Cooling holes in combustion liners are typically circular, as are the holes used for acoustic damping of combustor instabilities. This hole geometry is chosen for several reasons, which include manufacturability, stress concentration considerations, and because the behavior of round holes is well understood. However, developments in both manufacturing techniques and modeling methods now allow other hole geometries to be considered, such as perpendicular arrangements of narrow slots carefully shaped to prevent high stress concentrations (patent pending). The latter have been demonstrated to provide more efficient cooling and lower stress compared to round holes. However, the acoustic properties of such arrangements are not straightforward to model yet since it is not known how some important factors are affected by this geometry change.
In order to study the acoustic behavior of such arrangements of holes, a series of tests were conducted in an impedance tube (in reflection) for various slot shapes, width and spacing, with a backing cavity and purge flow (controlled via pressure drop). Some baseline measurements for circular holes were also obtained in an attempt to quantify any difference resulting from the change in hole shape.
The results have shown that at any pressure drop, the maximum absorption coefficient for narrow perpendicular slots and round holes is nearly equal when the hole area corrected by the discharge coefficient (i.e. the effective area) is also nearly the same, and that narrow perpendicular slots give more broadband absorption on the high frequency side of the absorption peak for a given cavity. It is suggested that since the maximum absorption between round holes and narrow perpendicular slots are nearly equal at an equivalent effective area, the resistance (real part of impedance) of the holes depends on the maximum flow velocity through the hole rather than the average velocity, for the particular combinations of hole dimension and sample plate thickness tested. It is also suggested that the greater broadband absorption at high frequencies for the narrow slots could be due to the fact that their high length to width ratio would result in a characteristic dimension dominated by their small width (similar to the airfoil thickness being the representative dimension of the size of the wake for an airfoil) hence a lower Strouhal number.