TED-AJ03-197 AN EXPERIMENTAL STUDY ON LOCAL FLAME DISPLACEMENT VELOCITY OF PREMIXED TURBULENT FLAME

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In our previous studies, the turbulent burning velocities of several mixtures having nearly the same laminar burning velocity were experimentally investigated. As a result, it becomes clear that the mean local burning velocity of turbulent flame can be changed from the original laminar burning velocity depending on the equivalence ratio, caused by the diffusional properties of reactants, i.e., the preferential diffusion effect. This change affects substantially the turbulent burning velocity characteristics. In order to clarify the influence of the local burning velocity on the turbulent burning velocity, an attempt is made in this study to examine directly the local flame propagation properties of premixed turbulent flames under weak turbulence condition where the continuous and thin reaction sheet is predominant. Several mixtures having nearly the same laminar burning velocity are prepared for experiments by adding nitrogen to hydrogen-air, methane-air and propane-air mixtures where the equivalence ratio is varied. A laser tomography technique is used to obtain the sequential two-dimensional flame shape and motion, and quantitative analyses are performed. Where, the local flame front curvature and the local flame displacement velocity are quantitatively obtained as the key parameters of the turbulent combustion. In the first place, the observation of sequential flame tomograms shows that the turbulent flame front can be classified into the active and inactive parts depending on its geometric configuration. In the next place, the local burning velocity and the local curvature are obtained directly by the flame front movement. The local flame displacement velocities of the turbulent flame are found to be not a constant value as the laminar burning velocity, but to be distributed over a wide range depending on the local curvature of the turbulent flame. In addition, the mean local flame displacement velocities of the hydrogen and methane mixtures have a tendency to become larger as decreasing its equivalence ratio, whereas those of the propane mixtures become smaller, even in Fig. A-1. These experimental results seem to be in disagreement with the conventional concept of turbulent burning velocity, where the local burning velocity is likely to be equal to the original laminar burning velocity under such weak turbulence condition. The discussion on the influence of preferential diffusion and Markstein number on the local flame displacement velocity is also made.[figure]
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TED-AJ03-197 AN EXPERIMENTAL STUDY ON LOCAL FLAME DISPLACEMENT VELOCITY OF PREMIXED TURBULENT FLAME

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