Influence of the chemical mechanism in the frame of diesel-like CFD reacting spray simulations using a presumed PDF flamelet-based combustion model

The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignitionprocess and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanismfor reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled.Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors butdirectly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further appliedto CFD engine simulations.This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustionmodel to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanismswidely accepted by the scientific community as a way to figure out the influence of chemistry in the keycharacteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spiteof providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimentalresults, the structure of the flame presents noticeable differences, especially in the low and intermediatetemperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has animpact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalenceratio-temperature maps. These differences are expected to be relevant considering the implications when couplingpollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidenceshow the observed differences are consistent and moderately dependent on the ambient conditions.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4746

CFD Predictions of CO Emission Trends in an Industrial Gas Turbine Combustor

CFD predictions of emissions such as NOx and CO in industrial lean-premixed gas turbine combustors depend heavily on the degree to which the complexity of turbulent mixing and turbulence-chemistry interaction in the flow-field is modeled. While there is much difficulty in obtaining detailed and accurate internal data from high pressure combustors, there is a definite need for accurately understanding the flow physics towards the improvement of design. This work summarizes some experience with using the RANS and LES approaches in a commercial code, Fluent 6.3, to predict CO emissions and temperature trends in the two-stage Rolls-Royce RB211-DLE combustor. The predictions are validated against exit emissions (obtained from exhaust gas analysis) and some thermal paint tests for qualitative agreement on flame-stabilization. The upstream geometry (plenum and counter-swirlers) was included in order to minimize the effect of boundary conditions on the combustion zone. The presumed pdf approach as well as finite-rate chemistry models using the eddy dissipation concept were used to compare the predictions. It was found that there was a very significant benefit in moving to more advanced turbulence modeling methods to obtain realistic predictions in a confined, swirling burner. Thermal paint tests indicated that flame stabilization and temperatures (and therefore CO) was incorrectly predicted in the RANS context. LES results, on the other hand, more accurately predicted flame stabilization with corresponding improvements in the exit CO predictions. Ongoing work focuses on the variations that can be expected by varying discretization schemes, combustion models and sub-grid turbulence models as well as obtaining detailed internal data suitable for LES comparisons.