Temperature Monitoring of Nozzle Guide Vanes (Ngvs) Using Ultrasonic Guided Waves

This poster considers the challenges involved in implementing an ultrasonic guided wave temperature measurement system on a Nozzle Guide Vane (NGV).

Real time condition monitoring of jet engines can lead to considerable improvements in efficiency, reducing pollution and fuel costs. In the case of Nozzle Guide Vanes (NGVs) the current methods of monitoring temperature are offline measurements utilising temperature sensitive paints or thermal barrier coatings, among others. These methods provide excellent temperature accuracy mapped across the surface of the blade but require frequent reapplication, can impact the aerodynamic properties of the blade, and cannot operate for extended periods of time.

Due to the extremely high temperatures (up to 1500°C) typical online temperature sensors are difficult to implement. Thermocouples can accurately monitor over a large temperature range but only provide a measurement at the point of contact with the blade. They would also be difficult to install without interfering with the operation of the part.

Acoustic methods are an alternative to traditional temperature measurement systems. Sensors such as Surface Acoustic Wave (SAW) devices are already used for condition monitoring applications but are not suitable for installation on an NGV as they suffer from the same limitations as thermocouples. Ultrasonic guided wave methods are more applicable for use on an NGV as the measurements can be carried out remotely with the sensors mounted away from the extreme conditions of the engine. This method may allow for temperature to be mapped across the surface of the NGV, utilising the reflections from the numerous holes of the cooling system. Other benefits include the potential for the transducers to be installed without affecting the operation of the engine. Their small footprint would not affect the vibrational characteristics of the vane, nor would they effect the pressurised cooling system that is present across the leading edge of the vane. The system could work over a large temperature range at an accuracy comparable to traditional temperature sensors.

Ultrasonic guided waves in the form of Lamb waves will propagate across the vane and can be used for temperature measurement if a suitable combination of transducers and signal processing techniques can be identified. Temperature changes will vary the velocity of guided waves which can be measured and compared to baseline results.

In this poster we investigate the feasibility of the method by reporting on initial experiments that investigate how Lamb waves can be selectively excited using wedge transducers in an aluminium plate. Waves are transmitted between two transducers and a measurement of wave velocity is carried out. This will inform the design of later systems, which will form the basis of the ongoing work into temperature measurement in this difficult environment.