Development of a particle image velocimetry technique for three-dimensional flame structure analysis

Abstract

A technique for the measurement of three-dimensional quantities in turbulent premixed flames was developed. The need for this information arises when a deeper understanding of the flame-flow interactions is sought. As yet, information was mostly obtained in a two-dimensional manner using planar laser light sheet based measurement techniques. Although they are well established, the data gained is only a projection of the reality into a two-dimensional plane. In an effort to gather truly three-dimensional quantities, four laser light sheets have been crossed in a single line and particle image velocimetry (PIV) has been performed in each of them. By using the vaporisation of seeded silicon oil droplets at the flame front, the flame structure can be extracted as additional information. Combining the information about velocity and flame structure, flame displacement speeds were deduced. For the separation of the four laser light sheets, different wavelengths and polarisations were used. The readily available frequency doubled output of a Nd:YAG laser at 532 nm was utilised to illuminate two of the light sheets, separating them by polarisation. A third light sheet was produced with the frequency-tripled output of another Nd:YAG laser at 355 nm. To create the fourth light sheet, a solid state external Raman laser with barium nitrate as the active material was set up. This quad-crossed plane PIV experiment was applied in a model flame stabilised in a diffuser type combustor, which involves decelerating the premixed methane/air flow to a point where the flow velocity matches the turbulent burning velocity and therefore results in a rather flat reaction zone. The diffuser was made from quartz glass to allow optical access. The information gained was compared to established theories and numerical simulation results. Furthermore, a comparison of three-dimensional and two-dimensional data was performed to critically analyse the significance of two-dimensional measurements.