KEYWORDS: Secondary flow losses; Transonic turbine stage; Leading edge fil-
let; endwall contouring.

The objective of this work is to study the effect of different end wall contours and
leading edge modifications on the performance of a highly loaded turbine with a special
emphasis to reduce the secondary losses, thus increasing the overall efficiency. Steady
and unsteady numerical investigations are carried out to predict the aerodynamic loss
mechanisms associated with the highly loaded low aspect ratio transonic turbine stage.
It is designed for a stage pressure ratio of 2.78 and stage loading of 1.9 operating at

a speed line of 15976 rpm. A computational fluid dynamics (CFD) method (ANSYS-
CFX 14.5) has been applied to predict the flow field in the turbine stage. Analysis is

carried out at different expansion ratios which represents the different stage outlet Mach
numbers ranging from subsonic to high transonic regime. Effect of expansion ratio
resulted in an increment in vane and rotor blade loading due to increment in oblique
nature of trailing edge shock wave as well as penetration length of secondary flow along
span is observed. Further, unsteady analysis is carried out at design point using Time
Transformation Technique to understand the effect of shock interaction between nozzle
guide vane and rotor blade.

Studies are carried out to understand the effect of constant and variable radii fil-
let on secondary flow losses in Nozzle Guide Vane of the transonic turbine stage. In

continuation the effects of it application is investigated in rotor passage also. Fillets
near the blade-endwall juncture are designed using shape parameters like leading edge
radius and included angle. Formation of horseshoe vortex and its transformation into
passage vortex is shown using streamline and vector plots. Results are discussed using
topological features of the flow field. Separation lines associated with inlet boundary
layer and passage vortex, saddle point and attachment nodes are resolved. Comparison
is made for exit flow angle variation and total pressure loss coefficient. Variable radii
fillet works better when compared to constant radii fillet applied around the blade for the both cases NGV and rotor endwall juncture.
Effectiveness of endwall contours on secondary flow field is investigated with and
without inclusion of fillet modifications. Axisymmetric variation on endwall profiles are

achieved by functional approximation and genetic algorithm based numerical optimiza-
tion method. A subset of control points are considered as the design variables having a

constraint to move in radial coordinates. Statistical tool Latin Hypercube Sampling is
adopted to explore the design space. Based on the values of objective functions obtained

from the numerical CFD calculations, functional approximation model is construed us-
ing the artificial neural network. Finally, a genetic algorithm is used to obtain the op-
timum solution and analyze the effect of the axisymmetric endwalls on secondary flow

losses. Design of endwall profile is studied in three different approaches by confining
the axisymmetric variation near hub, shroud and both endwalls. Based on the obtained
optimized profiles, secondary flow mechanisms occurring inside the NGV passage are
investigated. Reduction in total pressure loss coefficient and improvement in isentropic
total-total efficiency are observed for the optimized profiles. Further in its investigation,

axisymmetric contours having variation of its kind in its location in streamwise direc-
tion and the shape on the tip side is investigated. Maximum reduction of 30.6% in mass

averaged total pressure loss coefficient is achieved for endwall contour having "S-type"
variation along with fillet for the NGV. The performance improvement is measured in
terms of reduction in secondary losses. The combined effect of end wall and leading
edge fillets has been investigated for their effectiveness in reducing secondary losses.
These objectives are met during this present study with the numerical investigations.