Origin, Evolution, Decay and Break-Down of Rotating Instabilities in Low-Flow Turbine Operation – Part A: Time-Averaged Results

Conventional power plants remain a key component in enabling a stable and flexible power grid. Unlike before, peak efficiency is no longer the only objective in turbine design. The low-flow operating regime that limits the minimum performance range is, however, a challenging one. It features aerodynamic instabilities, increased temperature loads, and high blade stresses, which can lead to blade failure. With this two-part paper, we aim to to provide a qualitative understanding of the flow field in low-flow turbine operation, focusing on the origin, evolution and temporal behavior of rotating instabilities (RI) in a three-stage turbine rig. In Part A of this paper, the time-averaged characteristics of low-flow turbine operation and the rotating instabilities forming in such operating points are investigated. Steady-state and time-averaged, full-annulus unsteady flow simulations supplement the experimental data. The map of the turbine rig investigated is assessed experimentally and numerically for multiple speed lines and mass-flow rates. The key parameter dictating low flow is the flow coefficient φ = cax/u. It is found that steady-state simulations and even simplified mean-line considerations, using a model introduced in this paper, can predict the time-mean flow field within an error margin of 8%. Only for very low flow coefficients, larger deviations are observed, necessitating unsteady simulations. Using the flow coefficient, an onset condition independent of the absolute value of speed can be established for all speed lines. When decreasing the mass flow rate at constant speed, local diffuser separation is observed. In low flow, suction-side incidence results in large-scale flow separation on the pressure side. This induces backflow in the top-most section of the span and, by interaction with the main flow, induces a vortex structure extending from 90% to 100% span. The high incidence also results in a change in the direction of turning in the turbine passage. As a result, the enthalpy change reverses its sign and the work process is inhibited or even reversed.