Most advanced turbine blades used in power
generation are primarily designed using computational fluid dynamics
(CFD) to simulate the flow of gases over the blades. However, before a
CFD model can be used with confidence to evaluate blade geometry, it
must be validated by running a physical test with an identical blade
and operating conditions to make sure the simulation matches the test
results.

The problem with new generation turbine blades is that
their complexity has made it impossible to fully capture the critical
geometry with standard coordinate measuring machines (CMMs), laser
scanners, or white light scanners. CMMs capture one point at a time and
their touch probes are too large to measure very small fillet radii.
Their ball-offset geometry limits the ability to accurately measure
leading and trailing edges. And the three axes of motion with
conventional CMMs is often not enough to access the entire blade
geometry, including the root.

Laser and white light scanners
capture a large amount of data, but do not accurately define the
leading and trailing edges of the blades. Plus, for an accurate
measurement, these systems require that machined or polished surfaces
be first coated with a matt spray that varies in thickness between 1
and 5 thousands of an inch, which can compromise the accuracy of the
measured points to the same degree.

To address this issue,
some turbine blade manufacturers have worked with NVision’s contract
service division in Wixom, Mich. The engineers at NVision use the MAXOS
system, which measures using a concentrated light rather than a touch
probe. The system eliminates the inaccuracies inherent in contact probe
measurement on small radii or sharp edges caused by ball-offset
geometry, sometimes known as “the cosine error.” The system can measure
radii down to 0.004 in. It also measures actual surface material
without the need for a matt coating. The accuracy of the system is +/-
0.0004 in. on polished and machined services and the resolution between
the measured points down to 0.0001 in.

NVision’s non-contact
measurement helps ensure that CFD simulations precisely match the
physical tests used to validate the simulations. Turbine blade
manufacturers can optimize their blades and reduce the cost of
expensive physical testing.

NVision, Inc.
www.nvision3d.com

MPF