Large scale numerical simulation of flow and combustion in internal combustion engines
Direct numerical simulation (DNS) and large eddy simulation (LES) approaches that employ detailed chemistry and transport properties will be used to study the mechanisms responsible for the onset of auto-ignition, and the structures and dynamics of the reaction front propagation in partially premixed charge compression ignition (PPC) conditions. This project is motivated by the public concerns on global warming due to emissions of the green house gas CO2, as well as emission of pollutants (soot, NOx, CO, and unburned hydrocarbons) from fossil fuel combustion in internal combustion engines. The Swedish and international IC engine industry and research community have spent great effort in developing clean combustion engines using the concept of fuel-lean mixture and low temperature combustion such as PPC engines. There are several technical challenges in applying the PPC concept to practical engines running with overall fuel-lean mixture, low temperature combustion. It is not known what the optimized partially premixed charge is for a desirable ignition, while at the same time maintaining low emissions. It has been noticed experimentally that the PPC process is rather fuel dependent. Recent studies have shown that the process is rather sensitive to incylinder turbulence. The goals of the DNS work are (i) to achieve improved understanding of the physical and chemical processes in the PPC processes; (ii) to generate reliable data for validating simulation models for analysis of the class of combustion problems. The goals of the LES work is to make use of the DNS data to develop and validate sub-grid-scale combustion models for IC engine combustion, including PPC, and to apply the models to real engine simulations. This shall lead to development of new strategies to achieve controllable low temperature combustion IC-engines, while maintaining high efficiency and low levels of emissions (soot, NOx, CO and unburned hydrocarbons). The following fundamental issues are to be investigated: a) the onset of auto-ignition under different stratification in the charge and temperatures; b) the structures and dynamics of the reaction fronts under different stratification conditions; c) the effect of turbulence, the stratifications in temperature and charge, and the loads (mean temperature and pressure in the cylinder) on the auto-ignition and reaction front propagation; d) development of predictive tools for the analysis of lean PPC processes. Generic cases will be considered in the DNS study. The computational domain will be a cubic shaped constant volume enclosure with periodic conditions at the boundaries. The fuels will be generic but also of industrial relevance, e.g. hydrogen, syngas, methanol, and n-heptane, whose chemical kinetic mechanisms are mature enough to allow for generation of reliable DNS database. Real engine cases will be considered in the LES study. The fuels will be diesel and gasoline surrogate (e.g. PRF fuels) and methanol. The LES cases will be designed based on the engine experiments carried out at Lund University’s competence center for combustion process (KC-FP).