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.