In this paper, the detailed steady and unsteady numerical investigations of a 1.5-stage axial flow turbine are conducted to determine the specific influence of interplatform steps in the first stator—as caused by deviations in manufacturing or assembly. A basic first stator design and a design consisting of a bow and endwall contours are compared. Apart from step height, the position and geometry of the interplatform border are varied for the basic design. To create the steps, every third stator vane was elevated, together with its platforms at hub and shroud, such that the flow capacity is only little affected. The results show that the effects of steps on the platform borders in front and aft of the first stator can be decoupled from those occurring on the interplatform steps. For the latter, being the main contributor to the additional loss, the intensity of recirculation zones and losses increase substantially when the platform border is located close to the suction side. Using a relative step height of 1.82% span, the entropy production doubles when compared to a position close to the pressure side, which can be explained by differences in local flow velocity level. Regarding a circular-arc-shaped platform, the losses can be more than halved—mainly due to lower included angles between step and endwall flow streamlines. The findings can be explained by a nondimensional relation of the local entropy production using local values for step height and characteristic flow quantities. Furthermore, a reduction in step height leads to an attenuation of the otherwise linear relationship between step height and entropy production, which is mainly due to lower local ratio of step height and boundary layer thickness. In the case of laminar or transitional flow regions on the endwall, typical for turbine rigs with low inlet turbulence and low-pressure turbines under cruise conditions, the steps lead to immediate local flow transition and thus substantially different results.

The effect of hub platform misalignment in the first vane of a 1.5 stage axial test rig turbine on the efficiency is numerically analyzed. An investigation is made into how this misalignment, as caused for example by manufacturing deviation, impacts the intended 3D flow in an endwall-contoured design and how robust the design is compared to a uncontoured turbine. Axial misalignment was created by extending all platforms within the blade row in radial direction by up to 5.5 % of the channel height. In order to create circumferential steps, only every third platform was elevated. The results are based on steady and unsteady simulations with the DLR RANS solver TRACE. In general, both axial and circumferential steps alter the static pressure field and lead to flow separations bubbles. These effects lead to the creation of new vortices which interact with the classic turbine secondary flow. It turns out that increasing the step height generally reinforces the secondary flow intensity. In addition to local detrimental effects, these processes significantly alter the inflow conditions to the subsequent blade rows, leading to increased losses there. A comparison of the results for the uncontoured and the non-axisymmetric endwall shows that the beneficial effects of the latter, which are based mainly on radial homogenization of the outlet flow yaw angle in the first vane, still continue to exist in the presence of platform steps, although the overall efficiency is significantly reduced. An experimental validation of the platform effects is not included in this paper but will follow in the near future.

In this paper detailed steady and unsteady numerical investigations of a 1.5 stage axial flow turbine are conducted to determine the specific influence of inter-platform steps in the first stator — as caused by deviations in manufacturing or assembly. A basic first stator design and a design consisting of a bow and endwall contours are compared. Apart from step height, the position and geometry of the inter-platform border are varied for the basic design. To create the steps, every third stator vane was elevated, together with its platforms at hub and shroud — such that the flow capacity is only little affected. The results show that the effects of steps on the platform borders in front and aft of the first stator can be decoupled from those occurring on the inter-platform steps. For the latter — being the main contributor to the additional loss — the intensity of recirculation zones and losses increase substantially when the platform border is located close to the suction side. Using a relative step height of 1.82 % span, the entropy production doubles when compared to a position close to the pressure side, which can be explained by differences in local flow velocity level. Regarding a circular-arc shape platform, the losses can be more than halved — mainly due to lower included angles between step and endwall flow streamlines. The findings can be explained by a non-dimensional relation of the local entropy production using local values for step height and characteristic flow quantities. Furthermore, a reduction in step height leads to an attenuation of the otherwise linear relationship between step height and entropy production, which is mainly due to lower local ratio of step height and boundary layer thickness. In the case of laminar or transitional flow regions on the endwall — typical for turbine rigs with low inlet turbulence and low-pressure turbines under cruise conditions — steps lead to immediate local flow transition and thus substantially different results.