Numerical modeling of thermal stratification driven combustion regimes

This dissertation is related to the research areas of computational physics and chemistry. The purpose of the study is to determine the role of temperature stratification in the secondary combustion of gasoline surrogates within the end-gas of spark ignition (SI) engines. Specifically, it highlights the influence of the Negative Temperature Coefficient (NTC) on the prevalence of various combustion regimes, including spontaneous, subsonic, and supersonic ignition, detonation, and deflagration (flame), within one and two-dimensional (1D/2D) scenarios. Gaining a deeper understanding of these phenomena facilitates understanding of the interaction between pressure oscillations, reaction front propagation, and fuel chemistry at SI engine-relevant conditions. Computational fluid dynamics (CFD) simulations of reacting flows, incorporating finite rate chemistry, efficient simulation methods, and high-performance computing, enable the investigation of the underlying physical and chemical processes in such reacting flows. This dissertation consists of three appended journal publications and their brief summary. In Publication I, a diagnostic approach distinguishes between secondary spontaneous ignition and flame of primary reference fuel (PRF) and PRF-ethanol (PRF-E) blends, in 1D setups. In Publication II, established 1D regime diagrams (detonation peninsula) are reconstructed for the PRF and PRF-E mixtures analyzing induced pressure levels from hotspot ignition (i.e. subsonic/supersonic ignition, and detonation modes). In Publication III, 2D phenomena are explored involving converging shock/detonation fronts and shockwave reflection-induced detonation, using the novel ARCFoam numerical framework. In essence, Publication I explores the prevalence of combustion modes at two extremes within the range of studied modes, namely flame and spontaneous ignition in a 1D setting. Publication II focuses on the pressure oscillations induced by hotspot ignition regimes within the 1D framework. Lastly, in Publication III, further geometrical considerations are introduced by investigation of hotspot ignition in 2D scenarios. The main conclusions of this dissertation are as follows: 1) NTC chemistry of the studied fuels is the dominant factor in manipulating the reactivity, inducing secondary combustion modes, and amplified pressure levels. 2) Given the dominance of auto-ignition in the presence of NTC and the impact of thermal stratification on ignition characteristics, density-based 1D/2D CFD simulations are preferred for analyzing underlying combustion regimes. 3) While the detonation peninsula predicts the initial ignition regime induced by hotspot ignition, NTC chemistry and 2D effects such as front curvature and shock reflection can significantly alter combustion dynamics. 4) Apart from the complexity of the flow field and the employed numerical techniques, the resourceintensive nature of finite rate chemistry solution remains the primary bottleneck in such reactive CFD simulations.