Ignition Quality Tester (IQT) Investigation of the Negative Temperature

Article pubs.acs.org/EF

Ignition Quality Tester (IQT) Investigation of the Negative Temperature Coefficient Region of Alkane Autoignition Gregory E. Bogin, Jr.,*,† Eric Osecky,† Matthew A. Ratcliff,‡ Jon Luecke,‡ Xin He,‡ Bradley T. Zigler,‡ and Anthony M. Dean† †

Colorado School of Mines, 1610 Illinois Street, Golden, Colorado 80401, United States National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States

‡

ABSTRACT: The negative temperature coefficient (NTC) region of alkane autoignition was observed for the first time in the Ignition Quality Tester (IQT). The C7 isomers studied included n-heptane, 3-ethylpentane, 2,4-dimethylpentane, 2,3dimethylpentane, and 2,2,3-trimethylbutane. The temperatures of the fuel−air mixture ranged from 650 to 1023 K with pressures of 0.5, 1.0, and 1.5 MPa at equivalence ratios between 0.8 and 1.0. The longer autoignition times of increasingly branched isomers allowed the reacting mixtures sufficient time to reach a pseudohomogeneous state, so that the kinetic behavior was similar to that observed in homogeneous rapid compression machine (RCM) and shock tube experiments. Although the IQT produced longer ignition delays than RCM data, the order of ignition delays for the various isomers was the same; that is, isomers with more branching had reduced reactivity and the location of the methyl group among equally branched isomers also affected reactivity. The characteristic NTC region was observed from all of the fuels at 0.5 MPa, except for n-heptane which had ignition delays too short to overcome the effects of fuel−air heterogeneity on autoignition. However, reducing the pressure to 0.2 MPa further increased the ignition delay so that NTC behavior was observed for n-heptane. A computational fluid dynamics model was used to study fuel evaporation and fuel−air mixing, and a 0-D homogeneous batch reactor was used to model the ignition of the C7 isomers. The latter produced reasonable levels of agreement with experiments across the temperature range. The 0-D chemical kinetic model also successfully modeled hexadecane autoignition in the IQT at long ignition delays (>20 ms). However, coupled computational fluid dynamics/kinetic model may be required at short ignition delays (20 ms) have the potential to provide meaningful data to assist in the validation of combustion kinetic mechanisms.

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INTRODUCTION

The IQT is a constant-volume device capable of rapidly measuring ignition delay over a range of pressures and temperatures for a variety of liquid fuels. The IQT uses small amounts of fuel, making it ideal for measuring advanced alternative fuels that may only be available in small quantities. The IQT has been used by others to study the impact of fuel chemistry on ignition.5,6 It is demonstrated in this present work that, if suitably modified, the IQT can complement RCMs and shock tubes for validation of ignition mechanisms. This paper focuses on the study of C7 alkane isomers using an IQT with a new control system developed at the National Renewable Energy Laboratory (NREL). The new control system allows for a wider range of pressures, temperatures, and ignition delays to be used when investigating the effects of fuel molecular structure on autoignition. This expanded capability allows ignition delay studies under pseudohomogeneous conditions such that the negative temperature coefficient (NTC) behavior observed in other devices can also be achieved in the IQT.

Due to increasing global demand for liquid transportation fuels, the related greenhouse gas emissions from fossil fuels, and national security concerns from dependence on foreign petroleum, there is an incentive to develop more energyefficient engines and expand utilization of alternative sources of liquid fuels. Such developments will be facilitated by advances in comprehensive modeling of the combustion process within engines. Such models comprise device-specific computational fluid dynamic (CFD) submodels coupled with chemical kinetic submodels. Along with accurately capturing the fluid dynamics and spray physics, it is equally important for the combustion models to have reliable, validated chemical kinetic mechanisms. Such validation experiments are usually performed with rapid compression machines (RCM), shock tubes, and jet stirred reactors. Most studies with these devices have been limited to homogeneous mixtures with relatively high-volatility fuels. Recent advancements in fuel−air mixture preparations and heating have enabled some studies of lower-volatility fuels in shock tubes1,2 and RCMs.3,4 There remains a need for a device that is simple and capable of rapidly producing data. The motivation for the current study is the modification of the Ignition Quality Tester (IQT) to serve as an additional validation device. © 2013 American Chemical Society

Received: October 26, 2012 Revised: January 17, 2013 Published: February 20, 2013 1632

dx.doi.org/10.1021/ef301738b | Energy Fuels 2013, 27, 1632−1642

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used,11 their CNs and for completeness, the associated Research Octane Number (RON) and Motor Octane Number (MON) values. Most of the fuels were supplied by Sigma-Aldrich and have purities of ≥99%, except for 3-ethyl-pentane (≥96%, 3.5% 3-methylpentane, Chemsampco). Control System. A National Instruments (NI) CompactDAQ based data acquisition and control (DAC) system was developed to expand the capability of the standard IQT system. The new DAC system controls the charge air valve, chamber inlet valve, chamber exhaust valve, and injection release valve. It is also able to reset the pressure amplifier, which is needed for the high-speed chamber pressure measurement. All thermocouples in the IQT were replaced with Omega K-type dual thermocouples, with one thermocouple used for the standard IQT system and the other one used for the new DAC system. This allows simultaneous monitoring of the IQT temperature by both systems. The standard IQT system controls the heaters and cooling system and preserves all other IQT safety features. The high-speed needle lift and pressure signals were simultaneously measured using an NI 9215 analog input module with a sampling rate of 100 kHz. The static fuel injection pressure and chamber pressure were measured using an NI 9205 analog input module. An NI 9213 thermocouple input module was used to measure IQT temperatures. Two NI 9481 relay modules were used to control valves and reset the pressure amplifier. A LabView program was developed to not only duplicate the standard IQT test following the ASTM D6890 test standard for determination of ignition delay and Derived Cetane Number (DCN) but also to allow flexible control of the valve acting sequence and data log duration as needed. The new DAC system allows the user to operate across a wider range of temperatures and pressures with the ability to record relatively long ignition delays (up to ∼1s). Observation of the longer ignition delays is essential to achieve the pseudohomogeneous conditions of interest in this work. At the longer delays, the influences of the initial physics-dominated events (droplet breakup and subsequent evaporation) are diminished, with the result that the ignition delays are really controlled by the intrinsic chemical kinetics. The 0.21-L IQT chamber was pressurized to 0.5, 1.0, and 1.5 MPa (21% O2 in N2), with the charge air heated prior to injection of the fuel across a temperature range of 650−1023 K. Two methods were used in performing the temperature sweeps: (1) For the constant fuel mass case the vessel was heated to the highest temperature (1023K) and then the heaters were turned off, as the temperature dropped to 650 K the ignition delay was measured over multiple injections during this time period. To verify the validity of this mode of operation for the temperature sweeps, similar tests were performed with the heaters remaining on at select temperature points between 650 and 1023 K. The mass of fuel injected is 27, 55, and 84 mg for 0.5, 1.0, and 1.5 MPa, respectively. The variability of the mass between injections was ±2 mg based on 10 injections. (2) For the constant equivalence ratio case, the heaters remained on for each temperature point with several

EXPERIMENTAL METHODS

IQT Overview. The IQT is a constant-volume combustion apparatus with a spray injection system designed for the direct measurement of liquid fuel ignition delay as seen in Figure 1.

Figure 1. Schematic of the IQT combustion chamber. Experimental parameters such as initial charge-air pressure and temperature, chamber wall temperature, oxygen concentration, and mass of fuel injected are well controlled. A pressure transducer installed in the combustion chamber measures the pressure rise during the combustion event; the time interval between the start of injection and the rise in combustion pressure to the “pressure recovery point” (normally 138 kPa above the initial chamber pressure prior to injection) defines ignition delay.7,8 For this study, a 276 kPa pressure rise was used to compensate for low temperature, heat release-induced pressure increases to more accurately determine start of ignition. The IQT apparatus and setup was described in more detail previously.9,10 Fuel Specifications and Preparation. The C7 alkane isomers exhibit a wide range in cetane number (CN). Table 1 lists the isomers

Table 1. C7 Isomers Listed in Order of Increasing Cetane Numbers chemical

RON

MON

CN

2,2,3-trimethylbutane (2,2,3-TMB) 2,3-dimethylpentane (2,3-DMP) 2,4-dimethylpentane (2,4-DMP) 3-ethylpentane (3-EP) n-heptane (nH)

112 91 83 65 0

101 89 84 69 0

4 14 18 27 56

Figure 2. Scheme of the reaction pathways for R• + O2 reaction (adapted from ref 15). 1633


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measurements made at each temperature. The onset of ignition is defined in this study as 276 kPa above the initial charge pressure, which is twice the standard 138 kPa. This value was changed to account for the low temperature heat release effects observed at long ignition delays.

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RESULTS Low-Temperature Oxidation and Negative Temperature Coefficient Behavior. Detailed kinetic models are available to describe the ignition behavior of a wide range of hydrocarbon fuels with good accuracy.12−14 The reactivity of peroxy radicals (RO2) dominates the chemistry in the low and intermediate ranges of temperature, as shown in Figure 2.15 RO2 is formed by O2 addition to alkyl radicals (R) following H abstraction from the primary fuel. Besides redissociation, RO2 can isomerize to hydroperoxyalkyl radicals (QOOH) or undergo a concerted elimination reaction to form an olefin and HO2. There are several pathways for reactions of QOOH, including a chain branching reaction initiated by the addition of another O2 molecule. This chain branching step is recognized as a key reaction that leads to ignition. The NTC behavior is due to shifts in the various equilibria in this system.16−18 Chain branching reactions are favored at lower temperature and higher O2 concentrations, while the chain propagation reactions become more important as the temperature increases. As the temperature increases, this shift from chain branching to chain propagation leads to diminished rates and longer ignition delay. Thus, there are three distinct temperature regions for alkane ignition kinetics, leading to a two-stage ignition process (also seen in RCMs by Cox et al.19). The first stage is commonly referred to as “cool flame” or low temperature heat release (LTHR). This stage is terminated as the temperature increase moves the system into the NTC region. In this regime H2O2 is produced by the reaction RH + HO2 → H2O2 + R. As the temperature continues to increase and approaches 1000 K, the decomposition of H2O2, forming highly reactive OH radicals, takes place and the transition to the second stage (main ignition) occurs. Variations in fuel structure substantially affect the low temperature kinetics, resulting in significant variations in ignition delay and the time interval between first and second stage ignition. Considering heptane isomers, it is well-known that the branched isomers are less reactive than n-heptane.20 The isomers vary in the number of primary, secondary and tertiary C−H bonds as well as their location. The different bond strengths (and the location relative to the peroxy group) result in different rates for the various RO2 isomerizations as well as the subsequent reactions of the QOOH species. These subtleties have been incorporated into the detailed kinetic models developed to describe hydrocarbon ignition.21 Figure 3 shows evidence of low-temperature heat release (LTHR) in the pressure traces of the C7 isomers at 723K which is the beginning of the NTC region for majority of the isomers. Only results for the 0.5 MPa case are shown for brevity and because all of the isomers exhibited NTC at this condition. LTHR was also seen at 1.0 and 1.5 MPa for the isomers which had an observable NTC region (723−869 K). Injection of the fuel is followed by a drop in charge pressure due to fuel evaporative cooling, followed by reheating of the fuel−air mixture by the chamber heaters, and then LTHR prior to the main ignition event. Based on several tests it was determined that varying the mass of fuel injected had very little effect on the magnitude of the pressure drop (or temperature drop) due to

Figure 3. Pressure traces show evaporative cooling followed by LTHR before the main ignition event. Initial charge pressure is 0.5 MPa. Temperature is 723 K. 2,2,3-TMB (yellow circle), is plotted on the upper x axis while the other fuels (2,3-DMP (blue diamond), 2,4-DMP (green triangle), 3-EP (red ×), nH (■) are plotted using the lower x axis.

evaporative cooling. This is most likely due to the C7 isomers having a high volatility and due to the relatively small amount of fuel injected, but will be something to consider for lower volatility fuels. LTHR is apparent based on the 50−130 kPa increase in pressure above the initial charge pressure of 0.5 MPa before the main ignition event. The NTC region is important to low-temperature oxidation kinetics and the overall ignition of alkanes. Therefore, if the IQT is to be a complementary research apparatus to RCMs and shock tubes for validating chemical mechanisms of low volatility fuels, the ability to capture NTC and LTHR behavior is essential. Study of n-Heptane and C7 Isomers. The C7 alkane isomers were combusted in the IQT across a wide range of temperatures and pressures. The temperature range (650− 1023 K) was chosen to cover low to mid combustion temperatures including the NTC region, and is comparable to the range found in the literature for studies of n-heptane and C7 isomers in RCMs;21,22 most shock tube experiments11,23 are done at higher temperature where NTC behavior is not observed. The upper temperature limit for the current work was established based on physical limitations of the IQT and its heating system. Initially, a pressure of 1.5 MPa was selected to capture ignition characteristics at moderately high pressures commonly found in internal combustion engines; however, it was found useful to explore lower pressures as well. The ignition delay data at 1.5 MPa are shown in Figure 4. The variation observed among the various isomers relate directly to their different CNs (Table 1). Note that the two isomers with the longest ignition delays, 2,3-dimethylpentane and 2,2,3-trimethylbutane, clearly display NTC behavior with three distinct kinetic regions as indicated by the different slopes. The characteristic zero slope or negative slope in the NTC region for alkanes has been observed by other researchers.21,22 The absence of any NTC behavior for the other three isomers is suggestive of the importance of physicsdominated effects on ignition at shorter ignition delays. Under these conditions, physical effects related to droplet formation and breakup and fuel evaporation may overshadow the chemical kinetics of combustion. Endothermic pyrolysis reactions in extremely fuel-rich regions may also contribute to this difference from homogeneous combustion. To test this 1634


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Figure 4. Ignition delay, 1.5 MPa with constant mass (84 ± 2 mg/inj.) of fuel injected (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. 2,2,3-TMB (○), 2,3-DMP (◆), 2,4-DMP (△), 3-EP (×), and nH (■).


hypothesis, the charge pressure was reduced to increase the ignition delay, allowing the air-fuel mixture to become more homogeneous and perhaps facilitate the appearance of the NTC region, which is documented with these isomers under the homogeneous charge conditions present in RCM studies cited earlier. The charge pressure was first reduced from 1.5 to 1.0 MPa to increase ignition delays (Figure 5). This resulted in shifts of the

The continued absence of an NTC region for n-heptane again suggests this may be the result of ignition delays being too short. These experimental results also suggest that an ignition delay greater than 20 ms is required to minimize the impact of the injection process on the ignition event for the fuels studied. In the 1.0 MPa case (Figure 5), the high temperature data points for 2,2,3-trimethylbutane and 2,3-dimethylpentane overlap, which was not observed at the higher 1.5 MPa pressure. When the pressure is reduced to 0.5 MPa, the high temperature portions of the curves merge for all of the isomers except n-heptane. This indicates that the high-temperature kinetics of these fuels are very similar. This behavior is also observed in the homogeneous RCM experiments of Silke et al.,22 again indicating that combustion in the IQT is dominated by the intrinsic chemical kinetics at these relatively long durations. The impact of pressure on the ability to observe the NTC region for each fuel can be seen more clearly in Figures 7−11.


low temperature to NTC region transitions (inflection points) to slightly lower temperatures for 2,3-dimethylpentane and 2,2,3-trimethylbutane. These shifts to lower temperatures are expected, based on equilibrium of R• + O2↔ RO2.16−18,24 As seen in Figure 5, the NTC region for 2,3-dimethylpentane is now more apparent, while the qualitative behavior of the three isomers with the faster delays are similar to that observed at 1.5 MPa, although the ignition delay increased at the lower pressure. A further decrease in pressure to 0.5 MPa shows a much more dramatic impact (Figure 6). Here 2,4-dimethyl pentane and 3-ethylheptane now exhibit NTC behavior, while only the onset of NTC is observed for 2,2,3-trimethylbutane.

Figure 7. Ignition delay of n-heptane, 0.2−1.5 MPa, constant mass of fuel (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. nH1.5 MPa (◇), nH-1.0 MPa (□), nH-0.5 MPa (Δ), and nH-0.2 MPa (×). 1635


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Reducing the charge pressure to 0.5 MPa increased the ignition delay for n-heptane (Figure 7) to approximately 15 ms at 1000/ T = 1.2, but without the appearance of an NTC region. The charge pressure was reduced further to determine if sufficiently long ignition delays could be produced and eliminate spray physics effects on the ignition event. As seen in Figure 7, lowering the charge pressure to 0.2 MPa produced significant increases in the ignition delays and also resulted in the appearance of the NTC region for heptane. 3-Ethylpentane (Figure 8) and 2,4-dimethylpentane (Figure 9), which did not

Figure 10. Ignition delay of 2,3-dimethylpentane, 0.5−1.5 MPa, constant mass of fuel (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. 2,3-DMP-1.5 MPa (◇), 2,3-DMP-1.0 MPa (□), and 2,3-DMP-0.5 MPa (Δ).

Figure 8. Ignition delay of 3-ethylpentane, 0.5−1.5 MPa, constant mass of fuel (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. 3-EP-1.5 MPa (◇), 3EP-1.0 MPa (□), and 3-EP-0.5 MPa (Δ).

Figure 11. Ignition delay of 2,2,3-trimethylbutane, 0.5−1.5 MPa, constant mass of fuel (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. 2,2,3-TMB-1.5 MPa (◇), 2,2,3-TMB-1.0 MPa (□), and 2,2,3-TMB-0.5 MPa (Δ).

that the expected shift of the NTC region to lower temperatures (observed in the 1.0 and 1.5 MPa data) might be sufficiently large that it is not observed. Another possible reason is that the excessively long times (>1 s) required for ignition are sufficient for other events to occur, such as increasing heat transfer, making the IQT unsuitable to probe these conditions. At extremely long ignition delay there is also the possibility of wall ignition and other surface chemistry which could affect the ignition event. The data presented show that NTC is observable in the IQT for fuels that normally exhibit NTC in RCMs, provided sufficiently long ignition delay (>20 ms) can be attained. The data presented thus far were obtained with the mass of fuel injected held constant to allow rapid, high-resolution ignition delay measurements across the temperature range. This approach has the advantage of significantly reducing the time required to collect the data. Under constant fuel mass injected operation, the global equivalence ratio decreases from 1.0 ≤ Φ ≤ 0.8 as more air is charged into the chamber at lower

Figure 9. Ignition delay of 2,4-dimethylpentane, 0.5−1.5 MPa, constant mass of fuel (1.0 ≤ Φ ≤ 0.8). Each data point is a single injection event. 2,4-DMP-1.5 MPa (◇), 2,4-DMP-1.0 MPa (□), and 2,4-DMP-0.5 MPa (Δ).

exhibit NTC at the two higher pressures (1.0 and 1.5 MPa), had ignition delays below 20 ms when 1000/T < 1.2. However, once the ignition delays of these two isomers were increased above 20 ms by lowering the charge pressure to 0.5 MPa, the NTC region was observed. 2,3-Dimethylpentane (Figure 10) exhibited NTC across the range of pressures tested, however, 2,2,3-trimethylbutane (Figure 11) showed unexpected behavior at 0.5 MPa. No NTC was observed, although it was evident at higher pressures. One possibility to explain this observation is 1636


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that had the longest ignition delay. Further consideration of these data must await more detailed analysis, wherein a detailed kinetic mechanism is coupled to a CFD analysis. Previous work employing a CFD model of the IQT using a skeletal mechanism for n-heptane autoignition9 over a matrix of temperatures, pressures, and oxygen concentrations were in good agreement with experimental data; a similar investigation with the C7 isomers is currently underway. Modeling the IQT Using CFD and 0-D Chemical Kinetics Model. The observance of the NTC region presented above suggests that the IQT is reaching a pseudohomogeneous mixture at ignition delays greater than 20 ms. To support this hypothesis, a CFD model9 of the IQT was employed to investigate fuel spray break-up (FSB), evaporation and mixing of the C7 isomers. In addition, work is currently underway to reduce the size of the C7 isomers detailed chemical mechanisms to allow modeling their ignition within the CFD model, which is computationally too costly with the detailed mechanisms. FSB and evaporation was modeled (combustion chemistry was turned off) at the two temperature (650 and 1023 K) and pressure extremes (0.5 and 1.5 MPa) studied in the constant mass experiments described earlier (a detailed discussion of the CFD model along with a discussion on fuel spray break-up, the effects of mass of fuel injected on ignition, local equivalence ratio effects on ignition, and single droplet modeling is presented in ref 9). Table 2 summarizes these modeling results

temperatures. Even though this variation is relatively small, additional experiments were performed at constant equivalence ratios to measure the effect of such changes. Several ignition delay measurements were obtained for each fuel in which the mass of fuel injected varied with the mass of charge air to maintain a constant global equivalence ratio of Φ = 1.0 (based on a theoretical homogeneous fuel−air mixture). Figures 12

Figure 12. Ignition delay, constant mass vs constant equivalence ratio, 1.5 MPa. Constant mass [2,2,3-TMB (○), 2,3-DMP (◆), 2,4-DMP (Δ), 3-EP (×), and nH (■)]. Constant equivalence ratio [2,2,3-TMB (red circle), 2,3-DMP (red square), 2,4-DMP (red triangle), 3-EP (×), and nH (red square)].

Table 2. Fuel Spray Break-up and Evaporation Times Compared to Measured Ignition Delays for the C7 Isomers at 650 and 1023 K mass of injected fuel (g)

charge pressure (MPa)

CFD FSB and evaporation (ms) @ 650 K

0.0838 0.0272

1.5 0.5

15.9 25.8

measured ignition delays (ms) @ 650 K

CFD FSB and evaporation (ms) @ 1023 K

measured ignition delays (ms) @ 1023 K

∼40 to 90 ∼80 to 1000

5.51 14.1

∼3 to 10 ∼10 to 30

and compares them with measured ignition delay ranges of the C7 isomers (refer to Figures 3−5 for measured isomer ignition delays at the two temperatures: 1000/T = 1.54 and 0.98). The liquid fuel spray (modeled to match experimental fuel spray captured by high-speed imaging and nozzle measurements, outlined in ref 9) was tracked at each time step to determine when the liquid fuel was completely evaporated. As a sensitivity analysis, the thermo-physical properties of n-heptane were varied to match the slight differences in the thermo-physical properties for the C7 isomers; this produced little to no measurable effects on the predicted ignition delays. As seen from Table 2, the time for FSB and evaporation can be significantly less than the overall time for ignition, particularly at the low temperature condition which produced long ignition delays (>40−1000 ms) with fuel evaporation times predicted to range from 16 to 26 ms. The much shorter ignition delays produced at the high temperature (e.g., nheptane < 14 ms, 1023 K, 0.5 MPa) are consistent with CFD model predictions that liquid fuel may still be present during the start of ignition for n-heptane (the only fuel at this condition with an ignition delay less than 20 ms, with no observed NTC). These results support the hypothesis that at short ignition delays, the ignition process is greatly affected by the fuel spray, but increasingly less so at longer ignition delays

Figure 13. Ignition delay, constant mass vs constant equivalence ratio, 1.0 MPa. Constant mass [2,2,3-TMB (○), 2,3-DMP (◆), 2,4-DMP (△), 3-EP (×), and nH (■)]. Constant equivalence ratio [2,2,3-TMB (red circle), 2,3-DMP (red diamond), 2,4-DMP (red triangle), 3-EP (×), and nH (red square)].

and 13 show that the constant mass ignition delay data for all fuels are within the variability of the constant equivalence ratio data for both 1.5 and 1.0 MPa, with the exception of the 2,2,3trimethylbutane at 1.0 MPa. The constant equivalence ratio data for 2,2,3-trimethylbutane at 1.0 MPa have a slightly faster ignition delay curve compared to the constant mass conditions. It is interesting that this observation coincides with the isomer 1637


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Figure 14. 2-D contour plots of the equivalence ratios (top) and temperature (bottom). Global Φ = 0.6, initial charge pressure = 2.1 MPa, initial charge temperature = 860 K.

K below the initial chamber temperature (see lower right-hand chart in Figure 14), due to evaporative cooling as discussed earlier. Therefore, this reduced temperature (called Teff) was used as the initial condition for the zero-dimensional batch reactor model and for comparison with the measured ignition delays. The reduced temperature Teff used for the 0-D model is based on thermocouple readings in the IQT chamber. Because the equivalence ratio of the “well-mixed” combustion region was predicted by the CFD model to be ∼50% higher than the global equivalence ratio (see upper right-hand chart of Figure 14), the uncertainty in the equivalence ratio was bounded by running the 0-D model at the global equivalence ratio, and at double the global equivalence ratio. Some preliminary results are presented below in Figure 15a−e. Further CFD simulations across the temperature range for each pressure are required to determine the best single equivalence ratio to use for comparisons of the different experiment conditions. The 2,2,3-trimethylbutane 0-D model results are in surprisingly good agreement with the experimental results (Figure 15a), given the simplifications involved in the analysis. Both the high temperature slopes and NTC region of the experimental data are captured by the 0-D model. As the measured ignition delay become smaller for 2,3-dimethylpentane and 2,4-dimethylpentane, the model results begin to deviate from the data, as now may be expected. The 2,3dimethylpentane and 2,4-dimethylpentane modeling results (Figure 15b,c) predict that the NTC region occurs at a lower temperature compared to the experimental results. The measured NTC regions are also much broader than the 0-D predicted NTC regions. However, the model’s accuracy appears to improve at the lower temperatures for 2,3-dimethylpentane. The modeling results for 3-ethylpentane and n-heptane have reasonable accuracy at the lower temperatures, but overpredict ignition delays in the NTC region. The model predicts a strong NTC region for n-heptane at all three pressures, which was not observed experimentally. This is likely due to the short ignition delays for n-heptane at these pressures, resulting in a

(>20 ms). Beyond some ignition delay (>20 ms) the mixture becomes sufficiently homogeneous that chemical kinetics dominates the ignition process. The time for fuel−air mixing is characterized by the equivalence ratio and temperature distributions within the IQT. Two times are presented in Figure 14 that compare fuel− air homogeneity at normal DCN conditions and at the longer mixing time of 20 ms. The 4.1 ms time is the CFD model predicted ignition delay for n-heptane under normal DCN conditions for the IQT (compared to 3.78 ms measured in the IQT).9 The 20 ms time was shown earlier with experimental data to be the point above which most isomers displayed NTC behavior. At 4.1 ms there are steep gradients in the equivalence ratio (at a minimum 0.1−1.5) and temperature (at a minimum 700−860 K) in the region of combustion, whereas at 20 ms the CFD contours show that a pseudohomogeneous mixture exists in the main combustion chamber region where ignition occurs. This suggests that ignition delays greater than 20 ms could be modeled using a 0-D batch reactor. The CFD simulations were run out to 100 ms, producing fuel−air mixtures contours similar to those at 20 ms but with further improved homogeneity and the local equivalence ratio approaching the global equivalence ratio. With the CFD modeling confirming that the IQT reaches a pseudohomogeneous mixture 20 ms after injection, the next test was to determine if the IQT could be modeled as a homogeneous 0-D reactor at long ignition delays. 0-D modeling was performed using the CHEMKIN software package with the IQT modeled as a homogeneous batch reactor and was assumed to be adiabatic. The input conditions for the model were based on the conditions measured during the experiments and ignition was determined by the temperature inflection point. For n-heptane, the mechanism developed by Curran et al.25 was used, and the mechanisms developed by Westbrook et al.21 were used for the other isomers. The CFD results (with the chemistry turned-off) showed that the temperature of the “well-mixed” combustion region was ∼60 1638


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Figure 15. (a) 0-D modeling results for 2,2,3-trimethylbutane vs IQT measured ignition delay. 2,2,3-TMB-1.5 MPa (blue diamond), 2,2,3-TMB-1.0 MPa (red square), 2,2,3-TMB-0.5 MPa (▲), 0-D-model-global-phi (···), and 0-D-model-double-global-phi (line). (b) 0-D modeling results for 2,3dimethylpentane vs IQT measured ignition delay. 2,3-DMP-1.5 MPa (blue diamond), 2,3-DMP-1.0 MPa (red square), 2,3-DMP-0.5 MPa (▲), 0-Dmodel-global-phi (···), and 0-D-model-double-global-phi (line). (c) 0-D modeling results for 2,4-dimethylpentane vs IQT measured ignition delay. 2,4-DMP-1.5 MPa (blue diamond), 2,4-DMP-1.0 MPa (red square), 2,4-DMP-0.5 MPa (▲), 0-D-model-global-phi (···), and 0-D-model-doubleglobal-phi (line). (d) 0-D modeling results for 3-ethylpentane vs IQT measured ignition delay. 3-EP-1.5 MPa (blue diamond), 3-EP-1.0 MPa (red square), 3-EP-0.5 MPa (▲), 0-D-model-global-phi (···), and 0-D-model-double-global-phi (line). (e) 0-D modeling results for n-heptane vs IQT measured ignition delay. nH-1.5 MPa (blue diamond), nH-1.0 MPa (red square), nH-0.5 MPa (▲), 0-D-model-global-phi (···), and 0-D-modeldouble-global-phi (line).

equivalence ratio. (2) Inaccuracies in the global equivalence ratios used for the 0-D model. (3) Accuracy of the chemical mechanisms with respect to reaction rates, pathways, thermo, etc. To further test the validity of modeling the IQT with a 0-D model, another alkane was investigated with short ignition delay and much lower volatility than the C7 isomers. Hexadecane (n-hex) was chosen because of the fuel’s low volatility that makes studying ignition characteristics of the fuel

heterogeneous fuel−air mixture with liquid fuel still present, as discussed earlier. Work is currently underway to determine the cause for the discrepancy between the 0-D (homogeneous) model for the C7 isomers and the ignition delay measured in the IQT. Such discrepancies could result from the following: (1) At the higher temperatures (shorter ignition delays) the CFD model indicates substantial spatial variations in both T and 1639


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difficult with traditional systems such as shock tubes and RCMS, while being relatively easy to study with the IQT. Figure 16 demonstrates that hexadecane ignition in the IQT

Figure 17. Ignition delay, IQT vs RCM, 1.0 MPa, Φ = 1.0 (data from Westbrook et al.21 and Silke et al.22 extracted from figures in the respective references). 2,4-DMP-Westbrook (▲), 2,4-DMP-IQT (red triangle), nH-Westbrook (■), nH-IQT (red square), and nH-Silke (□).

Figure 16. 0-D model of hexadecane ignition delay within the IQT. Initial charge pressure is 2.1 MPa with a global equivalence ratio of 0.5. n-hex (■) and 0-D-model-global-phi (line).

Figure 18 compares data for various isomers collected at 1.5 MPa. The RCM data for 2,4- dimethylpentane show obvious

can be accurately modeled using a 0-D homogeneous batch reactor at ignition delays greater than the ∼20 ms (modeling was performed using a detailed chemical mechanism by Westbrook et al.26). Further modeling and experiments are currently underway with hexadecane at long ignition delays with reduced chemical mechanisms incorporated into the CFD model for the IQT. Comparison of the C7 Isomers in the IQT with RCM Studies. The ignition delay data measured in this study with the IQT were compared to RCM data reported by Silke et al.,22 and RCM data from Westbrook et al.21 The latter contains RCM data from Griffiths et al.27 and Cox et al.28 It was difficult to make extensive comparisons between the IQT ignition delays and RCM ignition delays because of 1) limited comparable cases, and 2) differences in physical process (e.g., fluid motion, compressing gases, spray dynamics, and heat transfer). Comparing data between different RCMs is also difficult, as discussed by Silke et al.22 and as can be seen by comparison of n-heptane data in Figure 17. Nevertheless, comparisons with RCM data have been attempted here because RCMs provide valuable data for validating chemical kinetic mechanisms for combustion, and therefore are an important benchmark. Figure 17 shows that n-heptane ignition delays measured in the IQT at 1.0 MPa are generally longer than those measured in the two RCM data sets, but the effect of temperature on the ignition delay is similar. The longer ignition delay in the IQT at the higher temperatures is not unexpected, because the very short ignition delay under these conditions (

Ignition Quality Tester (IQT) Investigation of the Negative Temperature

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