Application
Investigations about the aerodynamic noise source mechanisms are taking place at the FloCon-1 and FloCon-2 testrigs. In addition, active and passive flow control methods can be applied to demonstrate their ability of the noise reduction in turbomachinery components. Experiments are conducted at axial fan stages with maximum flow rates of 2.2 kg/s and blade tip Mach numbers of up to 0.23 at design conditions. Regarding to the noise source mechanisms, the available laboratory fan stages correspond to low pressure compressor components at subsonic operating conditions of aeroengines and stationary gas turbines respectively. The test facility enables extensive parametric studies due to its flexible options with a wide range of hardware configurations and with low costs during operation. The aeroacoustic sources can be investigated in detail by using various types of measurement equipment. The objective is to evaluate generally the noise generation at turbomachinery components and to identify the dominant source contribution. As a result, an insight into the physics of the noise generation is given. All the experimental data is used for a validation of numerical simulations with regard to noise prediction and in-duct propagation. In addition, passive, and adaptive, and active methods are used to both increase the aerodynamic performance, as well as to reduce the noise generation of the investigated components.
Measurement Techniques
The induced noise and the propagating sound field within the testrigs are typically investigated and characterized by using microphone array measurements in the duct. An acoustic mode analysis enables a decomposition of the sound field into acoustic waves of different aximuthal and radial mode orders. The resulting mode amplitudes can be utilized both for an interpretation of the noise source mechanisms as well as the determination of the propagated sound power. By using multiple microphone arrays with an enhanced analysis method based on correlations, a transmission and a reflection of the acoustic modes at the individual turbomachinery components can be determined. For both the testrigs FloCon-1 and FloCon-2, an anechoic determination is available as defined in the Norm ISO 5136. The unsteady aerodynamics inducing noise can be measured by using hot wire-anemometrie. Thereby, the turbulence intensity distribution can be derived from the experimental data. Such characteristics both upstream and downstream of a blade row is of importance in determining the individual noise source strength. Further, the hotwire data yield an insight into the secondary flow phenomena located in the blade tip region. Kulite sensors are integrated in the blade surfaces to measure the unsteady pressure characteristics within the rotating frame. A combined interpretation of the aerodynamic and acoustic data enables a quantification of various noise source mechanisms and evaluates their importance within the overall emitted sound power level of the investigated turbomachinery component. Noise source mechanisms are for example, the interaction between the rotor wakes and its corresponding turbulence characteristics with the downstream stator blades or the interaction between the turbulent boundary layer with the trailing edge of the rotor blades
Testrig- FloCon-1
The test rig FloCon-1 consists of an axial low speed fan. Due to its modular design, a wide range of hardware configurations can be tested for example with various rotor-stator configurations in combination with different tip clearance sizes. The main parameters for the reference configuration with 24 blades are given in table 1. Here, the rotor blade angle can be varied. Upstream of the rotor, the hub terminates with a hemispherical spinner. The design speed is 3000 rpm. The clearance size can be varied by using different outer casings resulting in 0.7%, 1.4%, 2.8% or 5,6% clearance ratio of the blade chord length. Three stator configurations with 16, 17, or 32 non profiled vanes are existent. An additional stator configuration is available with 30 NACA 9412 profiled vanes, where the stagger angle of the stator vanes can be adjusted in a range between -10° and +10°.
Most of the investigations at the testrig FloCon-1 focus on the secondary flow phenomena and their general impact on the operational performance and the acoustics. Further, this set-up is used to test new active and passive noise reduction techniques, which target to influence directly the corresponding aerodynamic sources. An outer casing with 24 equidistantly placed nozzles is available to influence the secondary flow field in the clearance region by using steady or unsteady pressurized air injection. Variable nozzle geometries are existent to adjust both the outflow angle as well as the axial position of the air injection. In addition, an air injection over the whole circumference is realizeable with a circumferential groove. Further this circumferential groove can be used for a boundary layer suction at various axial positions at the rotor blades. A special rotor blade design enables the air blowing out of each blade tip in the rotating frame with an angle of 45°. Another specialzed rotor with 18 blades is available, where pressurized air can be blown out of each trailing edge to fill up the wakes behind the rotor blades (details given in Experimental methods to reduce broadband noise in turbomachinery).
Testrig- FloCon2
The testrig FloCon-2 consists of an axial fan with the following specifications: Rotor diameter 358 mm, hub-to-tip ratio 0.62, NACA profiled blades, chord length at the blade tip 52 mm, maximum blade thickness 3 mm, design speed 4000 rpm, two rotor configurations with 16 or 18 blades, 16 non profiled vanes in the stator, the stagger angle of the rotor blades can be adjusted, the clearance size related to the chord length is 0.58%.
Most investigations at this set-up are aimed to reduce the rotor-stator interaction noise by using active flow control. In this case, the active flow control concept uses a stationary air injection through nozzles which are equidistanly placed between the rotor and the stator. The number of nozzles over the whole circumference is in accordance with the number of rotor blades. The amplitude of the induced secondary pressure field can be adjusted by varying the injected massflow. The phase of the secondary pressure field can be adjusted by varying the circumferential positions of the nozzles in relation to the stator vanes. Based on this approach, the testrig FloCon-2 consists of various traversable segments each of holding a ring of nozzles. Thereby, each ring of nozzles can be circumferentially placed seperately in relation to the stator vanes. Thus, the secondary pressure field excited by active flow control superimposes with the primary pressure field and ideally results in a cancelling of the rotor-stator interaction noise.