Computational Studies of Autoignition and Combustion in Low Temperature Combustion Engine Environments.

Combustion;Low Temperature Combustion Engines;Autoignition;Low Dimensional Manifolds;Direct Numerical Simulation;Computational Fluid Dynamics;Mechanical Engineering;Engineering;Mechanical Engineering

Bansal, GauravWooldridge, Margaret S. ;

University of Michigan

关键词: Combustion;    Low Temperature Combustion Engines;    Autoignition;    Low Dimensional Manifolds;    Direct Numerical Simulation;    Computational Fluid Dynamics;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/63726/gbansal_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Computational studies are performed on the autoignition and combustion characteristicsencountered in modern internal combustion (IC) engines in which combustion isachieved primarily by autoignition of the reactant mixture. High-fidelity computationaltools with varying levels of complexity are employed in order to systematically investigatethe phenomena under consideration.As a first baseline study, the effects of unsteady temperature fluctuations on the ignitionof homogeneous hydrogen-air mixture in a constant-volume reactor is studied both computationallyand theoretically using asymptotic analysis. It is found that ignition delay showsa harmonic response to the frequency of imposed temperature fluctuation and the responsemonotonically attenuates as frequency increases.The effects of spatial transport on the autoignition characteristics are next investigatedusing a one-dimensional counterflow configuration, in which unsteadyscalar dissipation rate represents the effects of turbulent flow field. A newly definedignitability parameter is proposed which systematically accounts for all the unsteady effects. n-Heptane, which exhibits a two-stage ignition behavior is studied next using similarconfiguration. Interestingly, two-stage ignition is observed even at significantly high initialtemperatures when the ignition kernel is subjected to unsteady scalar dissipation rate. Mechanism for the appearance of two-stage ignition in unsteady conditions is found to be not chemical but is attributed to the spatial broadening of the ignition kernel and subsequent radical losses.Guided by the above findings, multi-dimensional simulations are conducted to investigatethe effects of spatial fluctuations in temperature and composition. Non-reacting 3DRANS engine simulations are first conducted to investigate different mixture formationscenarios that might exist in LTC engines prior to autoignition. Small-scale effects of thesedifferent mixture formation scenarios on the autoignition and subsequent front propagationare then studied using high-fidelity direct numerical simulation (DNS).In the last part of dissertation, a novel principal component analysis (PCA) based approachis used to identify intrinsic low-dimensional manifolds in a complex autoignitingenvironment. A small number of principal components (PCs) are found to very well representthe complex reacting system. The approach thus provides a promising modelingstrategy to reduce the computational complexity in solving realistic detailed chemistry inmixed-mode combustion systems.