Abstract
Rainbow schlieren deflectometry was employed to characterize the flame structure of unsteady laminar liquefied petroleum gas (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. The a priori relationship between the refractive index difference and temperature was established using the opposed flow flame reactor concept. The peak temperature observed in the near-field region varies with axial location from 1000 K to 1700 K, which falls in the range of typical sooting flame. The sooting flame has depicted a typical flicker behavior of the unsteady laminar diffusion flame. Flicker behavior and flame puff formation could 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 (1700 K), which leads to better oxidation of carbon soot and its precursors at down-stream locations, a unique feature of sooting flames. Irrespective of whether space–time evolution at any axial location or frequency spectrum analysis of temporal signal of hue at any spatial location or the amplitude–time signal of decomposed mode from proper orthogonal decomposition (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 correspond to flow features indicating strong ambient air entrainment pointing towards the role of outer vortical structures on flame flickering phenomena.
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