Experiments on Front- and Aft- Disk Cavity Ingestion in a Subscale 1.5-Stage Axial Turbine

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.