Fabian Wartzek

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

Experimental and Numerical Investigation of Tip Leakage Flow in a 1 1/2 Stage Turbine Rig Comparing Flat and Cavity-Squealer Tip Geometries

Tip leakage loss is one of the main loss mechanisms in shroudless high-pressure turbines. Efficiency reducing phenomena are the reduction of mass turned by the blade, and the mixing of the tip leakage and passage flow. Applying a cavity squealer tip is a promising technique to reduce the tip leakage loss. The study investigates the influence of a cavity squealer on the flow field of a 3D high-pressure turbine.The 1 1/2 stage turbine test rig is equipped with steady and unsteady measurement techniques, like five-hole probe, 2-component hot wire, oil & dye visualization, unsteady wall pressure probes and particle-image-velocimetry measurements. To improve the flow phenomena interpretation, unsteady numerical simulations were carried out using a commercial solver.Copyright

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.

Blade Triggered Excitation of Periodically Unsteady Impinging Jets for Efficient Turbine Liner Segment Cooling

In the present study, an application for efficient cooling of turbine liner segments employing pulsating impinging jets was investigated. A combined numerical and experimental study was conducted to evaluate the design of a case cavity device which utilizes the periodically unsteady pressure distribution caused by the rotor blades to excite a pulsating impinging jet. Through an opening between the main annulus and a case cavity, pressure pulses from the rotor blades propagated into this cavity and caused a strong pressure oscillation inside. The unsteady computational fluid dynamics (CFD) results were in good qualitative agreement with the measurement data obtained using high-frequency pressure transducers and hot wire anemometry. Furthermore, the numerical study revealed the formation of distinct toroidal vortex structures at the nozzle outlet as a result of the jet pulsation. Within the scope of the measurements, the influence of the operating point on the pressure propagation inside the cavity was investigated. The dependence of shape and amplitude of the pressure oscillation on engine speed and stage pressure ratio was found to be in accordance with an analytical consideration.

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