Handbook of Turbomachinery Second Edition Revised - Ventech!

occur due to particles of unburned carbon emerging from the combustor or
dust and sand in the air in certain arid regions. The particles transported by
hot gases over blades do not follow the fluid streamlines since they are more
massive and the centrifugal forces tend to throw them toward the blade tips.
Hence, they often damage the blade tip or the suction surface so badly that
the performance is affected and the blade may need replacement. They may
damage the thermal barrier coat (if present) and expose the metal surface to
hot gas. While the presence of unburned carbon is determined by combustor
design, the inflow of sand particles may be minimized by using an
appropriate filtering medium or particle separator at the engine inlet.
However, it is difficult to prevent the erosion of helicopter blades due to
sand.
Corrective measures such as slight changes in geometry, introduction
of an insulating layer at appropriate locations, increase in cooling flow, etc.
may be initiated if the first design leads to a shorter than desired life. With
the completion of stress and vibration analyses and life evaluation, drawings
are prepared and released for the manufacture of the parts.
Validation and Certification Tests
It is usual to perform cold flow, spin-pit, etc. tests for items like blades and
nozzles, while rig tests are performed on individual components such as the
axial compressor, centrifugal compressor, HPT, or LPT. Scaled models may
also be used in some of the tests. The rig tests include complete
instrumentation and are used to validate the calculated performance of
the components. The mass flows of air, pressures, and temperatures as well
as rotational speeds are monitored at all the design points, ranging from idle
to rated maximum power (usually 110% of rated power). For turboshaft
engines, similar tests may be performed to determine the performance
parameters of other components such as recuperators. These tests, if
satisfactory, are followed by a full engine test, with operating conditions
ranging from idle to maximum power conditions. The Federal Aeronautics
Administration (FAA) specifies the operating cycle and the number of cycles
to which the engine must be subjected without deterioration before being
certified as airworthy for commercial or military use. The following is a brief
list of some of the tests.
Component Tests. These are performed on individual components such as
blades, vanes, shrouds, heat exchangers, etc. They may be run in laboratory
rigs to verify the validity of the assumptions used in design. The components
may be tested under simulated conditions close to those expected during
engine operation. These consist of:
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