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.