This paper describes experiments performed in a subscale 1.5-stage axial air turbine in which ingestion of mainstream air into the front and aft disk cavities was measured. The front disk cavity is upstream of the rotor, the aft disk cavity is downstream of the rotor. Both disk cavities contain a labyrinth seal at a radially inboard location; this seal divides the cavity into a ‘rim cavity’ and an ‘inner cavity’. The front rim cavity features a double seal with radial clearance and axial overlap at its periphery; the aft rim cavity double seal possesses axial gap. Results are reported for three experiment sets, each set defined by the main air flow rate and rotor speed. Furthermore, each set comprises four different purge air flow rates. The initial step in each experiment was the measurement of time-average static pressure distribution in the main gas annulus and the disk cavities to establish the steady-state pressure differentials that contribute to ingestion into and egress from the cavities. This was followed by the measurement of tracer gas (CO2) concentration distribution in the disk cavities to quantify the ingestion. In addition to the pressure and concentration (in terms of sealing effectiveness) results, the mass flow rates of ingested main air into the front and aft rim cavities are reported for the various experimental conditions. Both inner cavities were found to be completely sealed against ingestion. Ingestion and egress discharge coefficients based on an orifice model are reported for the front and aft double rim seals; their trends are quite different from those of single rim seals with axial overlap and radial clearance reported earlier.

The interaction between the mainstream and disc cavity purge flows in a turbine stage is an unsteady 360° phenomenon. Most of the current rotating rigs have used steady pressure transducers to measure the mainstream annulus pressure distributions as well as the pressure distribution in the disc cavity. Unsteady static pressure measurements in these regions using fast-response transducers have also been reported but to a much lesser degree, mainly at ASU, OSU, VKI, and ETH. To gain better insight into the prevailing unsteady flow phenomena, and to assess the difference between steady and time-averaged unsteady pressure data, new unsteady static pressure measurements were recently carried out at three locations in an ASU-Honeywell turbine stage, namely, in the main gas path on the outer shroud near vane trailing edge as well as on the vane platform lip, and on the stator surface rim seal. They are reported in this paper along with the comparative results of the corresponding URANS CFD simulation reported in an earlier publication. Experiments were carried out at five different purge air flow conditions for each of the two mainstream air flow rate and rotor speed combinations. The current unsteady measurements indicate that the rim cavity pressure frequency is governed by the blade passage frequency. The unsteadiness amplitude increases with purge flow in the main gas path, but decreases with increase in purge flow for the rim cavity where the sensitivity to change in purge flow is smaller at the lower mainstream flow rate. The difference in the ambient-corrected time-averaged static pressures between those evaluated from the current unsteady measurements and the previously published steady measurements are found to be within the measurement uncertainties.