Numerical Investigation of a 1-1/2 Axial Turbine Stage at Quasi-Steady and Fully Unsteady Conditions

Abstract

The analysis and design methods used for turbomachinery components are mostly based on steady aerodynamics, neglecting the important unsteady nature of the flow field. An improvement in performance can however be achieved with a prior understanding, evaluation and modeling of the main unsteady loss sources generated in rotor/stator interactions, through new advanced experimental data coupled to systematic and controlled numerical simulations performed at the full unsteady level of approximation. But such calculations are even nowadays challenges to the CFD community, due to their high requirement in computer resources. To investigate the importance of unsteady loss mechanisms, a 1-1/2 axial turbine stage has been resolved at both quasi-steady and fully unsteady levels of approximation. In order to reduce the demand on computer resources, a scaling procedure can be applied to retrieve equal pitch distance on both sides of each rotor/stator interface. The space and time flow periodicity are then uncoupled and the unsteady flowfield may be resolved on a reduced number of blade passages per row without having to consider any time periodicity in the boundary treatment. The grid scaling however affects the turbine total pressure ratio and the position and strength of secondary flows, as the pitch-to-chord ratio is not kept constant. This effect is analyzed in the paper, with the objective to assess the associated approximation errors. Steady and unsteady numerical simulations are compared with the experimental data along three measuring stations placed downstream of each blade row. Even if steady results are in good agreement and allow capturing the main flow structures of the turbine stage, only the fully unsteady calculation resolves the complex loss mechanisms encountered mainly in the rotor and downstream stator components. These unsteady interactions are observed through time variations of the entropy, absolute flow angle and static pressure.