Experimental investigations have been made with the objective of studying the effect of turbulence on the premixed and non-premixed jet flame. The turbulence is generated by means of a loudspeaker-driven cavity. Relatively low frequencies are used to induce one dimensional waves. The velocity is analyzed by using the wave equation and the flow behavior in oscillating pressure gradient. In case of premixed flame, the turbulence directly interacts with the premixed flame and the premixed flame velocity is changed due to the deformation of flame structure. But in case of non-premixed flame, the combustion intensity is changed by mixing time scale which is dominant time scale in the overall reaction time step. Thus it is necessary to investigate the turbulence effect on the flame structure in the premixed flame and the mixing in non-premixed flame.
Firstly, the interaction between a premixed flame and a flow field is investigated to understand a turbulent premixed flame. Low frequencies (less than 100 Hz) are applied to easily obtain the scales of flame structure from direct photographs. The flame front shapes, obtained in this experiment, can be grouped into three classes : conical flame, oscillating planar flame and oscillating cellular flame. The influence of flame shape on the flame velocity is visualized by the flame position relative to the initial flame position. The principle and method of stabilizing a premixed flame in the tube are described for the first time and demonstrated through a simple experiment. The influences of acoustic velocity on flame front behavior are experimentally examined. To qualitatively confirm the characteristic effect on flame behavior, a simple analysis is also carried out. It is shown that the acoustic pressure along the tube can be simply estimated only with the oscillating velocity evaluated from the direct photograph. A very interesting pattern of cell distribution appears while the equivalence ratio is changing slowly, and it has never been found before. Cell wavelengths are measured and modelled using the characteristic of radial velocity distribution. The flame extinction are evaluated by estimating flame stretch rate with direct flame areas and the effect of cell scale to the limit of flame propagation velocity is investigated.
Secondly, rich premixed flames are introduced to study the limits of mixing enhancement in a tone excited jet flame. The excitation frequency is chosen for the resonant frequency identified as a pipe resonance due to acoustic excitation. Methane, propane and butane are used to examine the effect of mixture property on the limit of equivalence ratio. Mixing is always enhanced in a methane/air flame as the excitation intensity increases. Constant lower limits of equivalence ratio for mixing enhancement are present in cases of propane/air and butane/air flames irrespective of mean mixture velocities. The equivalence ratio limits are also found to be related to the flame instability ; the lower Le, the higher the limit of equivalence ratio. Under the equivalence ratio limits, cellular flames are generated as the excitation intensity increases. The amplitude of oscillating velocity for generating a cellular flame in the equivalence ratio limit is proportional to a mean mixture velocity irrespective of mixture properties.
