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Energy & Fuels 1998, 12, 344-352
Computational Fluid Dynamics Simulations of Jet Fuel Oxidation Incorporating Pseudo-Detailed Chemical Kinetics J. S. Ervin* and S. Zabarnick University of Dayton Research Institute, Dayton, Ohio 45469-0210 Received July 29, 1997
Military jet aircraft circulate fuel as a cooling medium. Upon heating, dissolved oxygen reacts with the fuel forming insoluble deposits which can block fuel lines and disrupt operation of close tolerance valves. Thus, an understanding of how dissolved oxygen reacts with the fuel is important to aircraft designers. A pseudo-detailed kinetics model which considers antioxidant chemistry was incorporated in a computational fluid dynamics code, and dissolved oxygen and hydroperoxide concentration profiles along a stainless steel tube were calculated for both nearly isothermal and nonisothermal flowing systems. Flowing experiments were performed with both a severely hydrotreated fuel and a straight-run fuel, and the predicted dissolved oxygen and hydroperoxide profiles agreed reasonably well with the measured profiles over a range of bulk fuel temperatures and flow conditions. The present model offers an improved understanding of jet fuel oxidation and antioxidant chemistry.
Introduction
fuel + O2 f products
Aviation fuel is circulated in military aircraft for cooling. In the presence of heat and dissolved O2 at relatively low temperatures (120-300 °C), jet fuel degrades through a complex series of reactions forming surface deposits. Accumulated surface deposits impair engine performance by disrupting the normal fuel flow within fuel system components.1 Moreover, the fouling of close-tolerance valves may lead to catastrophic failure. Since the chemical reactions in which dissolved O2 is consumed are intimately tied to fouling processes, a fundamental understanding of dissolved O2 removal is imperative for aircraft design. For over five decades, the thermal and oxidative stabilities of aviation fuels have been studied by experiment, and the experimental results largely have been described using empirical relationships.1 Computational solutions of the Navier-Stokes, energy, and species conservation equations are more general and can be used to predict O2 consumption for different flow geometries, flow rates, and temperatures. Since fuel oxidation is complex, global-chemistry kinetics have been used in computational simulations.2-5 The global chemistry mechanisms used in computational fluid dynamics simulations generally employ one reaction to represent fuel autoxidation:
Although fuel autoxidation involves a series of reactions, the underlying assumption of eq 1 is that the overall reaction of a mixture of compounds can be represented by one rate equation. This rate equation is composed of a rate constant multiplied by concentrations with simple order dependence, such as in eq 2. It
* Corresponding author. E-mail:
[email protected]. (1) Hazlett, R. N. Thermal Oxidative Stability of Aviation Turbine Fuels; ASTM: Philadelphia, PA, 1991. (2) Krazinski, J. L.; Vanka, S. P.; Pearce, J. A.; Roquemore, W. M. ASME J. Eng. Gas Turb. Power. 1992, 114, 104-110. (3) Chin, L. P.; Katta V. R.; Heneghan, S. P. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 39, 19-25. (4) Katta, V. R.; Blust, J.; Williams, T. F.; Martel, C. R. J. of Thermophys. Heat Transfer 1995, 9, 159-168. (5) Ervin, J. S.; Williams, T. F.; Katta, V. R. Ind. Eng. Chem. Res. 1996, 35, 4028-4032.
-d[O2]/dt ) k[RH][O2]n
(1)
(2)
is further assumed that the rate constant, k, is defined by the product of an Arrhenius A factor and an activation energy term. Some engineering computational fluid dynamics codes which use global chemistry assume an O2 consumption reaction order of unity for simplicity.2 Others permit a step change in the global order.3,5 These models assume that the overall reaction order is zero until the dissolved O2 is reduced to some arbitrary critical concentration. At this low concentration, the order is set equal to unity. However, with autoacceleration, or if naturally occurring antioxidants or antioxidant additives are present, the explicit form for the global order is not simple.6 In previous studies which made use of global chemistry to predict the mass of carbon deposits accumulated along the length of heated tubes, it has been found that global models employed much beyond the temperature calibration regime poorly predict measured deposit mass profiles.3 One explanation for the poor prediction of most global models is that as the temperature is increased, the rate of hydroperoxide decomposition (6) Ervin, J. S.; Heneghan, S. P. The Meaning of Activation Energy and Reaction Order in Autoaccelerating Systems. Presented at the International Gas Turbine Institute Turbo Expo 97, Orlando, FL, June 1997. Paper 97-GT-224.
S0887-0624(97)00132-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/04/1998
Fluid Dynamics Simulations of Jet Fuel Oxidation Table 1. Characteristics of F2747 and F2827 Jet Fuels fuel type density at 25 °C (kg/m3) viscosity at 25 °C (kg m/s) refinery treatment total sulfur (%) mercaptan sulfur (%) aromatics (vol %) JFTOT breakpoint (°C) copper (ppb) iron (ppb) zinc (ppb)
F2747
F2827
Jet A-1 809.0 1.9 × 10-5 hydrotreated 0.004 0.000 19 332
