Effect of non-axisymmetric casing on flow and performance of an axial turbine

Advances in computer based optimization techniques can be used to enhance the efficiency of energy conversions processes, such as by reducing the aerodynamic loss in thermal power plant turbomachines. One viable approach for reducing this flow energy loss is by endwall contouring. This thesis implements a design optimization workflow for the casing geometry of a 1.5 stage axial flow turbine, towards mitigating secondary flow losses. In this thesis, a new non-axisymmetric endwall design method for the stator casing is implemented through a novel surface definition that draws from observations of the typical secondary flow pattern over the casing. The new casing design technique focuses on manipulating specific flow structures directly while also influencing the surrounding pressure field. This approach is tested on a three-dimensional axial turbine RANS model built in OpenFOAM Extend 3.2, with k-ω SST turbulence closure. Computer-based optimization of the surface topology is demonstrated towards automating the design process. This is implemented using Automated Process and Optimization Workbench (APOW) software. The designs are optimized using the total pressure loss across the full stage as the target function. The optimization and its sensitivity analysis give confidence that a good predictive ability is obtained by the Kriging surrogate model used in the prototype design process. The casing surface parametrization was shown to produce topologically smooth interfaces with the rest of the passage geometry. This was achieved by using the Beta distribution function to design a smooth casing groove path, which is a first application of the Beta distribution function to the contouring of a turbomachine casing. The flow analysis confirms the positive impact of the optimized casing groove design on the turbine isentropic efficiency compared to a reference diffusion based endwall design and compared to the benchmark axisymmetric design, at design and at off design conditions.