Flow Transition in the Wheel Side Space of a 1.5-Stage Test Turbine

Secondary air systems in gas turbines provide a pressurized purge for the protection of highly-stressed components. The selection of an appropriate purge air mass flow seeks to sufficiently reduce the ingestion of hot annulus gas into the side wheel space with minimal impact on gas turbine efficiency. This paper describes a computational study of the flow field in a wheel side space of a 1.5-stage test turbine under different operating conditions. Measurement data for the investigated test turbine shows the formation of large flow structures (low pressure areas) in the wheel side space for sufficiently low dimensionless purge air mass flow rates CW when using an axial sealing geometry. These structures are mainly apparent through their influence on the unsteady pressure field within the wheel side space. Large pressure inhomogeneities increase the ingestion of hot annulus gas thus reducing sealing efficiency. Furthermore, the high angular momentum of the ingested gas causes an increase in fluid core rotation which in turn results in large radial pressure gradients in the time-averaged flow field. Depending on operating conditions the flow transition (breakdown of large flow structures) can be observed around a characteristic CW,trans. This flow transition has a profound impact on pressure distribution and sealing efficiency. Contrary to the approach of high model complexity and large computational efforts for analysis of the highly intricate flow fields in wheel side spaces, this paper presents a methodology utilizing a highly simplified computational model with an (U)RANS approach. Despite the low model complexity, the simulation results shown in this paper show good agreement with measuring data pertaining to the flow field in the rotor-stator cavities and more specifically the flow transition. Possible reasons for deviations from measurement data as well as possibilities for further feasible simplification in the modeling approach are discussed. Furthermore, the presented simulation methodology is applied to a radial sealing geometry, which does not show a flow transition in experimental data. It is shown that the flow within the wheel side space can be modeled with comparatively small deviations from measurement data regarding pressure and sealing efficiency. To assert, whether a flow transition effect can be found outside of flow conditions measured in the experiment, simulations with very low purge air flow rates are conducted.