Lift-off height and flame curvature of triple flame with various sound pressures has been investigated. The triple flame was formed by multi-slot burner which can make the triple flame in a mixing layer two dimensionally without vortex interference in a shear layer. The mixing layer was observed by Schlieren system. The standing acoustic waves whose sound pressure is up to 0.12 kPa with 0.5 kHz were generated in a stainless resonance tube. As a result, the lift-off height significantly decreased with increase in the sound pressure, whereas the significant change of the flame curvature were not observed. The meandering of the mixing layer due to fluid displacement caused by velocity oscillation was observed by Schlieren images. From the above, the mechanism of lift-off height reduction can be explained that the fuel concentration gradient is made to gentle by meandering of mixing layer and the flame moves to the new position where the fuel concentration gradient is equal to the original fuel concentration gradient 'without sound.
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