A Unified Turbine Preliminary Design Study

Abstract

During the preliminary stages of design, designers must make decisions concerning size, speed and machine type. Misinformed choices in design parameters and architecture can lock in suboptimal performance. Design charts leveraging similitude are powerful tools that can help designers make better decisions at this point in the design process. The design charts used up until now are either partially or entirely based on lost correlations and do not include mixed flow architectures. On top of this many of the current charts contain little to no information about the various loss mechanisms, turbine characteristics and how both change through out the design parameter space. This thesis builds on the pre existing work by mapping out design parameter spaces using computation methods, more specifically using a Reynolds averaged Navier Stokes Equation Solver. This also allowed for the partial decoupling of loss mechanisms. This thesis includes a unified design methodology built around a unified set of design flow parameters. This was used to generate design charts for all architecture types, including radially fibred mixed flow architectures with fixed cone angle and radius ratio. Both total to static and total to total efficiency was presented as a function of loading coefficient and duty flow coefficient instead of the typically used non dimensional speed and diameter. This was done in order to help aid in the aerodynamic interpretation of the data. Results showed the shapes of the design spaces in the present framework can largely be explained by surface dissipation. That is, entropy generated at the surface is proportional to the surface velocity cubed with a fixed dissipation coefficient. A universal trend across all architectures was found. Turbines with high duty flow coefficient have characteristically high surface velocities and turbines with lower values of duty flow coefficient are characterised by high surface area. However, the balance between area and surface velocity of the different architectures differed significantly. Axial turbines have characteristically high surface velocities. Whereas radial turbines have characteristically high surface area and lower surface velocities. The lower surface velocities of the radial turbine was in part due to the centrifugal loading. It was shown that shifting loading from the relative acceleration term to the centrifugal term decreases relative passage velocities. It was shown that axial turbines have the broadest range across the design space, both the constrained mixed flow and radial turbines sit inside the axial space. In addition to this, axial architectures did not show a performance boundary at lower duty flow coefficients. This was attributed to the aspect ratio being fixed and other tolerances based geometric features scaling with the passage. The mixed and radial architectures were shown to suffer high losses at lower duty flow coefficients, this was attributed to the high end wall surface area, characteristic of designs in this region of the Balje chart. iv All turbines suffer a drop off in performance at high duty flow with all mechanisms con tributing to this. The characteristically high velocities of these designs result in high profile and end wall surface dissipation loss. In addition the leakage loss increases in part due to the high velocity with which the leakage flow is mixing and the high over tip driving pressure. Radial architectures show a sharper drop in performance with increasing duty flow. This was attributed to flow separation at the casing due to reducing radius of curvature. By virtue of having a lower cone angle, the mixed flow architecture have a significantly larger radius of curvature and did not exbibit any signs of flow separation. As expected the (radially fibred) mixed flow architectures have lower performance loss with increasing loading coefficients than the (radially fibred) radial architectures due to the inlet metal angle not being constrained. However axial architectures outperform both. When comparing the mixed and axial flow architectures the mixed flow architectures showed a sharper increase in surface area with increasing loading.