Multidimensional Simulation of Cool Diffusion Flame
Abstract
Understanding the behavior of spherical cool diffusion flames in microgravity conditions is crucial for advancing combustion research. This knowledge has significant implications for both fundamental science and practical applications, such as spacecraft fire safety and the development of advanced, high-efficiency engines that benefit from low-temperature chemistry. Spherical cool diffusion flames are typically characterized by peak temperatures between 500 and 1000K and are often followed by the radiative extinction of a hot flame. Some low-temperature ignitions can sustain indefinitely at around 700K, leading to chain-branching pathways that produce intermediate species such as C2H4, H2O2, HO2, CH2O, CH3O, and HCO. In some cases, the temperature continues to rise after the first stage ignition, and upon reaching a critical limit, a second stage ignition may occur, resulting in a hot flame. Controlling low-temperature ignition, cool flames, and the transition from cool to hot flames is therefore critical for ignition timing, engine knock prevention, and emissions reduction. Numerous experiments have been conducted aboard the International Space Station to investigate spherical cool diffusion flames in a microgravity environment. However, these experiments are often costly and associated with various uncertainties. Computational Fluid Dynamics (CFD) simulations, on the other hand, provide detailed insights into chemical kinetics, heat and mass transfer, and fluid dynamics. In this thesis, a burner-supported spherical hot and cool flame in a normal flame configuration was studied. Long-duration hot flames were observed for ethylene, while cool flames were observed for n-butane. An optically thin radiation model has been integrated to observe radiative extinction. To reduce computational expense and accurately capture the Negative Temperature Coefficient (NTC) region, a chemical kinetic model reduction was performed. A hot and cool flame flux analysis was conducted to identify key reaction pathways. The heating effect of the burner was found to play a crucial role in sustaining the cool flame, leading to the introduction of a conjugate heat transfer effect. Furthermore, this thesis extends the research by developing a CFD code to model droplet evaporation based on the Volume of Fluid (VOF) method. This establishes a foundation for future research, aiming to integrate a combustion solver with the current evaporation model to study cool flames in droplet combustion.