Experimental and Modeling Study of n-Propylcyclohexane Oxidation

Energy & Fuels 2009, 23, 2453–2466

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Experimental and Modeling Study of n-Propylcyclohexane Oxidation under Engine-relevant Conditions Thomas Dubois,†,‡ Nabiha Chaumeix,*,‡ and Claude-Etienne Paillard‡ TOTAL Raffinage Marketing, Direction des Recherches and CNRS Orle´ans-ICARE, Ae´rothermique, Re´actiVite´ et EnVironnement ReceiVed December 5, 2008. ReVised Manuscript ReceiVed March 4, 2009

The oxidation of n-propylcyclohexane (C9H18) was experimentally studied using the shock tube and the spherical bomb techniques. Both devices were heated in order to prevent wall adsorption. Ignition delay times, τi, of n-propylcyclohexane/O2/Ar mixtures based on OH* and CH* radicals chemiluminescence have been measured behind reflected shock waves over a large range of temperatures (1250-1800 K), pressures (10-20 bar), and equivalence ratios (0.2-1.5). Laminar flame speeds at an initial temperature of 403 K and an initial pressure of 1 bar were also determined using a stainless steel spherical bomb for equivalence ratios between 0.6 and 1.75. The visualization of the flame was achieved using a Schlieren system coupled to a high-speed camera. All the results obtained were successfully modeled based on a detailed kinetic mechanism (110 species and 690 reactions) built using two models from the literature.

1. Introduction During the few last decades, the international authorities have been publishing standards to reduce the emissions of some pollutants such as NOx, CO2, and particulate matter (PM) from diesel engines.1-4 Even though the global equivalence ratio inside a diesel engine is around 0.7, the poor mixing near the injection port is responsible for high equivalence ratio locally in the engine. These local rich areas are responsible for the soot inception. The necessity to reduce these emissions to meet the new standards has led car manufacturers to equip diesel engines with post-treatment systems, but research has also headed toward alternative combustion modes that can lead to an important reduction of engine-out emissions. One alternative combustion mode for diesel engines is the homogeneous charge compression ignition (HCCI). HCCI engines have the potential to meet future requirements concerning NOx and soot emissions from diesel-powered cars without the necessity of exhaust gas postprocessing. The principle of HCCI combustion is based on burning a lean homogeneous fuel/air mixture diluted in burned gas using exhaust gas recirculation (EGR). On one hand, the use of the dilution allows reduced combustion temperatures, thereby decreasing the formation of NOx during the process. On the other hand, the fuel-lean conditions used prevent the particulate matter formation. However, HCCI diesel engines have to be further developed before being * To whom correspondence should be addressed. E-mail: chaumeix@ cnrs-orleans.fr. † TOTAL Raffinage Marketing. ‡ CNRS Orle ´ ans-Institut de Combustion. (1) Regulation (EC) No 715/2007 of the European Parliament and of the Council of 20; June 2007. (2) Aceves, S. M.; Flowers, D. L.; Frias, J. M.; Smith, J. R.; Dibble, R.; Au, M.; Girard, J. SAE Paper 2001, 01-2077, 2001. (3) Ryan T. W., Callahan T. J. SAE Paper 961160, 1996. (4) Zhao, F.; Asmus, T. W. Assanis, D. N., Dec, J. E. Eng, J. A., Najt, P. M. Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and DeVelopment Issues; Society of Automotive Engineers: Warrendale, PA, USA, 2003; pp 11-12.

usable and, in order to limit research costs, car manufacturers couple CFD codes with detailed kinetic models. Therefore, detailed kinetic mechanisms able to reproduce the combustion process have to be developed. For their development, experimental data are needed. Nevertheless, very few results concerning the oxidation of surrogates for diesel fuels in the particular HCCI conditions are currently available. Thus, new data have to be obtained. Since naphthenes are a chemical family that can represent up to 35% of diesel fuels composition, it is important to get information on their oxidation to understand the behavior of diesel fuels during their combustion in an engine. However, very few studies have been carried out on heavy cyclic alkanes oxidation at high temperature. In the literature, n-propylcyclohexane is usually taken as a reference for the naphthenes family.5-9 Most of those studies5-8 deal with the oxidation of blends of chemicals representative of kerosene and diesel fuels on a jet-stirred reactor. Even though the results give interesting information on the oxidation of kerosene or diesel fuels, it is difficult to extract the contribution of the naphthenes family due to the presence of other species. Therefore, pure npropylcyclohexane was studied in this work to obtain more information on the oxidation of this hydrocarbon family. From the literature, only species profiles obtained in a jet-stirred reactor can be found for pure n-propylcyclohexane oxidation.9 Equivalence ratios of the mixtures studied varied from 0.5 to 1.5 in a temperature range of 950-1250 K and at atmospheric pressure. The results show that during n-propylcyclohexane oxidation, light species such as methane, ethane, ethylene, propene, butadiene, and formaldehyde are, with benzene, the main products. A detailed submechanism was developed and (5) Gue´ret, C.; Cathonnet, M.; Boettner, J. C.; Gaillard, F. Energy Fuels 1992, 6, 189–194. (6) Dagaut, P.; Gaı¨l, S. J. Phys. Chem. A 2007, 111, 3992–4000. (7) Dagaut, P.; El Bakali, A.; Ristori, A. Fuel 2006, 85, 944–956. (8) Mati, K.; Ristori, A.; Gaı¨l, S.; Pengloan, G.; Dagaut, P. Proc. Combust. Inst. 2007, 31, 2939–2946. (9) Ristori, A.; Dagaut, P.; El Bakali, A.; Cathonnet, M. Combust. Sci. Technol. 2001, 165, 197–228.

10.1021/ef801062d CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

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Dubois et al.

oped for n-propylcyclohexane oxidation was developed and validated during this work. 2. Experimental Setups and Procedure

Figure 1. Evolution of the pressure and emission signals recorded by the numerical oscilloscope as a function of the time. The mixture studied here is the following: 0.011% C9H18/0.489% O2/99.5% Ar; T5 ) 1520 K; P5 ) 19.2 bar.

added to an existing model in order to simulate the results. A good agreement with the experimental data is observed. Nonetheless, no validations of this model were done at temperature higher than 1200 K because no experimental data are yet available. Indeed, no studies dealing with ignition delay times or flame speeds measurements of n-propylcyclohexanehave have been performed. Only ignition delay times of methylcyclohexane/O2 mixtures have been measured using a shock tube.10 The results obtained show that ignition delay times (τi) decrease with decreasing equivalence ratio (from 2 to 0.5) and with increasing pressure (from 1 to 4 atm). These results could be used to compare with our product. In the present work, ignition delay times of n-propylcyclohexane/O2 mixtures diluted in argon were obtained using a heated shock tube. The ignition delay times were measured based on both OH* and CH* radicals chemiluminescence. In addition, flame speed measurements on n-propylcyclohexane/ air mixtures have been made using a heated spherical bomb. The kinetic model available in the literature9 was tested upon the experimental results, and a new semidetailed model devel-

2.1. The Heated Shock Tube. Experiments behind reflected shock waves were carried out in a heated stainless steel shock tube as previously documented.11 This shock tube has a 2 m long, 114.3 mm bore driver section and a 5.15 m long, 52.5 mm bore driven section. The shock wave is generated by the sudden burst of a couple of diaphragms mounted between the two parts of the shock tube. The driver gas used is helium (Air Liquide, 99.99%). Both sections are evacuated using two primary vacuum pumps to reach pressure around 10-5 bar for a leak-outgasing rate of 10-6 bar · min-1. The last part of the driven section (1.4 m long) is blackened with a special surface treatment to prevent multiple reflections of light near the measurement section. A previous study,12 by comparison with results from literature,13 has shown that this surface passivation has no impact on the chemical reactions that take place in the tube. To heat the shock tube, 12 separate heating zones and thermal insulation were used to obtain a homogeneous temperature ((3 K). The initial temperature at which the experiments were conducted is 363 K in order to prevent wall adsorption. Indeed, at this temperature, no n-propylcyclohexane wall adsorption nor condensation were observed. The reactive gas mixtures studied were prepared in a heated (up to 363 K) 30 L stainless steel tank adjacent to the shock tube. The maximum vapor pressure introduced in the tank is 0.4 kPa, and the saturated vapor pressure at 363 K is 2.55 kPa; hence, the fuel vapor pressure is far below the saturated value, and no condensation was likely to occur. This tank is equipped with a magnetic stirrer that ensured homogeneous mixtures after a 15 min mixing time. The mixtures were prepared using n-propylcyclohexane (Sigma Aldrich, 99+ %), oxygen (Air Liquide, 99.999%), and argon (Air Liquide, 99.999%). During the experiments, the shock velocity was measured via four piezo-electric pressure transducers equally spaced by 150 mm, with the last one being 10 mm away from the shock tube end wall. In the same section as the last pressure transducer, two fused silica windows (9 mm optical diameter and 6 mm thickness) are mounted across a photomultiplier tube HAMAMMATSU 1P28 equipped with a 306 nm centered narrow-band filter, characteristic of OH* radicals emission; and a photomultiplier tube HAMAMMATSU R928 equipped with a 431 nm centered narrow band filter, characteristic of CH* radicals emission. Both filters have a bandwidth of 10 nm. The pressure and chemiluminescence signals

Figure 2. Typical propagation of the flame during a spherical bomb experiment. The mixture studied here is the following: 1.80% C9H18/20.62% O2/77.58% N2 at initial temperature and pressure of 403 K and 1 bar, respectively.

Figure 3. Typical evolution of the pressure during a spherical bomb experiment. The mixture studied here is the following: 2.41% C9H18/20.49% O2/77.10% N2 at initial temperature and pressure of 403 K and 1 bar, respectively.

Oxidation of n-Propylcyclohexane

Energy & Fuels, Vol. 23, 2009 2455 Table 1. Experimental Conditions Used during n-Propylcyclohexane Oxidation in Shock Tube

Figure 4. Evolution of the flame radius as a function of time for a mixture 1.80% C9H18/20.62% O2/77.58% N2 at 403 K initial pressure and 1 bar initial pressure.

are transferred and recorded by two numerical oscilloscopes (Tektronix TDS5054B). Incident and reflected shock conditions (temperature, pressure, and density) were computed from the conservation equations assuming thermal equilibrium and no reaction before ignition. The method used for the calculations assumes that γ (the ratio between heat capacity at constant pressure and heat capacity at constant volume, Cp/Cv) varies with temperature and completely accounts for the effect of the fuel and oxygen concentrations on the reflected shock temperatures.14 This method allows determining the pressure and the temperature with a good accuracy as the uncertainties on temperature and pressure are estimated to be (1 and (1.5%, respectively. 2.2. Ignition Delay Time Calculation. Figure 1 shows typical OH* and CH* radicals chemiluminescence signals recorded by the numerical oscilloscope. From those signals it is possible to calculate the ignition delay time, which is defined as the time interval between the reflected shock wave passage, determined from the pressure trace, and 50% of the maximum OH* or CH* radicals emission. As can be seen in this figure, for n-propylcyclohexane, there is no significant difference between the ignition delay times based on

C9H18 ppmv

O2 % mol

Ar % mol

φ

T5 (K)

P5 (bar)

75-1000

0.45-6.8

93.1-99.5

0.2-1.5

1250-1800

10-20

CH* and OH* radicals. This tendency was observed during most of the experiments. Therefore, the results further presented concern only the OH*-based autoignition times, the CH*-based ignition delay times were used to verify the accurateness of the measurements. The uncertainties on the ignition delay times mainly depend on the record length of the numerical oscilloscope and on the signal form. The numerical oscilloscope records 10 000 points on the time scale and 256 points on the voltage scale. As the latter have been adapted during the experiments to obtain the best accuracy, the sampling rate varied between 20 ns · .pt-1 and 200 ns · pt-1 for the time and between 31.25 mV · pt-1 and 312.5 mV · pt-1. As in most of the experiments that were conducted, the sharp drop of the signal seen in Figure 1 has been observed. This led to a maximum uncertainty of around 10% for high temperature ignition delay times (700 µs). 2.3. The Spherical Bomb. The laminar flame speeds were determined using a heated stainless steel spherical bomb made of two concentric spheres. The internal sphere in which the experiments are taking place has an internal diameter of 476 mm. The internal surface has been blackened using a special surface treatment in order to limit multiple reflections of light. Between the two spheres, a heat transfer fluid heats the apparatus to the desired temperature. Thermal insulation ensures a homogeneous temperature ((1 K). The mixtures were prepared inside the spherical bomb using n-propylcyclohexane (Sigma Aldrich 99+ %) and dry air (21% O2 + 79% N2). The introduction of air created turbulences that ensured a good mixing. An extra 2 min mixing time was added to obtain homogeneous mixtures. The spherical bomb is equipped with two opposite quartz windows (100 mm diameter, 50 mm thick). Two thin (2 mm diameter) tungsten electrodes are used to ignite the mixture. They are linked to a high voltage source (primary voltage of about 10 kV) that produces a spark at the center of the spherical bomb. The intensity and voltage of the discharge are adjustable and are measured through a current and voltage probe. Therefore, it is possible to calculate and adjust the amount of energy used to ignite the mixture and to minimize it. The visualization of the flame was obtained using a classical Schlieren apparatus. A white continuous lamp (manufactured by Leybold Didactic GmBh) is used to illuminate the flame via two lenses (diameters of 75 and 20 mm

Figure 5. Evolution of pressure during a spherical bomb experiment. The mixture studied here is the following: 1.80% C9H18/20.62% O2/77.58% N2 at initial temperature and pressure of 403 K and 1 bar, respectively.

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Table 2. Experimental Ignition Delay Times for n-Propylcylohexane/Oxygen/Argon Mixtures Measured during This Study equivalence ratio

fuel (ppm)

O2 (%)

argon (%)

TRSW(K)

PRSW(kPa)

ignition delay (µs)

1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

500 500 500 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 500 500 500 500 500 500 500 500 500 500 345 345 345 345 345 345 345 345 345 345 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690 690

0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93

99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0

1424 1375 1727 1462 1682 1590 1529 1696 1475 1684 1774 1496 1460 1502 1423 1396 1647 1567 1605 1508 1543 1491 1557 1704 1547 1661 1622 1574 1472 1398 1453 1466 1523 1503 1397 1481 1474 1628 1397 1540 1537 1558 1400 1439 1609 1514 1435 1435 1589 1475 1474 1613 1423 1523 1469 1534 1602 1459 1429 1376 1533 1325 1490 1601 1649 1526 1458 1662 1676 1609 1511 1650 1625 1678 1312 1520

1051 1023 1018 1069 1050 998 1008 1025 1017 1201 1274 1047 1034 1025 989 932 1202 1030 1230 1035 1027 928 1104 1708 1909 1961 1984 2025 1974 1958 2037 2042 1962 1959 1970 1894 2029 2057 2022 2013 1943 1860 2078 2013 922 1027 990 1068 1173 934 1098 1449 1009 1103 1020 1337 1580 858 593 1066 2106 1068 1012 1085 1063 1008 933 1156 1086 1100 1179 1159 1222 1293 943 1103

695.00 1144.80 23.84 406.40 37.64 103.84 150.20 35.54 262.60 22.17 11.80 220.00 291.40 224.60 396.40 499.60 48.92 89.70 60.92 207.40 138.00 166.00 144.80 27.70 153.75 43.64 61.42 104.83 398.60 872.00 435.60 362.00 181.34 216.00 512.80 193.20 193.60 36.68 569.60 85.42 88.64 73.60 498.80 261.00 32.97 101.00 186.87 206.72 37.20 136.10 150.15 42.17 257.35 105.20 161.77 70.35 41.15 132.60 255.20 514.40 88.92 775.94 94.94 35.67 37.31 83.31 154.45 23.10 25.49 41.46 108.89 29.11 32.92 23.78 991.90 116.50


Energy & Fuels, Vol. 23, 2009 2457 Table 2. Continued

equivalence ratio

fuel (ppm)

O2 (%)

argon (%)

TRSW(K)

PRSW(kPa)


1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40

690 1000 1000 1000 1000 1000 1000 1000 1000 1000 960 960 960 960 960 960 960 960 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 360 178 178 178 178 178 178 178 178 178 178 290 290 145 145 145 145 145 145 145 145 145 290 290 290 290 290 290 290 290 290 290 290 290 290

0.93 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 2.55 2.55 2.55 2.55 2.55 2.55 2.55 2.55 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.48 0.48 0.48 0.48 0.48 0.48 0.48 0.48 0.48 0.48 0.97 0.97 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97

99.0 98.6 98.6 98.6 98.6 98.6 98.6 98.6 98.6 98.6 97.4 97.4 97.4 97.4 97.4 97.4 97.4 97.4 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.0 99.0 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0

1565 1312 1448 1648 1559 1559 1503 1437 1383 1478 1476 1270 1382 1433 1374 1264 1379 1348 1424 1444 1558 1490 1578 1467 1467 1566 1552 1518 1592 1486 1558 1547 1358 1370 1412 1629 1485 1655 1456 1287 1489 1607 1463 1423 1343 1299 1488 1520 1365 1570 1360 1447 1524 1600 1509 1346 1472 1308 1382 1358 1353 1429 1418 1575 1551 1344 1455 1509 1466 1567 1519 1476 1496 1428 1524 1456

1278 1027 1014 1059 1026 1072 1037 1070 1063 984 1041 1041 982 1013 1051 1059 1012 1093 927 1048 1317 1100 1106 1086 1048 1227 1224 1047 1068 945 1128 1146 1092 1079 1048 1104 1073 1130 1033 971 1078 1306 1987 2019 1944 1912 1852 2043 2027 2081 1926 2013 1083 959 1977 1953 1994 1967 1980 2038 2043 1974 2014 990 1067 1124 1072 1080 1066 1534 954 1090 1101 898 1056 1192

57.79 653.20 136.40 22.46 50.38 51.33 100.00 198.20 355.80 122.60 41.66 523.60 128.67 73.00 207.00 675.60 163.40 234.80 109.90 90.22 29.86 73.36 31.14 90.04 72.63 33.12 33.28 48.18 29.32 80.33 39.06 43.05 312.33 210.46 130.43 25.90 67.67 20.44 96.63 778.69 51.66 17.50 96.00 167.80 508.80 895.60 69.80 50.80 395.60 39.80 403.40 148.34 49.95 23.58 48.72 310.40 78.42 726.40 270.00 347.60 328.80 150.92 162.00 31.84 34.44 319.11 69.19 51.68 77.86 26.38 74.43 57.40 50.56 137.11 48.45 85.18

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Dubois et al. Table 2. Continued

equivalence ratio

fuel (ppm)

O2 (%)

argon (%)

TRSW(K)

PRSW(kPa)


0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

290 290 290 290 290 290 1000 1000 1000 1000 1000 1000 1000 1000 1000 110 110 110 110 110 110 110 110 1000 1000 1000 1000 1000 1000 1000 1000 1000 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 215 150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 150 75 75 75

0.97 0.97 0.97 0.97 0.97 0.97 3.36 3.36 3.36 3.36 3.36 3.36 3.36 3.36 3.36 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 4.49 4.49 4.49 4.49 4.49 4.49 4.55 4.55 4.55 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.50 0.50 0.50

99.0 99.0 99.0 99.0 99.0 99.0 96.5 96.5 96.5 96.5 96.5 96.5 96.5 96.5 96.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 95.4 95.4 95.4 95.4 95.4 95.4 95.4 95.4 95.4 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0 99.5 99.5 99.5

1758 1454 1500 1409 1373 1520 1329 1351 1458 1229 1325 1398 1442 1289 1267 1521 1503 1336 1349 1416 1459 1351 1464 1440 1253 1359 1319 1243 1236 1261 1305 1200 1414 1511 1463 1523 1403 1540 1474 1358 1478 1455 1434 1541 1700 1354 1462 1428 1639 1380 1577 1509 1541 1465 1499 1475 1430 1284 1489 1491 1406 1534 1431 1359 1354 1278 1339 1406 1469 1517 1514 1424 1465 1513 1465 1417

1271 1115 1105 1087 1110 1082 1084 1044 1053 1053 1105 1043 1064 1077 1074 1915 1964 2048 1977 2017 2022 1920 1960 1080 1016 1092 1035 1037 1087 1103 1054 1056 1055 1059 1100 983 934 1099 936 1087 1375 1255 1121 1157 1172 1126 1104 1066 1105 1088 1095 1071 1293 1050 1165 1066 1150 984 1347 1213 1044 1122 1117 996 1078 1002 1030 931 1103 992 1106 1021 1095 1952 1986 2001

8.90 69.54 50.63 142.28 209.28 48.48 168.60 131.83 40.70 754.80 266.40 105.42 54.42 368.40 499.60 31.24 39.63 400.40 319.44 139.75 76.50 262.20 77.58 32.00 347.80 101.00 172.42 494.20 550.00 474.00 198.60 900.00 76.95 32.04 83.07 38.15 119.19 43.46 60.54 186.38 62.23 62.69 66.16 27.74 11.09 180.41 65.12 80.26 18.93 132.01 23.17 41.41 25.62 73.42 48.38 47.02 78.76 456.00 37.24 36.56 78.19 22.35 55.21 118.00 189.20 495.60 200.20 79.10 41.41 28.12 26.93 61.36 46.60 29.58 52.58 125.67


Energy & Fuels, Vol. 23, 2009 2459 Table 2. Continued

equivalence ratio

fuel (ppm)

O2 (%)

argon (%)

TRSW(K)

PRSW(kPa)


0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20

75 75 75 75 75 75 75 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000

0.50 0.50 0.50 0.50 0.50 0.50 0.50 6.76 6.76 6.76 6.76 6.76 6.76 6.76 6.76 6.76 6.76

99.5 99.5 99.5 99.5 99.5 99.5 99.5 93.1 93.1 93.1 93.1 93.1 93.1 93.1 93.1 93.1 93.1

1347 1371 1295 1530 1462 1321 1325 1222 1264 1257 1369 1183 1336 1357 1283 1345 1389

1946 2124 2034 1921 2040 2017 1950 1092 1076 1011 1111 1064 1032 1158 1048 1107 1089

265.80 218.20 534.80 24.80 72.58 274.80 307.00 546.00 273.80 302.60 61.67 703.60 119.66 85.00 217.00 89.08 47.08

and focal lengths of 150 and 22 mm, respectively) and two concave spherical mirrors (diameter 250 mm, focal length 1 m). A numerical high speed camera (Photron APX) with an acquisition frequency set up to 4000 images per second records the Schlieren images of the growing flame. The frame size was fixed to a 512 × 512 pixels for a maximum radius of 97 mm. Each pixel corresponds to 0.19 mm. The images are then processed (Visilog 5.2 image processing) to obtain the radius of the flame as a function of time, which allows the calculation of the unstretched laminar flame speed S°l . 2.4. Laminar Flame Speed Calculation. Figures 2 and 3 show a typical record of noncellular and cellular flame propagation, respectively. The evolution of the flame radius as a function of time can be extracted from the images, and the spatial flame speed Vs can then be deduced (Vs ) drf/dt with rf being the flame radius). Figure 4 shows the processed data for the experiment presented in Figure 2. Depending on the speed of the flame, 50-150 frames were obtained. When the observation is limited to the initial stages of the flame expansion where the pressure does not vary, a simple relationship links the spatial flame velocity Vs to the fundamental flame speed Sl:

Fu Sl) ×Vs Fb where Fu and Fb are the densities of unburnt and burnt gases, respectively.

K)

1 dA A dt

As can be seen in Figure 5, during all the experiments carried

K)

Vs 2 dRf )2 Rf dt Rf

out, the pressure did not vary during the visualization of the flame. Therefore, it is possible to simply derive the laminar flame speed

V°s - Vs ) LK from the spatial flame speed. However, the flame, owing to its

( )

Rf Rf ) S°l × t - 2L' × ln Rfinal Rfinal

spherical geometry, is subject to a local stretch due to curvature and strain rate. The velocity at which the flame increases in size differs from the one defined in the case of a planar flame. Thus, it is necessary to apply a stretch correction to the flame velocity. The stretch of a flame could be defined as a function of the growth rate of its surface area A:15

In the case of a spherical propagation where A ) 4πr2, the stretch can be directly calculated: If the stretch remains weak (below 2000 s-1), the unstretched (V°s ) and stretched flame (Vs) velocities differ by a coefficient known as the Markstein length, L: Integrating this expression gives an expression from which the unstretched flame speed can be obtained: where Rfinal is the final radius measured and L’ ) (Fb/Fu) × L.

3. Results and Discussion 3.1. The Shock Tube Study. During the shock tube study, the temperature was varied between 1250 and 1800 K. Shocks were run at 10 and 20 bar to investigate the effect of pressure. Six different equivalence ratios (0.2, 0.3, 0.4, 0.5, 1.0, and 1.5) were tested in order to evaluate the effect of this parameter. The effect of the fuel and oxygen concentrations on the ignition delay times was also investigated. The experimental conditions used are listed in Table 1, and the experimental data are summarized in Table 2. In Figure 6, the evolution of ignition delay times at 10 bar as a function of the temperature inverse is presented for constant oxygen concentration mixtures. One can see that ignition delay times decrease with increasing temperature. Furthermore, the variation of the logarithm of ignition delay times with the temperature inverse is linear at a fixed equivalence ratio. This denotes an Arrhenius behavior that is generally observed for H + O2 f OH + O

(R1)

H + O9H18 f C9H17 + H2

(R2)

other hydrocarbons.10,17,19 The same observation can also be made for constant fuel concentration mixtures at 10 bar that (10) Orme, J. P.; Curran, H. J.; Simmie, J. M. J. Phys. Chem. A 2006, 110, 114–131. (11) Douce, F.; Djebaı¨li-Chaumeix, N.; Paillard, C. E.; Clinard, C.; Rouzaud, J. N. Proc. Combust. Inst. 2000, 28, 2523–2529. (12) Douce, F.; Djebaı¨li-Chaumeix, N.; Paillard, C.-E.; Clinard, C.; Rouzaud, J.-N. Proc. Comb. Inst. 2000, 28, 2523–2529. (13) Kellerer H., Koch R., Wittig S. Combust. Flame 2000, 120, 188199. (14) Paillard, C. E.; Youssefi, S.; Dupre´, G. Prog. Astronaut. Aeronaut. 1986, 106, 394–406. (15) Law, C. K.; Sung, C. J. Prog. Energy Combust. Sci. 2000, 26, 459– 505. (16) Horning, D. C.; Davidson, D. F.; Hanson, R. K. J. Propul. Power 2002, 18, 363–371. (17) Horning, D. C. Thesis, Stanford University: United States, 2001. (18) Tsang, W.; Lifshitz, A. Annu. ReV. Phys. Chem. 1990, 41, 559– 599. (19) Olchanski, E.; Burcat, A. Int. J. Chem. Kin. 2006, 38, 703–713.

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Figure 6. Ignition delay times variation with the temperature inverse for different equivalence ratio mixtures n-propylcyclohexane/O2 diluted in argon with constant oxygen concentration.

Figure 7. Ignition delay times variation with the temperature for different equivalence ratio mixtures n-propylcyclohexane/O2 diluted in argon with constant fuel concentration.

Figure 8. Effect of equivalence ratio on the evolution of ignition delay times with the temperature inverse for constant oxygen concentration mixtures.

are presented in Figure 7. Furthermore, if the curves obtained for each equivalence ratio are gathered in the same graph, one can see that the ignition delay times is decreasing with equivalence ratio both for constant oxygen concentration mixtures (Figure 8) and for constant fuel concentration mixtures

Figure 9. Effect of equivalence ratio on the evolution of ignition delay times with the temperature inverse for constant fuel concentration mixtures.

(Figure 9). This phenomenon was already observed for linear alkanes16 and for methylcyclohexane.17 The explanation given in the case of linear alkanes can be used to understand this phenomenon. It has been suggested that before ignition occurs, there is a competition between the fuel and oxygen for H radicals reaction.18 The two possible reactions are:


Because oxygen relative concentration increases with decreasing equivalence ratios, chain-branching reaction R1, which forms two reactive radicals, is promoted whereas reaction R2, which transform a reactive radical into an unreactive one and a molecular species, is disadvantaged. Thus, with decreasing equivalence ratio, the reactivity of the mixture increases. As a result, the ignition delay times decrease with decreasing equivalence ratios. To evaluate the effect of pressure on ignition delay times, some experiments were carried out at 20 bar. Figure 10 presents the results obtained for constant oxygen concentration mixtures diluted in argon up to 99.5% for different equivalence ratios. Using a 99.5% dilution in argon during 20 bar experiments

Energy & Fuels, Vol. 23, 2009 2461

τi ) 1.094 × 10-5 × [C9H18]0.82[O2]-1.22[Ar]0.39 × 196 600 2 exp r ) 0.97 RT

(

)

allowed obtaining similar fuel and oxygen molar concentrations behind the reflected shock wave as the 10 bar and 99% dilution in argon experiments. Figure 10 shows that there is no significant effect of pressure between 10 and 20 bar on ignition delay times. Indeed, according to the experimental uncertainties, the same ignition delay times are obtained for both 10 and 20 bar experiments. Using all the experimental data (250 ignition delay times) obtained during this study, a multiple regression was applied

Figure 10. Effect of pressure on the evolution of ignition delay times with the temperature inverse for constant oxygen concentration mixtures n-propylcyclohexane/O2/Ar at φ ) 1.5 (a), 1 (b), 0.5 (c), 0.4 (d), 0.3 (e), and 0.2(f).

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Figure 11. Comparison between correlated (dashed line) and experimental (symbols) ignition delay times evolution with the temperature inverse for n-propylcyclohexane/O2 mixture diluted in Ar up to 99%. Figure 13. Unstretched laminar flame speed obtained for n-propylcyclohexane/air (21% O2/79% N2) at initial temperature and pressure of 403 K and 1 atm, respectively, as a function of the equivalence ratio.

Figure 12. Comparison between methylcyclohexane ignition delay times from Orme et al.10 (open symbols) and n-propylcyclohexane ignition delay times at φ ) 1.

and allowed to derive a simplified expression for the ignition delay times determination according to the fuel, oxygen, and diluents (Ar) molar concentrations and the temperature. The -0.01

(RTP ) × 196 600 exp( RT )

τi ) 1.094 × 10-5 × X(C9H18)0.82X(O2)-1.22X(Ar)0.39

expression obtained is the following: where τi is in microseconds, the concentration is in mol · cm-3, the temperature is in K, and the activation energy is in J · mol-1. As can be seen from Figure 11, a very good agreement is obtained between the correlated and the experimental ignition delay times. The average error between the experimental data and this correlation is 14%. This expression shows an inhibitive effect of the fuel on its oxidation, and a promotive effect of oxygen that is generally observed for heavy hydrocarbons19 as well as lighter ones. Furthermore, if the correlation is expressed as a function of molar fraction and pressure, it confirms that the pressure has almost no effect on ignition delay times between 10 and 20 bar as experimentally observed:

Figure 12 compares the ignition delay times obtained for stoichiometric mixtures to the ones measured by Orme et al.10 for methylcyclohexane. This figure shows that ignition delay times are on the same order of magnitude for both fuels. However, it is to be noted that methylcyclohexane measurements have been done at much lower pressure (1-4 atm) than the n-propylcyclohexane experiments carried during this study (10-20 bar). If no pressure effect has been observed in our case between 10 and 20 bar it is nonetheless expected that 1 atm experiments would lead to longer delay. Indeed, Orme et al. observed an effect of pressure between 1 and 4 atm for methylcyclohexane. Therefore, it is expected that n-propylcyclohexane gives slightly longer ignition delays than methylcyclohexane for similar pressure conditions. 3.2. The Spherical Bomb Study. During the spherical bomb study, the initial temperature was kept constant at 403 K and all the experiments were carried out at an initial pressure of 1 bar. It is to be noted that particular attention was paid to those two initial parameters, which can have a high impact on the laminar flame speed. Different equivalence ratio mixtures (0.6 e φ e 1.75) were tested. Figures 2 and Figure 3 give typical examples of flame propagations recorded by the camera during an experiment. As one can see, the propagation of the flame is perfectly spherical. This was the case in every single experiment performed. Furthermore, Figure 5 shows that the pressure remains constant during the visualization of the flame propagation. Therefore, no effect of pressure is affecting its propagation. The results obtained for n-propylcyclohexane/air flames are given in Figure 13, and the experimental data are gathered in Table 3. As can be observed in this figure, the unstretched laminar flame speed (S°l ) is maximum for mixtures just above stoichiometry (1.05 e φ e 1.10) and decreases dramatically with either increasing or decreasing equivalence ratios. This tendency is usually observed for heavy hydrocarbons.20 In addition, during the experiments, no ignition was obtained for equivalence ratio mixtures below 0.55, even if the energy used to ignite the mixture is slightly increased. This allowed us to estimate the lower flammability limit around 1.05% (φ ) 0.70) (20) Zhao, Z.; Li, J.; Kazakov, A.; Dryer, F. L. Combust. Sci. Technol. 2005, 177, 89–106.



Table 3. Experimental Laminar Flame Speeds Measured for n-Propylcylohexane/Air Mixtures during This Study equivalence ratio

XFuel

XO2

XN2

cellular?

Sl°(mm/s)

expansion coefficient

PAICC (bar)

Pmax (bar)

0.61 0.70 0.75 0.78 0.82 0.86 0.87 0.88 0.95 0.96 1.02 1.05 1.06 1.10 1.13 1.18 1.19 1.20 1.25 1.28 1.36 1.44 1.59 1.74

0.94% 1.08% 1.15% 1.20% 1.25% 1.32% 1.33% 1.35% 1.46% 1.48% 1.56% 1.60% 1.63% 1.68% 1.73% 1.79% 1.76% 1.84% 1.91% 1.95% 1.98% 2.19% 2.41% 2.63%

20.81% 20.77% 20.74% 20.73% 20.73% 20.71% 20.71% 20.71% 20.68% 20.68% 20.65% 20.65% 20.64% 20.64% 20.63% 20.61% 20.62% 20.60% 20.59% 20.58% 20.58% 20.52% 20.49% 20.44%

78.25% 78.16% 78.11% 78.06% 78.02% 77.97% 77.96% 77.94% 77.86% 77.84% 77.79% 77.74% 77.73% 77.68% 77.64% 77.59% 77.62% 77.57% 77.50% 77.47% 77.44% 77.29% 77.10% 76.93%

no no no no no no no no no no no no no no no no no no no no yes yes yes yes

No ignition 375.1 442.0 500.3 508.7 517.7 535.1 543.9 590.0 579.9 603.0 588.7 583.7 593.8 596.8 565.1 553.0 544.0 514.1 485.6 397.7 329.8 214.8 143.3

5.08 5.30 5.46 5.60 5.77 5.80 5.85 6.06 6.09 6.21 6.26 6.28 6.31 6.31 6.30 6.31 6.29 6.26 6.31 6.23 6.11 5.98 5.85

5.99 6.21 6.37 6.51 6.66 6.7 6.19 6.94 6.98 7.1 7.17 7.2 7.26 7.3 7.35 7.32 7.35 7.36 7.36 7.36 7.29 7.18 7.05

5.43 5.75 6.01 6.08 5.98 6.36 6.28 6.39 6.47 6.53 6.58 6.85 6.7 6.71 6.72 7.23 6.65 6.74 6.67 6.65 6.61 6.59 6.57

of n-propylcyclohexane in air at initial temperature and pressure of 403 K and 105 Pa, respectively. On the other hand, for equivalence ratio mixtures above 1.40 the flame wrinkles during its propagation, which makes the calculation of S°l difficult. Indeed, due to the wrinkling of the flame, its surface is much higher than the envelope of the sphere of the same radius. Therefore, this, as well as the stretch generated by the spark in the first stages of the flame propagation, limits the number of images that can be used to derive the laminar flame speed. Because the flame becomes really cellular for φ g 1.75, it prevented the determination of the unstretched laminar flame speed. Figure 3 shows an example of a cellular flame. 4. Modeling and Discussion Very few modeling studies have been performed on npropylcylohexane oxidation so far. A comprehensive kinetic model for n-propylcyclohexane oxidation was first developed9 and validated against data obtained in a jet-stirred reactor between 950 and 1250 K at atmospheric pressure and for equivalence ratios from 0.5 to 1.5. Furthermore, the submechanism developed for n-propylcyclohexane has been used later in other models to simulate the results obtained in the same jet-stirred reactor for different diesel fuel surrogates mixtures.6,8 All three models6,8,9 were tested on the ignition delay times obtained in the current work. The calculations were performed with the SENKIN-CHEMKIN II package. Since all the models tested come from the same research team, only the results from the best model are presented here.9 Furthermore, they are compared with those obtained using the kinetic mechanism developed in this work. This model is based on an n-decane model developed and validated against laminar flame speed measurements20 and the submechanism for n-propylcylohexane developed by Ristori et al.9 However, some changes have been made on the obtained mechanism. To limit the number of reactions and species, the n-decane chemistry has been removed from the model. Furthermore, in the n-propylcyclohexane submechanism proposed by Ristori et al.,9 three reaction pathways are duplicated by the presence of both global steps and elementary reactions. Table 4 summarizes the reactions involved and Figure 14 describes the detailed reaction pathways. Those three global steps have been removed from the mecha-

Table 4. Reactions of the n-Propylcyclohexane Submechanism Involved in the Duplication of Three Reaction Pathways global steps CYC9H18 f 2C2H4 + C5H10 CYC9H18 f C2H4 + nC3H7 + C4H7 CYC9H18 f 3C2H4 + C3H6 elementary steps CYC9H18 ) CCYC9H17 CCYC9H17 f 2C2H4 + C5H9 CYC9H18 ) ECYC9H17 ECYC9H17 f C4H6 + C2H4 + nC3H7 CYC9H18 ) FCYC9H17 FCYC9H17 f C7H12 + C2H5 C7H12 f aC3H5 + C4H7

nism. The missing transport data necessary to carry flame simulation out were calculated and implemented in the model. The Lennard-Jones parameters based on the critical conditions were used to compute these data. The final model obtained is composed of 690 reactions involving 110 species and is perfectly suitable for flame calculations. This new model was tested on the JSR results. As it can be seen in Figure 15, the new model reproduces fairly well the species profiles for temperatures higher than 900 K at an equivalence ratio of 0.5. The same trends are observed for stoichiometric and rich mixtures. Figure 16 compares the experimental ignition delay times with the results predicted by the two models. As one can see, the model developed in the present work reproduces with a good accuracy the ignition delay times, especially those obtained with a dilution in argon above 98.5%. Furthermore, the model reproduces with a fairly good accuracy the activation energy. On the other hand, the Ristori et al. model9 is less accurate to simulate the ignition delay times and fails to reproduce the activation energy. This could be explained by the fact that this model was developed and constrained using JSR results at lower temperature than the current study. Nonetheless, the n-propylcyclohexane submechanism developed by Ristori appears to be a good basis for building a high-temperature model. Indeed, the mechanism presented in this study, constructed using this submechanism with a few modifications, is in good agreement with the experimental ignition delay times measured. If a multiple regression is applied to the results predicted by the models, an expression for the determination of the ignition delay time according to the fuel, oxygen, and argon

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Figure 14. Detailed pathways of the reactions involved in the duplicated pathways.

Figure 15. Comparison between the current model and the results obtained using the JSR results of Ristori et al.9 for a mixture of n-propylcyclohexane/ O2/N2 at an equivalence ratio of 0.5. The initial pressure is 1.013 MPa.

molar concentrations and the temperature can be obtained for each mechanism. Table 5 compares those expressions to the experimental correlation. As one can see, both models reproduce with a good accuracy the inhibitor effect of the fuel on its oxidation and the promotive effect of oxygen. Furthermore, the mechanism built during this study show a

good agreement in predicting the noneffect of pressure on autoignition delay times between 10 and 20 bar. It also does a much better job in reproducing the global activation energy of n-propylcyclohexane oxidation. The simulation of laminar flame speeds was done assuming a one-dimensional freely propagating flame using Cosilab 2



Figure 16. Comparison between experimental ignition delay times ((9) for constant fuel concentration experiments and (•) for constant oxygen concentration experiments) and results from the simulation ((s) for Ristori’s model and (- - -) for current model). Table 5. Coefficient Obtained from the Correlation τind ) A[HC]r[O2]β[AR]χ exp(E/RT) Derived from the Experimental Results and the Results from Ristori’s Model and the Model Built in This Study A experimental Ristori’s model current model

1.09 × 10-5 1.63 × 10-7 5.34 × 10-6

R

β

χ

E

0.82 0.82 0.74

-1.22 -1.40 -1.12

0.39 0.28 0.35

196 600 219 600 207 500

and Cantera 1.7.1. It is to be noted that both packages lead to similar results if the same convergence criteria are applied (differences below 1%). Figure 16compares the experimental results with the predictions from the simulation. This figure

shows that, despite the model overestimates, the laminar flame speeds above stoichiometry (by a factor of 10 to 15%) and the equivalence ratio at which the flame speed is maximum (1.13 instead of 1.08 experimentally) are in good agreement with the experimental data. In addition, the model predicts with a very good accuracy the burning velocities of lean mixtures that are used in HCCI combustion. Hence, despite the small discrepancy remaining on the rich-side mixtures for laminar flame speeds, the model has been successfully validated as it shows good abilities to predict the experimental data measured in this study.

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5. Conclusion The first set of experimental data on n-propylcyclohexane oxidation at high temperature and pressure was obtained during this study. Ignition delay times based on OH* and CH* radical chemiluminescence were measured. The results show that ignition delays decrease with decreasing equivalence ratio. Furthermore, no significant effect of pressure between 10 and 20 bar was observed. A simplified expression for ignition delay time calculation according to reactants concentrations and temperature was derived from all the experimental data. This correlation shows an inhibitive effect of the fuel on its oxidation and a promotive effect of oxygen that is usually observed for heavy hydrocarbons. In addition, laminar flame measurements were determined using the spherical bomb technique. The results obtained show that laminar flame speeds are maxima for equivalence ratio mixtures around 1.08. The values then decrease either for decreasing or increasing equivalence ratios. No

Dubois et al.

ignition was obtained below equivalence ratio of 0.55 at initial temperature and pressure of 403 K and 1 bar, respectively. This allowed us to estimate the lower flammability limit around 1.05% of n-propylcyclohexane in air (φ ) 0.70) for npropylcyclohexane. The experimental data, mandatory to the development and validation of kinetic models, have been used to build a semidetailed mechanism (690 reactions and 110 species) from two mechanisms available in the literature. A generally good agreement between the simulation and the experimental data is obtained for both ignition delay times and laminar flame speeds, which validate the model. Acknowledgment. We wish to kindly thank Total, PSAPeugeot-Citroen, and CNRS for the financial support they provided to this study in the framework of French program PNIR/CAM1. EF801062D

Experimental and Modeling Study of n-Propylcyclohexane Oxidation

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