An experimental investigation of the dynamical modes of propagation of a burner stablized, laminar, premixed methane-air flame

This investigation documents the observation of the dynamical modes of propagation of a burner stabilized, laminar, premixed methane-air flame and maps the associated stability boundary of each mode. The flame is stabilized on a water-cooled, stainless-steel, porous plug burner by delivering a flow of gas with a velocity equal, but in a direction opposite to the burn velocity of the mixture. The stability of the flame is improved with the addition of a wire mesh grid located above the flame in the burned gas mixture. By varying the equivalence ratio, the inflow velocity, chamber pressure, and screen height, the flame can be made to oscillate in one of several modes. These modes are characterized by variations in the position of the flame surface and intensity, similar in some respects to the modes of oscillation of a vibrating membrane. Recently, theoretical studies of burner stabilized flames and freely-propagating flames in channels have predicted similar modes of propagation. Each mode was characterized by using several critieria; an intensity signature, visual observation, and stop-action photography. The intensity signature was monitored by a photodiode whose time series output was provided to the input of a real-time spectrum analyzer. The time series were then Fast Fourier Transformed by the spectrum analyzer which generated a frequency signature for each regime. The stability boundary for each mode was then determined by examining a set of six (6) equivalence ratio/inflow velocity planes. Each of the planes were examined at a constant pressure and screen height. In each plane, the stability boundary of each mode was determined by observing the values of the equivalence ratio where the frequency signature changed significantly. In this way a group of six 2-dimensional slices were then used to build a 4-space picture of each mode and its boundaries.