Turbine Blade Aerodynamics

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4.3
Turbine Blade
Aerodynamics
Sumanta Acharya
Gazi Mahmood
Louisiana State University
CEBA 1419B, Mechanical
Engineering Department
Baton Rouge, LA 70803
phone: (225) 578-5809
email: acharya@me.lsu.edu
363 363
4.3-1 Introduction
The aerodynamics of the fl ow in a turbine stage (stator/rotor)
is rather complex and is still the subject of many ongoing research
activities in the gas turbine community. The fl ow is inherently
three dimensional due to the vane/blade passage geometry
with features such as twisting of the vane/blade along the span,
clearance between the blade tip and the shroud, fi lm cooling holes,
and end wall contouring1. The passage fl ow is characterized
by boundary layer effects, secondary fl ows generated by the
passage pressure gradients, and vortical fl ow structures such as
the leading edge horse-shoe vortices, tip-leakage fl ow vortices,
and corner vortices2. The effects of centrifugal-buoyancy, shock-
boundary layer interaction, and fl ow interactions between the
stator and rotor rows complicate the passage fl ow fi eld even
further. Along the end walls, the fl ow structure is strongly three-
dimensional with the passage vortex and coolant injection on the
hub side and the tip-leakage vortex on the tip side. In the mid-
span regions located away from the passage walls and outside
the viscous shear layer, the radial fl ow is almost negligible and
the fl ow is effectively two dimensional. The fl uid dynamics in
this region can then be based on two dimensional planar cascade
fl ow studies without any signifi cant loss of information. The
three dimensional complex fl ow structures near the hub endwall
region and in the blade tip-shroud clearance have been simulated
in annular vane/blade passages with and without rotating blade
row3. Studies of the complex end-wall fl ows have also been
performed in stationary cascades with three dimensional airfoil
shapes4. The qualitative features of the passage fl ows, which
comprise mainly of the passage crossfl ow (fl ow from the pressure
side of vane/blade to suction side of adjacent vane/blade) and
vortical fl ows induced by the leading edge, the corners, and the
injected coolant fl ows have been studied in detail in stationary
cascades and are considered to be similar in both stationary and
rotating blade rows. The primary difference in the secondary fl ow
structure between the blade passage and vane passage is that the
vortical fl ows and cross fl ows in the blade passage are stronger
because of higher turning of the fl ows along the blade passage.
Secondary fl ows are the major source of aerodynamic losses,
which account for 35%-40% of all losses5 and thermal loading
in the turbine passage, and thus require special considerations by
the turbine designers.
The primary objectives of this chapter are to present and
analyze the features of the fl ow fi eld in the turbine vane/blade
passage near the hub endwall and mid-span locations of the blade.
Toward this effort, reported measurements and computations of
pressure, velocity distributions, fl ow turning angles, turbulence
intensity, and vorticity distributions in the cascade test section
are presented. Recent efforts to reduce the secondary fl ows by
structural modifi cations in the passage are discussed. In this
chapter, basic fl uid dynamic principles and mathematical models
of the fl ow in the passage are not discussed, and the reader is
referred to notes 1, 2, and 6 for additional details6. Also details
on the aerodynamic design methodology for the vane/blade
passage are not presented.