60118 - Experimental Investigation of Acoustic Characteristic of Shaped Orifices With Bias Flow 

For the last three decades, combustion dynamics has been a significant problem for a lean, premixed, prevaporized (LPP) combustor. Understanding the acoustic characterization of the combustor component is essential to modeling thermoacoustic behavior in a gas turbine combustion system. Four different-shape orifices are experimentally investigated to understand the effect of the flow field on the acoustics characteristics. The various orifices used in the experiment represent simplistic modeled premixed swirl cups in LPP combustors. Previous studies have shown that bias flow through a square edge orifice makes the acoustic properties much more complicated compared to the well-known analytic theories with no bias flow. The experimental method uses 1-D acoustic wave theory and a multiple microphone method to study the wave properties inside a square duct. The predicted acoustic pressure is compared with the measured acoustic pressure. The result showed that the reconstructed waves have a measurement error of less than 3% downstream and less than 1% upstream from the orifice over the range of operating conditions. The wave properties that are focused on this paper are the acoustic resistance and the scattering matrix elements of the orifice. The measured impedance through an orifice showed that the resistance increases with the increase of bias flow, which is consistent with Howe’s theory. The orifices acoustic resistance increased linearly with respect to bias flow. However, the thickness of the orifice changed the trend of acoustic resistance for low bias flow. Different types of orifices were considered in this study that would behave like a thin orifice at high bias flow even though the discharge coefficients vary as much as 30% between them. The scattering matrix theory was used to study the transmission and reflection through and from the orifice. The reflection coefficients are higher than the transmission coefficients for any bias flow. As the bias flow rates increased, the reflection coefficients increased, and the transmission coefficients decreased.  The measured scattering matrix element results show that the orifice acoustic energy is 10% more absorbent for a diverged shaped orifice than a converged shaped orifice.