DETAILED CHEMISTRY SPRAY COMBUSTION MODEL FOR THE KIVA CODE

Till recently, the application of the detailed combustion chemistry approach as a predictive tool for engine modeling has been a sort of a ”taboo” motivated by different reasons, but, mainly, by an exaggerated rigor to the chemistry/turbulence interaction modeling. The situation has drastically changed only recently, when STAR-CD and Reaction Design declared in the Newsletter of Compuatational Dynamics (2000/1) the aim to combine multi-dimensional flow solver with the detailed chemistry analysis based on CHEMKIN and SURFACE CHEMKIN packages. Relying on their future developments, we present here the methodology based on the KIVA code. The basic novelty of the proposed methodology is the coupling of a generalized partially stirred reactor, PaSR, model with a high efficiency numerics based on a sparse matrix algebra technique to treat detailed oxidation kinetics of hydrocarbon fuels assuming that chemical processes proceed in two successive steps: the reaction act follows after the micro-mixing resolved on a sub-grid scale. In a completed form, the technique represents detailed chemistry extension of the classic EDCturbulent combustion model. The model application is illustrated by results of numerical simulation of spray combustion and emission formation in the Volvo D12C DI Diesel engine. The results of the 3-D engine modeling on a sector mesh are in reasonable agreement with video data obtained using an endoscopic technique. INTRODUCTION As pollutant emission regulations are becoming more stringent, it turns increasingly more difficult to reconcile emission requirements with the engine economy and thermal efficiency. Soot formation in DI Diesel engines is the key environmental problem whose solution will define the future of these engines: will they survive or are they doomed to disappear? To achieve the design goals, the understanding of the salient features of spray combustion and emission formation processes is required. Diesel spray combustion is nonstationary, three-dimensional, multi-phase process that proAddress all correspondence to this author. ceeds in a high-pressure and high-temperature environment. Recent attempts to develop a ”conceptual model” of diesel spray combustion, see Dec (1997), represent it as a relatively well organized process in which events take place in a logical sequence as the fuel evolves along the jet, undergoing the various stages: spray atomization, droplet ballistics and evaporation, reactant mixing, (macroand micromixing), and, finally, heat release and emissions formation. This opens new perspectives for the modeling based on realization of idealized patterns well confirmed by optical diagnostics data. The success of engine CFD simulations depends on submodels of the physical processes incorporated into the main solver. The KIVA-3v computer code developed by Amsden (1993, July 1997) has been selected for the reason that the code source is available, thus, representing an ideal platform for modification, validation and evaluation. For Diesel engine applications, the KIVA codes solve the conservation equations for evaporating fuel sprays coupled with the three-dimensional turbulent fluid dynamics of compressible, multicomponent, reactive gases in engine cylinders with arbitrary shaped piston geometries. The code treats in different ways ”fast” chemical reactions, which are assumed to be in equilibrium, and ”slow” reactions proceeding kinetically, albeit the general trimolecular processes with different third body efficiencies are not incorporated in the mechanism. The turbulent combustion is realized in the form of Magnussen-Hjertager approach not accounting for chemistry/turbulence interaction. This is why the chemical routines in the original code were replaced with our specialized sub-models. The code fuel library has been also updated using modern property data compiled in Daubert and Danner (1989-1994). The detailed mechanism integrating the n-heptane oxidation chemistry with the kinetics of aromatics (up to four aromatic rings) formation for rich acetylene flames developed by Wang and Frenklach (1997) consisting of 117 species and 602 reactions has been validated in conventional kinetic analysis, and a reduced mechanism (60 species, including soot forming agents and N2O and NOx species, 237 reactions) has been incorporated into the KIVA-3v code. This extends capabilities of the code to predict spray combustion of hydrocarbon fuels with particulate emission.