In service, the rotor blades of axial-flow compressors of turbine engines undergo high alternating aerodynamic forces that are exciting the blades to vibrate. Possible causes are the pressure distributions in the gas flow resulting from the engine intake geometry or the stator vane cascades or from aerodynamic instabilities like flutter and surge. The blades respond with vibrations, with the amplitudes in many cases significantly varying from blade to blade, due to manufacturing tolerances and blade-to-blade interactions. Severe vibratory loads may cause cracks and ultimately blade failure, which frequently leads to total engine failure.
While this “just” results in delays and extra costs during the development phase, engine failure in a production aircraft means a substantial risk to passengers. “Therefore, it is of paramount importance to monitor blade vibrations and potential blade crack formation during development,” said Michael Zielinski, Consultant for Special Measurement Technologies at the Engine Controls & Testing Division of MTU Aero Engines GmbH, based at Munich, Germany. “This particularly applies to initial runs when measuring results are not yet available and parameters like variable vane angles have not yet been optimized.”
The company has developed a combined noncontact blade vibration and tip clearance measurement system, called BSSM (Berührungslose Schwingungs- und Spaltmessung, or noncontact vibration and clearance measurement) to prevent damage from occurring on development engines and to facilitate reliable prediction of the blade life.
BSSM makes use of capacitive probes mounted in the compressor casing above the rotor blades. “Despite the wide signal shape of these probes, accurate time-triggering and voltage level measurement for the following vibration and tip clearance analyses are achieved by signal digitalization in conjunction with intelligent software algorithms,” explained Zielinski. Per rotor stage the system hardware includes six capacitive probes, high-frequency probe driver modules, and a personal computer with an analog digital converter board. The probe signals are directly read into the computer. Signal analysis, i.e., timing and amplituding, is carried out by software. The capacitive probes, also developed and manufactured at MTU, reliably work at up to 700°C without cooling.
“We are routinely using BSSM and its real-time capability as an instrument to monitor our compressors during rig and engine testing and are continuously developing the system that is currently in its fourth generation,” said Zielinski. Compared with strain gauges, this technique provides notable advantages: It is noncontacting, senses all blades, and reduces costs because it eliminates the need for rotor instrumentation and for telemetry. During several rig and engine tests, BSSM outputs were directly compared with strain-gauge results to validate the system precision, and the data matched within a 20% span.
“Measuring the gap between the blade tips and the compressor casing is also an important part of engine development,” Zielinski confirmed. As large clearances translate into lower efficiency caused by greater leakage flow, the clearance is designed as small as possible: Compressors of production engines are running with gap widths that are in the magnitude of just 1 mm or less to achieve good efficiency. “However, efficiency is just one important aspect. Optimizing the surge margin, which deteriorates as the gap grows, is another,” said the BSSM expert.
Since tip clearance is affected as a function of speed by centrifugal force and thermal effects, measurements are required to verify engine design. While the rotor diameters vary instantaneously with the centrifugal force, rotor disks, blades, and casings respond to temperature variations of the gas stream. “This results in a complex clearance behavior over speed and time. Casing distortions and radial rotor movements, which are a result of imbalances or acceleration forces in flight, can also affect tip clearance,” he noted. BSSM meets both purposes, vibration and clearance measurement, because the signal voltage variation generated by the blade tips when passing the probes can be correlated to blade tip clearances.
Two blade crack detection methods are also integrated in the system: One exploits the phenomenon that resonance positions shift in the presence of a crack. The other method uses the observation that cracked blades show characteristic deformation patterns at the blade tip under aero-loads and centrifugal force when compared to the undamaged blade condition.
The near-future challenge for BSSM occurs in the air, Zielinski added: “Currently we are preparing to use the BSSM system for a fully automated flutter detection application that monitors all low pressure compressor stages of a turboprop engine in the flying testbed.”