Verification of Low-Flow Conditions in a Multistage Turbine

Modern power plants face increasing problems with windage effects in high pressure steam turbines, due to the bigger size of the rotor blades and a more flexible demand of the electricity market, which may lead to more frequent operation at low-flow conditions. So far, no theoretical model exists to fully describe these flow phenomena which would help to prevent an overheating of the turbine blades and minimize the risk of damage. The main goal of this research project therefore is to predict the part-load behavior. Measurements of the flow field of a four-stage research air turbine were carried out at low Mach numbers to better understand the aerodynamic characteristics and the flow mechanisms at part-load. The experimental data such as temperature, pressure, velocity, and flow angles, measured in 6 different planes along the turbine annulus for different rotational speeds and different relative mass flows, have been compared with the numerical results of the CFDsolver TRACE. To obtain more realistic results than in computations published earlier, a newly generated finer grid and an extension of the computational domain at the outlet were used. It is shown that with the right initialization, the CFD-Solver is capable of providing converged calculation results even for low mass flows and high rotational speeds. The results are verified with experimental data e.g. by the temperature distribution within the four-stage turbine and the pressure and temperature profiles in the measurement planes. As a general result, the highest temperatures in the turbine do not occur behind the last stage, but in the downstream third of the machine, which agrees with experiences of damage observed in real turbines.Copyright © 2007 by ASME