"The continued combustion of fossil fuels to fulfill global energy demand is being questioned because of the well-known problem of greenhouse-gas (GHG) emissions, which introduces new carbon, in the form of carbon dioxide, into the environment causing climate change. However, the inherent advantages of combustion-based engines, e.g., energy and power densities, make it hard for other power systems to compete; hence, a leading strategy is to avoid burning fossil fuels by using alternative renewable fuels, such as hydrogen and renewable biofuels. Adaptability with alternative renewable fuels that have variable compositions is referred to as fuel flexibility, which is an important parameter of next-generation combustor design. However, fuel flexibility significantly affects combustor operability properties, such as blowout, flashback, and dynamic stability, mainly due to variations in turbulent burning rates. Changing the fuel and oxidizing-gas mixture composition affects flame characteristics and burning rates through changing: (1) mixture reactivity, which is represented by unstretched laminar flame speed, and (2) mixture diffusivity, i.e., the diffusivity of the deficient reactant and diffusivity of heat. The disparity between thermal and mass diffusivities at the flame front is known as "differential diffusion", which causes stretch sensitivity, and thermal-diffusive instabilities, in flame-front propagation, and is represented by Lewis number, a ratio of thermal-to-mass diffusivities.This thesis investigates the effects of differential diffusion and stretch sensitivity on propagation, stabilization, and structure of lean turbulent premixed flames in the thin reaction zone regime. In the context of fuel flexibility, various fuels and oxidizer-inert mixtures are used to form mixtures with distinct effective Lewis numbers, through changing both fuel diffusivity and thermal diffusivity of the mixture. In these experiments, the unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio, in order to minimize the effects of chemistry. Furthermore, bulk-flow properties and the temperature boundary condition are kept constant; hence, the study highlights the effects of differential diffusion. The experiments are carried out using strained counter-flow flames, in order to study the effects of both components of the flame stretch, i.e., hydrodynamic strain and curvature. Local instantaneous statistics of various flame parameters within the imaged plane are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at various levels of turbulence intensity. These statistics include flame location, flame velocity, and flame-front topology, such as flame stretch, flame-front curvature, and flame surface area.The statistics of various parameters of turbulent flames with distinct effective Lewis number show that the effects of differential diffusion on the burning rates and the structure of turbulent premixed flames are important in highly turbulent flames in the thin reaction zone of combustion. Furthermore, these results are not dependent on the fuel or oxidizing-gas mixture and can be described fully by the effective Lewis number and turbulence intensity. In addition, at constant turbulence intensities, differential diffusion increases the burning rate of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing the local flamelet displacement velocity, and (2) increasing the flame surface area. This thesis shows the need to advance the combustion theory to produce models that can capture the effects of differential diffusion for flames in real-world combustion systems, in order to predict the performance of future fuel-flexible combustors. The experimental results of this thesis provide a valuable dataset for the validation of such theories." --

This book explores the potential of hydrogen combustion in thermal engines and serves as a foundation for future research. Hydrogen, a well-established energy carrier, has been used in internal combustion engines for centuries, but despite progress and industry interest, hydrogen engines have yet to reach mass production. In light of recent efforts to combat climate change with clean energy and environmentally-friendly technologies, the use of hydrogen in thermal engines is gaining momentum. This book examines the unique challenges of hydrogen combustion due to its wide flammability limits, high auto-ignition temperature, and high diffusivity. It reviews current knowledge on the fundamental and practical aspects of hydrogen combustion and considers current developments and potential future advancement.

This book contains the successful submissions to a Special Issue of Energies entitled “Engineering Fluid Dynamics 2019–2020”. The topic of engineering fluid dynamics includes both experimental and computational studies. Of special interest were submissions from the fields of mechanical, chemical, marine, safety, and energy engineering. We welcomed original research articles and review articles. After one-and-a-half years, 59 papers were submitted and 31 were accepted for publication. The average processing time was about 41 days. The authors had the following geographical distribution: China (15); Korea (7); Japan (3); Norway (2); Sweden (2); Vietnam (2); Australia (1); Denmark (1); Germany (1); Mexico (1); Poland (1); Saudi Arabia (1); USA (1); Serbia (1). Papers covered a wide range of topics including analysis of free-surface waves, bridge girders, gear boxes, hills, radiation heat transfer, spillways, turbulent flames, pipe flow, open channels, jets, combustion chambers, welding, sprinkler, slug flow, turbines, thermoelectric power generation, airfoils, bed formation, fires in tunnels, shell-and-tube heat exchangers, and pumps.

Keywords: dilution, co-flow, non-premixed flame, diffusion flame, combustion, flame stability.

Keywords: mechanical engineering, flames, combustion.

Flame propagation in gaseous mixtures generally involve two length scales: one scale is associated with the diffusion processes and characterizes the flame thickness, and the other scale is associated with the underlying flow field. When the hydrodynamic length is larger than the nominal flame thickness, the flame can be viewed as a surface of density discontinuity, advected and distorted by the flow. The analysis of the internal structure of the flame provides expressions for the flame speed and temperature and jump conditions for the velocities and pressure across the flame. The resulting hydrodynamical model is valid for flames of arbitrary shape propagating in general fluid flows, being laminar or turbulent. The present work extends earlier studies by adopting a curvilinear coordinate system attached to the flame front, thus presenting a formulation in coordinate-free form, using a two-reactant scheme thus allowing for mixtures whose compositions vary from lean to rich including stoichiometric conditions, using non-unity and general reaction orders in an attempt to mimic a wider range of reaction mechanisms, allowing all transport coefficients to depend arbitrarily on temperature in order to better represent actual experimental conditions, and incorporating volumetric heat losses which may often lead to flame extinction.