Investigation of site-specific wind field parameters and their effect on loads of offshore wind turbines

The main contributing factors to unsteady loading of Offshore Wind Turbines (OWT) are wind shear, turbulence, and waves. In the present paper, the turbulence intensity and the wind shear exponent are investigated. Using data from the FINO 1 research platform, these parameters are analyzed and compared with the proposed wind field parameters in the IEC standard 61400-3. Based on this analysis, aeroelastic simulations are performed to determine the effect of wind field parameters on the fatigue and the extreme loads on the rotor blades. For the investigations, the aeroelastic model of a 5 MW OWT is used with a focus on design load cases in an operating state (power production). The fatigue loads are examinedby means of the damage-equivalent load-range approach. In order to determine the extreme loads with a recurrence period of 50 years, a peak over threshold extrapolation method and a novel method based on average conditional exceedance ratesare used. The results show that the requirements of the IEC standard are very conservative for the design of the rotor blades. Therefore, there could be a large optimization potential for the reduction of weight and cost of the rotor blades.Ministry for Science and Culture in Lower SaxonyFederal Maritime and Hydrographic Agency (BSH)Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU

Impact of turbocharger non-adiabatic operation on engine volumetric efficiency and turbo lag

Turbocharger performance significantly affects the thermodynamic properties of the working fluid at engine boundaries and hence engine performance. Heat transfer takes place under all circumstances during turbocharger operation. This heat transfer affects the power produced by the turbine, the power consumed by the compressor, and the engine volumetric efficiency. Therefore, non-adiabatic turbocharger performance can restrict the engine charging process and hence engine performance. The present research work investigates the effect of turbocharger non-adiabatic performance on the engine charging process and turbo lag. Two passenger car turbochargers are experimentally and theoretically investigated. The effect of turbine casing insulation is also explored. The present investigation shows that thermal energy is transferred to the compressor under all circumstances. At high rotational speeds, thermal energy is first transferred to the compressor and latter from the compressor to the ambient. Therefore, the compressor appears to be "adiabatic" at high rotational speeds despite the complex heat transfer processes inside the compressor. A tangible effect of turbocharger non-adiabatic performance on the charging process is identified at turbocharger part load operation. The turbine power is the most affected operating parameter, followed by the engine volumetric efficiency. Insulating the turbine is recommended for reducing the turbine size and the turbo lag

Early assessment of defects and damage in jet engines

The jet engine maintenance process is complex, expensive and time-consuming. It often requires engine disassembly or boroscopic examinations. In order to accelerate the process and reduce the down time of an engine we intend to develop a method to locate and characterize defects and damage at an early state, without having to disassemble the engine. The assumption is that various defects in the hot gas path of an engine have a noticeable impact on the spatial density distribution of the exhaust jet of an engine. The resulting differences in the exhaust jet pattern will be measured with the Background Oriented Schlieren method (BOS). We perform numerical simulations (CFD) in order to analyze the effects of various general defect types on the density pattern of the exhaust jet. The defects under investigation include the malfunction of one burner, the increase the turbine blade tip clearance and burned trailing edges of the blades. The changes in the pattern resulting from the defects are compared to the density distribution of the undamaged initial state. It is shown that different exhaust jet patterns can be linked to the investigated hot gas path defects. Furthermore, a BOS set-up is installed in a test cell of a helicopter engine with a twostage axial turbine to demonstrate the applicability of the BOS method for the measurement of small density gradients resulting from temperature non-uniformities. A cold streak was inserted into the exhaust diffuser to simulate an artificial defect. The completed measurements show that the BOS method is able to detect these small variations. The present paper summarizes the results of different investigations. It presents a combination of BOS measurements of the exhaust jet and CFD simulations of defects within the hot gas path as a promising approach for evaluating the condition of a jet engine.DFG/CRC/87

Experimental validation of a compact mixed-flow compressor for an active high-lift system

Compact, electrically-driven compressors are a core component of a novel active high-lift system for future commercial aircraft. A newly-developed aeromechanical optimization process was used to design the compressor stage. The optimization resulted in an unusual mixed-flow compressor design with very low aspect ratio blades and a high rotational speed of up to 60,000 rpm. Due to the unusual design, experimental validation of the performance predictions by means of CFD is necessary. This paper presents the first experimental results obtained using a preliminary prototype at part-speed, i.e. rotational speeds from 20,000 to 30,000 rpm. The experimentally-determined pressure ratios deviate up to 1.5 %, the polytropic efficiencies up to 4 percentage points from the CFD predictions. Besides the deficiencies of available turbulence models, the underestimation of overall losses is presumably due to the omission of the volute in the CFD model. An experimental validation of the CFD predictions at full-speed is under way

Effect of Geometric Uncertainties on the Aerodynamic Characteristic of Offshore Wind Turbine Blades

Offshore wind turbines operate in a complex unsteady flow environment which causes unsteady aerodynamic loads. The unsteady flow environment is characterized by a high degree of uncertainty. In addition, geometry variations and material imperfections also cause uncertainties in the design process. Probabilistic design methods consider these uncertainties in order to reach acceptable reliability and safety levels for offshore wind turbines. Variations of the rotor blade geometry influence the aerodynamic loads which also affect the reliability of other wind turbine components. Therefore, the present paper is dealing with geometric uncertainties of the rotor blades. These can arise from manufacturing tolerances and operational wear of the blades. First, the effect of geometry variations of wind turbine airfoils on the lift and drag coefficients are investigated using a Latin hypercube sampling. Then, the resulting effects on the performance and the blade loads of an offshore wind turbine are analyzed. The variations of the airfoil geometry lead to a significant scatter of the lift and drag coefficients which also affects the damage-equivalent flapwise bending moments. In contrast to that, the effects on the power and the annual energy production are almost negligible with regard to the assumptions made.Ministry for Science and Culture in Lower Saxony/ForWin

Aerodynamic behavior of an airfoil with morphing trailing edge for wind turbine applications

The length of wind turbine rotor blades has been increased during the last decades. Higher stresses arise especially at the blade root because of the longer lever arm. One way to reduce unsteady blade-root stresses caused by turbulence, gusts, or wind shear is to actively control the lift in the blade tip region. One promising method involves airfoils with morphing trailing edges to control the lift and consequently the loads acting on the blade. In the present study, the steady and unsteady behavior of an airfoil with a morphing trailing edge is investigated. Two-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations are performed for a typical thin wind turbine airfoil with a morphing trailing edge. Steady-state simulations are used to design optimal geometry, size, and deflection angles of the morphing trailing edge. The resulting steady aerodynamic coefficients are then analyzed at different angles of attack in order to determine the effectiveness of the morphing trailing edge. In order to investigate the unsteady aerodynamic behavior of the optimal morphing trailing edge, time-resolved RANS-simulations are performed using a deformable grid. In order to analyze the phase shift between the variable trailing edge deflection and the dynamic lift coefficient, the trailing edge is deflected at four different reduced frequencies for each different angle of attack. As expected, a phase shift between the deflection and the lift occurs. While deflecting the trailing edge at angles of attack near stall, additionally an overshoot above and beyond the steady lift coefficient is observed and evaluated.BMWi/Smart BladesGerman Aerospace Center (DLR)LUI

Recent progress in turbine blade and compressor blisk regeneration

The regeneration process of jet engines is a highly complex, expensive and time-consuming. Especially the regeneration of high pressure turbine blades and compressor blisks are at the border of what is technically feasible. These components are highly loaded and thus substantial wear occurs. The blades and blisks must be overhauled or replaced regularly. The existing repair methods for these parts are inflexible and cannot be applied in many cases, resulting in a large number of scrapped parts. Therefore a new turbine blade regeneration process is presented. The goal of the improved process is to reduce the scrap rate and cost. This process includes an early evaluation of the condition of the hot-gas path components before disassembly, new detection methods for defects on the turbine blades surfaces, and more flexible manufacturing processes. The process is supported by production process simulations and functional simulations to predict the optimal regeneration path depending on the blade condition and the business model of the customer. The paper also presents a new approach for compressor blisk regeneration. This process will be developed and validated in the next years. New challenges in structural mechanics, aerodynamics, and manufacturing must be addressed due to the complexity of blisks. As part of the ongoing research, three new blisks will be designed and subjected to the complete regeneration path, which is also supported by simulations. In order to validate the simulations, their results will be compared to experimental results of the regenerated components on a compressor test rig.DFG/SFB/87

Full-scale deformation measurements of a wind turbine rotor in comparison with aeroelastic simulations

The measurement of deformation and vibration of wind turbine rotor blades in field tests is a substantial part of the validation of aeroelastic codes. This becomes highly important for modern rotors as the rotor size increases, which comes along with structural changes, resulting in very high flexibility and coupling between different vibration modes. However, performing full-scale field measurements for rotor blade deformation is not trivial and requires high temporal and spatial resolution. A promising deformation measurement technique is based on an optical method called digital image correlation (DIC). Recently, DIC measurements on a Siemens Gamesa SWT-4.0-130 test turbine were performed on the tip of all blades in combination with marker tracking at the hub for the first time with synchronised measurement of the inflow conditions by a ground-based lidar. As the turbine was additionally equipped with strain gauges in the blade root of all blades, the DIC results can be directly compared to the actual prevailing loads to validate the measurement method. In the end, an example for a comparison of the measured deformations and torsion with aeroelastic simulations is shown in the time and frequency domain. All in all, DIC shows very good agreement with comparative measurements and simulations, which shows that it is a suitable method for measurement of deformation and torsion of multi-megawatt wind turbine rotor blades. © 2020 Author(s)

Design and test of a 10kW ORC supersonic turbine generator

Manufactures are searching for possibilities to increase the efficiency of combustion engines by using the remaining energy of the exhaust gas. One possibility to recover some of this thermal energy is an organic Rankine cycle (ORC). For such an ORC running with ethanol, the aerothermodynamic design and test of a supersonic axial, single stage impulse turbine generator unit is described. The blade design as well as the regulation by variable partial admission is shown. Additionally the mechanical design of the directly coupled turbine generator unit including the aerodynamic sealing and the test facility is presented. Finally the results of CFD-based computations are compared to the experimental measurements. The comparison shows a remarkably good agreement between the numerical computations and the test data.Forschungsvereinigung Verbrennungskraftmaschinen e.V. (FVV)Forschungsvereinigung Antriebstechnik e.V. (FVA)Antriebstechnik und Entwicklungs GmbHSchaeffler, Herzogenaurauch/GermanyBarden Corp, Plymouth/UKSieb & Meyer, Lueneburg/German

Non-contact test set-up for aeroelasticity in a rotating turbomachine combining a novel acoustic excitation system with tip-timing

Due to trends in aero-design, aeroelasticity becomes increasingly important in modern turbomachines. Design requirements of turbomachines lead to the development of high aspect ratio blades and blade integral disc designs (blisks), which are especially prone to complex modes of vibration. Therefore, experimental investigations yielding high quality data are required for improving the understanding of aeroelastic effects in turbomachines. One possibility to achieve high quality data is to excite and measure blade vibrations in turbomachines. The major requirement for blade excitation and blade vibration measurements is to minimize interference with the aeroelastic effects to be investigated. Thus in this paper, a non-contact-and thus low interference-experimental set-up for exciting and measuring blade vibrations is proposed and shown to work. A novel acoustic system excites rotor blade vibrations, which are measured with an optical tip-timing system. By performing measurements in an axial compressor, the potential of the acoustic excitation method for investigating aeroelastic effects is explored. The basic principle of this method is described and proven through the analysis of blade responses at different acoustic excitation frequencies and at different rotational speeds. To verify the accuracy of the tip-timing system, amplitudes measured by tip-timing are compared with strain gage measurements. They are found to agree well. Two approaches to vary the nodal diameter (ND) of the excited vibration mode by controlling the acoustic excitation are presented. By combining the different excitable acoustic modes with a phase-lag control, each ND of the investigated 30 blade rotor can be excited individually. This feature of the present acoustic excitation system is of great benefit to aeroelastic investigations and represents one of the main advantages over other excitation methods proposed in the past. In future studies, the acoustic excitation method will be used to investigate aeroelastic effects in high-speed turbomachines in detail. The results of these investigations are to be used to improve the aeroelastic design of modern turbomachines.Siemens EnergyDFG/SFB/87