G. Ludwig

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

On the Kinematics of Sheet and Cloud Cavitation and Related Erosion

The influence of flow parameters such as cavitation number and Reynolds number on the cavitating cloud behavior and aggressiveness is analysed in an experimental work. The focused geometry is a convergent-divergent nozzle with a given radius of curvature at the minimum cross section. By means of a high-speed camera the kinematics of cloud cavitation is visualized. The shape of the cloud is a horse shoe (U-shaped) with two legs ending at the material surface which is in agreement with the Helmholtz vortex theorem. Indeed it is worthwhile to look at the cavitation cloud as a ring vortex whose second half is a mirror vortex within the material. Due to the convection flow, the legs of the vortex are elongated and hence the rotational speed of the vortex core will increase. Thus cavitation bubbles will concentrate within the legs of the vortex and that behavior is observed in the cavitation experiments. The aggressiveness of the cloud is quantified by using soft metal inserts adapted on the nozzle geometry. The interpretation of the plastic deformation, called pits, is done with a 2-dimensional optical measurement system, which is developed to scan large and curved surfaces. In this way damage maps are obtained. Consequently dimensional analysis is used to analyse and generalize the experimental results. Thus a critical Reynolds number is found for the transition from sheet to cloud cavitation. Further an upper limit for the Strouhal number exists for the given geometry. A physical model for the critical Reynolds number is given elsewhere [1]. Also a model for the dynamics of the observed stretched cloud with circulation is published by Buttenbender and Pelz [2].

Experimental evidence of nucleation from wall-bounded nuclei in a laminar flow

The importance of nucleation from wall-bounded nuclei for cavitation and especially cavitation inception is undisputed. Although various theories and models found their way to standard literature, there is a lack of experiments allowing a closer look onto the process of nucleation. In the present paper we present a new experimental set-up that allows the investigation and analysis of nucleation from wall-bounded nuclei. The experimental findings support an extended understanding of nucleation as a self excited cyclic process. Impressive high-speed visualisations can be found in the supplementary material.

Design Factors Affecting Pump Efficiency

The investigations presented as follows were carried out in the frame of an EU funded SAVE project headed “Study on Improving the Energy Efficiency of Pumps”. One aim of this study was to quantify the effects of several design factors affecting pump efficiency of single-stage centrifugal pumps. The theoretical investigations that were carried out at the chair of Turbomachinery and Fluid Power at Darmstadt University of Technology where mainly focused on the influence of surface roughness, the aspect of smoothing several parts of pumps and the internal leakage flows corresponding to the sized gap clearances. The specific speeds of the considered pumps covered the range from ns= 10 min-1up to ns= 100 min-1which represents the typical field of application of standard centrifugal pumps The theoretical estimations were obtained by the aid of a special (non CFD) software tool, which was developed within the scope of a former research project named “Attainable Efficiencies of Volute Casing Pumps” sponsored by the Research Fund of the German Pump Manufacturer Association (VDMA).

Experimental and Numerical Research of the Effects of a Perturbed Inflow of a Centrifugal Pump

The influence of a disturbed inflow condition on a single-stage, end suction volute casing pump, as they are frequently found e.g. in water supply plants, was analyzed experimentally in a test bench and numerically using CDF simulation. The analyzed pump operates with water and has a specific speed (ns ) of 42 (metric units), an outside impeller diameter (D2 ) of 188 mm and an operating speed (n) of 2300 rpm. The measurements were carried out in a closed test loop where the operating conditions and the inlet piping can be varied to analyze the influence of bend and valve over the pump performance. For the 3-D numerical analysis two configurations were simulated. First the combination bend and butterfly valve was calculated and second the complete pump. Different coupling methods between fixed and rotating parts and some turbulence models were analyzed. For the comparison between experiment and simulation the characteristic curves of the pump are used. Also the calculated flow distribution directly upstream of the suction branch of the pump was compared with the velocity profile measured non-intrusively with a Laser Doppler Velocimeter (LDV). The aim of the study is to determine the effects of a disturbed flow at the inlet of a pump on its performance and to analyze the causes of these effects using CFD.Copyright

Experimental Investigations Concerning Erosive Aggressiveness of Cavitation at Different Test Configurations

The experimental results, which will be presented in this paper, demonstrate the significant influence of the flow velocity, respectively the rotational speed, on the erosive aggressiveness of cavitating flows. On two of the three investigated test objects, cavitation erosion can only be observed in the initial stage by the so-called pit-count evaluation method. Developed erosion with mass loss is impossible to measure because of the very long duration until mass loss appears. The third test rig generates a very aggressive type of cavitation, so that mass loss, depending on the tested material, will appear after relatively short durations. In addition, the initial stage of cavitation erosion can be observed. Three different techniques were applied to investigate cavitation erosion in the initial and developed stage. Thereby, the capability of methods to quantify erosive effects in dependence of influencing operating parameters has been proven.Copyright