Cavitation and Generic Flows in Turbo Machinery and Systems
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Unsurpassed dynamics: in no process energy is focused as strongly as in cavitation. Although this phenomenon has been known for a long time, it is still not fully understood. We research on this topic with laser measurement technology as well as high-speed visualization techniques.

Robert Andrews Millikan

Science walks forward on two feet, namely theory and experiment.

Cavitation occurs when the static pressure drops rapidly below a certain critical pressure. This leads to the formation of vapor-filled cavities (gas bubbles) in a liquid’s flow. The bubbles collapse again shortly after their formation, which leads to high pressure surges, material damage, sound emission and changes in the operating behaviour within the affected system. Cavitation is a research topic not only because of its negative effects in maritime applications and hydraulic machinery. It is also profitabely used for surface cleaning of solid surfaces by means of ultrasound. This effect is applied in the semiconductor industry and is based on the generation of cavitation by sound waves (acoustic cavitation). Furthermore, the cavitation’s destructive influence on bacteria and particles allows cavitation to be used for wastewater treatment. Considering lithotripsy (physical destruction of kidney stones), the effects of cavitation are also used in the medical field. We experimentally explore the nucleation process of cavitation, i.e. the formation of bubbles and nuclei, cavitation erosion and the dynamics of large cavitation structures (sheets, clouds). We also develop analytical models to describe these processes and validate them using the obtained experimental data.

Turbomachinery is an important element of our modern civilization. They consume about 30% of the electricity consumed in industry, but also generate, for example, over 50% of the UK's energy needs. For economic reasons, turbomachinery often does not operate at the design point where efficiency is best. In this part load operation regime, various secondary flow phenomena occur, such as cavitation, rotating stall, and part load recirculation that reduce both efficiency and operational reliability. Considering the significance of turbomachinery, we are dedicated to study these phenomena. Partial load recirculation occurs when the flow rate falls below a critical value, followed by separation and recirculation of the flow. Separation and recirculation lead to an undesired change of the velocity profile, shear stress, dissipation and turbulence of the boundary layer.

Analytical models are developed to describe the dynamics of cavitation clouds. These are based on continuity and balance of momentum as well as the Rayleigh-Plesset equation.

Material damage due to cavitation is quantified using the pit-count measurement system. In this process, the damaged surface is optically scanned and the damage energy is determined. The material damage on test specimen is generated on our cavitation test rig.

A high-speed camera and a solid-state laser or LED light source are used to visualize the cavitation regimes. The velocity field of the flow is determined by PIV measurement technique.

Laser Doppler anemometry is a non-contact measurement method to measure the flow velocity of particles. Two coherent laser beams are superimposed at one measuring point and the beat of the interference is measured by the Doppler shift. With the help of this technique it is possible to determine the flow velocity spatially resolved.

The aims of the project are to develop a calculation method for predicting the temporal course of cavitation erosion and to make it available for the industrial design process of pumps. The question to be answered is after which cumulative operating time the first erosion damage occurs. A combined experimental-numerical approach is chosen for the development of the calculation method. The numerical work will be carried out at the Chair of Hydraulic Fluid Machinery at the Ruhr University Bochum. For the validation of this calculation method, experiments for (i) flow visualisation by means of high-speed visualisation and (ii) erosion experiments by means of pit-count and mass loss measurements at the cavitation erosion channel are carried out at the Chair of Fluid Systems at the Technische Universität Darmstadt. Laser measurement techniques such as laser Doppler anemometry and particle image velocimetry are used to specify the boundary conditions for the numerics.

The aim of this project is to investigate the phenomenon of part-load recirculation using a general, generic model. The advantage of the generic model is that it is independent of the geometry of a specific turbomachine and thus represents the ideal test object. Explicitly, the effect of an accelerated or decelerated flow on the boundary layer and on the propagation of the flow separation in a rotating pipe is investigated. In order to be able to investigate the boundary layer and the flow separation, the method of laser Doppler anemometry is used in an open wind tunnel.

Topic
Testing and validation of cavitation models for calculation of the erosive aggressiveness of cavitating flows in radial flow centrifugal pumps Report (opens in new tab)
Numerical prediction of the erosion start due to impinging particles on a metal matrix composite Publication (opens in new tab)
Numerical and experimental analysis of influences of the experimental conditions in cavitation measurements on centrifugal pumps Report (opens in new tab)
The transient cavitation in a convergent-divergent nozzle Report (opens in new tab)
Theoretical investigations of energy bundling in cloud cavitation processes Poster (opens in new tab)
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