Francesco Contino

Simulations of advanced combustion modes using detailed chemistry combined with tabulation and mechanism reduction techniques

Multi-dimensional models represent today consolidated tools to simulate the combustion process in HCCI and Diesel engines. Various approaches are available for this purpose, it is however widely accepted that detailed chemistry represents a fundamental prerequisite to obtain satisfactory results when the engine runs with complex injection strategies or advanced combustion modes. Yet, integrating such mechanisms generally results in prohibitive computational cost. This paper presents a comprehensive methodology for fast and efficient simulations of combustion in internal combustion engines using detailed chemistry. For this purpose, techniques to tabulate the species reaction rates and to reduce the chemical mechanisms on the fly have been coupled. In this way, the computational overheads related to the use of these mechanisms are significantly reduced since tabulated reaction rates are re-used for cells with similar compositions and, when it becomes necessary to perform direct integration, only the relevant set of species and reactions is taken into account. The proposed approach named tabulation of dynamic adaptive chemistry (TDAC) has been implemented in the Lib-ICE code, which is a set of libraries and applications for IC engine modeling developed using the OpenFOAM® technology. In particular, a modified version of the in-situ adaptive tabulation (ISAT) algorithm has been developed for systems with variable temperature and pressure, and the directed relation graph (DRG) method has been used to reduce the mechanism at run-time. The validation has been carried out with HCCI and Diesel cases both using a simplified case to compare the results obtained with and without TDAC, and a detailed case that is validated with experimental data. For each tested condition, a detailed comparison between computed and experimental data is provided along with the achieved speed-up factors compared to the use of direct-integration.

Combustion and Emissions Characteristics of Valeric Biofuels in a Compression Ignition Engine

AbstractNew-generation biofuels are mainly produced from nonfood crops or waste. Although second-generation ethanol is one of the main options, valeric esters can also be produced from lignocellulose through levulinic acid. However, only few experimental results are available to characterize their combustion behavior. Using a traditional compression ignition (CI) engine converted to monocylinder operation, the engine performances and emissions of butyl and pentyl valerate (BV and PenV, respectively) were investigated. This paper analyses the experimental results for blends of 20%vol of esters in diesel fuel, taking diesel fuel as the reference fuel. The BV and PenV have a smaller cetane number and consequently the ignition delay of the blends is slightly longer. However, engine performances and emissions are not significantly modified by adding 20%vol of esters to diesel fuel. The BV and PenV then represent very good alternative biofuels for CI engines.

Study of the HCCI Running Zone using Ethyl Acetate.

HCCI mode has shown its potential to improve emissions and efficiency in internal combustion engines. In addition, it has open the possibility to use a wider range of fuels than in SI and CI engines. However, the engine running zone is still one of the main challenges that HCCI has to face. We have investigated this zone in the case of ethyl acetate using CFD simulations with a simplified combustion mechanism. This paper describes how ethyl acetate influences the running zone of HCCI engines compared to iso-octane. Biochemical conversion of fermentable biomass can produce large quantities of esters by the reaction of ethanol with volatile organic acids. Among them, ethyl acetate has a low vaporization temperature and a high auto-ignition temperature. Preliminary experiments on SI engines have shown that it ignites more slowly than gasoline even if their physical properties are similar. As fuel oxidation kinetics determine start of ignition, heat release rate and part of the emissions in HCCI engines, we used a detailed mechanism for ethyl acetate in a zero-dimensional analysis. Based on these results, we also developed a simplifed combustion mechanism for this molecule in order to simulate HCCI mode using a commercial CFD software (Fluent). Different engine settings are investigated in an axisymmetric geometry. They are also studied with a simplified mechanism for iso-octane in order to compare the extents of the running zones. Using ethyl acetate, the knock limit is, as expected, expanded to higher loads.

EXPERIMENTAL MEASUREMENTS AND MODELING USING CESFAMB TM SOFTWARE OF THE PRODUCT GAS COMPONENTS ON THE 2MW TH GASIFIER PLANT

The work assesses the performance of a prototype 2M Wth plant as an auxiliary source of energy based on biomass gasification using wood pellets as a fuel. During steady operation, process temperature, proce ss pressure and concentrations of components in the pr oduct gas have been measured the measurements are compared to the simulation results obtained with th e CeSFaMB software. The underlying model in this software is also used to determine the sensitivity of the simulated concentrations to various paramete rs of the gasification process. The results of modeling are i n general agreement with those obtained experimenta lly.