Unsteady Secondary Flow in an Engine-Representative Low Pressure Turbine

An effective method to reduce losses associated with secondary flows in turbines and thereby increase turbine efficiency is endwall contouring. Due to the number of parameters involved in the definition of the endwall geometry, automated optimisation algorithms are used in the design process. Steady simulations based on the Reynolds-averaged Navier-Stokes equations (RANS) allow for the sufficient number of iterations in the optimisation procedure. Nevertheless unsteady interactions alter the secondary flow field in a vane-rotor configuration considerably. Yet, the physical phenomena in the development of unsteady secondary flows, especially vane-rotor-vane interactions, are very complex and not fully understood. Thus, this thesis examines the development of unsteady secondary flow over multiple rows in a low pressure turbine. Experimental and numerical investigations of unsteady secondary flows were performed in a two-stage engine-representative low pressure turbine. Area traverses between each blade row provided time-resolved and time-averaged measurement data for the validation of an unsteady RANS (URANS) simulation. It was revealed that the simulation was capable of predicting the secondary flow system including vane-rotor-vane interactions. Subsequently, the unsteady simulation was used to examine the development of the secondary flow through the machine. The analysis of the unsteady flowfield revealed that secondary flow structures of vane 1 and rotor 1 influence the time-mean and time-resolved flow field at the exit of vane 2. The secondary flow system in the turbine differs both between hub and casing as well as from row to row due to unsteady interaction mechanisms. Those mechanisms were analysed and illustrated. Numerical studies were performed to transfer the improved understanding of unsteady secondary flows into turbine design recommendations. The accuracy of the predicted secondary flow field at vane 2 exit improved considerably with the number of upstream blade rows simulated with URANS. A clocking study revealed that the relative circumferential position of vane 1 on the secondary flow structures at vane 2 exit should be taken into account in the design process. Responsible for that were not only the secondary flows of vane 1, but also the unsteady effect of the first vane on the development of the secondary flow field of the rotor. This work concludes in a recommendation for the configuration of endwall design simulations based on URANS.