Experimental study on the effect of density ratio on the rotor-stator disc cavity purge flow

The ingestion of hot mainstream through the rim seal of a high-pressure turbine stage can be controlled by the so-called cavity purge flow. It is a cooling flow of pressurized air introduced into the wheel space between the stator and rotor disks, which then interacts with the main annulus flow via the rim seal. In gas turbines there is a temperature difference between these two flows and thus a density ratio that affects the effectiveness of the rim seal. Also, the purge flow reduces turbine efficiency. The purpose of this thesis is to study the density ratio influence of the cavity purge flow, mainly in terms of sealing effectiveness and the effect on other relevant cavity flow parameters. The study was carried out through the KTH Test Turbine, that is a joint venture between Siemens Energy and KTH. The investigated turbine stage is a low degree of reaction high pressure axial turbine and the useful flow details are taken through pneumatic and temperature probes. The stage was tested under varying purge flow densities created by flowing air (DR=1) or argon (DR=1.38) into the cavity, and for three different mass flow ratios (MFR), both at the design point and off-design. To evaluate the sealing effectiveness, trace gas measurements of CO2 are performed in the cavity. For the same value of purge mass flow (i.e. MFR), the use of a purge gas characterized by a higher density ratio results in lower effectiveness values. In off-design conditions, the sealing level increases. As for the efficiency, total-to-total and total-to-static efficiency have been plotted; the trend is decreasing with the increase of the MFR, as expected, regardless of the gas used as purge. For the same MFR, the use of an heavier purge gas appears to result in lower efficiency values, apart from the higher MFR, both at the design point and off-design. The experimental data obtained could be used to calibrate correlations and models developed in the past few years. In this regard, the Orifice Model was used to interpolate the experimental data in order to find the optimal value for the discharge coefficient, which characterizes the rim seal, confirming results in literature. As the last, the swirl ratio was calculated to check whether any compressibility effect occurs within the cavity; observing the values of the swirl Mach number, it can be concluded that no major compressibility effects occur. The results of this work can be useful to predict the appropriate amount of purge to achieve a good trade-off between efficiency and safe operating temperatures. Also, to extrapolate test rig results to engine operating conditions.