Controlling secondary flow development is key to high efficiency turbine stages that must employ low aspect ratio blades.
The design of a turbomachine is a trade-off between many disciplines. There are instances where the overall efficiency of a machine is optimised even though the aerodynamics of a particular component are challenged. Such a situation often arises in the first stage of the low pressure turbine (or the intermediate pressure turbine of a three shaft engine) of an aero-engine. A typical requirement for the stator row of these turbines is that they must be thick enough to allow a structural support or an oil service pipe to pass through the main flow path into the centre of the engine. This leads to thick profiles with a low aspect ratio (span-to-chord ratio). The aerodynamics of low aspect ratio blades, particularly high turning ones, are dominated by secondary flows and are expected to suffer an associated performance penalty.
When the vorticity present in an end wall (hub or casing) boundary layer enters a blade row, the vortex filaments are turned so that the vorticity has a streamwise component at row exit: "secondary flow". For a particular blade row and inlet boundary layer, the extent of the secondary flows away from the endwall will scale with blade chord. In low aspect ratio blades, where the span is comparable or less than the chord, these secondary flows dominate. Several projects have tackled this problem at the Whittle Laboratory, each taking a different route to control the secondary flows: three-dimensional design, delayed turning profiles, low turning profiles (for contra-rotating shafts), and swept blades.
In the current project, splitter blades have been used to control the secondary flows. One or more splitter blades - short chord (high aspect ratio) - are inserted between adjacent low aspect ratio blades. The whole pack of blades (low aspect ratio blade plus accompanying splitters) is designed together using an optimiser and rapid 3D steady computations (using Turbostream). The design process, and associated test and analysis programme, has lead to both improved performance and also new insights into the development and control of secondary flows in turbines.
Aerodynamic Design of High End Wall Angle Turbine Stages—Part II: Experimental Verification
A. W. Cranstone; G. Pullan; E. M. Curtis; S. Bather
J. Turbomach. 2013; 136(2):021007-021007-10.
Improving Intermediate Pressure Turbine Performance by Using a Nonorthogonal Stator
Sungho Yoon; John Denton; Eric Curtis; John Longley; Graham Pullan
J. Turbomach. 2013; 136(2):021012-021012-8.
Secondary Flows and Loss Caused by Blade Row Interaction in a Turbine Stage
Graham Pullan; Neil W. Harvey
J. Turbomach. 2004; 128(3):484-491.