Numerical modeling of ignition and flame propagation in gas engines

Presently, most of the world's energy is produced by combustion. Combustion related emissions, such as NOx and soot, are strictly regulated. In this context, natural gas (NG) is a promising fuel offering e.g. lower CO2 and soot emissions. However, compression ignition of NG in internal combustion engines (ICE) is a challenge due to its low reactivity. This issue is usually managed by spark ignition or by addition of high reactivity fuel into NG. Lean gas combustion is particularly sensitive to cycle-to-cycle variations (CCV). CCV refers to the non-repeatability of the combustion processes in ICEs, which may lead to increased fuel consumption and emissions. The background motivation of this thesis is on better understanding of CCVs. The present thesis seeks answers to some of the challenges related to lean gas combustion such as ignition control and CCV. In order to provide a better understanding of the underlying physical and chemical phenomena, the current multidisciplinary thesis employs elements from three main research areas: 1) applied computational fluid dynamics (CFD), 2) combustion chemistry, and 3) turbulent combustion modeling. The thesis consists of three journal publications. In larger scope, Publications I-III are related to a project on lean gas combustion. Furthermore, the chemical kinetics findings in Publication I were applied in Publications II-III. In Publication I, the research focus is in numerical simulation of the ignition process in single- and dual fuel methane-diesel combustion. Publications II and III focus on utilizing scale-resolved CFD modeling to premixed combustion of methane. The chosen configuration is motivated by previous, international research efforts by direct numerical and large-eddy simulations. In a larger context, the aim is to explore the usage of computational methods for improved prediction of spark ignited engine processes. The present thesis offers the following novel accomplishments. In Publication I, a comprehensive comparison between various state-of-the-art chemical mechanism is provided in dual fuel context. Additionally, details on chemical interactions between high and low reactivity fuels are discussed. In particular, the importance of radicals for dual-fuel ignition is noted. In Publications II and III, the effect of flow and thermal fields on the combustion process variations is assessed with a new level of detail. For example, a connection between initial local flow conditions and CCV was demonstrated. Also, the effect of engine speed on combustion rate and CCV has been discussed using the computational framework herein.