Automated Inspection of Cooling Holes in Turbine Blades

Let's clear things up: Cooling holes in turbine blades are hugely important. They may be tiny little holes, but they fulfill very important tasks. The fact is that current compressors achieve far higher outlet temperatures than would be beneficial to the material of the turbine blade. The blade material must therefore be protected. And that is exactly what these small holes in the turbine do. They ensure that film cooling is applied to the outer surfaces of the turbine blades.

Cooling holes on a turbine blade — Cooling Holes on a Turbine Blade

How to inspect Cooling Holes on a Turbine Blade areo-engine — Inspection of the Cooling Holes

Why do the Compressors run so Hot in the first place?

The high temperatures in an airplane's compressor provide more efficiency on the one hand and lower fuel consumption on the other. That both sounds good, doesn't it? However, if the small holes did not do their cooling work, the material would melt. So cooling holes are needed to prevent turbine failure and guarantee safety during flight. Incidentally, turbine blades with cooling holes are not only used in aviation. The energy sector also uses the small holes that protect the blades from heat.

Sophisticated Distribution System instead of "Just drill a hole!"

It is clear that these holes fulfill an important task by protecting the turbines from overheating damage. This also leads to the conclusion that the correct positions, shapes and distribution should not simply be left to chance. Cooling holes are quite a challenge for quality assurance for the following reasons:

They have very small diameters.
Their angle of inclination often varies greatly and they are generally drilled at an angle.
They are very closely distributed on the turbine blades.
They can have irregular shapes due to different production processes.
The turbine blade itself is strongly curved.

Various shapes of Cooling Holes in Aero-engines

Not all cooling holes are the same. We can classify them according to their shapes:

Diffuser hole
Round hole
Conical hole
Long hole

different shapes of cooling holes in turbine blades — Different shapes of Cooling Holes and the applicable measurement tasks

What Inspection tasks need to be performed on the Cooling Holes?

Depending on the shape of the cooling bore, the position, diameter, funnel geometry or minimum cylinder length must be checked. This is quite a challenge for measurement technology. In fact, very few systems are suitable at all. Purely tactile measuring methods fail not only because of the small diameters or the inclination of the bore, but also because of the curvature of the turbine blade itself.

So how can the Quality of these Cooling Holes be ensured?

Bruker Alicona has a clean solution for obtaining all the parameters of the cooling holes. A single device - the µCMM - combines two measurement technologies to obtain the desired 3D data. It works like this: Vertical Focus Probing is used to measure the cylindrical part of the cooling hole. The surface, i.e. the conical part of the hole, is the task of Advanced Focus Variation. By using a lens with a super-large working distance, the user does not have the slightest accessibility restrictions.

Smart Solution for complex Cooling Holes

So all you need to perform a safe, repeatable quality check on your cooling holes is the Bruker Alicona Cooling Hole package, which consists of the following components:

µCMM (hardware)
MetMaX (software)
Cooling Hole Add-on (software feature specifically for the cooling hole application)

This package not only makes it possible to inspect cooling holes, but also automates this complex application. You can see the processes in the graphic below. First, the cooling hole is aligned so that it is perfectly centered in the live view. In the next step, the measurement takes place and finally the relevant parameters are analyzed. That sounds like a pretty simple solution for such little lifesavers. In the next article, we will show you how easy and intuitive it is to use the Cooling Hole Application in MetMaX.