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CFD analýza kavitačních jevů v hydraulických strojích
Sedlář, M. ; Zima, Patrik
The paper deals with the CFD analysis of cavitation phenomena in the hydraulic machinery. Both the ANSYS CFX 11.0 package and an in-house software developed in the SIGMA Research and Development Institute and the Institute of Thermomechanics, AS CR v.v.i. are used to study three types of cavitation problems: calculation of the NPSH3 curves, modelling of the cavitation instabilities and prediction of the cavitation erosion.
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Kavitace a nukleace bublin ve vodě s příměsemi
Zima, Patrik ; Maršík, František ; Sedlář, M.
The latest advances in modeling of cavitation in water flows and experimental investigation of bubble nucleation in a cavitation nozzle will be presented. The emphasis will be put on presenting the results of the recent measurements with cavitation nozzle and the critical assessment of the applicability of the cavitation nozzle principle for determining the size of critical bubble radii. Some results of numerical modeling of cavitation in pumps will be summarized.
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Simulace kavitační nestability v induktoru se 3 lopatkami
Sedlář, M. ; Maršík, František ; Zima, Patrik
Axial-flow inducers are used to improve cavitation performance of radial-flow or mixed-flow pumps. There is a wide range of cavitation instabilities, which can be observed during inducer operation, even at the design flow rate. The type of cavitation instability is determined by the frequency analysis. Two cavitation models were used to determine the frequency range of four different types of cavitation instabilities – CEV and VOF built in the ANSYS CFX software – for a 3-blade inducer and suction impeller of a multistage pump. The calculated values of NPSH3 were compared with experimental data for SST, SST-SAS and RSM SSG turbulence models for fully unsteady solutions. It was found that the SST-SAS model provides the most realistic picture of the backflow vortices and their oscillations.
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Dynamics of a bubble in variable pressure and temperature fields of a liquid
Zima, Patrik ; Maršík, František
A study of nucleation and growth of bubbles in a liquid by means of a method based on the nucleation pulse in a shock tube is proposed. The nucleation of bubbles in the shock tube is initiated by a sudden depressurization due to the expansion of the liquid. It is assumed that the nucleation is homogeneous and the nucleation rate and the growth of bubbles can be evaluated using the light scattering method. Description of the experimental setup and analytical solution of the expansion of water in a shock tube are given. The theory of radial dynamics of a spherical vapor/gas bubble in a liquid is then employed in the form of the Rayleigh-Plesset equation and some aspects of inertially controlled bubble growth are discussed. The governing equation and conditions for mass transfer across the bubble boundary due to diffusion of the contaminant gas are given.
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Nucleation and dynamics of bubbles in binary water-gas solution
Zima, Patrik ; Maršík, František
The paper deals with nucleation and dynamics of bubbles in water with dissolved contaminant gas. The nucleation model assumes binary solution of water and pure gas. Some attention is paid to the study of the effect of the dissolved gas on the onset of nucleation (cavitation). In addition, the presence of the gas component is reflected in the Rayleigh-Plesset equation, which describes the subsequent growth (and dynamics) of a bubble in a liquid. The derived equation for bubble radius assumes mass transfer across the bubble boundary, which is in the contrary to the previous and widely used assumptions of boundary impermeability and which appears to provide more realistic insights into bubble behavior. The effect of mass transfer across the bubble boundary is studied analytically and numerically. The natural frequency of the bubble is also shown for the studied problem.
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