Impact of the Purge Flow Density Ratio on the Rim Sealing Effectiveness in Hot Gas Ingestion Measurements

The rim seals of gas turbines are used to control the ingestion of hot mainstream gas into the wheel space between the rotor disk and the stationary casing. Sealing air, which is generally used to pressurize the cavity space, flows through the seal clearance and then mixes with the flow path in the annulus. Predicting the correct quantity of purge flow necessary to prevent excessive ingestion of hot gases while, at the same time, minimizing the penalties in terms of engine efficiency and stage aerodynamics represents a great challenge for the designers and a crucial point for the design of reliable engines. Such estimate is governed by unsteady phenomena, and computational fluid dynamics (CFD) approaches are still expensive and time consuming, especially if 3D domains and unsteady conditions have to be simulated. Fundamental test cases, replicating actual engines geometries, are still a valid approach to calibrate correlations or simplified models such as the so-called orifice model. However, most of the experimental studies deal with test rigs at room temperature and do not take into account the effect of the density ratio (DR) between purge and main flows. To fill this gap, the present article reports the impact of the density ratio on the rim sealing effectiveness by performing a nonintrusive diagnostic based on the pressure-sensitive paint (PSP) technique on both the stator side and the rotor side. The analysis was performed on a cold rotating cavity rig, developed for the study of hot gas ingestion, where two different values of density ratios were tested by using N₂ (DR = 1) and CO₂ (DR = 1.52) as purge flow. The data extracted from the PSP seal effectiveness maps allowed to calibrate the orifice model for the stator side and to fit the coefficients of the buffer ratio model for the rotor surface at different flow conditions where the externally induced ingress is the dominant mechanism for gas ingestion. The results highlighted the impact of the DR on the seal effectiveness and on the low-order models considered for the data analysis. In the end, it is shown that the obtained results can be used to scale experimental data, generally collected at DR close to one, toward more representative engine values where the difference between the density of purge and main flows cannot be neglected.