Chemical Kinetics and Propagation Dynamics of Laminar and Turbulent Flames

Abstract

Recognizing the fundamental and practical importance of premixed flames in combustion research, the present dissertation consists of a series of studies on the chemical kinetics and propagation dynamics of both laminar and turbulent flames. First, starting with the relatively simple but fundamental configuration of the laminar unstretched flames propagating in quiescent environments, the laminar flame speeds of hydrogen flames have been measured, to validate and improve the corresponding kinetic models. Based on the updated model, the analysis on the kinetic networks shows the opposing role of the reaction H+O2(+M)=HO2(+M) under different conditions. Then, the dynamics of laminar flame propagation have been studied; on one hand, in a local perspective, the collision of flames has been studied with the emphasis on the dynamics and flow pattern of the flame corners generated. It is demonstrated that the flame corners are controlled by the interaction of preferential diffusion and kinematic restoration, generating the vortices of considerable strength relative to the turbulent eddies. On the other hand, in a global perspective, we have looked into the evolution of cellular instabilities, including hydrodynamic and diffusional-thermal modes. The influence of preferential diffusion as well as thermal expansion, on the onset of the instability and acceleration of flame propagation, has been studied. Next, recognizing the knowledge of laminar flames, the turbulent flames have been studied thereafter recognizing the effect of hydrodynamic instability. A scaling analysis has been performed on the interaction between the growth of hydrodynamic cells and the wrinkles by the turbulent eddies, within different regimes in the turbulent flame regime diagram. The turbulent flame speeds have been measured experimentally, and subsequently compared with their scaling law under various conditions with the diffusionally neutral mixtures. Furthermore, we shall demonstrate the essential role of the molecular preferential diffusion in the structure and propagation of turbulent flames. It is demonstrated that due to preferential diffusion, the mixtures, whose concentrations are near or beyond the conventional flammability limits, can be rendered to burn strongly in turbulence, with distinctive finger-shape structures on the flame surfaces. Finally, the key remarks in this dissertation have been summarized, with recommendations for further explorations.