Dimension Reduction is a chemistry acceleration method in addition to ISAT storage-retrieval, Dynamic Mechanism Reduction, and Cell Agglomeration, providing faster chemistry calculations with a corresponding loss of accuracy. To learn how to use chemical mechanism Dimension Reduction, see Dimension Reduction in the User's Guide.
Dimension Reduction solves a smaller number (
) of species
transport equations (see Equation 7–1) than the number of species in the full chemical mechanism (
). These are
termed the represented species. The remaining 
species are termed the unrepresented species. In Ansys Fluent,
the unrepresented species are reconstructed assuming that they are
in chemical equilibrium [527]. The Dimension
Reduction algorithm is implemented as follows:
Ansys Fluent solves transport equations for the mixture enthalpy and the
represented species mass fractions.
Transport equations are also solved for the unrepresented element
mass fractions, which are the cumulative mass fraction of each atomic
element in all the unrepresented species.For the reaction step, the initial unrepresented species in a cell are reconstructed by assuming that they are in chemical equilibrium at the cell pressure and enthalpy, subject to the constraints of the represented species mass fractions and the unrepresented element mass fractions. That is, the unrepresented species are determined as those that give maximum mixture entropy with the represented species held fixed, while also satisfying the unrepresented element mass fractions and mixture enthalpy. At the end of this reconstruction step, all
species mass
fractions of the full mechanism are available in the cell.Next, the full detailed reaction mechanism with all
species is integrated for the
reaction time step. Reaction mappings for the 
represented
species are available after the reaction step, and the mappings for
the unrepresented species are discarded.
As Dimension Reduction integrates the full chemical mechanism,
which is computationally expensive, its advantage lies in coupling
with ISAT. An ISAT table with 
dimensions will cover more
of the reaction manifold than ISAT tabulation of the full 
dimensions
in the same time. Consequently, a simulation with Dimension Reduction
is initially not much faster than a simulation with the full mechanism,
but iterations can be substantially faster at later times when the
ISAT table is retrieved. Additionally, the retrieval time with Dimension
Reduction enabled (proportional to 
), is much smaller than that with the Full
Mechanism (proportional to 
). For this reason, Dimension Reduction is only available with ISAT.
Note that Dimension Reduction allows CFD simulations with full chemical mechanisms consisting of more than the Ansys Fluent transported species limit of 700, as long as the number of represented species is less than 700.
For more information, see the following section:
Judicious selection of the represented species is important for simulation accuracy. Boundary and initial species must be included in the represented species list. Species of interest, especially species that are far from chemical equilibrium, such as pollutants, should also be included. Intermediate species that occur in large mass fractions relative to the fuel and oxidizer species should be included, as well as species important in the chemical pathway. For example, for methane combustion in air, CH3 should be included as a represented species since CH4 pyrolizes to CH3 first.
In Ansys Fluent, you will specify the number of represented species, as well as selected represented species, such as the boundary Fuel and Oxidizer species. When the selected represented species are less than the input number of represented species, the remaining represented species are taken from the full mechanism in the order they appear in the Mixture Species list.
After you obtain a preliminary solution with Dimension Reduction, it is a good idea to check the magnitude of all unrepresented species, which are available in the Full Mechanism Species... option in the Contours dialog box. If the mass fraction of an unrepresented species is larger than other represented species, you should repeat the simulation with this species included in the represented species list. In turn, the mass fraction of unrepresented elements should decrease.