Investigation of the Effect of Nitric Oxide on the

AIAA 2011-96

49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 4 - 7 January 2011, Orlando, Florida

Investigation of the Effect of Nitric Oxide on the Autoignition of JP-8 at Low Pressure Vitiated Conditions C. C. Fuller † , P. Gokulakrishnan ‡ , M. S. Klassen § and R. J. Roby

**

Combustion Science & Engineering, Inc., 8940 Old Annapolis Road, Suite L, Columbia, MD 21045.

B. V. Kiel †† Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, OH

Currently there is very little data available for jet fuel oxidation at low pressure, vitiated conditions found in some aircraft combustion systems. Due to the lack of this information, current kinetics models do not have the necessary data for validation within these combustions regimes. A previous screening study [1] by the authors has shown that the amount of NO present in the vitiated oxidizer significantly influences the ignition of jet fuel in addition to temperature and oxygen levels. The current study examines the effect of NO on the ignition of JP-8 in detail at temperatures (700 K - 900 K), pressures (0.5 atm and 1.0 atm) and oxygen levels (12% and 20%) relevant to vitiated combustion in aircrafts. Experimental results show that small amounts of NO (varied up to 1000 ppm) are capable of dramatically enhancing the oxidization of JP-8 with percent reduction in ignition delay time up to 80%. It is also found that significant coupling exists between NO and the other design variables (temperature, oxygen level and pressure) related to the effect of NO on ignition.

I. Nomenclature Xn IJign

= mole fraction concentration of specie n given [O2: mole%; NO: ppm] = ignition delay time [msec] or [sec]

II. Introduction

U

nderstanding the effect of vitiation on ignition and extinction of combustion processes is important in the development and validation of kinetics models that are used in the design and analysis of efficient combustion systems. Common forms of vitiated combustion include exhaust gas recirculation (EGR) systems applied to automobile engines [2] and furnaces for emission reduction [3]. It is also found in aircraft engines where fuel is injected into the turbine exhaust at low pressures to increase the thrust [4]. In most cases, vitiated air consists of an air stream mixed with the following combustion gas components: CO2, CO, H2O, and NOX. The effect of NOX on methane oxidation has been studied extensively over the years [5]-[14]. Earlier works showed that presence of NO promotes the oxidation of hydrocarbon fuels via the NO-NO2 catalytic cycle[6]. However, there has been little work reported in the literature on the effect of NOX on higher order hydrocarbons relevant to jet fuels at low-pressure. Kowalski [15], Moreac et al.[16][17], Dubreuil et al. [2], and Eng et al. [18] investigated the effects of NOX on hydrocarbon oxidation under high pressures relevant to gasoline engines. Kowalski [15] investigated the effect of small amounts of NO on a gasoline reference fuel blend and real gasoline in a high pressure flow reactor at 12.5 atm. It was found that NO inhibits the oxidation of gasoline and its surrogate fuel blend in the low-temperature oxidation

† Staff Engineer, AIAA Member ‡ Principal Engineer, AIAA Member § Principal Research Engineer, AIAA Member ** President and Technical Director, AIAA Member †† Augmentor Technology Lead, RZTC, 1950 5th Street, WPAFB, AIAA Senior Member 1 American Institute of Aeronautics and Astronautics

Copyright © 2011 by by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

regime while enhances the fuel oxidation under intermediate and high-pressure regimes. A similar observation was reported by Moreac et al. [16] and Dubreuil et al. [2] in their high-pressure Jet Stirred Reactor (JSR) studies at 10 atm with gasoline reference fuels such as n-heptane, iso-octane and toluene between 600 K and 1200 K. For single surrogate components [16], it was observed that NO inhibited the oxidation of n-heptane in the low-temperature regime between 550 K and 700 K, while it enhanced the oxidation in intermediate and high temperature regions. On the other hand, nitric oxide promoted the oxidation of iso-octane and toluene in all temperature regions above 600 K. Similar experiments were also performed with binary mixtures of n-heptane/iso-octane and n-heptane/toluene [2] that found both enhancement and inhibition of oxidization due to initial concentrations of NO in the oxidizer. All of these studies [2][15][16] conclude that the addition of NO significantly alters the nature of the negative temperature coefficient (NTC) oxidation region occurred in n-heptane and iso-octane by inhibiting and accelerating the fuel reactivity before and after the turn-over temperature, respectively. It was also found that NO addition had little effect on toluene oxidation in similar experiments at 10 atm [17]. The measurements of NO, NO2, and NOX in these tests indicate that the reaction pathways of NO-sensitized oxidation of hydrocarbons differ depending on the temperature regime. These observations have implications for jet fuels which consist of significant proportions of nparaffins, iso-paraffins, and aromatics. As there are very few experimental data with NOX interaction for higher order hydrocarbons relevant to jet fuels, the chemical kinetic effect of NO on jet fuel oxidation is not fully understood at low-pressures. In contrary to the high-pressure results discussed above, atmospheric pressure flow reactor experiments of Bromly et al. [19] with diluted mixtures of n-butane/NO in air found that small amount of NO promotes the oxidation in the lowtemperature regime. Chemical kinetic models [11][17] have made some progress in accounting the reaction pathways that are responsible for NO-sensitized oxidation of higher order hydrocarbons, however further work is required looking into nitroalkane chemistry and NO reactions with alkylperoxides and their derivatives that are relevant at low and intermediate temperatures. It is also noteworthy that most of the experiments used for current model validations were carried out using heavily diluted fuel/air mixtures. Previously, a screening study was performed by the authors at atmospheric pressure to examine the main and interaction effects that CO2, CO, H2O and NO, as well as temperature, oxygen reduction and equivalence ratios relevant to aircraft combustion conditions, have on ignition delay time [1]. The significance of the main effect of these variables as well as their two factor interactions are shown in Figure 1.

100

TͲNO

CumulativeProbability

75

O2

50

25 T

NO

0 Ͳ100% Ͳ50% 0% 50% 100% MainandTwoͲFactorInteractionEffectsBasedon theExperimentalData Figure 1: Normalized main factor and two-factor interaction effects of experimental design variables based on ignition delay time. O2 value is based on the reduction of from 21% to 15 %[1]. 2 American Institute of Aeronautics and Astronautics

A seven-variable Box-Behnken design of experiment was carried out by varying the vitiated air composition for NO, CO, CO2, H2O and O2 (balance N2) in addition to temperature and equivalence ratio to measure JP-8 ignition delay time. This series of experiments provided data to estimate the main effect of the 7 variables on the ignition delay time as well as 21 two-factor interaction effects on the JP-8 oxidation. The outliers in Figure 1, temperature, NO, O2 and the temperature-NO coupling were found to have a statistically significant influence on the JP-8 ignition delay time. The data that lie on a straight line as shown in Figure 1 follow a normal distribution within a 95% confidence interval which deems to have statistically insignificant effect on the ignition delay time. It is noteworthy that, aside from temperature, the presence of NO has the largest effect on ignition delay time; even with initial concentrations as low as 50 and 100 ppm in the oxidizer stream, followed by the effect of reduced O2 levels. The results of the screening study emphasize the significance that NO has on ignition of jet fuels compared to other vitiated air constituents and how its influence can vary significantly with temperature. In the current work, these results were used to develop a test plan to examine the effect of NO in vitiated combustion in more detail at lower pressures and temperatures

III. Experimental Setup In the current effort, experiments were performed using a flow reactor apparatus that has the capability to measure ignition delay times of vitiated and unvitiated fuel/air mixtures at atmospheric and sub-atmospheric pressures. The experimental apparatus used in this study is an expansion of the system used in the aforementioned screening study [1], with modifications made to allow for longer residence times, lower temperatures and subatmospheric pressures. The primary modifications included the extension of the flow reactor test section length from 40 inches to 208 inches as well as the addition of a vacuum system for sub-atmospheric pressure testing. Experiments were performed across temperatures ranging from 700 K to 900 K at pressures of 0.5 atm and 1.0 atm with 12% and 20% O2 in the oxidizer. Unlike the previous study [1], which used a Box-Behnken design of experiment technique to optimize testing of 7 variables, the current work examined the effect of NO by varying its initial concentration from 0 ppm to 1000 ppm in the oxidizer at various levels of temperature, O2 concentration and pressure. Due to the limitation on reactor length, ignition was not measured for certain test conditions where the ignition delay time exceeds the reactor residence time. The majority of the experimental measurements were taken using JP-8 as the fuel. Companion tests using n-decane were also performed for comparison purpose with and without initial concentrations of NO in the oxidizer at selected conditions. The JP-8 sample used in this study is 02POSF-4177 supplied by the Air Force Research Laboratory at Wright-Patterson Air Force Base, and its chemical and physical properties are provided in Table I. Table I: Physical and chemical properties of JP-8 Fuel Type WPAFB ID

JP-8 02-POSF-4177

Summarized D2425

(vol %)

D6379

(vol %)

Paraffins Cycloparaffins Dicycloparaffins Tricycloparaffins

51.3 18.0 11.8 1.6

Monoaromatics Diaromatics Total Aromatics Total Saturates

16.1 1.2 17.3 82.7

Alkylbenzenes

9.3

Indan and Tetralins Indenes CnH2n-10 Naphthalene

6.7