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The cavitating flow behaviour is very sensitive to nuclei content, which is undoubtedly dependent on the physical properties of the liquid. For water, it is believed that heterogenous nucleation initiators prevail over homogeneous nucleation and this is understood to be the reason why the classical nucleation theories are regarded as unreliable for the treatment of cavitation.
As a result, the problem of nuclei content is typically treated either empirically or experimentally. In this paper we show how the empirical approach can be used to obtain a useful picture of the cavitation flow aggressiveness (erosion potential) using numerical modelling of the turbulent cavitating flow. In addition, we present the latest advances in the understanding of the bubble nucleation process under cavitating conditions based on the modified binary nucleation theory.
As a result, the problem of nuclei content is typically treated either empirically or experimentally. In this paper we show how the empirical approach can be used to obtain a useful picture of the cavitation flow aggressiveness (erosion potential) using numerical modelling of the turbulent cavitating flow. In addition, we present the latest advances in the understanding of the bubble nucleation process under cavitating conditions based on the modified binary nucleation theory.
In this article we also shortly describe the experimental research of the cavitating flow aimed at the validation of the erosion potential model, development of the nuclei-content measurement and the validation of the bubble nucleation model. As far as the practical application of our work is concerned, the paper concentrates on an evaluation of the cavitation erosion potential in the hydraulic machinery, mainly water pumps and turbines.
Introduction
In general, there are three major reasons for numerical modelling of cavitation in industrial applications. The first reason is to predict changes in the flow field caused by the cavitation phenomena, which result mainly in a degradation of machine performance. The second reason is to determine cavitation instabilities, which can generate unwanted noise and vibrations. The third reason is to assess the potential of material erosion due to cavitation and to determine the areas on the blade surface (for example in pumps), which are most endangered with erosion. In all the above cases cavitation depends on many factors, which can be divided into two categories: hydrodynamic factors (such as flow parameters or turbulence level)and factors associated with the liquid properties (such as surface tension, bubble content and liquid composition).
Nevertheless cavitation is mainly treated as a solely hydrodynamic problem and the dependence on the physical properties of the liquid is typically neglected. Prediction of cavitation phenomena in hydraulic devices is usually based on the assumption that a given spectrum of cavitation
nuclei flow through the regions with rapidly changing static pressure, which results in a very complicated dynamic behaviour of the cavitation bubbles. From the point of view of an engineer one of the important challenges of the cavitation research is the determination of the number and size of the cavitation nuclei. These quantities naturally depend on the liquid properties; however, they are usually predicted empirically or by rather expensive and complicated measurements on the case-to-case basis. The consequences are most obvious in the case of water when the experimental measurements of the cavitation events under exactly the same hydrodynamic conditions can give very different results.
The main problem in the theoretical estimation of the cavitation nuclei spectrum in water is that the classical nucleation theory predicts only one (critical) bubble size and that the bubble nucleation rate is much higher than the experimentally observed rate. This is true mainly in the case of pure water. The situation is simplified when we consider that the water running trough the hydraulic machinery parts contains a known number of air-filled or vapour-filled microbubbles of known size distribution.
Summary and Conclusion
In this paper we have shortly described the numerical modelling as well as the experimental research of cavitation in hydraulic machines. The main interest was focused on the flow aggressiveness associated with cavitation. The presented numerical model predicts regions highly endangered by the bubble collapses as well as the energy of these collapses denoted as “erosion potential”. The first stage of experimental research in the cavitation tunnel was also presented.
This stage is aimed at validation of the erosion potential model, visualisation of the cavitation bubbles, development of the nuclei content measurement and the validation of the bubble nucleation model. Though the nuclei content in the numerical model was still determined empirically, the agreement of the theoretical results and the observations is encouraging. The measurements of the cavitation nuclei in water by light scattering and the acoustic spectrometry are being completed to provide accurate nuclei distribution. Finally, we have shown that the Classical Nucleation Theory can be improved to reliably predict the process of bubble nucleation under cavitation conditions. As far as the practical application of our work is concerned, the paper demonstrates the analysis of the cavitation erosion in the impeller of the mixedflow water pump.
Introduction
In general, there are three major reasons for numerical modelling of cavitation in industrial applications. The first reason is to predict changes in the flow field caused by the cavitation phenomena, which result mainly in a degradation of machine performance. The second reason is to determine cavitation instabilities, which can generate unwanted noise and vibrations. The third reason is to assess the potential of material erosion due to cavitation and to determine the areas on the blade surface (for example in pumps), which are most endangered with erosion. In all the above cases cavitation depends on many factors, which can be divided into two categories: hydrodynamic factors (such as flow parameters or turbulence level)and factors associated with the liquid properties (such as surface tension, bubble content and liquid composition).
Nevertheless cavitation is mainly treated as a solely hydrodynamic problem and the dependence on the physical properties of the liquid is typically neglected. Prediction of cavitation phenomena in hydraulic devices is usually based on the assumption that a given spectrum of cavitation
nuclei flow through the regions with rapidly changing static pressure, which results in a very complicated dynamic behaviour of the cavitation bubbles. From the point of view of an engineer one of the important challenges of the cavitation research is the determination of the number and size of the cavitation nuclei. These quantities naturally depend on the liquid properties; however, they are usually predicted empirically or by rather expensive and complicated measurements on the case-to-case basis. The consequences are most obvious in the case of water when the experimental measurements of the cavitation events under exactly the same hydrodynamic conditions can give very different results.
The main problem in the theoretical estimation of the cavitation nuclei spectrum in water is that the classical nucleation theory predicts only one (critical) bubble size and that the bubble nucleation rate is much higher than the experimentally observed rate. This is true mainly in the case of pure water. The situation is simplified when we consider that the water running trough the hydraulic machinery parts contains a known number of air-filled or vapour-filled microbubbles of known size distribution.
Summary and Conclusion
In this paper we have shortly described the numerical modelling as well as the experimental research of cavitation in hydraulic machines. The main interest was focused on the flow aggressiveness associated with cavitation. The presented numerical model predicts regions highly endangered by the bubble collapses as well as the energy of these collapses denoted as “erosion potential”. The first stage of experimental research in the cavitation tunnel was also presented.
This stage is aimed at validation of the erosion potential model, visualisation of the cavitation bubbles, development of the nuclei content measurement and the validation of the bubble nucleation model. Though the nuclei content in the numerical model was still determined empirically, the agreement of the theoretical results and the observations is encouraging. The measurements of the cavitation nuclei in water by light scattering and the acoustic spectrometry are being completed to provide accurate nuclei distribution. Finally, we have shown that the Classical Nucleation Theory can be improved to reliably predict the process of bubble nucleation under cavitation conditions. As far as the practical application of our work is concerned, the paper demonstrates the analysis of the cavitation erosion in the impeller of the mixedflow water pump.
by : Milan Sedlář, Patrik Zima, Tomáš Němec and František Maršík
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