| Summary: | Erosion of gas turbines operating in sandy or dusty environments can result in
serious damage to the engine components, particularly the compressor unit. This
phenomenon is a result of the ingestion of the sand particles into the engine and their
consequent abrasive impacts on the blade surfaces. In order to understand the
mechanism of sand ingestion and the resulting performance degradation, a general
methodology has been developed for predicting the trajectories of particles, the erosion
rates and blade profile changes, with predictive capabilities
for performance
degradations within more general configurations of turbomachines. This methodology
was applied to an axial fan with upstream guide vanes (contra whirl) and was supported
by experimental results.
The numerical models for calculating the particle trajectory are based on the Lagrangian
tracking technique and the eddy lifetime concept. The turbulence effect is assumed to
prevail as long as the particle eddy interaction time is less than the eddy lifetime, and
the displacement of a particle relative to the eddy is less than the eddy length. The flow
field was solved separately using the Navier-Stokes finite volume flow solver "
TASCflow " commercially available from ASC. The governing equations of the particle
motion are solved using the Runge-Kutta Fehlberg technique. The tracking of particles
and their locations is based on a finite element interpolation method. The developed
Fortran code for predicting particle trajectory and erosion due to particle
impact
accounts for different types of boundary conditions and handles different frames of
reference. The fragmentation of particles after rebound was also implemented.
The number of particles seeded upstream of the IGV blades can be determined either by
a user defined concentration profile or by a measured concentration profile. Also,
particles can be seeded separately in a group at a release position. In the present study,
the concentration profile and the initial particle velocity and angle of particle spread
were determined from a laser transit anemometer. Two types of particles were used, a
narrow size bandwidth (150-300micron) quartz particle and MIL-E5007E quartz
particle, both of which have a normal distribution. The global rate of erosion, the
reduced mass of blades and the changes of the blade geometry were predicted and
compared with experimental results at different concentration levels.
The baseline axial fan characteristics were measured at different mass flow conditions at
a constant speed of rotation. To assess the effects of erosion, the characteristic
measurement was repeated after each step of sand ingestion. The predicted aerodynamic
performance; adiabatic efficiency, pressure rise coefficient and stall margin before and
after erosion degradation were also determined from a developed Fortran program,
which is basically a mean line method that uses advanced correlations for aerodynamic
losses.
Prediction
of the particle trajectories show that high numbers of impacts (and
maximum erosion) occurred near leading edge and tip region, which were also borne
out by locally injected sand tests. The global rate of erosion and the consequent changes of the blade geometry were also predicted and compared with experimental results. The
erosion pattern at high concentration of MIL-E5007E sand particles depicts net loss of
material over the leading edge and the tip corner. The tip clearance increased markedly
with a rounding of blade leading edge, which is the main cause of the decrease in
efficiency, pressure rise, and surge margin. A parametric study with turbulence and
fragmentation effects show that both parameters can influence the erosion rate and blade
geometry deterioration. The results of the aerodynamic performance simulation using
mean line method, which includes an erosion fault model, show good agreement with
experimental results.
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