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Chemkin 4.1.1 Chapter 2: Combustion in Gas-phase Processes There are various ways of defining the ignition time, experimentally as well as computationally, for combustion applications. For example, it is often defined as the time at which either the maximum or onset of certain species concentrations is reached, the time at which a specified rate of increase of temperature occurs, the time at which luminous radiant output from the system is first observed, etc. The reported experimental data can vary greatly, depending on which definition was used in the experiments 5, 7 . Thus, it is often useful to select which ignition-delay time definition should be used in numerical computations. CHEMKIN 4.1 allows the user such flexibility. For example, in CHEMKIN'S homogeneous, perfectly-stirred reactor, the ignition time can be defined to be the time during which the maximum amount of heat is released during a combustion process (as indicated by the inflection point in the temperature profile), as well as the time of the maximum of a certain species concentration chosen by the user. CHEMKIN 4.1 further allows the user to input his/her specific definition of the ignition time via the Ignition Criterion User Routine. 2.3.3.2 Project Setup The project file is called closed_homogeneous__ignition_delay.ckprj. The data files used for this sample are located in the samples41\closed_homogeneous\transient\ignition_delay directory. This sample uses the mechanism and thermodynamic data from the University of California, San Diego 6, 8 . This reactor diagram contains only one closed homogeneous reactor. Open the project file. On the Reactor Physical Properties tab of the C1_Closed Homogeneous panel, the problem type is selected as a constant-pressure simulation where the energy equation will be solved (the default). The initial temperature (1000 K) is then input, along with the pressure (1 atm). A volume is not specified as it is not important for the results of this simulation, so the default value of 1 cm 3 will be used for the initial volume. Since this is an isobaric closed homogeneous system, the results in terms of species fraction and temperature will be the same, regardless of the volume value. If surface chemistry were included, the volume-to-surface ratio would be important, but in this case only gas-phase chemistry is present. On the Reactant Species sub-tab, the starting gas mixture is given as 0.02 C 3 H 8 , 0.05 O 2 , and 0.93 Ar. These rather dilute conditions are representative of shock-tube experimental conditions. 7. Combustion Theory, Second Edition, F.A. Williams, Addison-Wesley Publishing Company, The Advanced Book Program, Redwood City, CA, 1985 8. http://maemail.ucsd.edu/combustion/cermech/ © 2007 Reaction Design 28 RD0411-C20-000-001
Chapter 2: Combustion in Gas-phase Processes <strong>Tutorials</strong> <strong>Manual</strong> On the Basic tab of the Solver panel, the End Time of the simulation is set to 0.4 sec. It is important to check the resulting output file after the run is complete to make sure that the End Time is large enough to allow for ignition to occur. If no ignition time is provided at the end of the output file, ignition has not yet occurred and the End Time of the simulation should be adjusted. The Solver Maximum Step Time is set at 1e-6 sec. It is important to make sure that this number is small enough to produce smooth temperature and species profiles. If the profiles are rough, numerous wrong “ignition time” values (based on the numerical and not actual inflection points and peaks) might be printed at the end of the output file. The Time Interval is set at 0.4 sec for Printing and the Time Interval for Saving Data is set to 0.1 sec, which minimizes both the size of the text output file and the computational time. Since only the ignition delay times are of interest here, we do not need most of the CHEMKIN output. On the Output Control tab, on the Ignition Delay sub-tab, the Temperature Inflection Point box is checked as well as Species Maximum Fraction, representing computational ignition time criteria. Species Maximum Fraction is set to the OH species. This means the ignition time will be computed based on the maximum of the OH concentration. The user can choose any species from the pull-down menu. The ignition time will also be computed at the point where the rate of change of temperature with respect to time is the largest (Temperature Inflection Point criteria). The user can choose any or all of the definitions of ignition times provided by CHEMKIN. It is of interest to run the same problem, but vary the initial temperature in order to demonstrate ignition time dependence on temperature as well as its dependence on the chosen ignition time criteria. For this purpose CHEMKIN'S Parameter Study Facility is used. For each Parameter Study, the initial Temperature as well as the End Time, the Solver Maximum Time Step and Time Intervals for Printing and Saving Data are changed. The latter three parameters are changed to reflect the fact that ignition occurs faster at higher initial temperatures and thus less time is needed to obtain an ignition time. The Solver Maximum Time Step is refined accordingly. Please refer to Chapter 3 of Getting Started with CHEMKIN for guidance in setting up a Parameter Study. Once the project is run, the ignition times (based on peak OH concentration and temperature inflection point) for the three runs are printed in the text output files, stored in the IgnitionTimeSample__ folder, contained in your working directory. You can look at the output files by clicking on Click to View Results under Run Results in the Running Parameter Study window. RD0411-C20-000-001 29 © 2007 Reaction Design
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Tutorials Manual 4.1.1 Index Index
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