Vector-scalar imaging in combustion using PIV and LIF

Abstract

The effect of turbulent flow-fields on a premixed flame-front has been of considerable interest in the field of combustion research. Modern laser diagnostics allow the measurement of important parameters such as species concentration, temperature and flow velocity with an accuracy, resolution and rate of data acquisition that has not been available with physical probes. The ability to simultaneously gather data over a two or three-dimensional region permits powerful digital analysis of the fundamentals of combusting systems. The aim of this project was to demonstrate the possibility of combining twodimensional species concentration and velocity measurement techniques, in order investigate the interaction of a turbulent flow-field at a premixed flame-front. Digital Particle Image Velocimetry was investigated as a diagnostic for making velocity measurements in turbulent combustion environments. The standard monochromatic two-pulse technique produces a 180° ambiguity in the direction of each velocity vector. In order to resolve the velocity direction directly, a feature that would be important in combustion studies, a novel variation of PIV was developed. Three Pulse Digital PIV used three unequally spaced monochromatic laser pulses to code the flow direction into the images. A triple-correlation function extracted the correct flow direction and resulted in less vector drop-out, since noise correlations were greatly reduced. A complete analysis software package was written to extract the velocity information from both two and three pulse digital PIV images. The technique was tested on various cold flow, demonstrating its ability to resolve the flow direction. Planar Laser Induced Fluorescence was used to measure the concentration of the OH radical in turbulent premixed methane-air flames. Over four hundred experimental images were obtained and corrected for systematic errors. Fractal analysis of the flame-fronts in these images was used to extract the inner cut-off values, the smallest scale of wrinkling. The inner cut-off distributions were invariant with stoichiometry and maturity of flame. The maximum OH fluorescence signal at each point along a flame-front was used to give a measure of the local mass burning rate. Correlation of the reaction rate with flame curvature class revealed that convex regions had enhanced burning rates, while concave regions had suppressed burning rates. The two laser diagnostic techniques were combined in an experimental investigation of a triple flame in a counterflow burner. Joint PFF-velocity and joint OH-velocity images were generated, demonstrating the possibility of using the combination of PLEF and DPIV to study flame-turbulence interactions in premixed combustion.