(425f) Numerical Investigation of Advanced Engine Combustion with CFD and Detailed Chemical Kinetics Using On-the-Fly Reduction

In combustion systems, the charge often times has significant inhomogeneities due to various sources, including incomplete mixing of fresh charge with residuals, vaporization, heat transfer, and turbulent transport [1]. These charge inhomogeneities result in both temporally and spatially varying mixing effects and chemical kinetics in the combustor, which are important for the evolution of the combustion event. To achieve a better control of the combustion system, a thorough understanding of in-cylinder processes is required. Numerous experimental investigations of in-cylinder charge stratification were conducted using advanced laser-imaging diagnostics [2]. However, very few numerical investigations were conducted due to the complexity of integrating detailed chemical kinetics and complex CFD models. In recent studies, detailed chemical kinetic mechanisms have been developed to predict combustion behaviors including ignition delay, combustion modes, and pollutant formation [3, 4]. These detailed kinetic mechanisms provide a comprehensive description of fuel chemistry, however, they usually consist of hundreds of species and thousands of reactions. Therefore, integrating these detailed mechanisms in engine CFD computations are expensive, and often times prohibitive. Numerous efforts have been devoted to tackling the complexity of this integration. These efforts can be divided into two major categories: detailed chemical kinetics with simplified flow models, and complex CFD computations with simplified reaction schemes. Simplification of flow models is usually achieved using the so-called ?zone? approach [5] while approximation on chemistry relies on mechanism reduction [6-8]. However, existing approaches either lack the ability to dynamically develop accurate reduced mechanisms based on local conditions or are unable to provide a detailed characterization of reactive flow in each computational cell. Recent advances in experimental and kinetic modeling capabilities have provided new insights into combustion processes. This paper focuses on the integration of detailed chemical kinetics and CFD models with efforts on the development of an efficient on-the-fly kinetic reduction approach. In the proposed approach a reduced mechanism is identified based on local conditions for every computational cell and time step of the CFD calculation, realizing so-called on-the-fly reduction. The reduction methodology employs instantaneous element flux analysis to identify redundant species and reactions for given conditions. The fuel is n-heptane and a detailed mechanism including 653 species and 2827 reactions [4] is used in the study. KIVA-3V [9] is used as the CFD framework and CHEMKIN [10] is employed to formulate chemistry and transport. Homogeneous Charge Compression Ignition (HCCI) engine was selected as an illustrative case to demonstrate the on-the-fly kinetic reduction approach, with special attention on multiple ignition points and stratified charge in HCCI engines. The direct injection mode of HCCI engine using room temperature, atmospheric-pressure spray is investigated. The initial conditions of the numerical model simulate the in-cylinder situation observed in laser-induced exciplex-fluorescence (LIEF) experiments [2]. On-the-fly reduction predictions of species concentrations, temperature, and pressure are in excellent agreement with solutions obtained with the detailed mechanism but at tremendously reduced CPU time. By coupling detailed chemical kinetics and engine CFD, the proposed approach enables detailed characterization of in-cylinder combustion behavior of HCCI engines.

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