Karsten Knobloch

Exploratory Experiments on Machined Riblets for 2-D Compressor Blades

During the last decades, riblets have shown a potential for viscous drag reduction. Several investigations and measurements of skin-friction in the boundary layer over flat plates and on turbomachinery type blades with ideal riblet geometry have been reported in the literature. The purpose of the present study is to investigate whether laser machined and ground riblet-like structures could be successfully employed on conventional 2-D (NACA) compressor blades in order to assess the potential of industrial machining processes for the creation of the riblet effect. Perfectly trapezoid riblets were designed specifically for the flow parameters in the wind tunnel. Parameters describing the geometry and the deviation from ideal riblets are developed. Riblet machining by high precision material ablation has the potential of achieving micro-machining quality. In comparison to ns-laser processing using either Q-switched solid-state lasers or excimer lasers, the results for high precision material ablation show the enormous potential of ps-laser radiation and achieve the required quality, free of thermally induced defects and, consequently, with high reproducibility. For grinding riblets, geometrically defined microprofiles must firstly be generated via a profile dressing process and then ground onto the work piece surface. A precise adjustment of the grinding wheel system (grit, bonding) and the dressing/grinding conditions is necessary, in order to satisfy the opposing requirements at both dressing and grinding. The blade specimens were geometrically measured with a confocal microscope as well as secondary electron microscope using a specially developed riblet-oriented analysis. For verifying the measurement results, an Atomic Force Microscope was used. The specimens, i.e. flat plates and compressor blades, are aerodynamically tested in a cascade wind tunnel and properly scaled model surfaces were tested in an oil channel in order to quantify skin-friction reduction. Wake measurements of a cascade with NACA-profiles which have the resulting riblet-like structured surface show that the laser shaped as well as ground riblets reduce skin-friction almost as well as the ideal ones, which means a skin friction reduction of up to 7%.Copyright

Acoustic measurements of perforated liners in hot and pressurized flow

Thermo-acoustic instabilities in gas turbine combustors can prevent the implementation of modern combustion concepts, which are essential for higher efficiency and lower emissions. Perforated combustor liners, especially in combination with a bias flow through the liner, are able to suppress the instabilities by increasing the acoustic losses of the system. Some insight into the parameter dependencies of the acoustic absorption has been gained by means of atmospheric testing at ambient temperature. The next step towards realistic testing conditions is taking into account high temperature and high pressure, which increases the effort of the experimental tests and the complexity of their analysis significantly. Tests in a real combustor can serve as a quality check of a given liner design, but are not appropriate for parameter studies. So far, numerical models accurate enough to enable the design of hot stream liners are simply not available, so that the experimental investigation of the liner’s dependency on temperature and pressure is essential for the transfer of laboratory scale results to a real engine application.A new test rig has been designed to overcome these problems. The Hot Acoustic Test rig (HAT) enables the study of the influence of pressure and temperature on the damping performance in an acoustically well defined environment, although the high temperature and high pressure conditions are challenging in terms of accurate acoustic measurements.This paper introduces the Hot Acoustic Test rig with its features and limitations and shows first examples of test results. The focus lies on the hardware, instrumentation, and analysis techniques that are necessary to obtain high quality acoustic data in hot and pressurized flow environments.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.

Measurements of Density Pulsations in the Outlet Nozzle of a Combustion Chamber by Rayleigh-Scattering Searching Entropy Waves

The development of measurement techniques, which enable temporal and spatial highly resolved density investigations even in harsh environments, is essential. Rayleigh scattering is a noninvasive optical measurement technique permitting such investigations. A Rayleigh-scattering measurement system is set up, providing a new insight into fluid mechanical processes in turbomachines. In this paper, Rayleigh scattering is used for the detection of density oscillations in the optical accessible convergent-divergent outlet nozzle of a small scale combustion test rig at various power consumptions and equivalence ratios. Until now, this part of the combustion chamber is sparsely investigated due to the challenging measurement conditions. The temporal density oscillation inside the nozzle can be shown up to 4 kHz as well as its spatial distribution. Systematic errors of the setup are investigated. Spectra of pressure and density oscillations are compared. Measurements with nonreacting air flow are conducted to study flow induced density fluctuations. Entropy noise related correlations between density and pressure fluctuations are found. Therewith, the builtup Rayleigh-scattering system enables investigations of the presumed region of indirect noise generation.

Entropy Noise Generation and Reduction in a Heated Nozzle Flow

Core noise gained increasing interest concerning the total noise emission of aero-engines. Hereby, direct and indirect combustion noise plays an important role. While direct combustion noise has its origin by unsteady heat release directly in the reaction zone, indirect noise is generated by accelerated fluctuations of entropy or vorticity in the turbine nozzle guide vane (NGV) at the outlet of the combustion chamber. This paper investigates the noise generation of accelerated cold spots through a nozzle structure at the Hot Acoustic Test rig (HAT) at DLR Berlin. The nozzle setup is derived from the geometry of a full scale NGV concerning the flow gradients. The cold spots are produced by injection of air at ambient temperature into the heated mean flow. In a further step a first attempt for entropy noise reduction by bias-flow liners are examined.

Indirect combustion noise: Experimental investigation of the vortex sound generation mechanism

uctuations, originating from the unsteady combustion process, are convected with the ow and their acceleration in the outlet nozzle of the combustion chamber or in the turbine generates further noise - the indirect combustion noise. In the model experiment the controlled injection of additional air into a tube ow was used for the generation of the uctuating vorticity yielding temporally to a swirling ow. The sound generation during the acceleration of this articial vortex structure in a convergent-divergent nozzle was measured. The acoustic pressure waves were recorded downstream of the nozzle, simultaneously the velocity components upstream. The spatial and temporal resolution of the velocity eld and therewith of the vortex structure was determined with Hot-Wire Anemometry measurements. With the distance variation between the air-injection inlet and the nozzle the identication and separation of direct and vortex sound was achieved. In a parametric study, the intensity of the swirling ow was inuenced

EXPERIMENTAL ASSESSMENT OF NOISE GENERATION AND TRANSMISSION IN A HIGH-PRESSURE TRANSONIC TURBINE STAGE

The noise originating from the core of an aero-engine is usually difficult to quantify and the knowledge about its Generation and propagation is less advanced than that for other engine components. In order to overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate the processes associated with the generation of noise in the combustor, its propagation through the turbine and the interaction of these two components, which may produce additional - so-called indirect combustion-noise. In the current work, a transonic turbine stage installed at the Laboratorio di Fluidodinamica delle Macchine of the Politechnico di Milano was exposed to acoustic, entropic, and vortical disturbances. The incoming and outgoing sound fields were analyzed in detail by two large arrays of microphones. The mean flow field and the disturbances were carefully mapped by several aerodynamic and thermal probes. The results include transmission and reflection characteristics of the turbine stage, latter one was found to be much lower than usually assumed. The modal decomposition of the acoustic field in the upstream and downstream section show beside the expected rotor-stator interaction modes additional modes. At the frequency of entropy or respectively vorticity excitation, a significant increase of the overall sound power level was observed.

Noise Generation in Hot Nozzle Flow

Noise originating from the unsteady heat-release during the combustion process in the combustor of a gas turbine is well known. However, an effect known as indirect combustion noise has received considerable interest only recently. Indirect combustion noise will be generated, when entropy or vorticity fluctuations will be subject to a strong velocity gradient like in nozzle guide vane of the high pressure turbine. First experimental proof of this phenomenon could be obtained some years ago in a dedicated small-scale laboratory experiment. Recent experiments performed in the Hot Acoustic Test rig (HAT) by the German Aerospace Center (DLR) aim at further understanding of this phenomenon by investigating the sound propagation through a nozzle and sound generation when cold air spots are injected into a hot mean flow. The nozzle Mach number was varied from subsonic to sonic conditions. First results based on propagation time analysis reveal noise generation at the location of the nozzle. Parameter studies of nozzle Mach number, temperature of the cold streaks, and the way the cold air is injected (radial/axial blowing) have been performed.Copyright

Helmholtz Resonator Liner with Flexible Walls

Liners are part of every modern commercial aero-engine. Usually, they are installed in the engine intake; but also in the bypass duct or in the outlet some liners can be found. Despite the decrease of overall engine noise due to the increase of bypass-ratio (BPR), cut-off design for rotor-stator combinations, and various other means, there is an increasing demand for efficient broad-band noise absorption with the final goal of further overall noise reduction. This demand is mainly caused through the reduction of the dominating tonal components, but might be also connected to an increase in broadband noise itself. In addition, the increase in BPR requires shorter nacelles in order to reduce associated drag and weight penalties. This leads not necessarily to a smaller area for liner installation e.g. in the intake of the engine, but to a shorter length of the intake and thereby to a shorter propagation distance of emitted noise over a lined surface in axial direction. State of the art for inlet liners are singleand double-degree of freedom (SDOF and DDOF) liners consisting of cells of fixed size (for DDOF for instance with a septum dividing the individual cells) covered with a perforated face sheet, and a rigid back plate. The whole liner structure must be very robust, but at the same time of light weight, withstand various fluids and environmental conditions etc. Current liners are

Innovative liner concept for enhanced acoustic damping: hybrid Zero Massflow Liner (ZML)

Modern aero-engines require due to their architecture an enhanced damping performance of passive, acoustically absorbing wall treatment, called liner. The concept in thispaper aims on the improvement of the broadband damping compared to the narrow damping behavior of conventional Single-Degree-of-Freedom (SDOF) liner. This study investi- gates a new liner concept based on an additional acoustical excitation of the liner cavity below the liner face sheet. This so-called Zero-Mass ow-liner (ZML) concept was investigated earlier in the framework of combustion chamber liner design. Here, this concept was enhanced by combining a single cavity ZML liner with a honeycomb-structured SDOF liner yielding a hybrid ZML liner concept. Within this work, this concept was investigated in detail focusing on different parameters like the cavity excitation amplitude, the face sheet porosity and the grazing flow in uence. In conclusion, the hybrid ZML concept reveals a signiffcant potential for increased broadband dissipation behavior due to an additional damping effect based on the periodically excited flow through the liner facesheet.