The next step is to set up a mechanism reduction operation in Reaction Workbench. To set up a reduction operation, select the CHEMKIN project from the previous step as both the Initial CHEMKIN Project and Target CHEMKIN Project on the Session Control panel. The Target CHEMKIN Project should always be the project that uses the full mechanism. While the first reduction operation may use the same project as the Initial CHEMKIN Project, the Initial CHEMKIN Project will be different at later stages in the reduction, as it will use the result from a previous reduction step.
Note: Chemkin Project settings should always be identical for the initial and target projects.
The steps to set up a mechanism reduction operation are listed below.
Note: To avoid long names when setting up the initial mechanism reduction operation, provide a short name (for example, test) in the text box Reduction Root Name on the Session Control panel.
Workbench Preferences: Check the box that signifies that we only consider the last ignition point during reduction. The rationale is that you won’t get the final ignition right if the pre-ignition is wrong, so setting the last will assure the others.
Methods: When a high level of mechanism reduction is required, using one or two reduction methods is not sufficient to produce a very small mechanism even if the number of targets and their ranges are limited or tolerances are relaxed. In such situations, it is necessary to use multiple reduction methods in an iterative manner. As an example, it may take more than 20 iterations involving various reduction methods to reduce a gasoline mechanism to a size smaller than 300 or 400 species, depending on the targets. When setting up a new mechanism reduction project, the recommended sequence of reduction methods is populated in the Session Control panel. The reduction sequence can be repeated depending on the need for further reduction. In general, the recommended methods for reducing mechanisms are DRG, DRGEP, and DRGPFA. Avoid the Species Sensitivity analysis method if the mechanism is larger than 500 species due to long run times. After the initial reduction, the linear lumping method can be used. Once the mechanism gets smaller than about 500 species, the FSSA method can be used. We find the following order of methods to work effectively.
Use DRGEP and DRG several times at first. The DRGPFA method can be used in later iterations.
Use the lumping method. Repeat (a) after using lumping. Add the DRGPFA method when the mechanism has fewer than 1000 species.
Use DRGEP+Sensitivity, DRG+sensitivity, and DRGPFA+sensitivity, when the mechanism is lesser than 1000 species. Repeat (a) after each sensitivity method.
Repeat b and c several more times. This typically results in a total of 10+ iterations.
Use the FSSA method. Then repeat d. This will result in a total of 20+ iterations.
An example of the sequence of methods is shown in the snapshot below. You can select any variation of this sequence involving the methods, such as DRG, DRGEP, Lumping, DRG+Sensitivity, DRGEP+Sensitivity, and FSSA.
Note: We do not recommend using the PCA method for most applications. The PCA method is computationally expensive and the reduction is no greater than that gained through DRG or DRGEP. With PCA, a solution is not always guaranteed, especially for large mechanisms with more than 100 species.
Targets and Tolerances: The ignition time is an adequate target for capturing all the important kinetics pathways and for capturing combustion involving both ignition and flame propagation using the reduced mechanism. Additional targets are required to capture emissions.
Table 2.1: Recommended ranges for tolerances in reduced mechanisms and Table 2.2: Recommended tolerances for soot targets in reduced mechanisms below show some recommended ranges for tolerances determined to maintain sufficient accuracy in reduced mechanisms. Typically, more relaxed tolerances will result in a smaller reduced mechanism. Also, a narrower range of targets and fewer targets will result in smaller mechanisms.
Table 2.1: Recommended ranges for tolerances in reduced mechanisms
| Species Mole Fractions | ||||||
|---|---|---|---|---|---|---|
| Target | Ignition Time | CO | NO | NO2 | C2H2 * | C3H6 * |
| Absolute Tolerance | 1.E-6 | 1.E-4 | 1.E-5 | 1.E-5 | 1.E-4 | 1.E-5 to 1.E-4 |
| Relative Tolerance | 10% | 10-30% | 10-30% | 10-30% | 10-30% | 10-30% |
| Comparison setting | End point | Maximum | Maximum | Maximum | Maximum | Maximum |
| * Representative UHC species. Exclude if only NOx and CO are the emissions targets. | ||||||
Table 2.2: Recommended tolerances for soot targets in reduced mechanisms shows the recommended tolerances for soot targets.
Table 2.2: Recommended tolerances for soot targets in reduced mechanisms
| Species from the Soot Model* | ||||||
|---|---|---|---|---|---|---|
| Target | C2H2 | C3H3 & C4H2** | C6H5 & C6H6 | C6H5CH3 (toluene), Naph (naphthalene), A2R5 (acenaphthalene), A4 (pyrene), coronene ** | ||
| Absolute Tolerance | 1.E-4 | 1.E-4 | 1.E-5 | 1.E-6 | ||
| Relative Tolerance | 10-30% | 10-30% | 10-30% | 10-30% | ||
| Comparison setting | Maximum | Maximum | Maximum | Maximum | ||
| * Select all species that participate as reactants in the
soot nucleation and growth steps. Additional species may be required depending on the choice
of the soot model. If soot is not of interest, do not include these species as
targets. ** Select these species only if the MFL soot-surface mechanism is used. | ||||||
Tip: Hovering over the Target Tolerance % displayed for each reduced mechanism on the Run Reduction panel displays the Run number and Error for each target. Sometimes a target may not be relevant for the condition that results in a larger error during reduction. For example, soot precursors may not be relevant for reactor temperatures below 800 K, or a higher level of error tolerance may be acceptable for ignition time at low temperatures for certain applications. In those cases, disable certain runs either by clearing the check box on the Select Conditions panel, or by increasing the Relative Tolerance, without impacting conditions that matter for the end application.
It is good to begin with smaller values of tolerances. After several iterations of reduction methods and obtaining smaller mechanisms, the tolerances can be relaxed as needed to achieve your desired balance between accuracy and mechanism size.
Use default values on the Select Species panel and on the Select Conditions panel.
At this point, run the reduction operation. Note that the entire reduction session takes a fairly large amount of disk space.
Once the smallest reduced mechanism is obtained, consider relaxing the tolerances if further reduction is needed, and repeat the above steps.
Once you have reduced the mechanism to the desired size, we recommend archiving the resulting reduced chemistry set, so that it can be easily unarchived in Ansys Forte, Energico or Chemkin.
Refer to Figure 2.1: Mechanism Reduction session with typical operations to understand the effectiveness of various methods and their computation costs.