Unsteady Effectiveness Measurements in Rim Seals Through Fast Pressure-Sensitive Paint

In recent years, research on gas turbine rim seals has shown that the sealing performance is strongly influenced by the inherent unsteadiness of the problem, making it clear that a steady analysis is not sufficient to fully understand the complex behavior of rim seal flows. In particular, the presence of unsteady large-scale flow structures rotating within the wheel-space has been identified. Previous research suggested that this phenomenon may originate from Kelvin-Helmholtz instabilities, which supposedly arise due to the tangential shear between the egress and the annulus flows. Although a few numerical studies have highlighted the direct influence of these instabilities on the sealing effectiveness, there is still a lack of experimental evidence. To fill this gap, the rotating cavity rig developed at the University of Florence was employed to perform a detailed investigation on the role of unsteadiness. First, the presence of large-scale flow structures was verified using a high-frequency pressure probe. Then, the fast pressure-sensitive paint technique was employed to capture 2D effectiveness contours on the stator wall at two distinct values of rotational Reynolds number (Reφ=3.00×105-1.20×106) and at different levels of purge flow. The analysis of the time-resolved effectiveness maps highlighted the presence of large-scale rotating structures, which cause effectiveness deterioration especially at higher radius. The structures are then dispersed at a lower radius in a diffusion-like mechanism influenced by the disk rotation. Although these instabilities appear to be characterized by a high level of randomness, a statistical analysis of the contours showed that, as the sealing level of the cavity increases, the rotational speed of the structures remains relatively constant, while the average number of occurrences per revolution progressively decreases. Interestingly, the results appeared to be completely independent of the rotational speed of the rotor disk. In the end, the time-resolved description of the sealing effectiveness allows drawing general conclusions about the flow behavior, which, in turn, are expected to have a long-term impact on cavity flow modeling, from low-order methods to advanced CFD methodologies.