Experimental Investigations of Fluid-Chemistry Interactions

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Title: Experimental Investigations of Fluid-Chemistry Interactions
Author: Marley, Stephen K.
Advisors: Kevin M. Lyons, Committee Co-Chair
William L. Roberts, Committee Co-Chair
Abstract: 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 premixed flame kernel, interacting with a single axisymmetric vortex toroid of variable strength, was investigated to quantify the transient coupling of flame chemistry and stretch rate. High-speed (4500 frames/second) broadband chemiluminescence imaging of natural CH*/OH* flame emission was utilized to map the available parameter space and probe both flame-flow and flame-flame interactions. Both methane and propane fuels were used with nitrogen diluent to carefully control flame speed. Methane flames were studied at equivalence ratios ranging from 0.64 to 1.13, with all flames diluted to the Φ= 0.64 flame propagation rate observed in the absence of the vortex. Emphasis was placed on propane-air flames since the heavier hydrocarbon fuel is considered more suited to fundamental studies related to internal combustion engine applications. Therefore, the equivalence ratios investigated with propane range from 0.69 to 1.49 with dilution levels controlled to allow the observation of three flame propagation rates, corresponding to the undiluted displacement speeds for Φ= 0.69,0.87,1.00. Detailed discussion of the flame kernel-vortex results is provided only for the cases at the Φ= 0.69 flame propagation rate, although measurements of unstretched laminar burning velocity and Markstein number are provided for all mixtures. Characterization of the invading vortex ring was performed using Particle Image Velocimetry (PIV) under cold flow conditions. Three vortex strengths, corresponding to different rotational velocities, were chosen to interact with the growing spherical flame. In the weakest flames, the strongest vortex has the ability to penetrate completely through the flame kernel and initiate a second propagating flame that connects to the original flame surface. The added ability to control the timing of spark ignition relative to vortex generation facilitates a multitude of different interactions to be observed for a given set of experimental parameters. The CH4-O2-N2 flames exhibited a weak Lewis number dependence in most cases, while the C3H8-O2-N2 flame structures transitioned from thermo-diffusively stable to unstable behavior as the equivalence ratio was increased above unity. These cases of thermo-diffusive instability, characterized by cellular flame structures, are especially intriguing given the dramatic response of the kernel combustion to the perturbation of the vortex. The effect of unsteady stretch, induced by a vortex toroid, has been shown to greatly augment flame propagation in many of these laminar premixed flames, while both local and global extinction are possible under certain conditions.
Date: 2005-04-29
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/3093

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