Direct numerical simulations of gaseous combustion in complex geometry

Abstract

This dissertation aims at investigating turbulent combustion and ows associated with complex geometries. Direct numerical simulations (DNS) have been done using the in-house, low Mach combustion solver DINO, for all the simulations in the dissertation. Detailed physicochemical models are used so that the coupling between chemical reactions, turbulent transport, and heat exchange are solved accurately. In the first chapter, a general introduction to the scope of this study is given. In the second chapter, the fundamentals on DNS have been explained. The governing equations solved in DINO are listed. The numerical algorithms and models are clarified. The third chapter is focused on the immersed boundary method (IBM), which is used to deal with the complex geometries. The background of IBM is introduced. The drawbacks of the conventional ghost-cell IBM are discussed and finally a novel, efficient ghost-cell IBM is developed. Different benchmarks ranging from the ow around a cylinder in two dimensions to the pulsating ow inside a patient-specific, three-dimensional cerebral aneurysm have been checked and compared. The fourth and fifth chapters investigate different gaseous combustion applications in simple geometry by DNS. In the fourth chapter, the heat release rate markers for turbulent premixed syngas ame, hydrogen ame, and methane ame are investigated in detail. Systematic DNS results concerning spherically expanding ames are analyzed. Optimal chemical markers are finally found, corresponding to different ranges of mixture conditions. In the next chapter, the hotspot ignition issue in turbulence has been discussed. The relationship between ignition probability, ignition delay time and turbulence intensity has been studied. Many cases are systematically simulated by DNS to get a statistically correct relationship, due to the randomness of the turbulence. The sixth chapter presents turbulent combustion simulations in complex geometries, such as a simplified internal combustion engine (ICE). The coupling equation between the temperature and pressure in a closed domain has been explained. A simplified pre-chamber/ main chamber system is first studied for analyzing ignition by a hot jet. The characteristics of the pre-chamber hot jet and its contribution to the ignition event in main chamber has been investigated in detail. Then, a simplified ICE geometry is simulated for the intake stroke. Finally, a real industrial pre-chamber geometry has been simulated, for a selected range of crank angles. Conclusions and outlook of this dissertation are presented in the last section. Novelties and significance: 1. A novel, efficient immersed boundary method has been developed to solve by DNS turbulent reacting ows in complex geometries; 2. Optimal heat release markers for syngas ames, hydrogen ames, and methane ames have been found by analyzing the DNS results. This is relevant for experimental measurements of ame fronts; 3. Using DNS, safety-related hotspot ignition probability has been found to be strongly in uenced by the turbulence intensity. The relationship between ignition probability, ignition delay, and turbulence intensity has been found; 4. Large-scale DNS has been enabled in realistic, engine-related geometries. The results are useful to guide industrial design and increase engine efficiency.

Zunächst wird ein vereinfachtes Vorkammer/Hauptkammersystem zur Analyse des Problems der Heißstrahlzündung vor der Kammer untersucht. Die Eigenschaften des Vorkammer- Heißstrahls und sein Beitrag zum Zündereignis in der Hauptkammer wurden eingehend untersucht. Anschließend wird eine vereinfachte ICE-Geometrie für den Einlasshub simuliert. Schließlich wurde eine reale industrielle Vorkammergeometrie für einen ausgewählten Bereich von Kurbelwinkeln simuliert. Schlussfolgerungen und Ausblick dieser Dissertation werden im letzten Abschnitt vorgestellt. Neuheiten und Bedeutung: 1. Eine neuartige, effiziente Methode mit eingetauchten Grenzen wurde entwickelt, um turbulente DNS-Reaktionsströmungen in komplexen Geometrien zu lösen; 2. Durch Analyse der DNS-Ergebnisse wurden optimale Wärmefreisetzungsmarker für Synthesegasflammen, Wasserstoffflammen und Methanflammen gefunden. Dies ist relevant für die experimentellen Messungen der Flammenfronten; 3. Bei Verwendung von DNS wurde festgestellt, dass die sicherheitsrelevante Hotspot- Zündwahrscheinlichkeit stark von der Turbulenzintensität beeinflusst wird. Die Beziehung zwischen Zündwahrscheinlichkeit, Zündverzögerung und Turbulenzintensit ät wurde gefunden; 4. Großformatiges DNS wurde in realistischen, motorbezogenen Geometrien aktiviert. Die Ergebnisse sind nützlich, um das Industriedesign zu steuern und den Motorwirkungsgrad zu erhöhen.