Paper # 070LT-0077 Topic: Laminar & Turbulent Flames

8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013

Effects of Surrogate Jet-Fuel Composition on Ignition and Extinction in High Temperature Applications

Ruiqin Shan and Tianfeng Lu

Department of Mechanical Engineering University of Connecticut, Storrs, CT 06269-3139

Abstract A numerical study was conducted to investigate the effects of surrogate jet-fuel composition on limit flame phenomena, e.g. ignition and extinction, in auto-ignition and perfectly stirred reactors (PSR) at high-temperature and fuel-lean conditions. Three types of fuel components, namely n- alkanes, n-butyl-cyclohexane and toluene, and their blends were employed as surrogates of jet fuels in the study using the detailed JetSurF2.0 mechanism. It was found that n-butyl-cyclohexane and the high n-alkanes, including n-octane, n-decane and n-dodecane, feature mostly identical ignition and extinction behaviors and thus each of them can be chosen as a representative component for jet fuels for high temperature applications. Furthermore, while the limit phenomena for pure toluene are significantly different from those of the high n-alkanes and the cyclo-alkane, blending a small amount of toluene, say up to 20% in mole, to n-alkanes induces only minor discrepancies in the high-temperature ignition and extinction states, which are insignificant compared to the differences resulting from different kinetic mechanisms. Therefore, individual n- alkanes can be adequate jet-fuel surrogates in term of capturing the limit flame phenomena for high temperature applications.

1. Introduction Most conventional or alternative aviation fuels involve a large number of components with various chemical structures and different physicochemical characteristics. It is computationally expensive to include such large number of components and their associated reaction pathways in complex flame simulations. As such surrogate mixtures with one or a few fuel components are frequently employed to mimic the combustion of the real fuels (Colket et al. 2007; Violi et al. 2002). In particular, it was found that mixtures of n-alkanes, iso-alkanes, cycloalkanes, aromatics and alkenes are important components in jet fuel surrogates (Dooley et al. 2010; Violi et al. 2002; Wang et al. 2010; Wood et al. 1989). The oxidation pathways of n-alkanes and cycloalkanes been studied extensively over a wide range of molecular size and flame conditions (Davidson et al. 2011; Kurman et al. 2011; Müller et al. 1992; Westbrook et al. 2009). In contrast, aromatics show rather different chemical reactivity from alkanes, e.g. in inhibiting the oxidation of alkane compounds and producing soot precursor (Metcalfe et al. 2011). Mechanisms for toluene, as a simple alkylated benzene, has been compiled and validated (Emdee et al. 1992; Metcalfe et al. 2011; Mittal & Sung 2007; Sivaramakrishnan et al. 2005). Ignition and emission characteristics of toluene was experimentally studied in (Dagaut et al. 2002; Lindstedt & Maurice 1996), while the kinetics of aromatics is overall less understood than that of alkanes. Jet fuel surrogates involving individual alkanes and mixture of alkanes and aromatics have been studied (Dooley et al. 2012; Holley et al. 2007; Kahandawala et al. 2008; Kumar & Sung 2010), while the effects of the surrogate compositions on higher temperature ignition and extinction are not completely clear. In the present study, three different kinds of surrogate components, including n-alkanes, n- butyl-cycloalkane and toluene, were employed for jet fuels in a numerical study using the JetSurF 2.0 mechanism (Wang et al. 2010) to investigate the effects of jet-fuel compositions on auto-ignition and steady state flame extinction and ignition in perfectly stirred reactors (PSRs) at high temperature conditions, and to find simplest surrogate mixtures for ignition and extinction of jet fuels at such conditions.

2. Model Description The present study is performed for 0-D systems, including auto-ignition and steady-state PSR, which features a simple homogenous mixing term. The ignition and extinction in PSR are resulting from the competition between chemical reactions and the mixing process. The governing equations of unsteady PSR can be expressed as the following set of ordinary differential equations (ODEs):

dY m Y 0 Y i   i  i i , i=1, 2, …, K, (1a) dt  

0 0 m ihi Yi (hi  hi ) dT V   i1,K  i1,K ,   , (1b) dt cp cp M in where K is the number of species, Y is the mass fraction of a species, and T is temperature, ݉ሶ is the mass production rate of a species through chemical reactions, ߩ is density, h is the specific enthalpy, and ܿ௣ is the averaged specific heat capacity at constant pressure, ߬ is the residence time which is the timescale of the mixing process, ܯሶ ௜௡ represents the mass flow rate at the inlet. The subscript i indicates the ith species and the superscript 0 represents the inlet condition. In addition, the first terms in RHS of eq. (1) are attributed to the chemical reactions and the second terms are the mixing process. A steady-state PSR can be characterized by a canonical S-curve with ߬ being a parameter which involves three branches and two turning points (Law 2006). The upper turning point is typically defined as the extinction state and the lower turning point as the ignition state. In contrast, an auto-igniting system only involves chemical reactions and the ignition delay time for constant-pressure auto-ignition can be obtained through solving the revised governing equations in eq. (1) without the mixing terms.

3. Results and Discussion The effects of jet-fuel compositions are studied with the JetSurF 2.0 mechanism for high-temperature applications (Wang et al. 2010). Selected n-alkanes, including n-octane, n-decane and n-dodecane, n-butyl- cyclohexane and toluene are chosen as the surrogate components for jet fuels. Three n-alkanes, including n-octane, n-decane, and n-dodecane, are first studied. Figure 1a shows the ignition delay time with initial temperature between 1000 and 1600 K at pressure p = 10 atm under a fuel-lean condition with equivalence ratio ߶ = 0.7. It is seen in Fig. 1a that the ignition delays for these three n-alkanes are mostly identical, indicating that the reactivity of high-order n-alkanes for high temperature ignition is not strongly dependent on the chain length. This observation is consistent with the previous measurements (Davidson et al. 2010; Oehlschlaeger et al. 2009). Figure 1b shows the S-curves in PSR at pressure of 10 atm, equivalence ratio of 0.7 and inlet temperature of 1000 K. It is seen that the upper and lower turning points, i.e. the extinction and ignition states, are again mostly identical for the three n-alkanes. As such, the high-order n-alkanes feature similar chemical reactivity for both ignition and extinction at high temperatures. To further investigate the effects of fuels with different molecular structures on the limit phenomena, n- dodecane, n-butyl-cyclohexane and toluene were compared in Fig. 2. It is seen in Fig. 2a that toluene features significant longer ignition delay compared to that of n-dodecane and n-butyl-cyclohexane, as expected due to the strong ring-structure in toluene. In contrast, the ignition delay of n-butyl-cyclohexane is comparable to that of n- dodecane. Figure 2b further shows the S-curves for the three fuel components in PSR. It is seen that n-dodecane and n-butyl-cyclohexane feature mostly identical ignition and extinction states with only approximately a 25% difference in the extinction residence time, while the S-curve for toluene significantly deviates from the other two species. Therefore, pure n-butyl-cyclohexane and n-dodecane feature similar limit behaviors at high-temperatures, while that toluene is rather different. -2 10 2400 n-dodecane (b) n-decane (a) -3 10 n-octane 2000 n-dodecane n-decane n-octane -4 10 1600

 = 0.7 Temperature, K Ignition delay,s -5 10  = 0.7 1200 p = 10 atm p = 10 atm T = 1000 K in -6 800 10 -6 -5 -4 -3 -2 -1 0 0.6 0.7 0.8 0.9 1 1.1 10 10 10 10 10 10 10 1000/T, 1/K Residence time, s

Figure 1: (a) Ignition delay times with various initial temperatures for constant pressure auto-ignition under pressure p = 10 atm, equivalence ratio ߶ = 0.7, and (b) temperature as a function of residence time under pressure p = 10 atm, equivalence ratio ߶ = 0.7 and inlet temperature Tin = 1000 K, for n-dodecane, n-butyl-cyclohexane and toluene, respectively.

-1 10

 = 0.7 (a) 2400 -2 10 p = 10 atm (b) n-dodecane 2000 -3 n-butyl-cyclohexane 10 toluene

-4 1600 10  = 0.7 Temperature, K Ignition delay,s n-dodecane -5 1200 p = 10 atm 10 n-butyl-cyclohexane T = 1000 K toluene in -6 800 10 -6 -5 -4 -3 -2 -1 0 0.6 0.7 0.8 0.9 1 1.1 10 10 10 10 10 10 10 1000/T, 1/K Residence time, s

Figure 2: (a) Ignition delay times with various initial temperatures for constant pressure auto-ignition under pressure p = 10 atm, equivalence ratio ߶ = 0.7, and (b) temperature as a function of residence time under pressure p = 10 atm, equivalence ratio ߶ = 0.7 and inlet temperature Tin = 1000 K, for n-dodecane, n-butyl-cyclohexane and toluene, respectively.

Based on the above results, binary mixtures of n-alkanes and toluene can be adequate surrogates of jet fuels to predict limit flame behaviors at high temperatures. To further show the effects of the toluene concentrations in the binary mixtures on the limit phenomena, three n-doedecane and toluene mixtures, with 0, 20%, and 50% toluene in mole, respectively, are used for auto-ignition and PSR, while less than 20% toluene has been used in previous studies (Colket et al. 2007; Farrell et al. 2007). Figure 3a shows that 20% toluene in the mixtures does not significantly affect the ignition delay, which is different from that of pure n-dodecane by only less than 10%. When the amount of toluene is increased to 50%, the relative difference in ignition delay is approximately 30%. Figure 3b shows the S-curves for the mixtures with different compositions under high inlet temperature of 1000 K. It is seen that 50% toluene in the mixtures results in approximately 30% relative difference in the ignition and extinction residence time compared to that of pure n-dodecane, while for the mixtures with 20% toluene the S-curve is mostly identical to that of pure n-dodecane. The results in Fig. 3 therefore indicate that the ignition and extinction states are primarily controlled by the major fuel component, i.e. the n-alkanes, while a small amount of toluene, say up to 20%, only has minor effects on the limit flame behaviors at high-temperature conditions.

2400 2400 (b) (a)

2000  = 0.7 2000 n-dodecane p = 10 atm 80% n-dodecane+20% toluene 50% n-dodecane+50% toluene T = 1000 K 0 1600 1600

Temperature, K  = 0.7 Temperature, K 1200 n-dodecane 1200 p = 10 atm 80% n-dodecane+20% toluene T = 1000 K 50% n-dodecane+50% toluene in 800 800 -2 -1 0 -6 -5 -4 -3 -2 -1 0 10 10 10 10 10 10 10 10 10 10 Time, s Residence time, s

Figure 3: (a) Temperature profile with time for constant pressure auto-ignition, and (b) temperature as a function of residence time in PSR, at pressure of 10 atm, equivalence ratio of 0.7 and high initial/inlet temperature of 1000 K, for binary mixtures of n-dodecane and toluene with 0, 20%, and 80% toluene in mole, respectively.

To further compare the effects of toluene addition with the uncertainties induced by different chemical kinetic mechanisms, two other detailed mechanisms by Lawrence Livermore National Laboratory (LLNL) for n- alkanes are compared with JetSurF 2.0. In Fig. 4, “LLNL1” indicates the detailed mechanism for n-alkanes from n- octane to hexadecane with 2115 species and 8157 reactions (Westbrook et al. 2009), and “LLNL2” indicates the recently developed mechanism for 2-methyl-alkanes and n-alkanes up to C12 with 2755 species and 11173 reactions (Sarathy et al. 2011; Sarathy et al. 2011). It is seen in Fig. 4 that the effects of toluene addition on auto-ignition delay and ignition/extinction in PSR are in general smaller than the uncertainties induced by using different kinetic mechanisms. As such, pure n-dodecane can be an adequate surrogate for prediction of ignition and extinction behaviors of jet fuels at high temperature and fuel lean conditions.

2400 (a) 2400 (b) n-dodecane(JetSurF2.0) 80% n-dodecane+20% toluene n-dodecane(JetSurF2.0) 2000 (JetSurF2.0) 2000 80% n-dodecane+20% toluene n-dodecane(LLNL1) (JetSurF2.0) n-dodecane(LLNL2) n-dodecane(LLNL1) 1600 1600 n-dodecane(LLNL2)

Temperature, K = 0.7 = 0.7

 Temperature, K  1200 1200 p = 10 atm p = 10 atm T = 1000 K T = 1000 K 0 in 800 800 -3 -2 -1 0 -6 -5 -4 -3 -2 -1 0 10 10 10 10 10 10 10 10 10 10 10 Time, s Residence time, s

Figure 4: (a) Temperature profile with time for constant pressure auto-ignition, and (b) temperature as a function of residence time in perfectly stirred reactor, at pressure of 10 atm, equivalence ratio of 0.7 and initial/inlet temperature of 1000 K, for binary mixtures of n-dodecane and toluene with 0, 20%, and 80% toluene in mole, respectively.

To explain the minor effects of toluene addition on high-temperature ignition and extinction observed above, a bifurcation analysis (Shan et al. 2012) was employed to identify the controlling chemical processes at the ignition and extinction states for the binary mixtures of 80% n-dodecane and 20% toluene shown in Fig. 3b. A bifurcation index (BI) defined in (Shan & Lu 2013) was employed to quantify the contribution of each reaction to the zero-crossing of the eigenvalue that induces the singularity of the Jacobian at the turning points on the S-curve in Fig 3b. It is seen in Fig. 5 that the reactions for CO formation and chain branching through H radical, in addition to the mixing process, control the extinction state, while the ignitiion state is controlled by mixing and reactions involving C1-C3 species. As such the fuel cracking processes are not rate-limiting for both ignition and extinction at the high temperature condition.

Figure 5: Controlling reactions and the BI values for (a) the extinction state, and (b) the ignition state, for 80% n- dodecane and 20% toluene in mole, in PSR at pressure p = 10 atmm, equivalence ratio ߶ = 0.7 and inlet temperature Tin = 1000 K.

4. Concluding Remarks The effects of jete -fuel surrogate compositions involving n-alkanes, n-buutyl-cyclohexane, and toluene were numerically studied for auto-ignition and ignition/extinction in PSR at high-temperature and fuel-lean applications, using the detailed JetSurF 2.0 mechanism. Comparing single-component fuel, it was found that the ignition and extinction behaviors for higher n-alkanes, e.g. n-octane, n-decane, and n-dodecane, and n-buutyl-cyclohexane are identical while that of toluene is significantly different. However, bllending a small amount of toluene, say up to 20% in mole, to n-dodecane only induces minor differences in ignition and extinction, which are smaller compared with the uncertainties induced by using different detailed chemical kinettic mechanisms. Therefore, pure n-dodecane can be an adequate surrogate of jet fuels in terms of prediction of limit fflame behavior at high-temperature and fuel-lean conditions.

Acknowledgements This work was supported by the Air Force Office of Scientific Research under Grant No. FA9550-13-1- 0057.

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