Fuels and the detailed modeling of fuel chemistry have become increasingly important for a number of reasons: the usage of “E” and “B” fuel mixtures for ground transportation is growing throughout the world; there are huge reserves of natural gas and their low emissions characteristics make them ideal candidates for power generation and inshore marine applications and, last but not least, the requirement for ever increasing accuracy of combustion modeling and, in particular, emissions prediction dictates detailed chemistry is used to represent the fuel.
To address this, CD-adapco and DigAnaRS have introduced generalized detailed chemistry in which chemistry mechanisms developed in the DARS chemistry modeling software can be used in the combustion models in STAR-CD. This has the clear advantage of combining detailed chemistry within the framework of combustion models that explicitly recognize flame structure and turbulencechemistry interactions and which have been subjected to extensive validation.
Depending on the combustion model being used, both library-based and on-the-fly chemistry using either detailed or reduced mechanisms are utilised. Cell-clustering techniques are available to reduce computation time for users that wish to solve chemistry on a cell-by-cell basis. A major advantage of using library-based methods is that the complex chemistry is performed just once for each fuel and thereafter used as a multidimensional look-up table, substantially reducing run time.
In collaboration with DigAnaRS, usage of detailed chemistry mechanisms and libraries for an ever-increasing range of fuels are available using a DARS fuel licence or, for users wishing to use their own chemistry mechanisms, DARS can be used directly to generate all information required for the supported combustion models.
The ECFM (Extended Coherent Flamelet Model) family of models has been widely used to simulate diesel, gasoline, and gaseous fuelled engines operating under premixed, diffusion and homogeneous autoignited combustion regimes.
The model can be used with in-built libraries for diesel or gasoline or with more extensive fuel mechanisms generated using DARS. In addition to the main combustion event, NOx and soot missions can be calculated over a wide range of operating parameters, including high levels of EGR.
A more recent development of this model is the ECFM-CLEH (Combustion Limited by Equilibrium) model which offers advantages in its treatment of diffusion flames. Experience with this model, coupled to the ELSA spray model, indicates improved predictions of diesel engine combustion over a wide range of operating conditions.
In addition to the ECFM family of models, STAR-CD also incorporates a Progress Variable Model (PVM) which combines the level-set method (also referred to as the g-equation) and the Transient Interactive Flamelet (DARS-TIF) model for post-flame chemistry. Combining models in this way has removed the restriction to specify premixed or diffusion modes of combustion and both can
co-exist in the same simulation.
Both the ECFM-3Z and PVM models offer the possibility of modeling the combustion process as a 2-component mixture, as mentioned earlier. Here, the models allow a chemistry mechanism that correctly represents 100% of component 1 to 100% of component 2.
This situation may exist if the evaporation characteristics of the 2 components are widely different or in dual-fuel engines where the fuels are introduced separately, such as in gas engines operating with a diesel pilot injection. Either (or both) of the components can itself be a complex fuel molecule, for example combustion involving both diesel and gasoline simultaneously can be accommodated using the model.