FLICKERING CHARACTERISTICS OF PREMIXED FLAME WITH OUT INTERACTION WITH AMBIENT AIR

Abstract

A large number of experimental and theoretical studies have investigated the flame flicker in diffusion flame. It has been known that the flickering frequency ranges from 10 to 20 Hz for a wide variety of burner sizes, flow rates, and compositions. Buckmaster and Peters reported that a buoyancy driven Kelvin-Helmholtz(KH) type instability causes flickering phenomena via buoyancy-induced velocity field surrounding jet flow and predicted a low frequency oscillation around 17 Hz using a viscous stability analysis with the infinite candle model - an ideal plane diffusion flame in which the flow field is induced solely by buoyancy. The previous researches in diffusion and premixed flames investigated buoyancy driven KH instability in the shear layer between the buoyant hot products and the ambient air. But Kostiuk and Cheng showed that the even the geometry of flames could affect the flickering phenomena significantly and which inspired us to investigate the buoyancy driven KH instability in flame front not in the shear layer. For the purpose, the flame should have geometry which induces large buoyancy driven KH instability without the effect of the buoyancy driven KH instability in the shear layer. In the present research, the flame with the shape of long inverted cone was selected so that buoyancy driven KH instability could be generated easily through large baroclinic torque due to near perpendicularity between the direction of the density gradient and the buoyancy force. The overall objective of the present research is to understand the effect of boundary conditions (i.e. whether or not there is interaction with ambient air) on flickering features and to find out dominant parameter of flickering phenomena without interaction with ambient air in premixed flames of inverted cone shape. Burning velocity and flame angle were measured for flame-generated vorticity expressed also as modified Richardson number Ri* and flickering wavelength was measured. Finally, the empirical relation between Ri* and St was obtained and it was shown that the flickering phenomena of inverted cone shape without interaction with ambient air can be explained by buoyancy driven KH instability.