A test facility for aereolastic investigations has been installed at the chair of Aero Engines at the Technische Universität Berlin. The test rig provides data for tool and code validation and is used for basic aeroelastic experiments. It is a low speed wind tunnel which allows free and controlled flutter testing. The test section contains a linear cascade with eleven compressor blades. Nine of them are elastically suspended. The paper presents a detailed description of the test facility, results to evaluate the overall flow quality and an aeroelastic model to predict the flutter velocity and critical interblade phase angles. Hot-wire anemometry has been applied to examine the inlet flow for several Mach- and Reynolds numbers. The results show small turbulence intensities. The blade surface pressure distribution and the flow field of the blade’s suction and pressure side has been accessed by oil flow visualization.

Axial compressors in aero engines are prone to suffering a breakdown of orderly flow when operating at the peak of the pressure rise characteristic. The damaging potential of separated flows is why a safe distance has to be left between every possible operating point and an operating point at which stall occurs. During earlier investigations of stall inception mechanisms, a new type of prestall instability has been found. In this study, it could be demonstrated that the prestall instability characterised by discrete flow disturbances can be clearly assigned to the subject of "Rotating Instabilities". Propagating disturbances are responsible for the rise in blade passing irregularity. If the mass flow is reduced successively, the level of irregularity increases until the prestall condition devolves into rotating stall. The primary objective of the current work is to highlight the basic physics behind these prestall disturbances by complementary experimental and numerical investigations. Before reaching the peak of the pressure rise characteristic flow, disturbances appear as small vortex tubes with one end attached to the casing and the other attached to the suction surface of the rotor blade. These vortex structures arise when the entire tip region is affected by blockage and at the same time the critical rotor incidence is not exceeded in this flow regime. Furthermore, a new stall indicator was developed by applying statistical methods to the unsteady pressure signal measured over the rotor blade tips, thus granting a better control of the safety margin.

Especially in the turbulence research, it is necessary to record three velocity components concurrently. As a result, all six components of the Reynolds stress tensor and higher-order velocity correlations can be determined. This article assesses the three-dimensional flow field of a linear transonic compressor cascade. The velocity field within the blade passage is measured using a 3D-LDA system. This requires a coincident measurement of the velocity. However, this leads to a significant reduction in the data rate. To circumvent this problem there is the general approach of a record interval window to the subsequent generation of coincident data. In comparison to previous publications, a transonic flow is considered here. For a coincident measurement it is essential that all velocities are perpendicular to each other. Because of limited arrangement possibility of the probes the system measures three non orthogonal velocity components. Therefore, a coordinate transformation is necessary. Due to uncertainties in the exact laser alignments, the transformation matrix for this arrangement is determined experimentally. This paper deals with these two problems, for a case similar to gas turbine conditions. The influence of the time interval on the measured data is compared with coincident data. Within the cascade different measuring planes are considered and a time step study is carried out.

High-fogging is widely used to rapidly increase the power outputs of stationary gas turbines. Therefore, water droplets are injected into the inflow air, and a considerable number enter the compressor. Within this paper, the primary process of droplet evaporation is investigated closely. A short discussion about the influential parameters ascribes a major significance to the slip velocity between ambient gas flow and droplets. Hence, experimental results from a transonic compressor cascade are shown to evaluate the conditions in real high-fogging applications. The measured parameter range is used for direct numerical simulations to extract evaporation rates depending on inflow conditions and relative humidity of the air flow. Finally, an applicable correlation for the Sherwood number in the form of S h ( R e 1 / 2 S c 1 / 3 ) is suggested.

A test facility for aeroeolastic investigations has been set up at the chair of Aero Engines at the Technische Universität Berlin. The test rig provides data for tool and code validation, and is used for basic aeroelastic experiments. It is a low-speed wind tunnel, which allows free and controlled flutter testing. The test section contains a linear cascade with eleven compressor blades. Nine of them are elastically suspended. The paper presents a detailed description of the test facility results to evaluate the overall flow quality alongside an aeroelastic model to predict the flutter velocity and critical interblade phase angles (IBPAs). Furthermore, chordwise pressure distributions, measured with traveling wave (TW) mode experimental tests, are presented. These measurements have been carried out for a wide range of IBPAs and have been compared to numerical results. Hot-wire anemometry has been applied to examine the inlet flow for several Mach numbers and Reynolds numbers. The results show small turbulence intensities. The blade surface pressure distribution and the flow field of the blade's suction and pressure sides have been obtained by oil flow visualization.

A three-dimensional flow in a high speed (Main = 0.7) linear compressor cascade’s end wall region is presented in this paper. The attached blade geometry is of medium curvature with a deflection angle Δβ = 15°. To intensify the secondary flow structure the profiles were stalled deliberately and oil flow visualization was used to prove the presence of that hub-corner stall. Together with streamlines derived from velocities, measured by a three-channel laser Doppler anemometer (LDA), that allowed the identification of the secondary flow region’s key features and their extents into the flow field. A therefrom-derived topology is then compared to existing concepts. Moreover, an associated URANS simulation is set up using the k–ε model to verify its capabilities to reproduce the measured flow field. To confirm the previous observations the stall indicator is computed from the simulation results. It is further shown that the hub-corner stall dissolves when removing the side wall boundary layer. In summary, this paper combines elaborate laser Doppler measurements at both high Reynolds and Mach numbers combined with verified simulations allowing a detailed description of the end wall behavior within the cascade and further introduces a new flow topology concept.

Evaporation cooling increases gas turbine power output. Experimental results suggest an 8% increase of power when 1% of the overall mass flow is added via water droplets injected upstream of the compressor. However, water injection has an impact on the flow field, which requires experimental research involving probe measurements in the droplet-laden flow as well as reliable monitoring during operation, as the volumetric flow rate throughout the stages changes notably and deviates from (dry) design parameters. Measuring with a conventional pressure probe in two-phase flows is challenging because the droplet-laden flow can clog the pressure taps, thus effectively separating the sensor from the measurement location. This paper presents a consistent approach to measure stagnation pressure in a droplet-laden flow field. The probe was purged constantly to prevent droplets from clogging the tubing. The recorded pressure is then corrected using a transfer function to account for the purging pressure offset. A detailed description of how to obtain this function is given within the paper. With this setup, the flow field downstream of a blade cascade was measured at several water mass fractions and spray characteristics. The pressure measurements are compared with the usual LDA/PDA measurements in the wake of the cascade. Based on the test results, an evaluation of the change of total-head loss due to water injection and evaporation compared to dry operation can be performed.

The rapid regulation of gas turbine power plants is becoming increasingly important. In the field of stationary gas turbine power augmentation droplet-laden flows receive special attention as they cool the compressor’s inlet temperature. High fogging is an effective and economic method that secures power supply by reacting rapidly to fluctuating power generation. However, water droplets effect the aerodynamic performance of the compressor negatively. Therefore, the Laboratory of Turbomachinery investigated the interaction between droplets and blades at midspan by using diverse experimental methods to comprehend aerodynamic performance and water film induced losses. Additionally, the knowledge of the separation’s extent for three-dimensional flows with corner separation is of great importance as it leads to a blockage of the flow cross-section and therefore contributes to the losses. This paper enlarges this knowledge base by studying the interactions of the dispersed phase with the gas flow in three-dimensional flow structures as they are found in the near wall regions of every turbo compressor. By utilizing a three-dimensional Phase-Doppler Anemometer spatially resolved information on velocity mean vector, its higher moments as well as droplet diameters were extracted from the separated region for a compressor blade, in middle cross section design, with spray-injection. Streamlines that derive from the mean velocities visualize key features such as saddle and focal points. Then an integral approach is presented to evaluate the effect of the dispersed phase on the flow structure compared to dry flow and the impact back on different diameter classes. Finally, the velocity in streamwise direction shows the influence of the disperse phase on the aerodynamic blockage.