"This thesis examines preferential diffusion effects in laminar and turbulent premixed combustion. Stretched, fuel-lean, laminar flames of methane, propane and hydrogen are studied experimentally in a counter-flow flame configuration to investigate the effect of Lewis number on stretched flames. Laminar flame results show that a maximum reference flame speed exists for mixtures with Le >= 1 at lower flame-stretch values than the extinction stretch rate. A continually-increasing reference flame speed is measured for Le “ 1 mixtures until extinction occurs when the flame is constrained by the stagnation point.Turbulent counter-flow flame experiments are then performed for these mixtures, using high-blockage turbulence-generating plates to produce turbulence intensities on the order of u'/sLo = 1 to 10. Measurements of average and instantaneous velocity within the turbulent flame are performed by time-resolved particle image velocimetry measurements. Average and instantaneous flame front position is also measured by Rayleigh spectroscopy.Measurements of average turbulent burning velocity demonstrate the ambiguity in definitions of the burning velocity and the difficulty of examining turbulent flame chemistry using averaged measurements. Instantaneous statistics are shown to be superior tools for studying turbulent combustion. The probability-density functions (PDF) of the local flamelet burning velocities for Le >= 1 mixtures show that the instantaneous flamelet burning velocities increase with increasing turbulence intensity and flame stretch. The PDF for the Le ~= 1 mixture has a sharply skewed shape at high turbulence intensity and has a sharp drop-off in probability at a velocity that corresponds with the experimentally-measured maximum reference flame speed from the laminar flame experiments. In contrast, in the Le “ 1 turbulent flames, the most-probable instantaneous flamelet burning velocities increase with increasing turbulence intensity and can significantly exceed the maximum reference flame speed measured in counter-flow laminar flames at extinction.These results are reinforced by instantaneous flame position measurements. Flame front location PDFs show the most probable flame location to be linked to velocity PDFs. Furthermore, hydrogen PDFs are recognizably skewed as u'/sLo increases, indicating a tendency for the Le “ 1 mixture to propagate farther into the unburned reactants. These results support the leading edge theory of premixed turbulent flame propagation for flames in which preferential diffusion effects are expected.In the study of turbulent flames, this work promotes the use of local, instantaneous statistics as a tool for describing experimental results and studying fuel chemistry." --

The combustion of fossil fuels remains a key technology for the foreseeable future. It is therefore important that we understand the mechanisms of combustion and, in particular, the role of turbulence within this process. Combustion always takes place within a turbulent flow field for two reasons: turbulence increases the mixing process and enhances combustion, but at the same time combustion releases heat which generates flow instability through buoyancy, thus enhancing the transition to turbulence. The four chapters of this book present a thorough introduction to the field of turbulent combustion. After an overview of modeling approaches, the three remaining chapters consider the three distinct cases of premixed, non-premixed, and partially premixed combustion, respectively. This book will be of value to researchers and students of engineering and applied mathematics by demonstrating the current theories of turbulent combustion within a unified presentation of the field.

This monograph deals with the structure, generation and stability of flames from a mathematical point of view. It uses a specific mathematical approach to provide a unified theoretical description of fundamental flame phenomena. Its importance stems from the fact that it provides the first clear evidence that combustion can be legitimately treated as a mathematical science as well as an empirical one. The book will be of interest to researchers in combustion, fluid mechanics and applied mathematics, as well as to graduate students taking advanced courses in these areas.