Keywords: mechanical engineering, flames, combustion.

The unsteady process of head on quenching of two laminar premixed hydrogen-air flames in one-dimension by mutual annihilation is investigated numerically using a detailed chemical mechanism and realistic transport. The process of annihilation through interactions is inevitable in highly corrugated turbulent flames, and contributes to turbulent flame shortening. Processes leading to mutual annihilation involve interactions that take place in the following stages: (1) interaction of preheat zones, which corresponds to the transport of heat and reactants, (2) interactions of the reaction layers as the flames merge, and finally (3) the process of burnout. The primary objective of this work is to study the effects of differential diffusion during the various events that occur during the unsteady process of annihilation. For the stoichiometric condition two cases are considered namely; a case where transport is based on prescribing non-unity Lewis numbers for all the species and a case with unity Lewis numbers prescribed for all the species. The latter case provides with a reference problem for the other flames considered. Because of the importance of differential diffusion during thermo-diffusive interactions, which are owed to the transport properties of H2, relative to temperature and the oxidizer, two additional cases are considered. They correspond to lean and rich hydrogen-air flames. The results show that differential diffusion of H2 plays an important role in determining the composition of the reacting mixture and thus, affects the final temperature and composition of the products. The differential diffusion of H2 causes a deficiency of the fuel for the stoichiometric and lean cases thereby altering the rates of reactions involving H2 while merger. For the rich case the deficiency caused by the differential diffusion is offset by the presence of excess H2 in the reaction mixture. Due to these conditions for the rich flames and non-unity Lewis number case for the s.

The effects of differential diffusion in the numerical modelling of a turbulent non-premixed hydrogen-air jet flame using a Conditional Moment Closure (CMC) method are investigated. The CMC calculations, which are coupled with computational fluid dynamics (CFD) calculations, relax the commonly used assumption of equal species mass diffusivities. The focus is on the predictions of species mass fractions and temperatures, especially the production of NO. The results of the calculations are compared with available experimental measurements. The formulation of the CMC species transport equation including differential diffusion is presented and the closure of the terms are discussed. Further, the CMC equation for conditional enthalpy is also derived in the present study. The implementation of the CMC equations using two dimensional finite volume method is discussed, including a presentation of the discretised forms of the equations. The results of the CMC calculations including the effects of differential diffusion show that NO mass fractions are increased from the large underpredictions observed for equal diffusivity results near the jet nozzle. Improvements are also found for other species such as H2 and H2O. The results show physical behaviours, such as a shift in the location of the reaction zone and increased reaction rates due to increased diffusion rates of H2. It is also found that differential diffusion effects persist downstream from the nozzle, where the effects are expected to be small, and reasons for the discrepancies are discussed in the present study. The profiles obtained from the CMC calculations show large radial variations, much larger than in equal diffusivity calculations. An analysis isolating the differential diffusion effects of various species shows that the largest changes occur due to the accounting for the differential diffusivity of H2. A budget of the terms in the CMC equations for the differentially diffusing chemical species and enthalpy is also investigated.

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.

The properties of turbulent premixed flames were investigated both theoretically and experimentally. Attention was limited to hydrogen/air mixtures burning as either turbulent jet flames or a freely propagating flames in isotropic turbulence. The research has application to a variety to premixed turbulent combustion processes: underwater metal cutting at great depth, primary combustors for high-speed airbreathing propulsion systems, afterburners, fuel/ air explosions, and spark-ignition internal combustion engines. Major findings of this phase of the investigation are as follows: (1) effects of preferential diffusion are relevent for flames at high Reynolds number, retarding and enhancing the distortion of the flame surface by turbulence for stable and unstable conditions, respectively; (2) local turbulent burning velocity, flame brush thickness and the fractal dimension of the flame surface all increase with distance from the flameholder, with larger rates of increases at larger turbulence intensities; (3) estimates of flame properties using contemporary turbulence models were only fair because these methods cannot account for effects of preferential diffusion, distance from the flameholder and finite laminar flame speeds; and (4) the stochastic simulation duplicated measured trends of flame surface properties for neutral preferential diffusion conditions (the only case considered) but underestimated effects of turbulence (particularly near the flame tip) due to the limitations of a two-dimensional simulation.