Modeling heavy-duty diesel engines using tabulated kinetics in a wide range of operating conditions

Abstract

Fast and high-fidelity combustion models including detailed kinetics and turbulence chemistry interaction are necessary to support design and development of heavy-duty diesel engines. In this work, the authors intend to present and validate tabulated flamelet progress variable model based on tabulation of laminar diffusion flamelets for different scalar dissipation rate, whose predictability highly depends on the description of fuel–air mixing process in which engine mesh layout plays an important role. To this end, two grids were compared and assessed: in both grids, cells were aligned on the spray direction with such region being enlarged in the second one, where the near-nozzle and near-wall mesh resolution were also improved, which is expected to better account for both spray dynamics and flame–wall interaction dominating the combustion process in diesel engines. Flame structure, in-cylinder pressure, apparent heat release rate, and emissions for different relevant operating points were compared and analyzed to identify the most suitable mesh. Afterwards, simulations were carried out in a heavy-duty engine considering 20 operating points, allowing to comprehensively verify the validity of tabulated flamelet progress variable model. The results demonstrated that the proposed approach was capable to accurately predict in-cylinder pressure evolution and NO x formation across a wide engine map.

CFD analysis of combustion and emission characteristics of primary reference fuels: from transient Diesel spray to heavy-duty engine

“…Regarding the turbulent spray modeling, a set of models falling into such technique were compared in [29,30], including tabulated well-mixed model (TWM), tabulated representative flamelet interactive model (TRIF), tabulated presumed PDF approach (TPPDF), and the tabulated flamelet progress variable approach (TFPV). The results proved that the TFPV model based on approximated diffusion flamelets [31][32][33] performs better in the description of spray flames due to the consideration of turbulence-chemistry interaction and local distribution of scalar dissipation rate, which has also been comprehensively validated by authors in the modeling of spray flames with single and double injections [34,35], as well as light-and heavy-duty Diesel engines [29,36,37].…”

“…Besides, its in-cylinder evolution is also correctly described: NO x accumulates during the combustion process and stabilizes at its maximum value when in-cylinder thermodynamic conditions are not able to promote any additional NO x formation. Further validation of this NO x model can be found in [29,37]. Following such satisfactory accuracy of pressure, AHRR, soot, and NO x predictions, a numerical cooptimization of fuel auto-ignition quality and injection timing was performed for the conventional Diesel operating condition (HTSID) considering ten PRFs from PRF0 to PRF90 with 10% increment in iso-octane mass fraction.…”

“…4, where the red arrow depicts the injection direction. The mesh structure is similar to what is generally employed in practical IC engine simulations [44,37]: the grid is refined in the vicinity of injector and its resolution progressively decreases when moving downstream of the injector and the combustion vessel walls to reduce the computational time, having about 0.4 million cells with a minimum size of 0.2 mm. The applied turbulence and spray sub-models Fig.…”

“…Large temperature and pressure ranges were considered to take into account all the expected thermodynamic states encountered in IC engine simulations. Such choice of chemistry table discretization has been comprehensively validated and assessed in [37,36,29], which is a good compromise between accuracy and table size, and further refinement of the table resolution does not show any improvement in the results. Note that the unsteady diffusion flamelet calculations were performed only within a 750-1200 K temperature range to reduce the computational cost, and tables were later extended to 400 K with values from homogeneous reactor chemistry tables.…”

CFD Modeling of Reacting Diesel Sprays with Primary Reference Fuel

“…The total number of mesh cells is approximately 0.4 million with optimal cell sizes of 0.2 mm in the vicinity of the nozzle. The mesh structure is similar to what is generally employed in practical IC engine simulations [40,53]: the grid is refined near the injector and its resolution progressively decreases when moving downstream of the injector and the combustion vessel walls to save the computational time. The oxidation of PRFs was modeled using two reduced mechanisms involving the low-temperature chemistry: the POLIMI kinetic mechanism proposed by Frassoldati et al [54,55,56], consisting of 156 species and 3370 reactions; the LLNL kinetic mechanism developed by Mehl et al [57], containing 679 species and 5935 reactions.…”