On the Simulation of unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine, Part III: Comparison of Harmonic Balance and Full Wheel Simulation

Abstract

This is the third part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. The third part of the series deals with the detailed comparison of the harmonic balance calculations with the full wheel simulations and measurements for the two-stage low-pressure turbine. The harmonic balance calculations were taken into account turbulence and transition either only once in the 0th harmonic and the other times in all harmonics. The same Wilcox’s two-equation k − omega turbulence model along with Menter and Langtry’s two-equation  Gamma− Re_Theta transition model is used in the Harmonic Balance simulation as in the full wheel simulations. The measurements on the second stator of the low-pressure turbine have been carried out separately for downstream and upstream influences. Thus, a dedicated comparison of the downstream and upstream influences of the flow to the second stator is possible. In the Harmonic Balance calculations, the influences of the not directly adjacent blade, i.e. the first stator, were also included in the second stator. In the first analysis, however, it was shown that the consistency with the full wheel configuration and the measurement in this case was not as good as expected. From the analysis of the full wheel simualtion, we found that there is a considerable variation in the magnitude of the unsteady values in the second stator. In a further deeper consideration of the configuration, it is found that modes are reflected in downstream rows and influences flow in the second stator. After the integration of these modes into the harmonic balance calculations, a much better agreement was reached with results the full wheel simulation and the measurements. The second stator has a laminar region on the suction side starting at the leading edge and then transition takes place via a separation or in bypass mode, depending on the particular blade viewed in the circumferential direction. In the area of transition, the clear difference between the calculations without and with consideration of the higher harmonics in the turbulence and transition models can be clearly seen. The consideration of the higher harmonics in the turbulence and transition models brings about an improvement in the consistency.