Browsing by Author "Anker, Jan Eric"

Filter results by typing the first few letters
Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    The simulation of turbomachinery flows at arbitrary Mach numbers and the analysis of leakage flows in shrouded axial turbines
    (2013) Anker, Jan Eric; Casey, Michael V. (Prof., D. Phil.)
    In the present thesis a preconditioned solution scheme for the simulation of turbomachinery flow at arbitrary Mach numbers is presented. A time-derivative preconditioning technique is applied to an explicit, time-marching Navier-Stokes code, originally concipated for compressible, high-speed turbomachinery applications. Various aspects of the accurate simulation of incompressible or low Mach number, compressible flows are addressed. The preconditioning scheme is formulated for fluids with a general equation of state. As verified by several test cases, the use of preconditioning allows the code to simulate flows efficiently and accurately for all Mach numbers. Four different dissipation schemes have been developed and adapted for the use in combination with preconditioning. It is shown how the robustness of Liou’s improved Advection Upwind Splitting Method (AUSM+) scheme can be enhanced by controlling the anti-diffusive parts of the dissipative fluxes. The different dissipation schemes are assessed on several test cases in terms of accuracy, solution monotonicity, and robustness. Since turbomachinery computations often are performed on truncated domains, a non-reflecting boundary condition (NRBC) treatment should be used. Because preconditioning alters the dynamics of the Navier-Stokes equations, the state-of-the-art NRBC treatment of Giles and Saxer can not be applied. For this reason, a new NRBC treatment for preconditioned systems and general equations of state is developed. The effectiveness of the novel treatment is demonstrated on two turbomachinery test cases. After validation against available measurement data, the solution scheme is used for a computational study of the interaction of labyrinth seal leakage flow and main flow in an axial, low-speed turbine. The results are used to investigate leakage flow phenomena and their dependence on the geometrical parameters of the sealing arrangements. The most influential geometrical parameter, the clearance height, is varied between a vanishing and a realistic height to systematically identify the impact of the leakage flow on main and secondary flow as well as on the loss generation mechanisms. The computational results show that the ingress of shroud leakage flow from the rotor significantly influences the main flow. The high swirl of the leakage flow causes a suction side incidence onto the following stator row and consequently leads to an amplification of the upper secondary channel vortex and increased losses. The results imply that in order to reduce the losses in shrouded, axial turbines, the swirl of the re-entering leakage flow should be corrected by constructive means. If this is technically infeasible, then the blades in the following blade row should be twisted to ensure a correct incidence and avoid flow separation. Since the computational results also show that not only the leakage flow but also the mere presence of cavities induce aerodynamic losses, the geometry of the inlet and outlet cavities must be considered with care in the design and optimization process of sealing arrangements for axial turbines.