Quantitative measurements in an unsteady flame using high-speed rainbow schlieren deflectometry

Rainbow schlieren deflectometry was integrated with a high-speed digital imaging system to quantify the scalar flow structure of flames at a high temporal resolution. The schlieren image data were processed to determine the temperature field across the whole field assuming chemical equilibrium in the flame. First, shapes of steady hydrogen jet diffusion flames estimated from the schlieren technique were compared with those obtained by direct visualization. Next, the schlieren technique was applied to flame experiments at a jet exit Reynolds number of 300, which produced periodic oscillations or flame flicker. Rainbow schlieren images taken at acquisition rates of 30, 60, 125, 250, 500 and 1000 frames per second were analysed to construct the instantaneous temperature distributions across the whole field, the vortex convection velocity profiles, and the mean and root-mean-square temperature profiles. Results demonstrate that high-speed rainbow schlieren deflectometry is effective in quantitatively describing the transient flow structure of unsteady flames.