Note: We strongly recommend keeping the default options for the chemistry solver, as these have been tested to provide the best accuracy and speed.
Use this panel to define chemistry-solver options. These include solution tolerances, as well as options that provide maximum efficiency in terms of computational speed. The options are described briefly below:
Absolute Tolerance for Chemistry Solver: This option sets the absolute tolerance for the species mass fractions as they are solved with detailed kinetics. The default of 1.E-12 will assure that all major and minor species that can contribute to a variety of engine phenomena are considered throughout the simulation.
Relative Tolerance for Chemistry Solver: This option sets the relative tolerance for the species mass fractions as they are solved with detailed kinetics. The default of 1.E-5 roughly says that the error in the converged solution will be around the fifth significant digit of the value reported. This assures that major species are resolved sufficiently to predict their impact on minor or trace species.
Use Dynamic Adaptive Chemistry: This option turns on a dynamic adaptive chemistry option, which uses on-the-fly skeletalization of the detailed reaction-kinetics mechanism, based on the Directed Relation Graph with Error Propagation (DRG-EP) method, as described in the Ansys Forte Theory Manual. This option is off by default, but can be turned on when very large reaction mechanisms (that is, with greater than 100 species) are used. It can potentially reduce the computational time significantly. It assures that only the relevant chemistry terms are being calculated at any given time and at each location in the computational grid, based on local conditions.
Search Tolerance: Controls the level of accuracy. The default value of 0.0001 has been tested and shown to provide good accuracy for a wide variety of problems. A larger tolerance would tend to lower the accuracy but might speed up the computation.
Size Threshold for Activating DAC: This is a trigger for activating DAC. It represents the ratio of species in the reduced mechanism to the number of species in the full mechanism. DAC will be enabled when this ratio is smaller than the threshold set here. A value of 0.6 is recommended. A value of 1.0 means DAC is always enabled.
DAC Initial Species List: Select the initial species to be tracked by the DAC algorithm. Ansys recommends selecting all (premixed or injected) fuel species, plus CO and HO2. You may also want to include emission-related species, such as NO, NO2 and C2H2 to improve accuracy for emissions calculations. If you select the DAC option, you must select the initial species.
Premixed Fuel Composition: Define the premixed fuel composition. (See Mixture Editor for details.)
Use Dynamic Cell Clustering: This is another option that can greatly speed the computation, by assuring that chemistry computations are not unnecessarily repeated in cells that have the same thermochemical state. The determination of thermochemical state similarity is based on 2 features, by default, and these are fuel-air equivalence ratio within the cell and cell temperature. The default tolerances for these variables are recommended.
Use Autoignition-Induced Flame Propagation Model: Enables initialization and propagation of flames induced by a cluster of autoignited computational cells. Once such a flame is initialized, the heat release and species conversion rate in the flame-containing cells will be calculated either by flame propagation or by the perfectly-stirred-reactor assumption.
Use Turbulence Kinetics Interaction Model: Turns on the Turbulence Kinetics Interaction model, including an option for the Mixing Time Coefficient, which controls the turbulent mixing time scale. A larger value increases the effect of turbulent mixing on reaction rates.
Use Species Lumping When Chemistry is Deactivated : Species lumping can be used to lump the mass of a group of trace species whose mass fractions are below a threshold into one or more absorbing species when chemistry is deactivated. After the lumping, the mass of those trace species become zero and they will no longer participate in flow transport. Note that species coming into the computational domain through sprays or open boundaries and species involved in GT-Power co-simulation are always kept and transported. The purpose of species lumping is for reducing the computational cost associated with transporting large number of species. When species lumping is applied, the list of species kept in flow transport will be printed in FORTE.log. The number of transported species is also reported in chemsolver.csv.
Mass Fraction Threshold For Marking Trace Species: A species will be marked as trace species if its mass fraction in the whole computational domain is smaller than this threshold value. The mass of identified trace species will be lumped into the absorbing species in each computational cell.
Absorbing Species: The absorbing species are used to absorb the total mass of all identified trace species in each computational cell during species lumping. The total mass of the trace species is allocated among the absorbing species proportionally based on the mass fractions of the absorbing species. If this user-specified absorbing species list is empty, the species that has the largest mass fraction in the domain will be used as the absorbing species.
Core Species Set: The core species set contains species that will not be marked as trace species, even though their mass fractions inside the whole domain may be lower than the mass fraction threshold. Recommended core species include: N2, O2, CO2, H2O, CO, NO, NO2, OH, H, H2, C2H2, C6H6, soot. Note that species coming into the computational domain through sprays or open boundaries and species involved in GT-Power co-simulation are always kept as core species by default, even if they are not specified on this list.
Activate Chemistry: In some cases, there is no need to solve all the chemistry-related equations during certain portions of the simulation, because we know a priori that very little or no chemistry will take place. For example, prior to fuel injection or before the temperature reaches 600-700 K, we would expect little chemistry to happen. You can select to have chemistry Always On, Always Off, or activated Conditionally. If you choose Conditionally, you can activate chemistry when a certain temperature is reached, at a certain crank-angle interval time interval, at a certain time, or after full injection or the first spark event. When you choose Conditionally, you can select which criteria must be satisfied before chemistry is activated. You can choose to place criteria on a threshold temperature, at a certain crank angle interval, at a certain time interval, after fuel injection or after the first spark event. For a CA-interval based activation control used in engine cases, the user-supplied starting and ending crank angle values will be converted to fit in the range of [0, 720) °CA (for 4-stroke engines) or [0, 360) °CA (for 2-stroke engines). The activation interval will then be treated as cyclic and repeated on a 720-degree schedule (4-stroke) or 360-degree schedule (2-stroke). In multi-cylinder engine cases, the CA-interval based activation control is applied with respect to each cylinder's local time frame. Note that all the specified criteria must be satisfied before chemistry is activated for a given cell.
Note: The chemistry activation controls using spark timing or injection timing will be ignored after the first engine cycle. Because of this, Ansys recommends using controls based on crank-angle intervals for multicycle simulations.
Disable Chemistry in the Unburned Region During Flame Propagation: Selecting this option causes chemistry calculation to be disabled in the unburned region when flame propagation is used (initiated by spark-ignition), where the unburned region is identified as the region ahead of and outside of the propagating flame front (represented by the G=0 iso-surface). This option is typically useful in diagnosing autoignition during flame propagation.