A VISUALIZATION STUDY ON THE EFFECT OF FORCING AMPLITUDE ON TONE-EXCITED ISOTHERMAL JETS AND JET DIFFUSION FLAMES

An experimental study was conducted to investigate the effects of axia l forcing on the flow structures near the nozzle exit in coaxial isoth ermal jets and jet diffusion flames. The jet was excited by adding a p eriodic velocity fluctuation ranging from 0 to 400% of the mean jet ve locity at the tube resonating frequency. The phase-averaged axial velo city fluctuation at the jet centre was measured with a one-component L DV and phase-locked visualization using a light chopper and a phase-co nditioning circuit was performed. The changes of large-scale structure s in the near field of the jet are described from the visualization of horizontal and vertical cross-cut Mie scattering images. The flow str uctures of the forced isothermal jet are classified into three regions on the basis of the emergence of azimuthal structures and the periodi c behaviour of vortex structures. The jittering of azimuthal structure s was characterized by a forcing amplitude ratio and the velocity diff erence between the jet and the co-flowing fluid. In case of the forced reacting jet, flame heights were measured from video tape recordings of the sooting images of the flame. The dependence of flame height on the forcing amplitude ratio shows the existence of a flame-length elon gation and reduction region. The flame elongation is found to be relat ed to the suppression of the flame flickering by forcing. From the Mie scattering images and flame-length measurements, it is suggested that the intense mixing observed in the fully forced laminar jet and the r eduction of the flame length is closely related to the development of azimuthal structures. (C) 1998 John Wiley & Sons, Ltd.