Modelling and Measurement of Sealing Effectiveness and Heat Transfer in a Rotor-Stator System with Ingress

This thesis investigates, both theoretically and experimentally, the phenomenon of ingress through gas turbine rim seals. The work presented focuses on modelling and measuring the required sealing flow levels to purge the wheelspace against combined ingress and the effect of externally-induced ingress on the surface temperature and heat transfer to the rotor. Combined ingress is driven by a pressure difference between the mainstream annulus and wheelspace cavity resulting from the combination of the asymmetric external pressure profile in the annulus and the rotation of fluid in the rotor-stator wheelspace cavity. Ingress can be prevented by pressurising the wheelspace through the supply of sealant flow. The Owen (2011b) combined ingress orifice model was solved to predict the required levels of sealant flow to prevent ingress into the wheelspace. The model was validated using prepublished data and data collected experimentally over the course of this research. Gas concentration measurements were made on the stator of the Bath single-stage gas turbine test rig to determine the variation of sealing effectiveness with sealant flow rate for an axial clearance seal geometry at design and off-design operational conditions. The measured variation of the required sealant flow rate with the ratio of the external and rotational Reynolds numbers, ReW / Reϕ, was consistent with the findings of other workers: at low values of ReW / Reϕ, ingress levels were influenced by the combined effects of the disc rotation and the annulus pressure profile and were therefore considered to fall into the combined ingress region; the influence of rotation diminished as ReW / Reϕ increased and the ingress levels were dominated by the annulus pressure field (externally-induced ingress). The orifice model was in good agreement with the experimental measurements and the prepublished experimental data. Thermochromic liquid crystal (TLC) was used to determine effect of ingress on the heat transfer coefficient, h, and adiabatic wall temperature, Tad, on the rotor of the Bath gas turbine rig. Concurrent gas concentration measurements were made on the stator to compare the effects of ingress on the two discs. Data was collected at the design condition, where ReW / Reϕ = 0.538 and at an overspeed off-design condition, where ReW / Reϕ = 0.326. The comparison between a newly defined adiabatic effectiveness, ead, on the rotor and the concentration effectiveness, ec, on the stator, showed that the rotor was protected against the effects of ingress relative to the stator. The sealing air, which is drawn into the rotor boundary layer from the source region, thermally buffers the rotor against the ingested fluid in the core. A thermal buffer ratio, η, was defined as the ratio of the minimum sealant flow required to purge the stator against ingress to the minimum sealant flow required to purge the rotor against ingress. The thermal buffer is dependent upon the flow structure in the wheelspace, which itself is governed the turbulent flow parameter, λT. A hypothesis relating η to λT was developed and shown to be in good agreement with the experimental data. The local Nusselt numbers, Nur, on the rotor were shown to be fairly constant with radius and increased as λT was increased. The latter finding can be explained by the flow structure in the wheelspace: as λT is raised, the swirl in the fluid core reduces, which results in an increase in the moment coefficient and Nur on the rotor. Difficulties in measuring Tad during the experiments suggested a new technique from which to solve for h and Tad using TLC surface temperature measurements. The solution Fourier’s equation for a step-change in the temperature of a fluid flowing over a solid of semi-infinite thickness (the ‘semi-infinite solution’) is limited to relatively low Fourier numbers if Tad is to be calculated accurately. A two-layer composite substrate made from, for example, polycarbonate and Rohacell, could be used to achieve accurate estimates of h and Tad over a larger range of Biot numbers than for a single material substrate. TLC could be used to measure the surface temperature history of the composite substrate during an experiment; this would allow h and Tad to be solved from the numerical solution of Fourier’s equation or from a combination of the semi-infinite and steady-state solutions. The work presented in this thesis has uncovered some interesting findings in areas where research was limited. The measurements of the minimum sealant flow required to purge the wheelspace at off-design operation for a rotor-stator system with blades and vanes and the measurements of the adiabatic effectiveness on a rotating disc affected by ingress are unique and provide a platform for further experimental studies and validation of CFD models.