Benjamin Pardowitz

Experiments on an Axial Fan Stage: Time-Resolved Analysis of Rotating Instability Modes

Rotating Instability (RI) occurs at off-design conditions in axial compressors, predominantly in rotor configurations with large tip clearances. Characteristic spectral signatures with side-by-side peaks below the blade passing frequency are typically referred to RI located in the clearance region next to the leading edge (LE). Each peak can be assigned to a dominant circumferential mode. RI is the source of the clearance noise and an indicator for critical operating conditions. Earlier studies at an annular cascade pointed out that RI modes of different circumferential orders occur stochastically distributed in time and independently from each other, which is contradictory to existing explanations of the RI. Purpose of the present study is to verify the generality with regard to axial rotor configurations.Experiments were conducted on a laboratory axial fan stage mainly using unsteady pressure measurements in a sensor ring near the rotor LE. A mode decomposition based on cross spectral matrices was used to analyze the spectral and modal RI patterns upstream of the rotor. Additionally, a time-resolved analysis based on a spatial Discrete-Fourier-Transform was applied to clarify the temporal characteristics of the RI modes and their potential interrelations. The results and a comparison with the previous findings on the annular cascade corroborate a new hypothesis about the basic RI mechanism. This hypothesis implies that instability waves of different wavelengths are generated stochastically in a shear layer resulting from a backflow in the tip clearance region.Copyright

Core noise - Identification of broadband noise sources of a turbo-shaft engine

This paper presents the investigation of turbo-shaft engine noise with a special focus on the identification of the different core noise sources and their contribution to the total noise emission. The analysis is based on comprehensive acoustic measurements of a full scale turbo-shaft engine with microphone sensors in different internal engine zones up to far field positions. A variety of different analysis methods were applied in order to evaluate the different noise generation mechanisms and to identify their contribution to the total emitted engine noise. Following the examination of the coherence function between sensors of the different engine locations certain frequency bands of different behavior could be categorized. With the evaluation of the corresponding phase relation functions varying propagation time values were derived. Therewith, the discrimination between sound propagation and convective transport effects was made possible. Further the application of several coherence-based noise-source identification techniques revealed dominant noise sources at specific frequency ranges. As a result the noise contributions associated to different engine components and distributed over the different frequency bands could be assessed.

Rotating Instability in an Annular Cascade: Detailed Analysis of the Instationary Flow Phenomena

Rotating instability (RI) occurs at off-design conditions in compressors, predominantly in configurations with large tip or hub clearance ratios of s* �> 3%. RI is the source of the blade tip vortex noise and a potential indicator for critical operating conditions like rotating stall and surge. The objective of this paper is to give more physical insight into the RI phenomenon using the analysis results of combined near-field measurements with high-speed particle image velocimetry (PIV) and unsteady pressure sensors. The investigation was pursued on an annular cascade with hub clearance. Both the unsteady flow field next to the leading edge as well as the associated rotating pressure waves were captured. A special analysis method illustrates the characteristic pressure wave amplitude distribution, denoted as “modal events” of the RI. Moreover, the slightly adapted method reveals the unsteady flow structures corresponding to the RI. Correlations between the flow profile, the dominant vortex structures, and the rotating pressure waves were found. Results provide evidence to a new hypothesis, implying that shear layer instabilities constitute the basic mechanism of the RI.

Technique to Analyze Characteristics of Turbomachinery Broadband Noise Sources

elds into their mode constituents is applied to two laboratory scale experimental setups. The broadband mode analysis technique is based on spectral coherences. In both cases, incoherent broadband sound elds were excited by means of a loudspeaker array. The results derived from the basic setup S1 were validated successfully with analytical ndings. The second setup S2 mimics the fundamental arrangement in an aero-engine exhaust, consisting of a broadband sound source (an annular combustor ring) and a turbomachinery stage (the LP turbine). In between, microphone arrays are arranged to acquire the broadband sound elds. Sample results of setup S2 are presented analyzed by means of the new technique. The experimental outcome proves the usefulness of the analysis technique to help interpreting and understanding broadband sound propagation in turbomachinery ow ducts.

Separation of broadband noise sources in aeroengine ducts with respect to modal decomposition

Two methods are presented that enable the separation of di erent broadband noise sources in turbomachinery ducts. These methods do not apply to simple measured point spectra, but take into account a complete radial{azimuthal decomposition of the sound eld into modes. In the rst method, source separation is achieved by placing a reference sensor near one of the sources and correlating the measured signal with the output of a complete microphone array. In this manner, the modal spectrum obtained from the array can be separated into the components originating from the source near which the reference sensor is placed and into the components originating from other sources. The method is designed and theoretically justi ed for a single source near the reference sensor, but measurements in a test set-up revealed that it is also valid for two sources and (to a lesser degree) for four sources. The second method for source separation is based on the investigation of coherences between di erent propagating waves in a duct. It is shown how such coherences can be used to determine the distance of two rings of incoherent sources.

Experimental identification of rotating instability of an axial fan with shrouded rotor

The rotating instability phenomenon is investigated experimentally on a laboratory low-speed axial fan. Unsteady pressure measurements are performed close to the rotor leading edge, in the far-field in the upstream-duct section, and within the rotating frame by using Kulite sensors near the blade tip. As reference, the occurrence of rotating instability is proven for a rotor configuration with large clearance size. Existing explanations assume that rotating instability is induced by the tip clearance vortex. Following these models, the characteristic spectral and modal signatures of rotating instability are generated by the circumferentially coupled unsteady vortex systems within the individual blade passages. However, some early findings on a shrouded rotor setup and a study on an annular cascade without clearance have raised questions about the validity of the existing theories. To answer these questions, a second configuration is investigated using the identical rotor in a shrouded version. Here, a shrouding band covers the blade tips over the whole circumference. By using a comprehensive sensor arrangement in a wide variation of operating conditions, the existence of spectral and modal rotating instability signatures is proven on the shrouded-rotor configuration, as well. In this case, the tip clearance vortex system and, respectively, the leakage flow are suppressed. The present study underlines that novel explanations are required to describe the physical mechanism that generates rotating instability.

On the Excitation of a Zero Mass Flow Liner for Acoustic Damping

The Zero Mass Flow Liner is able to convert a perforated liner with minimal damping qualities into a very effective sound absorber. The damping improvement results from an additional acoustic excitation within the Zero Mass Flow Liner. This paper studies the damping performance of the Zero Mass Flow Liner while changing the parameters of this excitation. A strong dependency on the sound pressure level and frequency is revealed from the experiments. Though, a mode structure with a distinct pressure pattern seems to be of minor importance. However, it is of great benefit to operate the actuator at its natural resonances. This leads to a very good damping performance from a minimum of input power.

The Application of an Aeroacoustic Actuator in a Zero Mass Flow Liner for Acoustic Damping

The Zero Mass Flow Liner (ZMFL) utilizes high amplitude sound to produce an oscillating bias ow through the ori ces of a perforated liner to improve its damping performance. While a loudspeaker would be the obvious source of sound, it is not suitable for harsh environments, e.g. gas turbine combustors. An aeroacoustic actuator, as simple as a jet-edge con guration, is suggested to enable the industrial application of the ZMFL concept. A demonstrator has been designed and the damping performance of the new setup has been evaluated experimentally. The ZMFL allows a reduction of 60 % of the driving mass ow rate compared to a conventional steady bias ow liner.

Advanced Stereo High-Speed PIV in an Annular Cascade without Clearance: Evidences of Rotating Instability

Rotating Instability (RI) induces noise, triggers blade vibrations and is a potential indicator for critical operating conditions in axial compressors. Despite numerous studies, the source of RI is not completely understood. The objective of the present study is to give further insight into the basic mechanism of RI by means of advanced Stereo High-Speed Particle Image Velocimetry (PIV) applied to an annular compressor cascade without clearance. In particular, results of the PIV measurements visualize the predominant flow mechanism corresponding to RI. The experiments were conducted at an inflow Mach number of Ma = 0.4. Additional reference sensors captured the time-resolved pressure fluctuations synchronously to the optical measurements. By using correlation techniques between the PIV flow field and the reference sensor data, discrete vortex structures corresponding to the RI modes could be identified and localized. As a verification of the PIV results, the steady PIV flow velocity vectors are compared to results from an oil flow visualization technique. Overall, the present investigations point out that the general flow mechanism of RI is similar in compressor cascades with and without tip clearance.

Experiments on an Axial Fan Stage: Time Resolved Analysis of Rotating Instability Modes

Rotating Instability (RI) occurs at off-design conditions in axial compressors, predominantly in rotor configurations with large tip clearances. Characteristic spectral signatures with side-by-side peaks below the blade passing frequency are typically referred to RI located in the clearance region next to the leading edge (LE). Each peak can be assigned to a dominant circumferential mode. RI is the source of the clearance noise and an indicator for critical operating conditions. Earlier studies at an annular cascade pointed out that RI modes of different circumferential orders occur stochastically distributed in time and independently from each other, which is contradictory to existing explanations of the RI. Purpose of the present study is to verify the generality with regard to axial rotor configurations.Experiments were conducted on a laboratory axial fan stage mainly using unsteady pressure measurements in a sensor ring near the rotor LE. A mode decomposition based on cross spectral matrices was used to analyze the spectral and modal RI patterns upstream of the rotor. Additionally, a time-resolved analysis based on a spatial Discrete-Fourier-Transform was applied to clarify the temporal characteristics of the RI modes and their potential interrelations. The results and a comparison with the previous findings on the annular cascade corroborate a new hypothesis about the basic RI mechanism. This hypothesis implies that instability waves of different wavelengths are generated stochastically in a shear layer resulting from a backflow in the tip clearance region.Copyright