MEASUREMENTS OF CONDITIONAL VELOCITIES IN TURBULENT PREMIXED FLAMES BY SIMULTANEOUS OH PLIF AND PTV

Joint velocity/scalar imaging measurements are performed in turbulent premixed natural-gas/air flames to better characterize the turbulent f lux of mean reaction progress variable, pu '' c ''. Simultaneous two-d imensional measurements of the velocity field and the relative OH conc entration 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 produ cts provides an indication of whether turbulent diffusion of the mean progress variable occurs in the gradient or counter-gradient sense. Fi ve flames having different ratios of rms velocity, u', to laminar flam e speed, S-L, are considered. The results indicate that a transition f rom counter-gradient to gradient diffusion occurs as u'/S-L is increas ed. Such a transition has previously been predicted by direct numerica l simulation (DNS) but has not been observed experimentally. The exist ence of this transition must be taken into account when developing mod els of turbulent premixed combustion. (C) 1999 by The Combustion Insti tute