Experimental and Numerical Investigations of Flowpath Profiling on Secondary Flow Losses in a Turbine Control Stage

Abstract

The numerically and experimentally investigated industrial steam turbine control stage is derived from a real design. Due to the production process and costs of the guide vanes for control stages of steam turbines the flowpath profiling is rotationally symmetric. However the combination of the two-dimensional shroud contour and the flow deflection in the guide vane results in a fully three-dimensional end wall contour having a strong influence on the secondary flow features in the turbine control stage. To obtain an improved profile for the nozzle shroud the reduction of the total pressure loss over the guide vanes is taken as an optimization criterion. The three-dimensional contour generates a diffuser flowpath between the suction and the pressure side of two guide vanes perpendicular to the main flow direction. This diffuser geometry affects the pressure distribution over the guide vane and therefore the formation mechanisms of secondary flows. For the experimental and numerical investigations a baseline shroud design and two additional profiled contours are analyzed in detail. The control stage test rig is operated with air and is capable to represent a wide range of operating conditions. The measurements show a considerable increase of the stage efficiency and power output. The effect of the flowpath profiling on the pressure distribution over the guide vane is clearly proved.

Experimental investigation on performance of a control stage turbine under partial admission

“…For the experiment, total inlet temperature of the Air (drawn from the environment) is set to 320.15K in the settling chamber and the inlet pressure is determined by the pressure ratio with environmental pressure at the outlet. As mentioned by Moser et al (2013), under full admission average Mach numbers reach from 0.67 to 1.1 and Reynolds numbers lay between 5.6 • 10 5 and 8.2 • 10 5 . The transition from a circular cross section in the settling chamber to the ring-shape of the inlet is realized with curved structures, including a korbbogen head, not reproducing the typical structure of block nozzle segments.…”

“…An axial-flow air turbine containing a control stage, located at the laboratory of turbomachinery of the Helmut Schmidt University in Hamburg is utilized for the experiment, as displayed in Figure 1 (a) and (b). The control stage had already been investigated under full admission with a plane and a contoured rotationally symmetric shroud sided endwall by Moser et al (2013), as seen in Figure 1 (d). The plane endwall variant serves as a reference to the contoured endwall variant.…”

“…The efficiency η s is determined as described with eq. ( 1) previously used by Moser et al (2013): the quotient of the performance P t and the isentropic enthalpy difference between inlet and outlet ∆H s . The mass flow through the air turbine is measured with an orifice plate upstream of the control stage and, the specific isentropic enthalpy difference is determined via pressure and temperature indications at the turbine.…”

“…Preceding investigations were directed to the numerical optimization of endwall contouring on the stator's shroud side, using evolutionary algorithms (Moser et al 2011). Following this, Moser et al (2013) experimentally investigated endwall contouring with the test rig, also applied in this work. An increase in efficiency for sub-critical pressure ratios was depicted, in comparing a plane and a contoured endwall design.…”

Optimization of the non-axisymmetric stator casing of a 1.5 stage axial turbine

“…Moser et al [8] designed an optimized radially profiled axisymmetric casing for a guide vane of a steam turbine stage that reduced the stage loss over a wide range of pressure ratios. Kumar and Govardhan [9] applied a streamwise end wall fence to reduce secondary flow losses in a linear turbine cascade.…”