Measurements of Conditional Velocities in Turbulent Premixed Flames by Simultaneous OH PLIF and PIV

Joint velocity/scalar imaging measurements are performed in turbulent premixed natural-gas/air flames to better characterize the turbulent flux of mean reaction progress variable, ru0c0. Simultaneous two-dimensional measurements of the velocity field and the relative OH concentration are obtained by particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of the OH radical, respectively. The data are used to obtain conditional mean velocities in both reactant and product regions. The relative mean velocity of reactants and products provides an indication of whether turbulent diffusion of the mean progress variable occurs in the gradient or counter-gradient sense. Five flames having different ratios of rms velocity, u9, to laminar flame speed, SL, are considered. The results indicate that a transition from counter-gradient to gradient diffusion occurs as u9/SL is increased. Such a transition has previously been predicted by direct numerical simulation (DNS) but has not been observed experimentally. The existence of this transition must be taken into account when developing models of turbulent premixed combustion. © 1999 by The Combustion Institute