Bernd Stoffel

Experimental and Numerical Studies in a Centrifugal Pump With Two-Dimensional Curved Blades in Cavitating Condition

In the presented study a special test pump with two-dimensional curvature blade geometry was investigated in cavitating and noncavitating conditions using different experimental techniques and a three-dimensional numerical model implemented to study cavitating flows. Experimental and numerical results concerning pump characteristics and performance breakdown were compared at different flow conditions. Appearing types of cavitation and the spatial distribution of vapor structures within the impeller were also analyzed. These results show the ability of the model to simulate the complex three-dimensional development of cavitation in a rotating machinery, and the associated effects on the performance.

Experimental Investigation of the Clocking Effect in a 1.5-Stage Axial Turbine—Part I: Time-Averaged Results

Comprehensive experimental investigations were conducted to get deeper insight into the physics of stator clocking in turbomachines. Different measurement techniques were used to investigate the influence of varying clocking positions on the highly unsteady flow field in a 1.5-stage axial low-pressure (LP) turbine. A Reynolds number typical for LP turbines as well as a two-dimensional blade design were chosen. Stator 2 was developed as a high-lift profile with a separation bubble on the suction side. This paper presents the results that were obtained by means of static pressure tappings and five-hole probes as well as the time-averaged results of unsteady x-wire measurements. The probes were traversed in different measuring planes for ten clocking positions. Depending on the clocking position, a variation in total pressure loss for Stator 2, a change of the rotor exit flow angle, and a dependency of the Stator 2 exit flow angle were found. The influence of these parameters on turbine efficiency was studied. Three main factors affecting the total pressure loss could be separated: the size of the separation bubble, the production of turbulent kinetic energy, and the strength of the periodic fluctuations downstream of Stator 2.

Detailed Analysis of Experimental Investigations on Boundary Layer Transition in Wake Disturbed Flow

In the framework of a co-operation between the University of Genoa and the Darmstadt University of Technology measurement data of a former investigation at Darmstadt, comprising measurements with surface-mounted hot-film sensors on the boundary layer transition in wake disturbed flow, were transferred to Genoa, then re-evaluated and in great detail analyzed, much further than the original data evaluation. In these experimental investigations at Darmstadt, the boundary layer transition with and without transitional separation bubbles was studied on a circular cylinder in cross flow. The comparison of hot-wire traverses with the surface-mounted hot-film distributions clearly indicated that the surface-mounted hot-film technique is a very suitable measurement technique to obtain reliable information on transition and separation phenomena with both high spatial and temporal resolution. The new data evaluation techniques applied to these data at Genoa further enhanced the insight into the details of the boundary layer transition and separation process. The surface-mounted hot-film data were evaluated by means of time-space diagrams for the first three statistical moments (mean, RMS and skewness), with which the origin and the extent of unsteady separation bubbles clearly could be seen. The results obtained from these data analyses on the one hand yield a considerable enhancement of the understanding of the periodically unsteady boundary layer transition process and on the other hand they form the basis for the application of surface-mounted hot-film sensors in more complex flow situations like e.g. in cold flow multistage turbine or compressor test rigs or even in the hostile environment of real aero engine compressors or turbines.Copyright

Comparison of Different Methods for the Evaluation of Cavitation Damaged Surfaces

The experimental data which will be presented in this paper are the results of the comparison between different methods for evaluating damaged surfaces by cavitation erosion. The different methods are partly working in the initial stage of cavitation erosion and partly at developed cavitation erosion, where mass loss occurs. The used test rig consists basically of a rotating disc with a diameter of 500 mm on which four holes are located. Each hole generates a cavitation zone while the disc is rotating. The test objects are material specimens made of copper. Copper was used as test material in respect to reasonable durations for the tests. The specimen can be implemented in the casing of the test rig directly across the rotating disc on the diameter where the holes are located. This rotating disc test rig generates a very aggressive type of cavitation, so that mass loss, of course depending on the tested material, will appear after relatively short durations. Also the initial stage of cavitation erosion can be observed. The used test rig is very interesting regarding the possibility to apply different measuring techniques to characterize the erosive aggressiveness of cavitation. These techniques are at first the so-called Pitcount-Method, which allows investigations of cavitation erosion in the initial stage. The second one is an acoustic method, which is based on a structure-borne noise sensor and a specially developed signal processing system. The third method is the measuring of mass loss of the material specimen after several time steps. With the help of a CCD-camera and special digital image processing software, images of different cavitation conditions were recorded. The information obtained from these images should serve as support for the evaluation of the other used methods. After the evaluation with the above mentioned methods, the specimens were evaluated with a special device which works with the help of a white light interferometer. With this evaluation method three-dimensional information can be obtained in respect to the actually eroded volume of the specimens. With this information the lost mass of the specimens could be calculated directly. Especially the comparison of the results obtained from the Pitcount-Method, which is a two-dimensional evaluation method, and the three-dimensional results of the white light interferometer is an important point of the work within this paper.Copyright

Clocking effects in a 1,5-stage axial turbine : boundary layer behaviour at midspan

This paper presents an experimental and numerical investigation on the influence of clocking on the boundary layer behaviour of the second stator in a 1.5-stage axial low pressure turbine. Surface mounted hot-film sensors were used to measure the quasi shear stress on the second stator and static pressure tappings to obtain the pressure distribution. All experiments were carried out at midspan for different clocking positions. The supporting numerical calculations were conducted with a two-dimensional Navier-Stokes solver using a finite volume discretization scheme and the v′2 f turbulence model.Copyright

CFD Simulation of a Hydraulic Servo Valve With Turbulent Flow and Cavitation

In this paper the internal flow in a hydraulic servo valve was investigated. The objectives of this research work were to investigate the turbulent flow and the cavitation phenomenon in the valve by using numerical simulation method in the form of CFD (Computational Fluid Dynamics). The geometry of this valve is described by a 3D model and hybrid mesh elements — tetrahedron and hexahedron — are applied to generate the necessary computational meshes for the further calculation. Using the CFD commercial code FLUENT, the internal flow was studied for the turbulent flow situation. In addition, cavitation phenomenon was also investigated with the multiphase model and standard k-e turbulence model. Simulation results show that the flow in this valve is characteristically turbulent and the cavitation emerges actually in the small gap region between the spool and the valve base.Copyright

NUMERICAL SIMULATION OF UNSTEADY AND TRANSITIONAL FLOWS PERTAINING TO TURBINE CASCADES

ABSTRACT Focusing on a linear and a non-linear k −e model as well as on the v 2 − f model, the capability of recent eddy-viscosity turbulence models to capture unsteady and transitional effects typically encountered in axial turbine cascades is investigated in this paper. First, a comprehensive validation procedure of the numerical scheme is presented for three challenging test cases, demonstrating the necessity to take sufficient spatial and temporal resolutions into account. Numerical simulations of a linear turbine cascade periodically disturbed by impinging cylinder wakes are evaluated next. As comparisons with detailed boundary layer measurements show, all models are able to reproduce important features associated with boundary layer transition and flow unsteadiness, while general deficiencies can be identified on the blade pressure surface. For this test case, time-accurate effects of transition are best predicted with the v 2 −f model.

Maßstabseffekte bei der Strömungskavitation

ZusammenfassungBei Kavitationserscheinungen werden Abweichungen von der klassischen Definition der dimensionslosen Kavitationszahl nach Thoma infolge unterschiedlicher Wasserqualität (Zugspannungsfestigkeit, Keimspektrum) und Reynoldszahl (Größe, Strömungsgeschwindigkeit, Viskosität) unter dem Begriff Maßstabseffekte zusammengefaßt. Diese Maßstabseffekte sind insbesondere bei der Übertragung von Modellversuchsergebnissen auf die reale Ausführung zu berücksichtigen. Die dargestellten Ergebnisse, welche an stillstehenden Körpern ermittelt und an einem Pumpenlaufrad überprüft wurden, zeigen, daß klare Beziehungen für die Maßstabseffekte abgeleitet werden können, wenn der Einfluß der Zugspannungsfestigkeit berücksichtigt wird.AbstractScale effects on cavitation phenomena are departures from the classical similarity rules (Thoma number) due to variations in water quality (tensile strength, number and size of nuclei) and Reynolds number (size, flow velocity, viscosity). These scale effects are especially important in the prediction of the prototype cavitation behaviour based on model tests. The presented results obtained on different bodies in test-sections and verified on an impeller show that, after water quality effects have been removed from experimental investigations, clear scaling relations are revealed.

Experimental Investigation of the Clocking Effect in a 1.5-Stage Axial Turbine—Part II: Unsteady Results and Boundary Layer Behavior

Comprehensive experimental investigations were conducted to get deeper insight into the physics of stator clocking in turbomachines. Different measurement techniques were used to investigate the influence of varying clocking positions on the highly unsteady flow field in a 1.5-stage axial low-pressure (LP) turbine. A Reynolds number typical for LP turbines as well as a two-dimensional blade design were chosen. Stator 2 was developed as a high-lift profile with a separation bubble on the suction side. This paper presents the results that were obtained by means of unsteady x-wire measurements upstream and downstream of Stator 2 and surface mounted hot-film measurements on the Stator 2 suction side. It was found that for the case when the Stator 1 wakes impinge close to the leading edge of Stator 2 the interaction between the Stator 1 and the rotor vortical structures takes place in proximity of the Stator 2 boundary layer, which leads to a shift of the transition point in the upstream direction. The major loss parameter concerning the Stator 2 aerodynamic performance could be attributed to the strength of the periodic fluctuations within the Stator 2 suction side boundary layer. A phase shift in the quasiwall shear stress signal in the front region of the Stator 2 vane was observed for different clocking positions.

Impeller integrated measurement of cavitation erosive aggressiveness

For the online measurement of erosive cavitation aggressiveness in hydraulic turbomachinery a system is developed, which is based on the measurement of the structure born noise. The direct coupling of the piezoelectric sensor on the exposed impeller provides a minimisation of the disturbance of the signal. A signal processing device and a transmission system have to be integrated into the rotating system too. This system may be used to detect critical cavitating operation conditions or for active control.