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Simulation of methane autoignition in a rapid compression machine with creviced pistons
Brief Communication Simulation of Methane Autoignition in a Rapid Compression Machine with Creviced Pistons L. BRETT, J. MACNAMARA, P. MUSCH, and J. M. SIMMIE* Department of Chemistry, National University of Ireland, Galway, Ireland
INTRODUCTION A rapid compression machine (RCM) is an instrument to simulate a single cycle of an internal combustion engine, thus allowing the study of spontaneous ignition under more favorable conditions than those existing in real engines. The RCM used for this project was originally built in the mid sixties by Shell at their Thornton Research Centre, Chester, England. The motivation was to investigate the chemical processes that precede ignition and result in an induction or ignition delay time. The machine was able to compress and to heat mixtures of fuel and oxidizer to conditions near those in a knocking engine. This RCM is of novel design using two opposed, pneumatically driven pistons to compress such a mixture [1]. It has an associated hydraulic synchronizing system for initializing the compression, and a unique position indicating system. This design facilitates very short compression times, of ⬇10 ms, without high mechanical stress, and is probably still mechanically superior to any other RCM built so far. Computer Modeling Computer modeling is essential for a proper understanding of a rapid compression event, because, for example, accurate temperature measurement is problematic [2, 3]. Despite efforts [4, 5], it is not currently [6] possible to model the process by computational fluid dynamics including full chemistry; however, Lee and Hochgreb [7] have made some progress by redesigning the piston head to simplify heat transfer. They have applied their code, as a * Corresponding author. E-mail: [email protected]. 0010-2180/01/$–see front matter PII S0010-2180(00)00193-0
subset of the chemical kinetic application Chemkin-II [8], to model hydrogen autoignition only [9] in a single-piston RCM. We have extended this pioneering effort into a preliminary study of methane autoignition, and, rewritten the code to provide a fully functional (with access to the thermodynamic and transport properties databases) extension to Chemkin, for a rapid compression machine [10]. The model assumes that the system is divided into three interacting regions: the core, the boundary layer, and the crevice volume. Furthermore, it is assumed that no reaction occurs outside the core or during compression, that the thermal boundary layer thickness is small compared to the chamber dimensions and smaller than the entrance gap of the piston head crevice, and that the transfer is controlled by heat conduction from the core gas to the wall with no convective motion along the piston motion.
EXPERIMENTAL Apparatus The essential features of the rapid compression machine have been described [1]. The pressure in the reaction chamber was measured using a PCB voltage-mode pressure transducer (Model M113A20) mounted flush with the chamber wall, and the signal was fed to a digital oscilloscope (Hitachi VC-7104). The piston’s position and velocity were also monitored. The volume of the precompression reaction chamber was 395 cm3, and the compression ratio used was either 12.0 or 13.4. The initial temperature of the reaction chamber varied between 329 and 357 K. Two Electrothermal heating tapes were wrapped around the reaction chamber and conCOMBUSTION AND FLAME 124:326 –329 (2001) © 2001 by The Combustion Institute Published by Elsevier Science Inc.
SIMULATION OF METHANE AUTOIGNITION
327
trolled by an Electrothermal Digital controller governed by a platinum probe positioned between the heating tape and the chamber wall. New piston heads were machined, incorporating a crevice (2.8 mm wide and 8.6 mm deep with a 3° tapered lead in section 4-mm long), based upon the best design available, and scaled in proportion to the cylinder radius from the MIT rapid compression machine [11]. However, no attempt was made to optimize the crevice design for this twin-piston machine. Mixture Preparation
Fig. 1. CH4/O2/Ar ⫽ 1/2/7, p i ⫽ 0.050 MPa, T i ⫽ 338 K (creviced piston heads): e— experiment; L & GRI—modeling simulations.
Fuel mixtures were prepared by standard manometric methods in 5 liter glass bulbs. A Wallace and Tiernan Precision Dial Manometer (FA-145, 0 –2 bar) was used for metering all components in the mixture. All gases were obtained from Irish Industrial Gases at a purity of greater than 99.5%, and were used without further purification. Mixtures were allowed to mix for at least 1 h before use to ensure homogeneity. The reaction chamber was filled to an initial pressure of 0.050 MPa, and the charge compressed to constant volume in under 17 ms. Compressed pressures around 1.60 MPa and temperatures between 985 and 1060 K were achieved.
RESULTS AND DISCUSSION
Temperature Determination
Hydrogen Oxidation
The compressed gas temperature, T c , was calculated from the equation of state for an ideal, adiabatic, isentropic compression using the measured pressures at the start, p i , and end, p c , of compression and the initial chamber temperature, T i via:
The ignition of 2:1:5 ⫽ H2:O2:(Ar ⫹ N2) mixtures was studied in the RCM using both flat piston heads with no appreciable crevice volume and also using creviced piston heads. The results for each were compared with a model whose simulations used a mechanism for hydrogen oxidation by Marinov et al. [12]. Typical pressure curves obtained for each case are shown in Figs. 2 and 3. In both cases the model overpredicts the compressed pressure obtained, and hence, calculates a shorter ignition delay time than was observed. However, the difference was greatly reduced when the creviced piston heads were used. This observation is in agreement with the findings of Hochgreb and Lee [7], and supports the hypothesis that the development of the corner vortex is suppressed, leading to a well-defined core gas region with reduced heat
冕
Tc
Ti
␥ d ln T ⫽ ln 共 p c/p i兲 ␥⫺1
where ␥ is the temperature-dependent specific heat ratio, C P /C V , for the mixture. Ignition Delays Ignition delay times were measured as a function of the compressed gas temperature, and ranged between 30 and 175 ms. The ignition
delay time was defined as the time difference between the end of compression and the maximum rate of pressure rise during ignition. A typical pressure–time trace for a 1:2:7 ⫽ CH4: O2:Ar mixture, which was compressed from an initial temperature of 338 K and an initial pressure of 0.050 MPa, is shown in Fig. 1. The postcompression peak values of pressure and temperature are calculated as 1.60 MPa and 1016 K against a measured peak pressure of 1.598 ⫾ 0.05 MPa.
328
Fig. 2. H2/O2/N2/Ar ⫽ 2/1/3/2, p i ⫽ 0.050 MPa, T i ⫽ 345 K (flat piston heads): e— experiment; m—model.
transfer when creviced piston heads are used. A lower peak pressure was obtained when using the creviced piston heads, as a result of them having a lower compression ratio than the flat piston heads (12.0 compared with 13.4). A higher proportion of argon was used in these experiments to compensate for the lower compressed temperature obtained with a lower compression ratio. These observations were consistent over the whole temperature range (955–1040 K) examined. Methane Oxidation A comparison between the experimental and calculated pressure histories for the rapid compression of pure methane using the creviced piston heads, initially at 0.060 MPa and 293 K, is
Fig. 3. H2/O2/N2/Ar ⫽ 2/1/2/3, p i ⫽ 0.050 MPa, T i ⫽ 344 K (creviced piston heads): e— experiment; m—model.
L. BRETT ET AL.
Fig. 4. Pure methane, p i ⫽ 0.060 MPa (creviced piston heads): m—GRI and Leeds mechanisms; e— experiment.
shown in Fig. 4. Generally the postcompression peak pressure of methane is slightly overpredicted by the model with an increasing deviation for increasing peak pressures. The steady level of the pressure after compression demonstrates the integrity of the reaction chamber. Reactive compressions were then performed on 1:2:7 ⫽ CH4:O2:Ar mixtures. Two complete mechanisms for methane oxidation were used to run the model: a Leeds mechanism [13] considers 35 reactive species and 190 reactions, while the Gas Research Institute [14] mechanism, GRIMech 2.11, which includes the possible formation of nitrogen-containing compounds, has 279 reactions of 48 reactive species (it would be possible to choose either better or improve the existing chemical kinetic mechanisms for this temperature range). The computed postcompression peak pressures calculated by both mechanisms are in very good agreement with the experimental pressure traces in Fig. 1. Of course, because the model assumes “frozen chemistry” during compression, one would expect essentially identical results for both mechanisms. There are, in fact, very slight differences between the predictions of the two mechanisms, but these are due to differences in the thermodynamic database of each mechanism. Figure 5 compares the predictions of each model with the experimentally observed ignition delay time as a function of temperature. The results are more closely predicted by the model based on the GRI mechanism, particularly at high temperatures. At low temperatures, ⱕ1000
SIMULATION OF METHANE AUTOIGNITION
329
edge support from Enterprise Ireland under their Research Scholarship and International Collaboration Program. REFERENCES Affleck, W. S., and Thomas, A., Proc. Inst. Mech. Eng. 183:365–385 (1969). 2. Desgroux, P., Gasnot, L., and Sochet, L. R., Appl. Phys. B 61:69 –72 (1995). 3. Desgroux, P., Minetti, R., Sochet, L. R., Combust. Sci. Technol. 113–114:193–203 (1996). 4. Griffiths, J. F., Rose, D. J., Schreiber, M., Meyer, J., and Knoche, K. F., Combust. Flame 91:209 –212 (1992). 5. Griffiths, J. F., Jiao, Q., Schreiber, M., Meyer, J., and Knoche, K. F., Proc. Combust. Inst. 24:1809 –1815 (1992). 6. Personal communication from Prof. Dr. Ju ¨rgen Warnatz, Interdisciplinary Center of Scientific Computing, Heidelberg University (1999). 7. Lee, D., and Hochgreb, S., Combust. Flame 114:531– 545 (1998). 8. Kee, R. K., Rupley, F. M., and Miller, J. A., CHEMKIN II: FORTRAN chemical kinetics package for the analysis of gas-phase chemical kinetics, Sandia Report, SAND89-8009 (1990). 9. Lee, D., and Hochgreb, S., Int. J. Chem. Kinet. 30:385– 406 (1998). 10. Musch, P., Report CCW99/06, National University of Ireland, Galway (1999). 11. Lee, D. (1997). Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, USA. 12. Marinov, N. M., Westbrook, C. K., and Pitz, W. J., in Transport Phenomena in Combustion, vol. 1 (S. H. Chan, Ed.), Taylor and Francis, Washington, DC, 1996, pp 118 –141. 13. Methane oxidation mechanism. Version 1.3, http:// www.chem.leeds.ac.uk/Combustion/Combustion.html. 14. Bowman, C. T., Hanson, R. K., Davidson, D. F., Gardiner, W. C., Jr., Lissianski, V., Smith, G. P., Golden, D. M., Frenklach, M., and Goldenberg, M., Methane oxidation mechanism, Release 2.11, http:// www.me.berkeley.edu/GRL_mech/releases.html. 1.
Fig. 5. Ignition delay times, , vs. peak temperature, T (creviced piston heads); 1:2:7 ⫽ CH4:O2:Ar, GRI and Leeds (L) mechanisms, ● Experiment.
K, the observed ignition delays far exceed those predicted by either model. CONCLUSION Redesigned piston heads do allow for much better agreement between simulation and experiment. However, the simulation still does not give a complete picture of the rapid compression event, and more work is required to improve our understanding of principally the physics and also the chemistry of the autoignition of fuels. We are very grateful to Prof. Simone Hochgreb (Combustion Research Facility at Sandia National Laboratory, Livermore, CA) for the provision of computer codes; to Dr. Chris Morley (Shell Thornton Research Centre) for the RCM, and to Prof. John Griffiths (University of Leeds) for encouragement and assistance. We also acknowl-
Received 11 April 2000; revised 30 June 2000; accepted 14 August 2000
INTRODUCTION A rapid compression machine (RCM) is an instrument to simulate a single cycle of an internal combustion engine, thus allowing the study of spontaneous ignition under more favorable conditions than those existing in real engines. The RCM used for this project was originally built in the mid sixties by Shell at their Thornton Research Centre, Chester, England. The motivation was to investigate the chemical processes that precede ignition and result in an induction or ignition delay time. The machine was able to compress and to heat mixtures of fuel and oxidizer to conditions near those in a knocking engine. This RCM is of novel design using two opposed, pneumatically driven pistons to compress such a mixture [1]. It has an associated hydraulic synchronizing system for initializing the compression, and a unique position indicating system. This design facilitates very short compression times, of ⬇10 ms, without high mechanical stress, and is probably still mechanically superior to any other RCM built so far. Computer Modeling Computer modeling is essential for a proper understanding of a rapid compression event, because, for example, accurate temperature measurement is problematic [2, 3]. Despite efforts [4, 5], it is not currently [6] possible to model the process by computational fluid dynamics including full chemistry; however, Lee and Hochgreb [7] have made some progress by redesigning the piston head to simplify heat transfer. They have applied their code, as a * Corresponding author. E-mail: [email protected]. 0010-2180/01/$–see front matter PII S0010-2180(00)00193-0
subset of the chemical kinetic application Chemkin-II [8], to model hydrogen autoignition only [9] in a single-piston RCM. We have extended this pioneering effort into a preliminary study of methane autoignition, and, rewritten the code to provide a fully functional (with access to the thermodynamic and transport properties databases) extension to Chemkin, for a rapid compression machine [10]. The model assumes that the system is divided into three interacting regions: the core, the boundary layer, and the crevice volume. Furthermore, it is assumed that no reaction occurs outside the core or during compression, that the thermal boundary layer thickness is small compared to the chamber dimensions and smaller than the entrance gap of the piston head crevice, and that the transfer is controlled by heat conduction from the core gas to the wall with no convective motion along the piston motion.
EXPERIMENTAL Apparatus The essential features of the rapid compression machine have been described [1]. The pressure in the reaction chamber was measured using a PCB voltage-mode pressure transducer (Model M113A20) mounted flush with the chamber wall, and the signal was fed to a digital oscilloscope (Hitachi VC-7104). The piston’s position and velocity were also monitored. The volume of the precompression reaction chamber was 395 cm3, and the compression ratio used was either 12.0 or 13.4. The initial temperature of the reaction chamber varied between 329 and 357 K. Two Electrothermal heating tapes were wrapped around the reaction chamber and conCOMBUSTION AND FLAME 124:326 –329 (2001) © 2001 by The Combustion Institute Published by Elsevier Science Inc.
SIMULATION OF METHANE AUTOIGNITION
327
trolled by an Electrothermal Digital controller governed by a platinum probe positioned between the heating tape and the chamber wall. New piston heads were machined, incorporating a crevice (2.8 mm wide and 8.6 mm deep with a 3° tapered lead in section 4-mm long), based upon the best design available, and scaled in proportion to the cylinder radius from the MIT rapid compression machine [11]. However, no attempt was made to optimize the crevice design for this twin-piston machine. Mixture Preparation
Fig. 1. CH4/O2/Ar ⫽ 1/2/7, p i ⫽ 0.050 MPa, T i ⫽ 338 K (creviced piston heads): e— experiment; L & GRI—modeling simulations.
Fuel mixtures were prepared by standard manometric methods in 5 liter glass bulbs. A Wallace and Tiernan Precision Dial Manometer (FA-145, 0 –2 bar) was used for metering all components in the mixture. All gases were obtained from Irish Industrial Gases at a purity of greater than 99.5%, and were used without further purification. Mixtures were allowed to mix for at least 1 h before use to ensure homogeneity. The reaction chamber was filled to an initial pressure of 0.050 MPa, and the charge compressed to constant volume in under 17 ms. Compressed pressures around 1.60 MPa and temperatures between 985 and 1060 K were achieved.
RESULTS AND DISCUSSION
Temperature Determination
Hydrogen Oxidation
The compressed gas temperature, T c , was calculated from the equation of state for an ideal, adiabatic, isentropic compression using the measured pressures at the start, p i , and end, p c , of compression and the initial chamber temperature, T i via:
The ignition of 2:1:5 ⫽ H2:O2:(Ar ⫹ N2) mixtures was studied in the RCM using both flat piston heads with no appreciable crevice volume and also using creviced piston heads. The results for each were compared with a model whose simulations used a mechanism for hydrogen oxidation by Marinov et al. [12]. Typical pressure curves obtained for each case are shown in Figs. 2 and 3. In both cases the model overpredicts the compressed pressure obtained, and hence, calculates a shorter ignition delay time than was observed. However, the difference was greatly reduced when the creviced piston heads were used. This observation is in agreement with the findings of Hochgreb and Lee [7], and supports the hypothesis that the development of the corner vortex is suppressed, leading to a well-defined core gas region with reduced heat
冕
Tc
Ti
␥ d ln T ⫽ ln 共 p c/p i兲 ␥⫺1
where ␥ is the temperature-dependent specific heat ratio, C P /C V , for the mixture. Ignition Delays Ignition delay times were measured as a function of the compressed gas temperature, and ranged between 30 and 175 ms. The ignition
delay time was defined as the time difference between the end of compression and the maximum rate of pressure rise during ignition. A typical pressure–time trace for a 1:2:7 ⫽ CH4: O2:Ar mixture, which was compressed from an initial temperature of 338 K and an initial pressure of 0.050 MPa, is shown in Fig. 1. The postcompression peak values of pressure and temperature are calculated as 1.60 MPa and 1016 K against a measured peak pressure of 1.598 ⫾ 0.05 MPa.
328
Fig. 2. H2/O2/N2/Ar ⫽ 2/1/3/2, p i ⫽ 0.050 MPa, T i ⫽ 345 K (flat piston heads): e— experiment; m—model.
transfer when creviced piston heads are used. A lower peak pressure was obtained when using the creviced piston heads, as a result of them having a lower compression ratio than the flat piston heads (12.0 compared with 13.4). A higher proportion of argon was used in these experiments to compensate for the lower compressed temperature obtained with a lower compression ratio. These observations were consistent over the whole temperature range (955–1040 K) examined. Methane Oxidation A comparison between the experimental and calculated pressure histories for the rapid compression of pure methane using the creviced piston heads, initially at 0.060 MPa and 293 K, is
Fig. 3. H2/O2/N2/Ar ⫽ 2/1/2/3, p i ⫽ 0.050 MPa, T i ⫽ 344 K (creviced piston heads): e— experiment; m—model.
L. BRETT ET AL.
Fig. 4. Pure methane, p i ⫽ 0.060 MPa (creviced piston heads): m—GRI and Leeds mechanisms; e— experiment.
shown in Fig. 4. Generally the postcompression peak pressure of methane is slightly overpredicted by the model with an increasing deviation for increasing peak pressures. The steady level of the pressure after compression demonstrates the integrity of the reaction chamber. Reactive compressions were then performed on 1:2:7 ⫽ CH4:O2:Ar mixtures. Two complete mechanisms for methane oxidation were used to run the model: a Leeds mechanism [13] considers 35 reactive species and 190 reactions, while the Gas Research Institute [14] mechanism, GRIMech 2.11, which includes the possible formation of nitrogen-containing compounds, has 279 reactions of 48 reactive species (it would be possible to choose either better or improve the existing chemical kinetic mechanisms for this temperature range). The computed postcompression peak pressures calculated by both mechanisms are in very good agreement with the experimental pressure traces in Fig. 1. Of course, because the model assumes “frozen chemistry” during compression, one would expect essentially identical results for both mechanisms. There are, in fact, very slight differences between the predictions of the two mechanisms, but these are due to differences in the thermodynamic database of each mechanism. Figure 5 compares the predictions of each model with the experimentally observed ignition delay time as a function of temperature. The results are more closely predicted by the model based on the GRI mechanism, particularly at high temperatures. At low temperatures, ⱕ1000
SIMULATION OF METHANE AUTOIGNITION
329
edge support from Enterprise Ireland under their Research Scholarship and International Collaboration Program. REFERENCES Affleck, W. S., and Thomas, A., Proc. Inst. Mech. Eng. 183:365–385 (1969). 2. Desgroux, P., Gasnot, L., and Sochet, L. R., Appl. Phys. B 61:69 –72 (1995). 3. Desgroux, P., Minetti, R., Sochet, L. R., Combust. Sci. Technol. 113–114:193–203 (1996). 4. Griffiths, J. F., Rose, D. J., Schreiber, M., Meyer, J., and Knoche, K. F., Combust. Flame 91:209 –212 (1992). 5. Griffiths, J. F., Jiao, Q., Schreiber, M., Meyer, J., and Knoche, K. F., Proc. Combust. Inst. 24:1809 –1815 (1992). 6. Personal communication from Prof. Dr. Ju ¨rgen Warnatz, Interdisciplinary Center of Scientific Computing, Heidelberg University (1999). 7. Lee, D., and Hochgreb, S., Combust. Flame 114:531– 545 (1998). 8. Kee, R. K., Rupley, F. M., and Miller, J. A., CHEMKIN II: FORTRAN chemical kinetics package for the analysis of gas-phase chemical kinetics, Sandia Report, SAND89-8009 (1990). 9. Lee, D., and Hochgreb, S., Int. J. Chem. Kinet. 30:385– 406 (1998). 10. Musch, P., Report CCW99/06, National University of Ireland, Galway (1999). 11. Lee, D. (1997). Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, USA. 12. Marinov, N. M., Westbrook, C. K., and Pitz, W. J., in Transport Phenomena in Combustion, vol. 1 (S. H. Chan, Ed.), Taylor and Francis, Washington, DC, 1996, pp 118 –141. 13. Methane oxidation mechanism. Version 1.3, http:// www.chem.leeds.ac.uk/Combustion/Combustion.html. 14. Bowman, C. T., Hanson, R. K., Davidson, D. F., Gardiner, W. C., Jr., Lissianski, V., Smith, G. P., Golden, D. M., Frenklach, M., and Goldenberg, M., Methane oxidation mechanism, Release 2.11, http:// www.me.berkeley.edu/GRL_mech/releases.html. 1.
Fig. 5. Ignition delay times, , vs. peak temperature, T (creviced piston heads); 1:2:7 ⫽ CH4:O2:Ar, GRI and Leeds (L) mechanisms, ● Experiment.
K, the observed ignition delays far exceed those predicted by either model. CONCLUSION Redesigned piston heads do allow for much better agreement between simulation and experiment. However, the simulation still does not give a complete picture of the rapid compression event, and more work is required to improve our understanding of principally the physics and also the chemistry of the autoignition of fuels. We are very grateful to Prof. Simone Hochgreb (Combustion Research Facility at Sandia National Laboratory, Livermore, CA) for the provision of computer codes; to Dr. Chris Morley (Shell Thornton Research Centre) for the RCM, and to Prof. John Griffiths (University of Leeds) for encouragement and assistance. We also acknowl-
Received 11 April 2000; revised 30 June 2000; accepted 14 August 2000