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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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Výpočet hustotních profilů nano-kapiček použitím gradientní teorie: numerické řešení v transformovaných proměnných
Hrubý, Jan
The Cahn-Hilliard gradient theory extends the continuum fluid mechanics and thermodynamics to objects of typical dimensions of 1 nm. The present investigation concerns the computation of the density profile of a equilibrium spherical case (droplet, bubble). The solution for this case reduces to a single second-order ordinary differential equation (ODE) with boundary conditions. To overcome computational difficulties due to the tremendous difference of thermodynamic properties of the liquid and vapor phases at low temperatures, we suggest some mathematical transforms. Using these transforms, the right-hand side of the ODE becomes well-behaved and the numerical solution is converging.
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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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Rosný bod spalin: teoretické modelování a předběžné měření
Krejčí, A. Petr ; Hrubý, Jan ; Ždímal, V. ; Trávníček, Zdeněk
Based on industrial needs, the dew point temperature of flue gases is re-examined, both theoretically and experimentally, with mixture of sulphuric acid and water vapors as a flue gas model. This work is motivated by inconsistency and unsufficient accuracy of previous works, which differ significantly in data on sulphuric acid saturation pressure. Experimental setup based on the Laminar co-flow tube is used to express saturation pressure of sulphuric acid via measurements of binary homogeneous nucleation rate.
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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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Kalibrace clonkové tratě rotačním průtokoměrem a rezonance systému
Randa, Zdeněk ; Tesař, Václav ; Hrubý, Jan
Rotary piston flowmeters (RPF) are widely used for accurate measurement of gas flow rates. We used it to calibrate a Koennecke-type quadrant nozzle flowmeter. The evaluated discharge coefficient exhibited strange local maxima, even exceeding unity. The phenomenon was sensitive to the length of the tubes connecting the RPF with the calibrated quadrant nozzle. One possible explanation, discussed in the present paper, is the local maxima to be due to acoustic resonance in the system, excited by periodic character of the flow through the RPF. Another possibility is that the also the inertia of the pistons plays a role, the air providing the spring action.
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