Abstract
Hot streaks can cause local hot spots on the blade surfaces of
high-pressure turbine stages, resulting in locally higher thermal
loads. These local loads represent a potential source of blade life
reduction and blade failure. The blade regions exposed to higher
thermal loads are determined by the effect of the unsteady blade
row interaction on the migration path of the hot streaks. In order to
improve understanding of these effects an experimental study on
the effect of shaping the inlet temperature distortion has been
undertaken.1
The experimental investigations have been performed in the axial
turbine facility “LISA” at ETH Zurich. The test configuration
consists of a one-and-1/2 stage, unshrouded, highly loaded axial
turbine with a hot streak generator placed upstream of the first vane
row. The latter is designed to provide different shapes of the inlet
temperature distortion, as well as different circumferential and
spanwise positions. The steady and unsteady aerodynamic effects
are measured respectively with pneumatic probes and the in-house
developed Fast Response Aerodynamic Probe (FRAP) technology.
The unsteady thermodynamic effects are measured in a time
resolved manner with the in-house developed Fast Response
Entropy Probe (FENT). The time resolved measurements are made
in planes at the inlet to the first vane row as well as downstream of
it and downstream of the rotor.
The current paper presents the results of the first shaped hot streak
injection and analyzes the mechanisms involved in the convection
and the migration of the hot streak through the bladed rows.
The effect of the first stationary blade row on the path of the hot
streak is explained by an analysis of the flow field and temperature
field at the exit of the first nozzle guide vane row. Mixing and heat
conduction as well as the unsteady effect of the downstream rotor
cause the total temperature distortion to diminish thus generating a
more uniform distribution.
The effect of the rotating blade row is shown with the flow field and
the temperature field at the exit of the rotor. The measurements
reveal a radial migration of the hot streak which is confined in
circumferential direction by the pressure side of the rotor wake
causing the fluid to partially go into the tip leakage vortex.
Furthermore, at the suction side of the rotor blade the hot gases are
confined in between the passage vortices of the row. The root mean
square of the unsteady pressure signal acquired can be used for
tracing the mixing process and losses showing the interaction of the
hot streak with the secondary flow structures.
