Characterization of fluctuating hydroxyl concentrations in a turbulent nonpremixed hydrogen–nitrogen jet flame

Two-point OH time-series measurements using a high-speed, laser-induced fluorescence system have been performed in a turbulent nonpremixed jet flame to obtain both radial and axial space–time correlations. Turbulent OH structures in such flames are found to undergo convection both axially and radially, but OH convection does not satisfy the ‘frozen-turbulence’ hypothesis owing to various turbulent interactions and chemical reactions. While axial OH convection occurs at approximately the local mean bulk velocity, radial convection is largely compromised by strong turbulent mixing along the same direction. The hydroxyl integral length scale can be interpreted as the typical dimension of a convective OH structure, which is axially elongated and becomes more isotropic in the post-flame region. The hydroxyl integral time scale can be interpreted as approximately the ratio of an axial integral length scale to a corresponding local mean flow velocity. In general, macroscale fluctuations of OH are dominated by large-scale turbulence, with little contribution from small-scale turbulence and OH chemistry.