The dynamic structure of turbulent V-shaped premixed flames

Abstract

A new technique, LARS (Linear Array for Rayleigh Scattering), for the investigation of the dynamic structure of premixed turbulent flames is described. The Rayleigh light scattered from a segment of a continuous laser beam is imaged, through an intensifier, onto a 512 element linear photo-diode array. The array is scanned at line rates up to 10 KHz and 2560 lines can be stored in a dual ported memory. The space-time images produced are manipulated by an image processor which normalizes the data and can perform a variety of image enhancement techniques. Methane/air premixed turbulent v-shaped flames at an approach velocity of 2 m/s and a range of inlet turbulence levels and equivalence ratios from 5%–8% and 0.6–0.8, respectively, were studied. The statistics of the flame front position were obtained by extracting the flame boundary from the images and generating a probability density function of its distance from the mean. The standard deviations of these distributions varied from 0.31mm–1.38mm and show that increases in inlet turbulence and equivalence ratio thicken the turbulent flame zone by amplifying the flame motions. These results lend support to various modeling assumptions. Comparison of the spectra of the flame front oscillations and the cold flow velocity indicates that the inlet turbulence controls flame motion at frequencies between 100 and 1000 Hz. Changes in heat release for a given inlet turbulence increase the amplitude of the flame fluctuations without modifying the spectral characteristics.