Hydroxyl time-series measurements in laminar and moderately turbulent methane/air diffusion flames

Picosecond time-resolved laser-induced fluorescence (PITLIF) is a developing technique used to probe minor-species concentrations in flames at rates sufficient for the study of turbulent fluctuations. This method has previously been applied to the measurement of CH signals in laminar and low- Reynolds number turbulent diffusion flames. In the present work, Laser- induced Fluorescence (LIF) measurements of OH are obtained in the same flames. The peak concentrations of the hydroxyl radical provide a signal-to- noise ratio sufficient to extend the measurements to a variety of axial and radial locations relative to the flame front. The time-series measured are used to compute power spectral densities (PSDs) which provide frequency information not available from more typical probability density functions. The PSDs are compared to those from the previous CH work and to expectations for scalars in moderately turbulent flames from the literature. These measurements provide the first known PSDs of the hydroxyl radical in a low- Reynolds number turbulent nonpremixed flame.