Flow Patterns in High Pressure Steam Turbines During Low-Load Operation

Abstract

Due to the legislative efforts of promoting renewable energy sources, electricity from these sources is preferentially fed into the electrical grid. This requires more frequent part- and low-load operation of peak- and even of base-load power plants to compensate for the varying energy output of renewable energy sources. These requirements ultimately lead to an increased part- and low-load operation not only of low pressure (LP) steam turbines but also of high pressure (HP) steam turbines, putting them at risk of damage due to windage, i.e., strongly separated flow with associated heat generation. For the first time measurements of the steady-state flow field in a 7-stage model air turbine with a modern HP steam turbine blading are conducted in order to extend the understanding of the part- and low-load operation from LP to HP steam turbines. In comparison with LP steam turbines, similar flow fields develop during windage. However, differences exist especially concerning the vortex development in front of the turbine vane rows. The present, geometrically realistic 7-stage turbine, unlike other turbines investigated before, does not show these vortices, which is explained by the shape of the vane passages of this turbine blading. Furthermore, steady-state flow field measurements at different rotor speeds show that the flow coefficient can be used as a nondimensional parameter for maintaining flow field similarity even in part- or low-load operation. Additionally, unsteady circumferential pressure measurements show the existence of pressure perturbations moving circumferentially in front of the stage 7 blades. Seven pressure perturbations moving at 60% of the rotor speed are identified. Due to the shrouded design of the HP steam turbine blading used, the pressure perturbations are not due to tip leakage vortices. Hence, they are identified as features which are similar to “Rotating Stall” cells known from compressors.