Experimental investigation of the response of laminar premixed flames to equivalence ratio oscillations

A detailed experimental analysis of laminar premixed methane/air flames subject to equivalence ratio oscillations is presented. The equivalence ratio of methane/air flames was periodically modulated by the addition of methane into the stationary fuel/air flow of a lean premixed flame. The reaction of the flame as a function of amplitude and frequency of the modulation was investigated in a parametric study. The flames were operated at reduced pressure (70 mbar) in order to increase the spatial resolution across and within the flame front and to approximate the time and length scales of the flame and the induced equivalence ratio oscillations in an order of magnitude of 10 Hz. The detection and evaluation of OH*-chemiluminescence revealed the shape and position of the flame front. One-dimensional laser Raman scattering was applied in order to simultaneously and quantitatively determine the concentration profiles of the major species and the temperature along the symmetry axis of the flame. The results show that several interacting effects occur, which have different dependencies on the oscillation frequency: first, a “macroscopic” reaction of the flame arises from the variation of the laminar flame speed and the flow velocity and, hence, its location. Second, the structure of the flame front is affected by the diffusional transport processes and the influence of the oscillation on the species concentrations. At low frequencies, the flame can follow the oscillations reaching temporarily quasi-steady states at the minimum and maximum fuel fraction levels. At sufficiently high frequencies, the flame tip location remains virtually unchanged. However, the species profiles across the flame front show a significant phase shift relatively to each other and cannot be described by steady states throughout the oscillation period.