Felix Holzinger

Aeroelastic Investigation of a Transonic Research Compressor

The trend in modern compressor design is towards higher stage loading and less structural damping, resulting in increased flutter risk. The understanding of the underlying aeroelastic effects, especially at highly loaded BLISK rotors, is small. This paper reports on the analysis of flutter phenomena in a modern transonic compressor.The geometry examined here is the one-and-a-half stage transonic research compressor operated by Technische Universitat Darmstadt. High blade deflections recorded during throttling measurements point to an aerodynamic excitation. Therefore, numerical investigations are carried out using the CFD-Code TRACE developed at the German Aerospace Center (DLR). Simulations are compared to measured compressor speed lines to validate the steady state results. The open source Finite Element code CalculiX is used to simulate the rotor blade eigenmodes and -frequencies. The results are then used in time-linearized calculations to determine the onset of flutter. These calculations confirm that there is an aerodynamic excitation of the first torsional eigenmode and blade flutter is at risk.A sensitivity study is carried out to further investigate the aerodynamic conditions under which structural vibrations become unstable and to identify influencing factors.Copyright

Mechanism of Nonsynchronous Blade Vibration in a Transonic Compressor Rig

This paper presents a numerical study on blade vibration for the transonic compressor rig at the Technische Universit€at Darmstadt (TUD), Darmstadt, Germany. The vibration was experimentally observed for the second eigenmode of the rotor blades at nonsynchronous frequencies and is simulated for two rotational speeds using a time-linearized approach. The numerical simulation results are in close agreement with the experiment in both cases. The vibration phenomenon shows similarities to flutter. Numerical simulations and comparison with the experimental observations showed that vibrations occur near the compressor stability limit due to interaction of the blade movement with a pressure fluctuation pattern originating from the tip clearance flow. The tip clearance flow pattern travels in the backward direction, seen from the rotating frame of reference, and causes a forward traveling structural vibration pattern with the same phase difference between blades. When decreasing the rotor tip gap size, the mechanism causing the vibration is alleviated.

Realistic Inlet Distortion Patterns Interacting with a Transonic Compressor Stage

The formation and the interaction of inlet distortions is a safety risk in the operation of an aircraft engine. The numerical simulation of an aircraft, including the engine nacelle and the turbo-machine inside, is not possible during the design process as it is too time-consuming. To gain insight into the effects, and the impact on the engine, in particular, experiments are necessary. Due to the complexity of generating and measuring distortion patterns screens are usually used. The screens generate a total pressure drop that is constant in space and time. In this paper the interaction of a transonic compressor stage with two complex, but more realistic distortion patterns is investigated. A delta wing represents a longitudinal vortex, which is representative of e.g. a ground vortex. A stalled engine inlet is modelled by a bevelled beam that generates a massive separation bubble, which is ingested into the rotor. The interaction of the distortion and the compressor is measured at different speeds and operating points. The influence of the delta wing seems small and is difficult to measure due to the small size of the distorted area. In contrast, the beam causes a global alteration of the flow. It changes the behaviour of the rotor around the whole circumference and along the whole span.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Rotating Instabilities and Flutter

This paper investigates the vibrations that occurred on the blisk rotor of a 1.5-stage transonic research compressor designed for aerodynamic performance validation and tested in various configurations at Technische Universitat Darmstadt.During the experimental test campaign self-excited blade vibrations were found near the aerodynamic stability limit of the compressor. The vibration was identified as flutter of the first torsion mode and occurred at design speed as well as in the part-speed region. Numerical investigations of the flutter event at design speed confirmed negative aerodynamic damping for the first torsion mode, but showed a strong dependency of aerodynamic damping on blade tip clearance.In order to experimentally validate the relation between blade tip clearance and aerodynamic damping, the compressor tests were repeated with enlarged blade tip clearance for which stability of the torsion mode was predicted.During this second experimental campaign, strong vibrations of a different mode limited compressor operation. An investigation of this second type of vibration found rotating instabilities to be the source of the vibration. The rotating instabilities first occur as an aerodynamic phenomenon and then develop into self-excited vibration of critical amplitude.In a third experimental campaign, the same compressor was tested with reference blade tip clearance and a non-axisymmetric casing treatment. Performance evaluation of this configuration repeatedly showed a significant gain in operating range and pressure ratio. The gain in operating range means that the casing treatment successfully suppresses the previously encountered flutter onset. The aeroelastic potential of the non-axisymmetric casing treatment is validated by means of the unsteady compressor data.By giving a description of all of above configurations and the corresponding vibratory behavior, this paper contains a comprehensive summary of the different types of blade vibration encountered with a single transonic compressor rotor. By investigating the mechanisms behind the vibrations, this paper contributes to the understanding of flow induced blade vibration. It also gives evidence to the dominant role of the tip clearance vortex in the fluid-structure-interaction of tip critical transonic compressors. The aeroelastic evaluation of the non-axisymmetric casing treatment is beneficial for the design of next generation casing treatments for vibration control.Copyright

A Laser-Optical Sensor System for Simultaneous Tip Clearance and Vibration Monitoring of Compressor Rotor Blades

In order to improve the efficiency and increase the life-time of compressors and turbines, online monitoring of operating parameters is an essential tool. One aim is to predict critical events like stall or flutter by observing blade vibrations and deformations. Due to superior material properties ceramic, carbon-fiber and glass-fiber reinforced composite blades become more and more popular. Hence, conventional measurement systems like capacitive probes are not able to deliver the necessary precision or, in the worst case, are not able to measure at all. Therefore, we developed a fiber-coupled, laser-optical sensor, named laser Doppler distance sensor (LDDS), which overcomes this drawback. The sensor is able to resolve the circumferential blade tip velocity as well as the radial expansion of each blade. Moreover, conventional blade tip timing measurements are possible as well.Our aim is to provide a universal stall prediction and monitoring sensor system that is applicable to all types of blades.Copyright

THE DEVELOPMENT OF AN AIR INJECTION SYSTEM FOR THE FORCED RESPONSE TESTING OF AXIAL COMPRESSORS

A phase-controllable air injection exciter system was developed to enable measurement of the forced response properties of a transonic axial compressor blisk. The project was performed as part of the FP7 European framework programme project FUTURE. The eventual aim of this project is to improve existing turbomachinery blade flutter prediction methods.The development and manufacturing of the exciter system was performed by the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa. The exciter system consists of 15 air injectors, each with its own servo motor and controller. The injectors consist of a small rotating disc with a specific number of holes equispaced around the periphery, rotating within a pressurised volume. When the holes are rotated, using a servo motor, past an exit tube an air pulse is generated that is injected upstream of the compressor. The controllers enable adjustment of the relative phase angle between the exciters and in this way a pattern that resembles different nodal diameters can be excited on the rotor blisk.Once the construction of the system was completed, it was transferred to Stellenbosch University, South Africa for sub-scale testing on a low speed compressor. The purpose of the sub-scale tests was to commission and verify the operation of the exciter system. The tests started with simple in-phase tests and then worked towards more complex test parameters that included frequency sweeps through the natural frequency of the compressor blades. The tests showed that it is possible to generate a blade response of different nodal diameters using the exciters. The blade response was also found to vary depending on the number of rotor holes, air supply pressure and sweep rate used for the exciters.Following completion of the sub-scale tests, the completed system was transferred to the transonic compressor test facility of the Technical University Darmstadt (TUD) where both free flutter and forced response experiments were performed on a purpose-designed blisk in the transonic compressor test rig. The experimental campaign was successfully completed with the forced response experiments showing that the air injection system could be used to measure the response characteristics of the blisk.Copyright