Experimental Investigation of Acoustic Characteristic on Orifice Shaped With Bias Flow

Combustion dynamics has been a significant problem for a lean, premixed, prevaporized (LPP) combustor. Understanding the acoustic characteristics of combustor components is essential to modeling thermoacoustic behavior in a gas turbine combustion system. Acoustic characteristics such as impedance and scattering matrix elements are experimentally determined for different-shape orifices with an emphasis on the effect of the flow field on them. These orifices are used to represent premixed swirl cups in LP combustors. The validity and limitation of two different methodologies are evaluated by comparing measured results with those of others. Consistent with analytical predictions, the measured resistance through an orifice increases as the bias flow increases. Different types of orifices considered in this study behave similarly to a thin orifice at high bias flow even though the discharge coefficients vary as much as 30% between them. The conventional method produces impedance values independent of waves reflected from the end boundary condition only when the scattering elements at the orifice downstream are roughly equal to those upstream of the orifice. However, the scattering matrix method produces impedance values that are not affected by the source or reflected waves at the system’s boundary. The scattering matrix measurements show that the reflection and transmission elements increases and decreases, respectively, as the bias flow through an orifice increases.