Non-contact measurement fine tunes turbines

Edited by the Engineeringtalk editorial team May 19, 2008

NVision uses its Maxos system to measure the critical blade geometry, ensuring the prototypes blades meet the design intent and enhance simulation accuracy.

NVision's noncontact measurement services are helping several major manufacturers of turbine blades improve blade performance by increasing the accuracy of flow simulations used in the design process.

In the past, the blade manufacturers could not accurately measure prototype blades, especially the leading and trailing edges.

Therefore they could not be sure that physical tests matched simulations, making it very difficult to validate the test results.

NVision uses its Maxos system to measure the critical blade geometry, ensuring the prototypes blades meet the design intent and enhance simulation accuracy.

Today's most advanced turbine blades used in power generation are primarily designed by using computational fluid dynamics (CFD) to simulate the flow of gases over the blades.

Simulation gives engineers the ability to quickly and inexpensively evaluate the performance of many different blade designs using virtual prototypes.

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 their complexity has made it impossible to fully capture the critical geometry with standard co-ordinate measuring machines (CMMs), laser scanners or white light scanners.

CMMs only capture one point at a time and their touch probes are too large to measure very small fillet radii.

Additionally, ball-offset geometry limits the ability to accurately measure leading and trailing edges.

Another problem with conventional CMMs is three axes of motion are often not enough to access the entire blade geometry, including the root.

Laser and white light scanners capture a large amount of data, but once again struggle to accurately define the and trailing edges.

Another issue with laser and white light is the inability to measure machined or polished surfaces without first coating them with a matt spray that varies in thickness between 25.4 and 127um, and compromises the accuracy of the measured points to the same degree.

The turbine blade manufacturers have addressed this problem by working with NVision's contract service division in Wixom, Michigan, which has a long history of working with turbine OEMs and their suppliers.

NVision uses 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".

In fact, the Maxos can measure radii down to 101.6um.

Unlike other noncontact measurement systems, the Maxos measures actual surface material without the need to spray it with a matt coating.

With NVision now measuring the critical blade geometry to high levels of accuracy, the blade designers can squeeze the last bit of performance out of their blades.

Current blades in production can also benefit from more accurate measurements.

NVision's noncontact measurement helps ensure that CFD simulations precisely match the physical tests used to validate the simulations.

The end result is that the turbine blade manufacturers can optimise their blades to higher levels of performance and reduce the cost of expensive physical testing.