Keywords: flame kernel, turbulent combustion, vortex, Lewis number.

Chemistry-turbulence interactions play a critical role in most practical combustion environments. Understanding the interaction between a flame kernel and a vortex is an important fundamental step. This dissertation presents high-speed movies of combustion luminosity during the interaction of a laminar vortex toroid with a spark -generated premixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts. This dissertation also describes experiments that were conducted to study the Lewis number effect on the flame kernel-vortex interaction. The influence of a time varying strain rate on kernel growth was investigated by studying both lean methane-air (thermo-diffusively unstable) and lean propane-air (thermo-diffusively stable) flame kernels, using both natural CH/OH emission image sequences acquired by a high-speed intensified camera to show details of the disturbed flame kernel growth, and OH-PLIF images to determine the true two-dimensional nature of the interaction. Significant differences are observed in the highly curved regions on the backside of the invading vortex in the two different mixtures. Lewis number effects on local burning rate variations, flame front wrinkling, and pocket formation are reported, and in general, the results are in agreement with predictions from asymptotic theory assuming low stretch rates. Local mixture enrichment by direct injection in the vicinity of the spark plug at the time of ignition can affect flame kernel development and extend the lean limit of flammability of a fuel/air mixture. In the third set of the experiments, flame kernels were ignited in a lean premixed CH4/air mixture with an equivalence ratio of 0.6, while CH4/air mixtur.

Keywords: double flame, flame kernel, vortex, laminar, turbulent, spray combustion.

The response of premixed flames to unsteady stretch is studied via kernel-vortex interactions. In this configuration a spark ignited kernel interacts with a vortex pair (in 2D) or a toroidal vortex (in 3D) of variable strength. Both detailed and simple chemistry approaches are explored. In the detailed chemistry effort a dilute Hydrogen-air mixture is used. The vortex causes significant distortion of the kernel topography. Two distinct regimes; "Breakthrough" and "Extinction" are observed. A continuous increase in flame area and volumetric reaction rate values are observed throughout interactions in the breakthrough regime. However, corresponding consumption speed values are lower than 1-D laminar flame speed values. Detailed chemistry analysis of downstream interaction at the leading edge is carried out. These interactions lead to mutual annihilation at the leading edge in the "Breakthrough" regime. During intermediate stages of the interaction, the mixture in between the interacting flames shows rich burning conditions. As the interaction proceeds the pool of products expands against the counter velocity gradient imposed by the vortex. The decrease in the temperature causes a steady decrease in the rates of reaction of the chain branching reactions causing. The behavior of various reaction layers is dictated to a large extent by their arrangement across the region of interaction. A simple two-step global reaction mechanism is formulated for lean methane combustion. These simple chemistry computations are carried out in an axis-symmetric configuration. Four distinct regimes of interaction: (1) the "laminar kernel" regime, (2) the "wrinkled kernel" regime, (3) the "breakthrough" regime, and the (4) "global extinction" regime are observed. Interactions in the laminar kernel regime show only minor deviations from unperturbed kernel values. Vortices in the wrinkled kernel regime impose substantial stretch on the kernel causing major deviations from unperturbed kernel values. A sharp drop in the flame surface area and the integrated reaction rate is observed during breakthrough. The primary mechanism governing global extinction is downstream flame interactions. A turbulent combustion diagram was derived for kernel-vortex interactions, which delineates conditions at each regime.

Investigations into the complex interaction between combustion chemistry and the hydrodynamic flow field have been performed in both laminar and turbulent flames. Lifted turbulent spray flames were studied to gain insight into the role of oxidizer entrainment and mixing in the development of double flame structures in polydisperse ethanol sprays. OH Planar Laser-Induced Fluorescence (PLIF) has been used to demarcate reaction zone contours, while smoke visualization illuminates the dynamics between entrained oxidizer and the evaporating fuel spray. Results show that the double flame structure consists of an outer diffusion flame with an inner structure that transitions from mixing controlled to partially premixed combustion downstream of the leading edge. Without air co-flow, the inner branch of the double structure burns intermittently with large regions of local extinction often observed, resulting from a high droplet flux and possibly high strain/scalar dissipation rates. Addition of 0.29 m/s co-flow lifts the flame base enough to increase air entrainment and enhance inner zone combustion. The inner zone burns continuously, with no apparent local extinction, due to turbulent mixing between entrained oxidizer and fuel vapor generated by easily vaporized droplets present in the recirculations along the shear layer. The polydisperse spray distribution yields larger droplets which are able to cross the inner reaction zone and vaporize in the hot region bounded by the double flame structure. This region serves as a fuel source to feed both the stable outer diffusion flame and the diffusive structures of the inner zone. In both cases, the flame leading edge stabilizes in the low-speed flow just outside the periphery of the spray cone, where flame propagation against the incoming flow is possible. The second phase of the research analyzed the response of laminar hydrocarbon-air flames to unsteady stretch via flame kernel-vortex interactions. A spark-ignited laminar premi.

The understanding of complex premixed combustion reactions is paramount to the development of new concepts and devices used to increase the overall usefulness and capabilities of current technology. The evolution from laminar spherically propagating flames to turbulent chemistry is a logical and necessary process to study the complex interactions which occur within any modern practical combustion device. Methane-air flames were chosen to observe the mild affects of thermo-diffusive stability. Five primary propane equivalence ratios were utilized for investigation: 0.69, 0.87, 1.08, 1.32, and 1.49. The choice of equivalence ratio was strategically made so that the 0.69/1.49 and 0.87/1.32 mixtures have the same undiluted flame propagation rate, dr/dt. Therefore, in the undiluted case, there are two flame speeds represented by these mixtures. Three vortices were selected to be used in this investigation. The vortex rotational velocities were measured to be 77 cm/s, 266 cm/s and 398 cm/s for the “weakâ€, “medium†and “strong†vortices, respectively. Ignition of the flame occurred in two ways: (1) spark-ignition or (2) laser ignition using an Nd:YAG laser at its second harmonic in order to quantify the effect of electrode interference. Accompanying high-speed chemiluminescence imaging measurements, instantaneous pressure measurements were obtained to give a more detailed understanding of the effect of vortex strength on reactant consumption rate over an extended time scale and to explore the use of a simple measurement to describe turbulent mixing. Further local flame-vortex interface analysis was conducted using non-invasive laser diagnostics, such as particle image velocimetry and planer laser induced fluorescence of the OH radical. The dependence of heat release rate on temperature provides an estimation of the strain rate dependence of the reaction rate.