A rapid compression machine study of the oxidation of propane in the negative temperature coefficient regime

Combustion and Flame 153 (2008) 316–333 www.elsevier.com/locate/combustflame

A rapid compression machine study of the oxidation of propane in the negative temperature coefficient regime S.M. Gallagher a , H.J. Curran a,∗ , W.K. Metcalfe a , D. Healy a , J.M. Simmie a , G. Bourque b a Combustion Chemistry Centre, National University of Ireland, Galway, Ireland b Rolls-Royce Canada, Montreal, Canada

Received 14 May 2007; received in revised form 9 August 2007; accepted 10 September 2007 Available online 13 November 2007

Abstract The oxidation of propane has been studied in the temperature range 680–970 K at compressed gas pressures of 21, 27, and 37 atm and at varying equivalence ratios of 0.5, 1.0, and 2.0. These data are consistent with other experiments presented in the literature for alkane fuels in that, when ignition delay times are plotted as a function of temperature, a characteristic negative coefficient behavior is observed. In addition, these data were simulated using a detailed chemical kinetic model. It was found that qualitatively the model correctly simulated the effect of change in equivalence ratio and pressure, predicting that fuel-rich, high-pressure mixtures ignite fastest, while fuel-lean, low-pressure mixtures ignite slowest. Moreover, reactivity as a function of temperature is well captured, with the model predicting negative temperature coefficient behavior similar to the experiments. Quantitatively the model is faster than experiment for all mixtures at the lowest temperatures (650–750 K) and is also faster than experiment throughout the entire temperature range for fuel-lean mixtures. © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Propane; Oxidation; RCM; Experiments; Modeling

1. Introduction

Propane is the main component of liquefied petroleum gas, which is widely used as an automotive fuel as well as in petrochemical production and home heating. An excellent account on the properties its properties is given in Chapter 20 of the “Automotive Fuels Reference Book” [1].

* Corresponding author.

E-mail address: [email protected] (H.J. Curran).

Much of the work to date on propane oxidation has concentrated on high temperature (1200–2000 K), chemistry at relatively low pressures, that is, at pressures less than 6 atm [2]. However, Newitt and co-workers [3–5] studied the oxidation of propane in the “slow combustion” and the “cool flame” region at atmospheric and reduced pressures in the temperature range 548–698 K. The formation of cool flames was identified with the presence of a critical concentration of higher aldehydes in the reacting medium. Moreover, it was shown that the reaction of an equimolecular propane– oxygen medium showed negative temperature coefficient behavior. Newitt and Thornes [5] found it nec-

0010-2180/$ – see front matter © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2007.09.004

S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333

essary to distinguish four types of reaction, which they depicted by reference to an ignition temperature– pressure diagram. These four types were defined as (a) high-temperature slow reactions, (b) cool-flame reactions, (c) low-temperature slow reactions, and (d) a region in which spontaneous ignition always occurs after a comparatively short induction period. Thereafter, Pease [6] confirmed the results of Newitt and co-workers, observing cool flames and a negative-temperature-coefficient (NTC) regime. It was shown that the rate of reaction passed through a minimum at 633–653 K. It was suggested that a lowtemperature branched-chain process is suppressed in the NTC region and gives way to an unbranched chain. Ignition delay times of propane–oxygen–argon mixtures were measured behind reflected shock waves by Burcat et al. [7,8], in the temperature range 1200– 1700 K, at pressures from 2 to 15 atm, and in the equivalence ratio range 0.125  φ  2.0. Cathonnet et al. [9] studied propane oxidation in a laminar flow quartz reactor at pressures of 1 and 6 bar near 1000 K, over a wide variety of equivalence ratios, 0.05  φ  25. Modeling of propane was also presented and good agreement was found between experiment and model. Hoffman et al. [10] presented a model validated against experimental results from an atmosphericpressure flow reactor (1000–1148 K) at fuel–air equivalence ratios of 0.07–1.59 [11], and shock tube data within the temperature range 1250–1700 K and a pressures of 2 to 15 atm [11]. This model was then compared to the authors’ own experimental results via the reactor at pressures of 3, 6, and 10 atm, at 850 and 900 K, φ = 0.3. The model successfully predicted fuel consumption and major species concentration versus residence time for the high pressure (6 and 10 atm) conditions; however, lower pressure results (3 atm) were less reproducible. Dagaut et al. [12] also developed a detailed mechanism and simulated experimental data obtained in both a high-pressure jet-stirred reactor and a shock tube, over a combined temperature range of 900– 1700 K, a pressure of 1–10 atm, and an equivalence ratio range of 0.15–4.0. Validation of the model was also performed against flow-reactor molecular species concentration profiles and flame speeds, where good agreement was found. Koert et al. carried out an experimental [13] and a modeling study [14] of propane oxidation. Species concentration profiles were obtained from experiments carried out in a flow reactor at pressures of 10 and 15 atm, within the low- to intermediate (600– 900 K)-temperature regime, at an equivalence ratio of 0.4. These studies focused on the chemistry occurring within the NTC behavior, between 640 and 770 K.

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The model was found to predict the NTC region quite well and reasonably simulate the species observed in the experiments. Ranzi and co-workers [15] presented a modeling study covering a wide range of temperatures and compared the mechanism with experimental data obtained over a wide range of operating conditions, concluding that there remained discrepancies in the overall scheme of C1 –C4 hydrocarbon oxidation. A modeling study was carried out by Qin et al. [16] within the temperature range of approximately 1250–2000 K. This model was validated against atmospheric flame speeds [17,18] and ignition data [19–21]. The work of Cadman et al. [22] explores propane oxidation, measuring ignition delay times at elevated pressures of 5–40 bar and temperatures of 850– 1100 K for a fuel-lean (φ = 0.5) mixture. The results showed ignition delay times shorter than expected based on extrapolation of delay data obtained at temperatures higher than 1200 K. This was reported to be due to changes in activation energy obtained in the range 850–1100 K, and further work was required in order to improve the overall mechanism. Herzler et al. [23] used a shock tube to measure ignition delay times of lean (φ = 0.5) propane–air mixtures in the temperature range 740  T  1300 K at pressures of 10 and 30 bar, respectively. The results obtained were in very good agreement with measurements of Cadman et al. [22]. The experiments confirmed that the activation energy of the ignition delay time decreases at around 1050 K and a linear extrapolation is not possible. Reaction schemes used in the literature could not predict the ignition delay times at these low temperatures. Recently Zhukov et al. [24] measured ignition delay times behind reflected shock waves for lean (φ = 0.5) propane–air mixtures in the temperature range 800–1500 K and in the pressure range 2–500 atm. The effect of pressure on the mechanism of autoignition was demonstrated. In addition, a detailed kinetic model was constructed for propane ignition, including mechanisms for low, intermediate, and high temperatures. Most recently Lamnaouer et al. [25] measured ignition delay times behind reflected shock waves for lean (φ = 0.5) propane–air mixtures in the temperature range 870–1330 K and at a reflected shock pressure of 30 atm. These results were in good agreement in those recorded by Cadman et al. and Herzler et al. at lower temperatures (870–1000 K) and were slightly slower than those measured by Herzler et al. at temperatures above 1000 K. Kim and Shin [26] obtained ignition delay data for the temperature range 1350–1800 K, pressures of 0.75–1.57 bar, and φ = 0.5–2.0. Modeling work was

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also presented and it was found that the mechanism agreed well with experimental data. Davidson et al. [27] and Horning et al. [28,29] measured ignition delay times by means of OH emission at 306.5 nm and CH emission at 431.5 nm over the temperature range 1300–1700 K, for 0.5  φ  2.0, at pressures between 1 and 6 atm, for propane, n-butane, n-heptane, and n-decane. Within this regime the authors found a marked similarity of the ignition delay time characteristics among the four alkanes and present a single expression for the stoichiometric ignition time data. Within the same equivalence ratio range, Lamoureux and co-workers [30] presented data at temperatures of 1200–2700 K, reaching elevated pressures, varying from 1 to 10 bar, for methane, ethane, and propane. Here a correlation between ignition delay time, temperature, pressure, and concentration was proposed and was found to estimate ignition delay times to within 20% of experimental measurements for all ranges. Results are also compared to a kinetic model of alkane oxidation. Low-pressure propane modeling and experiments are also reported by Levitsky et al. [31] at 586, 613, and 658 K, Freeman and Lefebvre [32] between 940 and 1050 K, and Hidaka and co-workers [33] in the temperature range 1300–1800 K. Mantashyan et al. [34] studied oxidation at 603 K and 270 Torr, while Wilk et al. [35] studied temperatures of 563–743 K. The shock tube ignition delay measurements of Cadman et al. [22], Herzler et al. [23], Zhukov et al. [24], and Lamnaouer et al. [25] were all recorded under fuel-lean (φ = 0.5) in air conditions. The current study reports on ignition delay times measured in a rapid compression machine (RCM) in the temperature range 680–970 K, at compressed gas pressures of 21, 27, and 37 atm, and at equivalence ratios of 0.5, 1.0, and 2.0. Such a comprehensive set of ignition delay data at low temperature and high pressure has not previously been presented in the literature, and it greatly extends the range of experimental data available for propane oxidation. A detailed chemical kinetic model is used to simulate this data with reasonable agreement between model predictions and experimental measurements. Ignition delay times measured for the 2.06% C3 H8 , φ = 0.5 in air mixture are almost two orders of magnitude longer in our RCM compared to shock tubes [22,23] under almost identical conditions.

2. Experimental A rapid compression machine (RCM) is a laboratory device that simulates the compression stroke of a single engine cycle and so allows autoignition

phenomena to be studied in a more ideal, constant, and controllable environment than a reciprocating engine [36]. The fundamental objective of an RCM is to raise the test gas as rapidly as possible to a high temperature and pressure with minimal heat losses. The NUI Galway RCM is different from most other RCMs in that it has a twin-opposed piston configuration described previously [37], resulting in a fast compression time of a little more than 16 ms. In addition, creviced piston heads are used to improve the postcompression temperature distribution in the combustion chamber [38,39]. This particular design was adopted following studies at MIT [40–42] that found that the creviced design resulted in an almost homogeneous temperature field in the postcompression period; that obtained using flat piston heads is far less homogeneous. In a previous study [43] the commercial software STAR/KINetics was applied to simulate the combustion of hydrogen in our twin-piston RCM and reasonably good agreement with experimental measurements was achieved. However, computations took five days using a mechanism of 10 reactions and 19 species, making it prohibitive to progress to more complex fuels. Experiments were carried out at a compression ratio, defined as the ratio between the volume before compression and at the end of compression, of approximately 10:1. The effective compression ratio is lower using argon compared to nitrogen as diluent gas, due to its higher thermal diffusivity and lower heat capacity. The compression time was approximately 16.6 ms. To vary the compressed gas temperature, TC , the proportions of the diluent gases (N2 , Ar) were varied to alter the overall heat capacity of the fuel and “air” mixture. Using only nitrogen as the diluent allows lower compressed gas temperatures, while pure argon reaches much higher temperatures following compression due to its lower heat capacity. In addition, an electrothermal digitally controlled heating blanket surrounds the combustion chamber of the machine so that the initial temperature may be varied up to a maximum operating temperature of 393 K. Sufficient time was allowed for the chamber temperature to stabilize after a change was made to the thermostat setting. Varying the diluent gas composition and using the heating device resulted in a compressed gas temperature range of 680–970 K to be studied. However, as the gas composition varied, this temperature range changed. In particular, the maximum achievable compressed gas temperature dropped with increasing fuel content. Pressure–time data were measured using a pressure transducer (Kistler 603B) and transferred via an amplifier to the oscilloscope and ultimately recorded digitally on computer. The ignition delay time, defined as the time from the end of compression to the

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319

Table 1 Mixture compositions and postcompression temperatures achieved Mix

Mole fraction

1 2 3 4 5 6 7

φ

C3 H8

O2

N2

Ar

Temperature range (K)

0.5 0.5 1.0 1.0 1.0 2.0 2.0

0.0206 0.0206 0.0403 0.0403 0.0403 0.0775 0.0775

0.2057 0.2057 0.2015 0.2015 0.2015 0.1937 0.1937

0.7738 0.2579 0.7582 0.2527 – 0.2429 –

– 0.5158 – 0.5055 0.7582 0.4859 0.7288

683–800 805–969 707–766 757–847 863–955 685–751 722–831

maximum rate of pressure rise during ignition, was measured using two vertical cursors on the oscilloscope. In general, we found that the ignition delay times were reproducible to within 10% of each other at each TC . The compressed gas pressure was measured using two horizontal cursors. The primary experimental data comprised the pressure–time record, but it was more practical to assimilate and present the results in terms of the overall dependence of ignition delay on the compressed gas temperature. The gas mixtures and the initial conditions are documented in Table 1. The compressed gas temperature, TC , was calculated using the initial temperature, Ti , pressure, pi and reactant composition and the experimentally measured compressed gas pressure, pC , defined as the maximum pressure immediately after compression, and employing the adiabatic compression/expansion routine in Gaseq [44], which uses the temperature dependence of the ratio of specific heats, γ , according to the equation  ln

pC pi



TC = Ti

γ dT , γ −1 T

while assuming frozen chemistry during compression. The compressed gas temperature is then plotted against the measured ignition delay time to obtain overall reactivity profiles of the propane mixtures. The installation of a fast-response thermocouple was not attempted in this experimental study, as such thermocouples have very narrow diameters and are very fragile—in the RCM environment they would be subjected to very high stress. Chien et al. [45] have used an intrusive resistance wire method to measure temperature with some success, while the Desgroux et al. [46] and Griffiths et al. [47] groups have used Rayleigh scattering and acetone laser-induced fluorescence temperature measurements, but optical access is currently not possible in our RCM. Gases used, nitrogen (CP Grade) 99.95%, argon (Research Grade) 99.9995%, and oxygen (Medical Grade) 99.5%, were supplied by BOC Ireland and

were used without further purification. Propane gas was obtained from Aldrich at 98% purity and used without further purification. The mixtures were prepared manometrically in a stainless steel container and allowed to homogenize. The mixtures, Table 1, were left overnight before use, allowing on average 15 h for homogeneous mixing to occur. Before compression the gas mixture was introduced in to the preheated compression cylinder from the mixing tank at a known temperature and pressure. Compression was achieved by simultaneous movement of the twin opposed pistons. The pressure–time history was recorded during and after compression until autoignition occurred, within a 400-ms timescale. The time for compression is fast, 16.6 ms, with most of the rapid rise in pressure and temperature taking place in the last 2–3 ms of compression; therefore heat losses during compression are low but do exist. For a period following compression the gases experience a high degree of heat loss owing to the swirl experienced within the chamber. Heat losses continue from the core gas during the constant volume period. Even though ignition delays were observed up to 400 ms following compression, repeat experiments with ignition delay times greater than 100 ms showed larger percentage variations in measured ignition delay times than those with ignition delay times below 100 ms.

3. Kinetic modeling The chemical kinetic mechanism was developed and simulations were performed using the HCT (hydrodynamics, chemistry, and transport) program [48]. The detailed chemical kinetic reaction mechanism used in these calculations was based on the hierarchical nature [49] of reacting systems. The hydrogen submechanism is based on that which has recently been validated by O’Conaire et al. [50] in the temperature range 298–2700 K at pressures from 0.05 to 87 atm, and equivalence ratios from 0.2 to 6.0. The kinetic mechanism employed for the methane/ethane

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Fig. 1. C3 H8 oxidation, τR = 198 ms, P = 10 atm. Experimental percentage fuel conversion (symbols) and model simulation (line).

system is based on that published by Fischer and coworkers [51,52] in their dimethyl ether study. The C3 submechanism is based on the modeling work of Curran et al. [53,54] using the thermochemical parameters and rate constant rules described in their work on isooctane oxidation. This mechanism has recently been used successfully to simulate the oxidation of methane/propane mixtures at high pressures [55]. The complete kinetic mechanism consists of 118 different chemical species and 663 elementary reactions and is available at http://www.nuigalway.ie/ chem/combust.htm#mecs. In order to validate the mechanism the flow reactor experiments of Koert et al. [13] were first simulated and the mechanism then applied to the rapid compression machine data. 3.1. Flow–reactor simulation Koert et al. [13] studied propane oxidation in a high-pressure flow reactor. Experimental conditions ranged from 10–15 atm and 650–800 K and have a residence time, τR , of 198 ms for propane– air mixtures at an equivalence ratio of 0.4. The experiments clearly showed negative temperature coefficient (NTC) behavior. Intermediate species concentration profiles were measured quantitatively and recorded versus temperature. The current detailed chemical kinetic mechanism was used to simulate these experiments and the results of this analysis are depicted in Figs. 1–3. The overall reactivity of the system is well reproduced by the mechanism. However, at the lowest temperatures (675–725 K), the model slightly underpredicts the degree of fuel consumption. At temperatures above this, the simulated fuel profile agrees well with the experimentally measured profile. More-

Fig. 2. C3 H8 oxidation, τR = 198 ms, P = 10 atm. (2) CO; ( ) C3 H6 ; ( ) CO2 . Experimental mole fraction (symbols) and model simulations (lines).

Fig. 3. C3 H8 oxidation, τR = 198 ms, P = 10 atm. (2) CH3 CHO; ( ) C2 H4 ; ( ) CH3 OH, ( ) C2 H5 CHO. Experimental fuel mole fraction (symbols) and model simulation (lines).

over, the classical NTC behavior is well reproduced in that in the temperature range 640–740 K there is an increase in reactivity with temperature, and above approximately 740 K, the reactivity of the system slows down, with less fuel being consumed as the temperature increases. Fig. 2 depicts the experimental and model-predicted profiles for carbon monoxide, propene, and carbon dioxide. Carbon monoxide is underpredicted by the model by approximately 20% at 740 K and propene is slightly overpredicted, while carbon dioxide is well reproduced by the model. Fig. 3 depicts the experimental and model-predicted profiles for acetaldehyde, ethylene, methanol, and propanal. These species are well reproduced, with the exception of acetaldehyde, which is overproduced by almost a factor of 3.

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3.2. RCM ignition delay simulations

Table 2 Parameters used in simulating unreactive C3 H8 /Ar mixtures

Rapid compression machine experiments by their nature involve heat losses and it is necessary to account for these in performing model simulations. Therefore, a series of experiments were undertaken whereby oxygen was replaced by nitrogen in the test mixture in order to record pressure profiles for “unreactive” mixtures. It was observed that, because the heat capacity and thermal diffusivities of nitrogen and argon are different, heat losses using various compositions of these diluent gas are also different, with those in pure argon greater than in pure nitrogen. HCT treats the RCM as a homogeneous reactor (zone) with a rigid left boundary and a right boundary that moves at any specified velocity (piston velocity). A positive piston velocity indicates compression and a decrease in volume while a negative velocity corresponds to an expansion process in which the volume increases. HCT was used to simulate all experiments in three phases: (i) the compression phase, (ii) a phase immediately after the end of compression where the greatest drop in pressure was observed in each experiment, and (iii) a phase after this to infinite time with a more constant, and lesser, pressure decrease. Phases (ii) and (iii) were simulated as an adiabatic expansion process, with a greater expansion in phase (ii) compared to phase (iii). This type of simulation was applied by Mittal et al. [56] in simulating their H2 /CO RCM experiments. A series of conditions of initial pressure and temperature were tested using (i) pure argon diluent, (ii) 66% argon/33% N2 diluent, (iii) 33% argon/66% N2 diluent, and (iv) pure nitrogen diluent. It was found that different input parameters to HCT had to be employed in the simulation depending on the diluent composition used, but not on the initial conditions of pressure and temperature. A comprehensive comparison of simulations versus experimental measurements of unreactive mixtures was undertaken and many comparisons of simulations versus experimental results are provided as supplemental material to this paper. After the successful simulation of the pressure profiles of “unreactive” mixtures, the same procedure was applied to reactive mixtures. Although this simulation is physically unrealistic, this treatment allows for heat losses using a zero-dimensional simulation and a more realistic simulation of the experimental conditions within the RCM.

Phase

Zone width (cm)

Piston velocity (cm s−1 )

Time (ms)

(i) (ii) (iii)

9.7 × 10−4 1.0 × 10−4 1.0 × 10−4

5.4375 × 10−2 −5.0 × 10−4 −8.0 × 10−5

0–16 16–55 55+

3.2.1. Simulating mixtures with Ar-only diluent In these simulations the geometric compression ratio was reduced from its measured value of 11.0 to an “effective” value to account for heat loss during compression. For mixtures in which Ar was the only diluent an “effective” compression ratio (CR)

of 9.7 was used. Thus, using the initial experimental temperature and pressure, it was possible to simulate successfully the experimentally measured pressure versus time profiles for unreactive mixtures using the conditions specified in Table 2. For phase (i), with the zone width set at 9.7 × 10−4 cm and a velocity of 5.4375 × 10−2 cm s−1 , the time taken for compression is 16.0 × 10−3 s, resulting in a final zone width of 1.0 × 10−4 cm, and thus a CR of 9.7. For phase (ii), which shows the largest drop in pressure immediately after compression, the zone width was retained at 1.0 × 10−4 cm, but a negative piston velocity was chosen so that the simulated pressure profile reproduced that observed experimentally. In this way we are able to simulate the pressure drop in the RCM as an adiabatic expansion process. It was necessary to simulate this phase from 16 ms (end of compression) to 55 ms. For phase (iii), from 55 ms to infinite time, there is a lower pressure loss and thus a lower heat loss. Therefore, we simulated this phase in like manner to phase (ii), using a zone width of 1.0 × 10−4 cm but with a piston velocity of −8.0 × 10−5 cm s−1 , in order to reproduce the experimental pressure profile. Fig. 4 shows a comparison of the HCT simulation versus experimentally measured pressure profile for an unreactive propane/Ar mixtures at φ = 1.0. 3.2.2. Simulating mixtures with 66% Ar/33% N2 diluent For mixtures containing 66% Ar/33% N2 as diluent, an effective compression ratio of 9.8 was used and together with the conditions specified in Table 3, the experimentally measured pressure versus time profiles for unreactive mixtures were successfully simulated. For phase (i), with the zone width set at 9.8 × 10−4 cm and a velocity of 5.5 × 10−2 cm s−1 , the time taken for compression is 16.0 × 10−3 s, resulting in a final zone width of 1.0 × 10−4 cm, and thus a CR or 9.8. Phases (ii) and (iii) were simulated in like manner to that described for propane/Ar diluent mixtures. However, it was necessary to simulate the second phase from 16 ms (end of compression) to 50 ms, and the third phase from 50 ms to infinite time (taken to be 450 ms). The slightly higher compression ratio

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S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333 Table 5 Parameters used in simulating unreactive C3 H8 /N2 mixtures

Fig. 4. Comparison of experimental (red and black points) versus model prediction (blue line) pressure profiles for unreactive C3 H8 /Ar mixture, “φ = 1.0,” Ti = 290 K, Pi = 0.722 bar. CR = 9.7. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 Parameters used in simulating unreactive C3 H8 /66% Ar/ 33% N2 mixtures Phase

Zone width (cm)

Piston velocity (cm s−1 )

Time (ms)

(i) (ii) (iii)

9.8 × 10−4 1.0 × 10−4 1.0 × 10−4

5.5 × 10−2 −5.0 × 10−4 −8.5 × 10−5

0–16 16–50 50+

Table 4 Parameters used in simulating unreactive C3 H8 /33% Ar/ 66% N2 mixtures Phase

Zone width (cm)

Piston velocity (cm s−1 )

Time (ms)

(i) (ii) (iii)

9.8 × 10−4 1.0 × 10−4 1.0 × 10−4

5.5 × 10−2 −3.5 × 10−4 −7.5 × 10−5

0–16 16–50 50+

(9.8 compared to 9.7 for the Ar-only diluent mixtures) and shorter time in the second phase of the simulation (16–50 ms compared to 16–55 ms) are consistent with lower heat losses using 66% Ar/33% N2 diluent mixture compared to pure argon. 3.2.3. Simulating mixtures with 33% Ar/66% N2 diluent For mixtures containing 33% Ar/66% N2 as diluent, an effective compression ratio (CR) of 9.8 was used with successful simulation of experimentally measured pressure versus time profiles for unreactive mixtures using the conditions specified in Table 4. For phases (i)–(iii) the conditions used in the simulation were almost identical to those for 66% Ar/

Phase

Zone width (cm)

Piston velocity (cm s−1 )

Time (ms)

(i) (ii) (iii)

10.1 × 10−4 1.0 × 10−4 1.0 × 10−4

5.6875 × 10−2 −2.6 × 10−4 −7.2 × 10−5

0–16 16–46 46+

33% N2 diluent mixtures. However, in phases (ii) and (iii) the zone velocities used are slightly lower, corresponding to reduced heat losses due to the presence of the higher concentration of N2 diluent gas. 3.2.4. Simulating mixtures with N2 -only diluent For mixtures with N2 -only diluent, an effective compression ratio (CR) of 10.1 was used with successful simulation of experimentally measured pressure versus time profiles for unreactive mixtures using the conditions specified in Table 5. Phases (ii) and (iii) were simulated in like manner to that described for propane/Ar diluent mixtures. However, it was necessary to simulate the second phase from 16 ms (end of compression) to 46 ms, and the third phase from 46 ms to infinite time (taken to be 450 ms). The higher compression ratio of 10.1 and shorter time in the second phase of the simulation are consistent with the lower heat losses for mixtures with N2 -only diluent. It should be noted from Tables 2–5 that the “effective” compression ratio increases from 9.7 using argon-only diluent, through 9.8 using varying compositions of argon and nitrogen, and increases to 10.1 using nitrogen-only diluent. This is due to lower heat losses in nitrogen diluent compared to argon, which depends on the thermal diffusivities of these species. The effect of diluent gas on heat loss and ignition delay times is the subject of another publication currently in press [57].

4. Thermal boundary layer model When a gas is compressed the temperature distribution in the reaction chamber is not uniform, because a certain amount of gas close to the chamber walls is cooler than the inner regions of the chamber. During the process of compression this cooler gas, the thermal boundary layer, mixes with the hotter core gas. The roll up of the thermal boundary layer during compression, and the vortex that arises, results in a non-uniform temperature distribution in the reaction chamber.

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In an attempt to counter this effect Lee and Hochgreb [40–42] developed a piston head with a crevice large enough to accommodate the complete volume of the thermal boundary layer that is created during the compression process. This suppresses the formation of the corner vortex and offers uniform temperature conditions that can be expressed in the form of a core region and a thermal boundary layer. With these basic foundations an application, RCMBL, was developed that included a more complete kinetic treatment of the reactions taking place in a RCM. Its assumptions and the equations on which it is based are described by Lee and Hochgreb [41]. This approach was the first that included an accurate treatment of the compression process and the subsequent ignition of a hydrogen mixture. It was implemented as a module of the well-known CHEMKIN [58] application. The program units that describe the process of compression perform the calculation of the pressure and temperature profile and the development of the thermal boundary layer. Additionally, heat transfer is controlled by heat conduction from the core gases to the wall. The application was generalized by Musch and Simmie [59] to enable the modeling of any combination of fuels and diluents with known thermodynamic and transport data, since the original application developed by Lee and Hochgreb was hard-coded for hydrogen fuel only. 4.1. Validation The RCMBL application requires the input of the physical geometry of the RCM as well as the initial conditions of pressure, temperature, and mixture composition. The most important parameters were found to be (i) the distance between the pistons before and after compression, i.e., the compression ratio, (ii) the volume of the piston crevice, and (iii) the piston radius. These parameters were measured for the NUI Galway rapid compression machine and the RCMBL application was tested against an experimentally measured pressure profile of pure argon. RCMBL did not perform well, overpredicting the compressed gas pressure. However, this was probably due to the chamber having some dead volume, mainly where the ports break through into the chamber, which cannot be accounted for, since the simulation assumes cylindrical symmetry. To counter this, the values for the crevice volume and the stroke length were altered until good agreement with the argon pressure profile was achieved. Once RCMBL reproduced the compressed gas pressure well, the pressure drop due to heat loss was examined. This required the adjustment of the heat

323

Fig. 5. Comparison between experiment and simulation results for an unreactive propane mixture, 4.03% C3 H8 , 40.42% N2 , 50.55% Ar. Pi = 1.6 bar, Ti = 290 K. Black line: experiment, red line: simulation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

loss coefficient until satisfactory agreement with experiment was obtained, once again for pure argon. With the initial parameters determined, the model was tested over a range of Ar/N2 unreactive mixtures, the results of which are given in the supplementary material. In all cases the agreement between simulated and measured compressed gas pressures is good and the rate of heat loss is well predicted. All of the measurements were taken at an initial temperature of approximately 293 K and at an initial pressure of 500 millibar. The performance of the code was also compared with unreactive propane mixtures. Once again the model shows good agreement with experiment, including matching a compressed gas pressure of over 20 bar, Fig. 5. The propane mixtures were measured over a longer timescale than the N2 /Ar examples, so it can be seen that the model accurately predicts the heat loss over a long timescale (>400 ms). It should be noted that heat losses in our rapid compression machine are specific to our machine. Thus, the procedures described here in using either the HCT code or the RCMBL model can be applied to other similar RCM facilities, but heat loss parameters have to be adjusted using unreactive fuel mixture profiles measured in them.

5. Results A compressed gas temperature range of 700– 970 K was studied for each mixture at a compressed gas pressure of 21, 27, and 37 atm and at an equivalence ratio of 0.5. Fig. 6 shows the plot of the logarithm of the ignition delay time, τ , versus the

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S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333 Table 6 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 20 atm

Fig. 6. Effect of pressure on ignition delay times during propane oxidation in “air,” φ = 0.5. (2) 21 atm, ( ) 27 atm, ( ) 37 atm. Solid lines: HCT simulations; dashed lines: simulations using RCMBL.

reciprocal of the compressed gas temperature for the various compressed gas pressure regimes. Ignition limits for each of the three conditions vary, though a significant difference may be observed between the 21- and 27-atm data. At 37 atm, ignition is observed at 745 K; 28.5 atm at 750 K; while this temperature limit rises dramatically to 915 K in the case of the 21.5-atmosphere data. However, analysis of the data in the higher temperature region show that the three sets of data do converge so that their ignition delay times become almost identical at 950 K. The negative temperature coefficient (NTC) region does not present itself in the lower pressure experiments, whereas the 27- and 37-atm data show very similar trends in ignition delay times. The NTC region in the 30-atm plot appears to show an increase in ignition delay with increasing temperature. However, the delay times through the NTC for the 37-atm data remain constant with increasing temperature. 5.1. Effect of pressure on ignition 5.1.1. 21 atm The minimum temperature at which ignition was observed following compression to 20 atm was found to be approximately 850 K. Therefore the range of temperatures available for examination was limited (due to the limit on the higher achievable temperatures), so that a total of only 28 experiments were performed under these conditions, Table 6. The compressed gas pressure over the range of these experiments was 21 ± 1 atm. Reproducibility was quite good, particularly at shorter delay times. No NTC region was observed experimentally, Fig. 6, over the temperature range analyzed. This may be explained by comparison to the higher pressure results, which

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79 0.79

293 293 300 310 312 314 314 317 317 317 319 319 319 319 319 319 322 322 322 322 324 324 329 329 341 341 354 354

20.2 20.2 22.0 21.7 21.7 22.0 22.0 20.2 20.8 20.5 21.4 21.7 21.7 19.9 19.9 20.2 21.4 21.7 21.7 21.7 22.0 22.0 21.7 21.7 21.7 21.7 21.7 21.7

800 800 829 851 855 863 863 850 857 854 869 872 872 852 852 855 876 879 879 879 888 888 896 896 925 925 956 956

No ign. No ign. No ign. No ign. 400.0 325.0 353.0 287.0 262.0 239.0 191.0 183.0 218.0 185.0 169.0 152.0 117.6 120.8 124.0 126.0 92.4 91.2 47.2 49.2 20.8 20.8 7.1 6.9

Note. Nitrogen–argon (1:2) diluent, φ = 0.5.

show the NTC region in the range of 730–850 K, that is, below the lowest temperature at which this mixture composition ignites at this pressure. The range studied does show a positive temperature dependence: the ignition delay times decrease with increasing temperature. However, it is almost certain that, had experiments been performed at lower temperatures, we would have again been able to record ignition times as the system fell into a region of higher reactivity. This is borne out by the comparison of the model predictions with the experimental data. Calculations using both the HCT simulation method and RCMBL predict that ignition is observable at lower temperatures. Simulations performed using HCT are in very good agreement with the experimental results, but the model does predict ignition delay times shorter than those measured experimentally in the temperature range 850–900 K. Results using the boundary layer model over the same temperature range are faster than those predicted using the HCT model.

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325

Table 7 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm

Table 8 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.20 1.25 1.25 1.30 1.24 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25

313 317 317 317 317 320 320 320 320 320 326 326 329 329

27.3 27.4 28.5 28.5 27.9 29.7 29.7 29.7 29.7 29.7 30.2 30.0 27.3 27.6

713 714 721 714 719 735 735 735 735 735 751 749 738 740

400.0 218.0 193.0 200.0 203.0 169.0 160.0 149.0 145.0 142.0 107.0 103.2 100.0 100.0

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

293 293 293 295 295 296 296 296 296 300 300 300 300 300 307 307 307

25.4 25.4 25.4 26.5 26.2 26.5 26.5 26.2 26.2 27.4 27.4 27.4 25.9 25.9 27.0 26.7 27.0

791 791 791 806 803 808 808 806 806 826 826 826 812 812 806 807 806

152.4 152.5 152.4 149.0 148.0 169.0 166.0 164.0 165.0 187.6 183.2 184.8 175.0 174.0 59.0 98.0 97.0

Note. Mixtures prepared with nitrogen only as diluent, φ = 0.5.

5.1.2. 27 atm Due to a slightly more pronounced NTC region, 49 experiments were performed at a compressed gas pressure of 27 ± 1.5 atm, φ = 0.5, Tables 7–9. The minimum temperature at which ignition was observed was found to be 750 K, where a delay of 400 ms was observed. Within the temperature range studied, most of the delay times were found to be greater than 100 ms, therefore each experiment was repeated up to six times to ensure reliable results. For ignition delay times of greater than 100 ms, agreement was found to be within 6–8 ms, while for shorter delay times, that is, <100 ms, agreement was found to be within 1 ms. The shape of the ignition delay time versus temperature profile, Fig. 6, for propane oxidation at a compressed gas pressure of 27 atm is very similar to that obtained at 37 atm. Within the compressed gas temperature range of 730 to 750 K the ignition delay times decrease with increasing temperature. An NTC region is thus observed between 750 and 830 K, inside which the ignition delay times increase with increasing temperature. Positive temperature dependence is reestablished at compressed gas temperatures above 830 K. Fig. 7 shows the reproducibility obtained at a compressed gas pressure of 27 atm; the delay times were 75.2 and 76.4 ms. Simulations performed using HCT are in good agreement with the experimental results at temperatures above approximately 900 K, but the model predicts shorter ignition delay times compared to experiment from 700–900 K. In addition, the HCT model predicts reactivity to begin just below 700 K, which is lower in temperature than observed in the experiment at 713 K. Results using the boundary layer model

Note. Nitrogen–argon (1:2) diluent, φ = 0.5.

Table 9 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.00 1.00 1.00 1.00 1.00 1.00 1.15 1.15 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

310 310 310 310 310 310 315 315 320 320 320 329 329 341 341 354 354 354 354

27.1 27.1 27.1 27.4 27.4 27.4 27.2 27.4 27.1 27.1 27.1 26.5 27.1 27.1 27.1 25.9 27.1 27.4 27.4

847 847 847 850 850 850 860 862 871 871 871 887 893 922 922 940 952 955 955

140.4 140.4 146.8 148.8 141.2 142.0 106.8 106.8 75.2 76.4 75.6 36.5 7.6 14.2 14.2 4.7 5.2 4.8 5.0

Note. Nitrogen–argon (1:2) diluent, φ = 0.5.

are qualitatively similar to those calculated using the HCT model over the entire temperature range but are slightly faster at lower temperatures (700 K), slower from 720 to 800 K, and then faster than HCT from 800 to 900 K. At temperatures above this the model predictions essentially converge.

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S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333 Table 10 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 38 atm

Fig. 7. Propane φ = 0.5, mix No. 2; TC = 871 K, PC = 27.1 atm.

5.1.3. 37 atm Forty experiments were carried out on the lean propane mixtures, φ = 0.5, reaching a compressed gas pressure of 37 ± 1 atm and 690  TC  944 K, with ignition delays ranging from 3 to 270 ms. Experiments were performed at least twice for each temperature, ensuring reproducibility. At the longer ignition delay times, that is, greater than 100 ms, heat losses become a significant factor, such that reproducibility of within 8 ms was observed. At delay times in the range of 50–100 ms, good reproducibility was found within 2 ms. As the delay dropped, with increasing temperature, below 50 ms, the reproducibility was found to be within 1 ms. Conditions and results for this set of experiments are documented in Tables 10 and 11. The ignition limit at this pressure was observed at 715 K; below this temperature ignition did not occur. Over the following 40 K the ignition followed positive temperature dependence; that is, as the temperature increased, the ignition delay time decreased. However, between 750 and 840 K, a negative temperature coefficient region was observed. Within this temperature regime the ignition delay remained constant as the temperature increased, as shown in Fig. 6. Once the temperature reached 840 K, the ignition delay time again decreased with increasing temperature. Simulations performed using HCT and RCMBL are in good agreement with the experimental results in the temperature range 800–900 K, Fig. 6. At higher temperatures both codes predict ignition delay times that are slower than those measured experimentally. Similarly to the comparison of the mechanisms with the data recorded at 30 atm, reactivity is predicted to begin just below 700 K, which is lower than the temperature of 713 K observed experimentally. Results using RCMBL are qualitatively very similar to those calculated using HCT over the entire tempera-

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6

296 296 310 310 314 314 314 317 317 317 324 324 324 326 326 326 341 341 342 354 354

38.4 38.4 38.1 38.1 36.7 37.0 37.0 37.0 36.6 36.7 36.7 36.4 36.4 38.1 38.4 38.4 38.7 38.1 38.1 37.0 37.0

689 689 717 717 719 720 720 726 725 725 739 738 738 750 752 752 784 781 783 801 801

No ign. No ign. 270.0 263.0 145.0 139.4 131.9 96.8 108.4 98.8 66.6 66.5 62.5 61.0 59.0 56.8 47.7 47.3 48.0 43.0 42.6

Note. Nitrogen-only diluent, φ = 0.5. Table 11 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 40 atm Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

293 293 293 310 310 310 320 320 320 329 329 329 341 341 341 341 354 354 354 354

37.4 37.4 37.4 37.9 37.4 37.4 37.4 37.4 37.4 37.4 36.8 35.6 36.8 36.8 36.8 36.8 37.9 36.2 36.8 36.8

805 805 805 847 843 843 868 868 868 889 885 877 914 914 914 914 952 940 944 944

49.2 47.2 47.2 50.0 48.8 47.6 34.8 31.8 31.8 12.8 17.8 18.7 8.1 6.0 7.5 7.7 2.4 2.8 3.0 2.5

Note. Nitrogen–argon (1:2) diluent, φ = 0.5.

ture range but are slightly faster at lower temperatures (700 K), slower from 730 to 810 K, and thereafter slightly faster than HCT from 810 to 900 K. At tem-

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327

Table 12 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm

Fig. 8. Effect of equivalence ratio on ignition delay time for propane oxidation in “air,” PC ≈ 30 atm. (2) φ = 0.5; ( ) φ = 1.0; ( ) φ = 2.0. Solid lines: HCT simulations, dashed lines: RCMBL simulations.

peratures above this the model predictions again converge. Overall, the qualitative agreement between the models and experiment is excellent, with the negative temperature coefficient behavior being well reproduced. In addition, the magnitude of the effect of pressure on ignition delay times is well reproduced by the models. However, the models are overall faster than experiment at lower temperatures (700–900 K) and are slower than experiment at temperatures above 900 K. 5.2. Equivalence ratio effect The effect of equivalence ratio on ignition delay times for three propane mixtures, Table 1, was examined at a compressed gas pressure of approximately 30 atm, Fig. 8. 5.2.1. Stoichiometric mixture A total of 37 experiments were carried out on the stoichiometric mixture, φ = 1.0, reaching a compressed gas pressure of 28.5 ± 1.5 atm, Tables 12–14. The temperature range studied spanned from 652 to 955 K. The minimum temperature for ignition was observed at 710 K. The majority of the measured ignition delay times were found to be less then 100 ms and were repeatable to within 1 ms. As the temperature increased between 710 and 760 K, the ignition delays decreased. An NTC region was observed, similarly to the lean mixture, between approximately 760 and 850 K. This NTC behavior is less pronounced than that of the fuel-lean mixture, with ignition delay times increasing very slightly with increasing temperature. Above 850 K, ignition delay times decrease as the temperature rises.

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.28 1.28 1.28 1.28 1.28 1.28 1.28 1.28 1.28 1.28 1.28 1.28

293 310 323 323 323 335 335 335 335 354 354 354

29.7 28.6 29.7 29.1 29.7 29.7 29.7 29.1 29.7 29.1 29.1 29.1

652 679 710 707 710 733 733 730 733 766 766 966

No ign. No ign. 160.0 144.8 176.8 59.2 61.2 60.8 60.4 37.2 36.4 38.4

Note. Nitrogen-only diluent, φ = 1.0. Table 13 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08 1.08

293 293 310 310 310 317 317 323 323 329 329 341 341

28.4 28.4 28.4 28.4 28.4 27.1 27.1 27.8 27.8 27.1 27.1 26.6 26.6

757 757 794 794 794 800 800 818 818 826 826 847 847

78.4 78.4 38.0 38.0 38.4 42.8 42.8 48.8 48.8 52.4 52.0 34.4 34.4

Note. Nitrogen–argon (1:2) diluent, φ = 1.0. Table 14 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02 1.02

310 310 310 323 323 323 335 335 335 354 354 354

27.7 27.7 27.7 26.6 27.1 27.1 27.1 27.1 27.7 27.1 26.6 26.6

863 863 863 883 889 889 917 917 922 961 955 955

38.0 37.4 38.0 15.6 16.0 16.4 6.6 6.8 6.4 1.7 1.8 1.9

Note. Argon-only diluent, φ = 1.0.

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Table 15 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm

Table 16 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 30 atm

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.258 1.263 1.263 1.263 1.263 1.263 1.263

300 303 307 309 309 309 310 310 316 317 317 317 328 328 328 338 338 338

28.5 28.5 28.5 27.7 27.7 27.7 28.6 28.6 28.5 28.4 28.4 28.4 25.1 25.1 25.1 27.4 27.9 27.9

685 691 698 698 698 698 705 705 715 716 716 716 715 715 715 748 751 751

No ign. No ign. No ign. 318.0 229.0 221.0 123.0 117.0 77.0 60.4 61.2 60.0 27.6 28.4 27.6 18.8 19.2 19.2

1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.174 1.184 1.184 1.184 1.184 1.184 1.184 1.204 1.234 1.234

293 293 293 293 293 303 303 303 314 314 314 323 323 323 335 335 335 335 354 354 354

29.0 28.4 28.4 28.4 28.4 28.4 28.4 28.4 27.9 28.4 28.4 27.9 28.4 28.4 28.4 28.4 28.4 27.9 27.9 28.5 28.5

725 722 722 722 722 741 741 741 759 763 763 777 779 779 802 802 802 798 831 831 831

99.6 98.0 117.6 103.2 110.4 32.4 31.6 32.8 22.0 21.6 22.0 19.6 19.2 19.0 18.4 18.8 19.8 19.8 16.2 14.8 15.0

Note. Nitrogen–argon (1:2) diluent, φ = 2.0.

5.2.2. Fuel-rich mixture The minimum temperature at which ignition was observed for the fuel-rich, φ = 2.0, propane mixture was 698 K. Thirty-nine experiments were carried out over the temperature range 685–830 K, achieving a compressed gas pressure of 28 ± 1 atm, Tables 15 and 16. The NTC region spans from 750 K to approximately 800 K. Unfortunately, due to the increase in heat capacity of the fuel-rich mixture (this is caused by the increased volume of propane), the maximum achievable temperature under these condition was 830 K. Within this regime of temperature and pressure the richer mixture, that is φ = 2.0, was observed to be most reactive, with reactivity decreasing with equivalence ratio, Fig. 8. This is as expected in the low-temperature regime, as the chain-branching mechanism depends on the fuel concentration, and the higher the fuel concentration the more reactive the fuel. An NTC region is observed in each case between approximately 740–850 K. However, the leaner mixture shows more pronounced NTC behavior. Under fuel-rich conditions the ignition delay remain considerably constant over the NTC range. It has been shown therefore that within this regime of temperature and pressure, increasing the fuel concentration, while the oxygen content remained relatively constant, leads to a decrease in ignition delay times. Overall, the qualitative agreement between the simulations and experiment is excellent, with the negative temperature coefficient behavior being well re-

Note. Argon-only diluent, φ = 2.0.

produced. In addition, the magnitude of the effect of equivalence ratio on ignition delay times is well reproduced by the models. However, the models are overall faster than experiment at lower temperatures (700–750 K) and in good agreement with experiment, particularly under stoichiometric and rich conditions for temperatures above 750 K, Fig. 8. 5.3. Fuel-rich, high-pressure data The effect of pressure on fuel-rich, φ = 2.0, propane mixtures was examined. Fig. 9, shows that the general trends for the 28- and the 38-atm data (see Tables 17 and 18) is quite similar. However, the higher-pressure delays are shorter than those observed at 30 atm, though not substantially. This shows that the effect of increasing the final pressure by 10 atm does not make a significant difference to the delay times and the trends observed. Overall, the qualitative agreement between the HCT model and experiment is excellent; the model predicts the reactivity to start at 700 K which coincides with the experimental results. The reactivity increases with temperature to 770 K, and thereafter there is a decrease in reactivity that is well reproduced by the model. However, the model is faster than experiment throughout the range of temperature measured at both 28 and 38 atm.

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329

Table 18 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 40 atm

Fig. 9. Effect of pressure on propane-rich, φ = 2.0 in “air,” ignition delay times. (2) 28 atm, ( ) 38 atm. Solid lines: HCT simulations. Table 17 Experimental conditions and results for propane oxidation in RCM, reaching compressed gas pressure of 40 atm Pi (atm)

Ti (K)

PC (atm)

TC (K)

τ (ms)

1.65 1.65 1.66 1.66 1.66 1.67 1.67 1.67 1.67 1.68 1.68 1.68 1.68 1.68 1.68 1.69 1.69 1.69

296 296 302 302 302 307 307 310 310 314 314 314 317 317 317 323 323 323

39.3 38.7 38.7 38.7 38.7 38.7 39.3 38.7 39.3 38.7 39.3 38.7 39.3 39.3 39.3 38.8 38.8 39.3

686 683 694 694 694 702 702 708 710 714 717 714 723 723 723 730 730 733

395.0 400.0 165.0 163.0 161.0 104.0 100.0 63.2 62.8 49.2 48.0 47.6 31.6 33.6 33.2 20.8 21.6 21.2

Note. Nitrogen–argon (1:2) diluent, φ = 2.0.

6. Comparison with literature data Cadman et al. [22], Herzler et al. [23], and Lamnaouer et al. [25] all measured ignition delay times for propane oxidation for a fuel-lean (φ = 0.5) mixture in an overlapping temperature range of 750–1300 K and at reflected shock pressures of 5 to 40 bar. The results obtained by Lamnaouer et al. were in good agreement with those reported by Herzler et al. at 30 atm and were in very good agreement with the measurements of Cadman et al. at 20 and 40 atm. However, Cadman et al. only presented three experimental points for propane oxidation at 40 bar and therefore trends, such as the NTC region, were not observed in their study. The experiments confirmed

Pi (atm)

Ti (K)

PC (atm)

TC (K)

1.55 1.55 1.55 1.48 1.50 1.55 1.55 1.55 1.57 1.57 1.57 1.55 1.57 1.59 1.59 1.59

303 303 303 310 310 317 317 317 329 329 329 335 335 341 341 341

36.3 36.3 36.9 34.5 35.1 35.8 36.3 36.3 36.4 36.4 36.4 35.8 35.8 36.4 36.4 36.4

735 735 738 748 749 760 763 763 784 784 784 795 792 804 804 804

τ (ms) 28.8 28.8 27.6 18.4 17.2 12.6 12.2 12.0 9.2 9.0 8.8 10.2 10.0 9.6 9.6 8.8

Note. Argon-only diluent, φ = 2.0.

that the activation energy of the ignition delay time decreases at around 1050 K and a linear extrapolation from high to intermediate/low temperature is not possible. Fig. 10 shows the comparison between the ignition delay times of Cadman et al., Herzler et al., and Lamnaouer et al. in comparison to the present results. The results obtained in the shock tube disagree considerably with the present results, varying by almost two orders of magnitude. As may be seen from Fig. 10, within the temperature range of 750–1000 K, the variation in ignition delay time with temperature follows a similar trend for both the present experiments and the predicted results. Moreover, considering the HCT simulations which include heat losses and those assuming adiabatic conditions, Fig. 10 does not account for this difference in measured ignition delay times. It is unclear as to why there is such a large difference. Both simulations, including and ignoring heat losses, tend to suggest that the RCM experiments are closer to reality. It is well known that shock tubes struggle to provide test times in excess of a few milliseconds. Normally special procedures are required to extend observational times but it is quite difficult to do without incurring additional nonideal effects [60]. It can be argued that our kinetic mechanism is not accurate in favoring the RCM data over those obtained in shock waves. However, significant alterations are required in order to improve agreement with the shock tube experiments. For example, agreement can be forced by simultaneously multiplying (i) all propyl alkyl radical and (ii) hydroperoxyl alkyl

330

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Fig. 10. Comparison of ignition delay times versus temperature for propane oxidation in “air” at φ = 0.5. (2) Cadman 20 atm, ( ) Cadman 40 atm, ( ) present RCM data: 30 atm, ( ) Herzler 30 atm in Ar, ( ) Herzler 30 atm in N2 , ( ) Lamnaouer et al. 30 atm in N2 . (—) HCT model prediction, (- - -) HCT model prediction assuming adiabatic conditions, (- · -) HCT model modified to fit shock tube data.

(QOOH) additions to molecular oxygen by a factor of 2, (iii) all alkyl peroxy (RO2 ) to hydroperoxyl alkyl radical (QOOH) isomerizations by a factor of 4, (iv) all peroxy alkylhydroperoxide radical (O2 QOOH) isomerizations to carbonyl hydroperoxide (keto) + hydroxyl radicals by a factor of 4, and (v) all carbonyl hydroperoxide (keto) species decompositions to a hydroxyl radical and another carbonyl alkoxy radical by a factor of 4; see the dot-dashed line in Fig. 10. However, we believe that such changes are unwarranted, since using this amended mechanism to simulate the flow reactor experiments of Koert et al. [13] results in very poor agreement. Throughout the entire temperature range (650–820 K), all fuel is predicted to be consumed, with the exhaust reactor temperature predicted to be between 1593 and 1742 K, which is a gross overprediction of reactivity and is not justified by the experimental results in the flow reactor. In a further attempt to explore the large differences in propane reactivity and to test whether the same discrepancies occur with other fuels, we have compared ignition delay times for n-heptane oxidation recorded in our RCM [61,62] with those of Griffiths et al. [63,64] in the Leeds RCM and with shock tube ignition delay times recorded by Ciezki and Adomeit [65], Fig. 11. All data were recorded for stoichiometric (1.87%) n-heptane in “air” mixtures, with both sets of RCM data taken at compressed gas pressures of 10 atm, while the shock tube data were measured at a reflected shock pressure of 13.5 atm. It can be seen that all data are in very good agreement with each other. In particular, our data are in almost perfect agreement with the shock tube data. We have used

Fig. 11. Comparison of ignition delay times versus temperature for n-heptane oxidation at φ = 1.0 in “air,” P ≈ 10 atm. (2) NUIG RCM data [61,62], (!) Leeds RCM data [63,64], (×) Ciezki and Adomeit [65] shock tube data.

identical procedures to record both our n-heptane data and the propane data presented in this study. Recently, syngas measurements at elevated pressure and low temperature showed the same discrepancy, in which chemical models predicted ignition delay times that were orders of magnitude longer than those measured experimentally. Petersen et al. [66] recorded shock tube ignition delay data for the oxidation of fuel-lean (φ = 0.5) syngas/air mixtures at low temperatures (940–1150 K) and at elevated pressures (18.7–29 atm) and refer to work published by Peschke and Spadaccini [67] in a continuous-flow reactor under similar conditions of pressure and concentration but in the temperature range 633–771 K. Petersen et al. used five chemical kinetic models (four of which were published in the past two years) containing full CO/H2 chemistry to simulate all of the above syngas data. These models include Davis et al. [68], RAMEC (based on GRI-Mech) [69], Saxena and Williams (San Diego Mechanism) [70], Sun et al. [71], and Li et al. [72]. All simulations showed a clear and disturbing disagreement between experiment and model, with the model simulations two to three orders of magnitude slower than the experimental data in the temperature range 700–800 K. Even under the shock-tube conditions near 950 K, the model predicts ignition delay times an order of magnitude higher than the data. Petersen et al. [66] also state that recent shock-tube kinetics experiments by Sivaramakrishnan et al. [73] for CO/H2 mixtures at pressures up to 450 atm and temperatures between 1000 and 1500 K show reasonable agreement with current models, suggesting that the differences observed may be due more to the lower temperatures than the elevated pressures. The disagreement presented by Petersen et al. for syngas mixtures and the disparity between the experimental results for propane presented here may

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or may not be related to the same problem with experiments and/or simulations. Currently we have no plausible explanation for the observed discrepancy in RCM and shock tube ignition times. However, research is ongoing and this problem is the subject of another publication presented at the recent International Symposium on Shock Waves [25]. It is shown in this paper that, for CH4 /C2 H6 mixtures at lower temperatures, experimental data are faster than chemical kinetic model predictions. However, the experimental data showed that there were in some cases, large pre-ignition pressure rises. As mentioned by Fieweger et al. [74], when such a pressure increase occurs, the ignition delay data can be made to agree with the zero-dimensional kinetics by taking into account the corresponding temperature rise due to the compression. When such an adjustment was made, the corrected data showed very good agreement with the chemical kinetic model. However, the cause of the large preignition pressure rises remains unresolved. It should be noted that Griffiths et al. [63] studied the oxidation of a series of n-alkane fuels, n-butane, n-pentane, n-hexane, and n-heptane, for stoichiometric fuel-in-“air” mixtures at a compressed gas pressure between 7.5 and 9.0 bar. We believe that the RCM ignition delay times reported here are consistent with the measurements of Griffiths et al., who showed in Fig. 1 of their paper that n-butane is significantly slower to react than the other larger n-alkanes, with ignition delay times in the range of 45–65 ms throughout the NTC region (750  T  850 K) and as long as 160 ms at 700 K. Griffiths et al. did not present any data for propane. However, we would speculate that, because propane has a higher octane number than n-butane, it is even less reactive than butane. Moreover, even taking differences in pressure (20 atm for some of the Cadman et al. propane data versus 9 atm for the Griffiths et al. butane data) and equivalence ratio (φ = 0.5 for the propane data and φ = 1.0 for the butane data) into account, both of these factors should be almost self-compensating in terms of ignition delay measurements, one would anticipate ignition delay times longer than 45–65 ms for propane oxidation in the temperature range 750–850 K presented in Fig. 10, consistent with the RCM data. It is still uncertain how to resolve this disagreement. The chemical kinetic mechanism reproduces the flow reactor data of Koert et al. [14] and the RCM data presented here quite accurately. All the experimental shock tube data that are not well reproduced are at low temperatures. It is possible that there are dynamic process in these low-temperature shock tube experiments that are not adequately captured by the assumption of constant-volume ignition made in the modeling simulations. Further examination of the conditions of these low-temperature shocks is war-

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ranted to resolve these differences so that the correct physical conditions can be used in simulating these experiments.

7. Conclusions The oxidation of propane has been studied in the temperature range 680–970 K, at compressed gas pressures of 21, 27, and 37 atm, and at varying equivalence ratios of 0.5, 1.0, and 2.0. These data are consistent with other experimental data presented in the literature for alkane fuels in that, when ignition delay times are plotted as a function of temperature, a characteristic negative coefficient behavior is observed. In addition, these data were simulated using a detailed chemical kinetic model. It was found that qualitatively the model correctly simulated the effect of change in equivalence ratio and pressure, predicting that fuel-rich, high-pressure mixtures ignite fastest, while fuel-lean, low-pressure mixtures ignite slowest. In addition, the reactivity as a function of temperature is well captured, with the model predicting negative temperature coefficient behavior similar to the experiments. Quantitatively the model is faster than experiment for all mixtures at the lowest temperatures (650–750 K) and is also faster than experiment throughout the entire temperature range for fuel-lean mixtures. One important outcome of the current work is that the ignition delay times recorded here in the rapid compression machine are almost two orders of magnitude longer than those reported by Cadman et al. [22], Herzler et al. [23] and Lamnaouer et al. [25]. Further analysis needs to be carried out to determine the source of this disagreement in experimental results.

Acknowledgments We thank Dr. Eric Petersen and his group at the University of Central Florida for allowing us to publish their shock tube ignition delay times prior to publication. We also thank Dr. Charles Westbrook and Dr. William Pitz at Lawrence Livermore National Laboratory for helpful discussions.

References [1] K. Owen, T. Coley, Automotive Fuels Reference Book, second ed., Soc. Automot. Eng., Warrendale, PA, 1995. [2] J.M. Simmie, Prog. Energy Combust. Sci. 29 (2003) 599–634. [3] D.M. Newitt, L.S. Thornes, J. Chem. Soc. (Resumed) (1937) 1656–1665.

332

S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333

[4] D.M. Newitt, W.G. Schmidt, J. Chem. Soc. (Resumed) (1937) 1665–1669. [5] D.M. Newitt, L.S. Thornes, J. Chem. Soc. (Resumed) (1937) 1669–1676. [6] R.N. Pease, J. Am. Chem. Soc. 60 (1938) 2244–2246. [7] A. Burcat, A. Lifshitz, K. Scheller, G.B. Skinner, Proc. Combust. Inst. 13 (1971) 745–755. [8] A. Burcat, A. Lifshitz, K. Scheller, G.B. Skinner, Combust. Flame 19 (1971) 29–33. [9] M. Cathonnet, J.C. Boettner, H. James, Proc. Combust. Inst. 18 (1981) 903–913. [10] J.S. Hoffman, W. Lee, T.A. Litzinger, D.A. Santavicca, W.J. Pitz, Combust. Sci. Technol. 77 (1991) 95–125. [11] C.K. Westbrook, W.J. Pitz, Combust. Sci. Technol. 37 (1984) 117–152. [12] P. Dagaut, M. Cathonnet, J.C. Boettner, Int. J. Chem. Kinet. 24 (1992) 813–837. [13] D.N. Koert, D.L. Miller, N.P. Cernansky, Combust. Flame 96 (1994) 34–49. [14] D.N. Koert, W.J. Pitz, J.W. Bozzelli, N.P. Cernansky, Proc. Combust. Inst. 26 (1996) 633–640. [15] E. Ranzi, T. Faravelli, P. Gaffuri, G.C. Pennati, A. Sogaro, Combust. Sci. Technol. 100 (1994) 299–330. [16] Z. Qin, V.V. Lissianski, H. Yang, W.C. Gardiner, S.G. Davis, H. Wang, Proc. Combust. Inst. 28 (2000) 1663– 1669. [17] C.M. Vagelopoulos, F.N. Egolfopoulos, C.K. Law, Proc. Combust. Inst. 25 (1994) 1341–1347. [18] C.M. Vagelopoulos, F.N. Egolfopoulos, Proc. Combust. Inst. 27 (1998) 513–519. [19] H.J. Curran, J.M. Simmie, P. Dagaut, D. Voisin, M. Cathonnet, Proc. Combust. Inst. 26 (1996) 613–620. [20] Z. Qin, H. Yang, W.C. Gardiner, Combust. Flame 124 (2001) 246–254. [21] Z. Qin, Ph.D. dissertation, University of Texas at Austin, USA, 1998. [22] P. Cadman, G. Thomas, P. Butler, Phys. Chem. Chem. Phys. 2 (2000) 5411–5419. [23] J. Herzler, L. Jerig, P. Roth, Combust. Sci. Technol. 176 (10) (2004) 1627–1637. [24] V.P. Zhukov, V.A. Sechenov, A.Yu. Starikovskii, Kinet. Katal. 46 (3) (2005) 319–327. [25] M. Lamnaouer, J. de Vries, E. Petersen, H.J. Curran, J.M. Simmie, M. Fikri, C. Schulz, G. Bourque, in: Int. Symp. Shock Waves, Göttingen, Germany, July 15–20, 2007. [26] K. Kim, K.S. Shin, Bull. Kor. Chem. Soc. 22 (2001) 303–307. [27] D.F. Davidson, J.T. Herbon, D.C. Horning, R.K. Hanson, Int. J. Chem. Kinet. 33 (2001) 775–783. [28] D.C. Horning, D.F. Davidson, R.K. Hanson, J. Propuls. Power 18 (2) (2002) 363–371. [29] D.C. Horning, D.F. Davidson, R.K. Hanson, in: Int. Symp. Shock Waves, Penticton, British Columbia, September 29–October 4, 2002, paper 5732. [30] N. Lamoureux, C.-E. Paillard, V. Vaslier, Shock Waves 11 (2002) 309–322. [31] A.A. Levitsky, S.S. Polyak, V.Y. Shtern, Int. J. Chem. Kinet. 16 (1984) 1275–1285. [32] G. Freeman, A.H. Lefebvre, Combust. Flame 58 (1984) 153–162.

[33] Y. Hidaka, A. Ikoma, H. Kawano, M. Suga, Int. J. Mass Spectrosc. Ion Phys. 48 (1983) 71–74. [34] A.A. Mantashyan, P.S. Gukasyan, R.H. Sayadyan, React. Kinet. Catal. Lett. 11 (1979) 225–228. [35] R.D. Wilk, N.P. Cernansky, R.S. Cohen, Combust. Sci. Technol. 49 (1986) 41–78. [36] J. Würmel, J.M. Simmie, H.J. Curran, Int. J. Vehicle Design 44 (1/2) (2007) 84–106. [37] W.S. Affleck, A. Thomas, Proc. Inst. Mech. Eng. 183 (1) (1968–1969) 365–385. [38] L. Brett, Ph.D. thesis, National University of Ireland, Galway, 1999. [39] L. Brett, J. MacNamara, P. Musch, J.M. Simmie, Combust. Flame 124 (2001) 326–329. [40] D. Lee, Ph.D. thesis, Massachussetts Institute of Technology, Cambridge, MA, USA, 1997. [41] D. Lee, S. Hochgreb, Combust. Flame 114 (1998) 531– 545. [42] D. Lee, S. Hochgreb, Int. J. Chem. Kinet. 30 (1998) 385–406. [43] J. Würmel, Ph.D. thesis, National University of Ireland, Galway, 2004. [44] C. Morley, Available from http://www.c.morley. ukgateway.net/gseqrite.htm. [45] X.J. Chien, K. Wakai, S. Takahashi, Meas. Sci. Technol. 16 (3) (2005) 707–715. [46] P. Desgroux, L. Gasnot, L.R. Sochet, Appl. Phys. B 61 (1995) 69–72. [47] J.F. Griffiths, J.P. MacNamara, C.G.W. Sheppard, D.A. Turton, B.J. Whitaker, Fuel 81 (2002) 2219–2225. [48] C.M. Lund, L. Chase, HCT—A general computer program for calculating time-dependent phenomena involving one-dimensional hydrodynamics, transport, and detailed chemical kinetics, Lawrence Livermore National Laboratory Report UCRL-52504, revised, 1995. [49] C.K. Westbrook, F.L. Dryer, Prog. Energy Combust. Sci. 10 (1) (1984) 1–57. [50] M. O’Conaire, H.J. Curran, J.M. Simmie, W.J. Pitz, C.K. Westbrook, Int. J. Chem. Kinet. 36 (2004) 603– 622. [51] S.L. Fischer, F.L. Dryer, H.J. Curran, Int. J. Chem. Kinet. 32 (2000) 713–740. [52] H.J. Curran, S.L. Fischer, F.L. Dryer, Int. J. Chem. Kinet. 32 (2000) 741–759. [53] H.J. Curran, P. Gaffuri, W.J. Pitz, C.K. Westbrook, Combust. Flame 114 (1998) 149–177. [54] H.J. Curran, P. Gaffuri, W.J. Pitz, C.K. Westbrook, Combust. Flame 129 (2002) 253–280. [55] E.L. Petersen, D.M. Kalitan, S. Simmons, G. Bourque, H.J. Curran, J.M. Simmie, Proc. Combust. Inst. 31 (2007) 447–454. [56] G. Mittal, C.J. Sung, M. Fairweather, A.S. Tomlin, J.F. Griffiths, K.J. Hughes, Proc. Combust. Inst. 31 (2007) 419–427. [57] J. Würmel, E.J. Silke, H.J. Curran, M. O’Conaire, J.M. Simmie, Combust. Flame (2007), doi:10.1016/ j.combustflame.2007.06.010. [58] R.K. Kee, F.M. Rupley, J.A. Miller, CHEMKIN II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics, SAND89-8009, Sandia Report, 1990.

S.M. Gallagher et al. / Combustion and Flame 153 (2008) 316–333 [59] P. Musch, J.M. Simmie, Hydrogen and methane oxidation in an opposed-piston rapid compression machine; computer simulation and model verification, CCW99/06, National University of Ireland Report, 1999. [60] R.L. Petrov, J. Eng. Phys. Thermophys. 31 (3) (1976) 1106–1110. [61] E.J. Silke, Ph.D. thesis, National University of Ireland, Galway, 2005. [62] E.J. Silke, H.J. Curran, J.M. Simmie, Proc. Combust. Inst. 30 (2005) 2639–2647. [63] J.F. Griffiths, P.A. Halford-Maw, D.J. Rose, Combust. Flame 95 (1993) 291–306. [64] J.F. Griffiths, K.J. Hughes, M. Schreiber, C. Poppe, Combust. Flame 99 (1994) 533–540. [65] H.K. Ciezki, G. Adomeit, Combust. Flame 93 (1993) 421–433. [66] E.L. Petersen, D.M. Kalitan, A.B. Barrett, S.C. Reehal, J.D. Mertens, D.J. Beerer, R.L. Hack, V.G. McDonell, Combust. Flame 149 (2007) 244–247.

333

[67] W.T. Peschke, L.J. Spadaccini, Determination of autoignition and flame speed characteristics of coal gases having medium heating values, Report No. EPRI AP-4291, Electric Power Research Institute, 1985. [68] S.G. Davis, A.V. Joshi, H. Wang, F. Egolfopoulos, Proc. Combust. Inst. 30 (2005) 1283–1292. [69] E.L. Petersen, D.F. Davidson, R.K. Hanson, Combust. Flame 117 (1999) 272–290. [70] P. Saxena, F.A. Williams, Combust. Flame 145 (2006) 316–323. [71] H.Y. Sun, S.I. Yang, G. Jomaas, C.K. Law, Proc. Combust. Inst. 31 (2007) 439–446. [72] J. Li, Z. Zhao, A. Kazakov, M. Chaos, F.L. Dryer, J.J. Scire, Int. J. Chem. Kinet. 39 (2007) 109–136. [73] R. Sivaramakrishnan, A. Comandini, R.S. Tranter, K. Brezinsky, S.G. Davis, H. Wang, Proc. Combust. Inst. 31 (2007) 429–437. [74] K. Fieweger, R. Blumenthal, G. Adomeit, Combust. Flame 109 (1997) 599–619.