Modal Analysis of Turbine Blades by Means of Distributed Optical Fiber Sensors

In this paper, a new method for the modal analysis of turbine blades is proposed and analysed. In detail, this paper studies the possibility of using optical fiber sensors placed on the blade to reconstruct the curvature and displacement modal shapes. The advantage of using this kind of sensors is due to the large amount of information available on a single fiber wire (the measurement can be reasonably approximated to a continuous strain measurement along the fiber path) and to the possibility of embedding this type of sensor into the blade structure.

The main disadvantage is related to the fact that this technology is relatively new and the accuracy and robustness of the measurement, especially when considering high-vibration environments, is not completely studied and guaranteed. Moreover, even if the number of measurement points is very high (in the order of 1 each few millimetres of fiber), the quality of the single sensor is relatively low. Anyway, this technique has been used with success in many field of vibration engineering, from structure monitoring and identification to vibration control and thus the opportunity of using it for turbine blades should be analysed in detail.

This could be, for example, a very interesting approach for the last stages of steam turbines, where the requirement of large size, together with the obvious need of structural resistance open the door to the possible use of carbon-fiber blades. This solution, even if not yet applied in commercial turbines, has been already investigated from a research point of view. In this case, the proposed method wold be useful both for a preliminary modal identification and for a continuous blade monitoring during the normal operations of the blade. Indeed, optical fiber sensors can be easily embedded into a carbon fiber structure during the lamination process and the signal coming from the fiber can be brought to the ground (i.e. outside from the rotating part of the machine) by means of optical slip rings.

In this paper, a measurement setup for modal identification on a turbine blade is presented. We discuss in detail how to derive the displacement modal shapes of the blade starting from fiber measurements, showing then an experimental application. The case study is a 3D printed full-scale model of a steam turbine blade, having the same shape and mass distribution (and so the same modal shapes), but with a lower global mass, corresponding to lower frequencies. This is due to a current limitation of the optical interrogator of such fibers, that we expect to be overcomed in the next years. The results obtained from the fiber measurements are compared with those coming from a 3D laser scanner, showing the promising effects of this approach and also discussing drawbacks and limitations.