INVESTIGATION OF LPG SOOTING DIFFUSION FLAME BY RAINBOW SCHLIEREN DEFLECTOMETRY

Abstract

Rainbow schlieren deflectometry was employed to characterize the flame structure of Laminar unsteady LPG jet diffusion flame. The schlieren images were acquired at 4000 Hz sampling rate for sooting LPG flame at Re = 500, and Abel inversion was employed to estimate local field refractive index difference. A priori relationship between refractive index difference and temperature was established using the opposed flow flame reactor concept. Peak temperature observed in the near field region is around 1300 K, which falls in the range for a typical sooting flame. Although the sooting flame was observed it depicted typical flicker behavior of the unsteady laminar diffusion flame. Flicker behavior and flame puff formation can be attributed to the interaction of buoyancy sustained outer vortical structure with the flame surface. It was observed that additional air entrainment during puff formation leads to higher temperatures in the puff, which can be attributed to better oxidation of carbon soot and its precursors at downstream locations, a unique feature of sooting flames. Irrespective of whether spacetime evolution at any axial location or frequency spectrum analysis of temporal signal of hue at any spatial location or the amplitude-time signal for decomposed mode from POD analysis of color schlieren images, is employed the global unique flicker frequency of 13.33 Hz was observed in the near field LPG diffusion flame. Interestingly the first two dominant energy modes of POD analysis corresponds to flow features indicating strong ambient air entrainment.