While it is widely recognized that ingestion of hot gas from the main annulus of axial gas turbine stages into rotor-stator disk cavities depend strongly on the unsteadiness of the prevailing flow field, the large computational effort needed to simulate the flow field renders its use in the design of turbine internal air system and seals difficult. As an alternative, considerable effort has been devoted in recent years to develop simple orifice models of disk cavity rim seals based on time-averaged flow information; these models contain empirical discharge coefficients for ingestion into and egress from the cavities. The present experimental work in a subscale axial turbine stage reports a simple orifice model of an axially-overlapping radial-clearance seal at the disk cavity rim and values of the discharge coefficients over a range of purge flow rate supplied to the cavity. In the experiments, the ingestion process was dominated by the main gas annulus flow. Time-averaged static pressure distribution was measured in the main annulus and in the disk cavity; the driving force for ingestion and egress was taken to be the pressure differential between the main annulus and the rim cavity at prescribed locations. Time-averaged ingestion was measured using the tracer gas technique; the pressure and ingestion data were combined to obtain the ingestion and egress discharge coefficients at several purge flow rates. The location on the vane platform 1mm upstream of its lip represented the main gas annulus pressure in the calculation of discharge coefficients. In the rim cavity, two locations on the stator, one in the ‘seal region’ and the other slightly inward radially, were prescribed to represent the rim cavity pressure as well as the sealing effectiveness. Two corresponding sets of ingestion and egress discharge coefficients are reported for the various purge flow rates. The ingestion discharge coefficient obtained using the seal region location in the rim cavity decreased as the purge flow rate increased; the corresponding egress discharge coefficient increased with purge flow rate. For the rim cavity location slightly inward radially from the seal region, the egress discharge coefficient maintained the same trend; however, the ingestion discharge coefficient decreased only slightly as the purge flow rate increased. It is suggested that the seal region location in the rim cavity is the more appropriate location in calculating the rim seal discharge coefficients. The ratio of ingestion to egress discharge coefficients exhibited considerable variation with purge flow rate.

This paper describes experiments in a subscale axial turbine stage equipped with an axially overlapping radial-clearance seal at the disk cavity rim and a labyrinth seal radially inboard which divides the disk cavity into a rim cavity and an inner cavity. An orifice model of the rim seal is presented; values of ingestion and egress discharge coefficients based on the model and experimental data are reported for a range of cavity purge flow rate. In the experiments, time-averaged pressure distribution was measured in the main gas annulus and in the disk cavity; also measured was the time-averaged ingestion into the cavity. The pressure and ingestion data were combined to obtain the discharge coefficients. Locations on the vane platform 1 mm upstream of its lip over two vane pitches circumferentially defined the main gas annulus pressure; in the rim cavity, locations at the stator surface in the radially inner part of the “seal region” over one vane pitch defined the cavity pressure. For the sealing effectiveness, two locations in the rim cavity at the stator surface, one in the “mixing region” and the other radially further inward at the beginning of the stator boundary layer were considered. Two corresponding sets of ingestion and egress discharge coefficients are reported. The ingestion discharge coefficient was found to decrease in magnitude as the purge flow rate increased; the egress discharge coefficient increased with purge flow rate. The discharge coefficients embody fluid-mechanical effects in the ingestion and egress flows. Additionally, the minimum purge flow rate required to prevent ingestion was estimated for each experiment set and is reported. It is suggested that the experiments were in the combined ingestion (CI) region with externally induced (EI) ingestion being the dominant contributor.