mechanisms of oxidation and deposit formation, in contrast to proposed ionic mechanisms of oxidation. A general theory of oxidation of hydrocarbons has been incorporated to account for the observed oxygen-dependences, involving a free-radical, autoxidation, chain mechanism. It is proposed that the presence of naturally occurring antioxidant molecules plays an important role in both inhibiting the oxidation of the fuel and forming deposit precursors. Some properties (concentration, reactivity) of these antioxidant molecules and the implications of the theory are discussed.

Analysis of slit function errors in single‐shot coherent anti‐Stokes Raman spectroscopy (CARS) in practical combustors

The temperature determined by a single‐shot coherent anti‐Stokes Raman spectroscopy (CARS) system is directly related to the half width at half maximum of the instrument slit function. Therefore, an accurate knowledge of the instrument slit function is necessary to determine temperature with CARS. However, in turbulent systems, the input slits of the spectrometer may be removed in order to guarantee signal throughput and establish the necessary dynamic range. In this case, the physical input slits of the spectrometer are replaced with apparent slits created by focussing the input beams near the entrance plane of the spectrometer. The slit function will then depend on the physical relationship among all of the optical components, the probe volume, and the dispersive performance of the spectrometer and detector, as well as the optical path through density and temperature gradients which may not be invariant in a turbulent system. The presence of high temperatures and turbulence levels can effect the size of...

Studies of Jet Thermal Stability in a Flowing System

A flowing, single-pass heat exchanger test rig, with a fuel capacity of 189 litres, has been developed to evaluate jet fuel thermal stability. This so called, “Phoenix Rig” is capable of supplying jet fuel to a 2.15 mm I.D. tube at a pressure up to 3.45 MPa, fuel temperature up to 900K, and a fuel-tube Reynolds number in the range 300–11,000. Using this test rig, fuel thermal stability (carbon deposition rate), dissolved oxygen consumption, and methane production were measured for three baseline jet fuels and three fuels blended with additives. Such measurement were performed under oxygen-saturation or oxygen-starved conditions.Tests with all of the blended fuel samples showed a noticeable improvement in fuel thermal stability. Both block temperature and test duration increased the total carbon deposits in a nonlinear fashion. Interestingly, those fuels that need a higher threshold temperature to force the consumption of oxygen exhibited greater carbon deposits than those that consume oxygen at a lower temperature. These observations suggested a complicated relationship between the formation of carbon deposits and the temperature-driven consumption of oxygen. A simple analysis, based on a bi-molecular reaction rate, correctly accounted for the shape of the oxygen consumption curve for various fuels. This analysis yielded estimates of global bulk parameters of oxygen consumption. The test rig yielded quantitative results which will be very useful in evaluating fuel additives, understanding the chemistry of deposit formation, and eventually developing a global chemistry model.Copyright

Scalar Measurements in Bluff Body Stabilized Flames Using Cars Diagnostics

Measurements of mean and rms temperature fluctuations were performed in confined turbulent premixed methane-air flames, stabilized on a conical flameholder. A CARS system was used for these measurements. These tests employed flameholders of different blockage ratios (13% and 25%), and mixtures with different equivalence ratios (0.56, 0.65, 0.8, and 0.9) and approach turbulence intensity (2%, 17%, and 22%).It was found that the recirculation zone closely resembles a perfectly well-stirred reactor. Blockage ratio, equivalence ratio, or approach turbulence intensity did not alter the scalar field. The turbulent flame structure enveloping the recirculation zone comprises: (i) an ignition/thin flame region in the vicinity of the flameholder base, (ii) a reacting shear layer region of large-scale coherent structures, and (iii) a thick flame region where entrainment is the dominant mechanism. Finally, analysis suggests that the scalar gradient-diffusion relationship is valid and areas of non-gradient diffusion, if any, are probably small.Copyright

Advanced Jet Fuels — JP-4 Through JP-8 and Beyond

Jet fuel requirements have evolved over the years as a balance of the demands placed by advanced aircraft performance (technological need), fuel cost (economic factors), and fuel availability (strategic factors). In a modern aircraft, the jet fuel is the primary coolant for aircraft and engine subsystems and provides the propulsive energy for flight. To meet the evolving challenges, the U.S. Air Force, industry and academia have teamed to develop new and improved fuels that offer increased heat sink and thermal stability, properties that will enable improved aircraft design and decrease fuel system maintenance due to fuel fouling/coking. This paper describes the team effort to develop improved JP-8, named “JP-8+100”, that offers a 55C (100F) improvement in thermal stability and a 50% increase in heat sink.The government, industry, and academia team has made numerous advances in the development of JP-8+100 with a more complete understanding of the fundamental processes of deposition, new approaches to reducing fouling/coking, and new tests and models to assist the designers of aircraft and engine fuel systems. Some of the principal advances are: new quantitative research devices and fuel system simulators that provide thermal stability information that cannot be obtained using the standard JFTOT test; new techniques to measure oxygen consumption and fuel degradation pathways; a free radical theory to explain behaviors such as the inverse relationship between thermal and oxidative stability, advanced CFD models with coupled degradation chemistry, and a new thermal stability ranking scale for jet fuels. The insight obtained has been applied to the development of an additive package for JP-8 that shows thermal stability improvements equal to or greater than the stated goal and enables the development of even higher thermal stability fuels such as JP-900.Copyright

Simple determination of the width of the slit function in single‐shot coherent anti‐Stokes Raman spectroscopy

The determination of temperatures using single‐shot coherent anti‐Stokes Raman spectroscopy normally requires an accurate measurement of the instrument slit function. This slit function is normally determined at a known temperature, and then assumed to be the applicable function at all temperatures and independent of the optical path which varies with density or temperature gradients. In this communication we show a simple method of determining the width of the slit function from the collected data at the actual temperature and turbulence level. This method depends on local thermodynamic equilibrium and is a generally applicable technique to determine the instrument slit function. It is limited to temperatures in excess of 1100 K, as it requires that there be a nonzero signal in the v=1 vibrational level of N2.

The Effects of Dissolved Oxygen Concentration, Fractional Oxygen Consumption, and Additives on JP-8 Thermal Stability

Since dissolved oxygen participates in fuel deposit formation, knowledge of the effects of dissolved oxygen concentration on fuel thermal stability is critical for fuel system design. In this work, the combined effects of dissolved oxygen availability and additives on jet fuel thermal stability are studied. Experiments with JP-8 jet fuel were conducted in a three-part heat exchanger which simulated a complex thermal and flow environment. The dissolved oxygen content at the flow inlet was varied, and deposition was studied under conditions of either fractional or complete oxygen consumption. The effects of a thermal stability additive package were also studied. An intriguing result found with JP-8 fuels is an increase in deposits formed in heated regions for decreased oxygen consumption, but inverse behavior with the additive package.Copyright

Designing high thermal stability jet fuels for the 21st Century

Recently, the US Air Force has flown a new JP-8 fuel formulation known as JP-8+100. This new fuel provides a 50% increase in heat sink capability over standard JP-8 and allows the fuel to be stressed to a bulk temperature of 218/spl deg/C (425/spl deg/F) and a wetted wall temperature of 260/spl deg/C (500/spl deg/F). Despite this advance, fuel system design and engine manufacturers will continue to push the limits of fuel thermal stability. Therefore, additional heat sink capacity will be needed in future jet fuels. This paper describes the current understanding of jet fuel thermal oxidative stability, discusses the role that additives play in increasing the heat sink of current JP-8 fuels, highlights possible schemes for significantly increasing jet fuel heat sink capability, and finally, presents some of the research challenges faced in developing a jet fuel for the 21st Century.

Surface Effects on Deposits From Jet Fuels

Flow experiments in a single-pass heat exchanger using JP-8 and certain additives were initiated under controlled conditions to explore the effects of a metal surface on deposition. The experimental apparatus permitted a unique viewing of the time evolution of deposits at different axial locations under conditions of limited oxygen availability somewhat similar to that in jet aircraft. Scanning electron microscopy was used to examine deposit microstructure. In addition, x-ray photoelectron spectroscopy and Auger electron spectroscopy determined the chemical composition of the deposits. Oxygen concentration measurements in the bulk flow were also performed, and the observed transient oxidation behavior was related to measured time-dependent changes in the deposit.Increasing dissolved oxygen levels and large changes in deposition were characteristic of the induction time. Mechanisms of fouling in the heated and cooled sections were different. Spectroscopic analysis indicated that deposits formed in the heated section had chemical compositions different from those formed in the cooled section. Scanning electron microscopy revealed differences in microstructure between the heated and cooled sections: more uniform deposits formed in the cooled section as a result of once-soluble species becoming insoluble at low temperatures. In addition, the JP-8 additives significantly reduced fouling in the heated section, but their effectiveness in the cooled section, especially after large periods, was unclear.Copyright

Effects of Oxygen and Additives on the Thermal Stability of Jet Fuels

A previously developed flowing single-pass heat-exchanger test rig (Phoenix rig) has been used to evaluate the effectiveness of various additives and the kinetic mechanism of both deposit formation and oxygen consumption. The Phoenix rig has been modified to include not just a heated single tube, but also a cooling test section and both hot and cold filters. The effects of flow conditions, antioxidants, and metal deactivator additives on the location and amount of the deposit are discussed. In general, antioxidants were effective at reducing the deposits on the hot test section, but almost invariably caused increased plugging of cool downstream filters. Downstream plugging of cool filters also increased with decreased temperatures in the heated section or with increased flow. Tests with both oxygen-saturated and oxygen-depleted fuels have shown that the solubility of oxygen is linearly related to the fraction of oxygen in a sparge gas, and that the amount of deposit is linearly related to the total quantity of dissolved oxygen passed. Finally, in contrast to initial modelling efforts, the consumption of oxygen is shown to be significantly more complex than a simple bimolecular, pseudo-first-order in oxygen, process. It is found to be much closer to pseudo-zero-order in the early stages, decaying to pseudo-first-order when the oxygen nears depletion. NOMENCLATURE A = pre-exponential E = activation energy [F] = concentration of fuel or a fuel component [02] t = concentration of oxygen at time t R = universal gas constant T temperature t = time = residence time L = Liters g = grams sec = seconds a, 11, y, = reaction orders AMW = average molecular weight k = rate constant V = Volume f jo 2 k [02] o