Demin Liu

LES numerical simulation of cavitation Bubble shedding on ALE 25 and ALE 15 hydrofoils

A cavitation calculation scheme is developed and applied to ALE 15 and ALE 25 hydrofoils, based on the Bubble Two-phase Flow (BTF) cavity model with a Large Eddy Simulation (LES) methodology. The Navier-Stokes equations including cavitation bubble clusters are solved through the finite-volume approach with a time-marching scheme. Simulations are carried out in a 3-D field with a hydrofoil ALE 15 or ALE 25 at an angle of attack of 8° and cavitation number σ=2.3 with a 2×106, meshing system. With the time-marching, the cavitation bubble gradually grows to a steady lump shape and then produces an irregular small bubble behind the main cavitation bubble, finally shedding from the leading edge of the cloud cavitation structure. The calculated results including velocity field and pressure field are consistent with experiment data at the same Reynolds number and cavitation number. The vortex and reverse flow are observed on the hydrofoil surface.

Runaway transient simulation of a model Kaplan turbine

The runaway transient is a typical transient process of a hydro power unit, where the rotational speed of a turbine runner rapidly increases up to the runaway speed under a working head as the guide vanes cannot be closed due to some reason at the load rejection. In the present paper, the characteristics of the runaway transient of a model Kaplan turbine having ns = 479(m-kW) is simulated by using a time-dependent CFD technique where equation of rotational motion of runner, continuity equation and unsteady RANS equations with RNG k- turbulence model are solved iteratively. In the calculation, unstructured mesh is used to the whole flow passage, which consists of several sub-domains: entrance, casing, stay vanes + guide vanes, guide section, runner and draft tube. And variable speed sliding mesh technique is used to exchange interface flow information between moving part and stationary part, and three-dimensional unstructured dynamic mesh technique is also adopted to ensure mesh quality. Two cases were treated in the simulation of runaway transient characteristics after load rejection: one is the rated operating condition as the initial condition, and the other is the condition at the maximum head. Regarding the runaway speed, the experimental speed is 1.45 times the initial speed and the calculation is 1.47 times the initial for the former case. In the latter case, the experiment and the calculation are 1.67 times and 1.69 times respectively. From these results, it is recognized that satisfactorily prediction will be possible by using the present numerical method. Further, numerical results show that the swirl in the draft-tube flow becomes stronger in the latter part of the transient process so that a vortex rope will occur in the draft tube and its precession will cause the pressure fluctuations which sometimes affect the stability of hydro power system considerably.

Numerical analyses of pressure fluctuations induced by interblade vortices in a model Francis turbine

Interblade vortices can greatly influence the stable operations of Francis turbines. As visible interblade vortices are essentially cavitating flows, i.e., the ones to cause interblade vortex cavitations, an unsteady simulation with a method using the RNG k-ε turbulence model and the Zwart-Gerber-Belamri (ZGB) cavitation model is carried out to predict the pressure fluctuations induced. Modifications of the turbulence viscosity are made to improve the resolutions. The interblade vortices of two different appearances are observed from the numerical results, namely, the columnar and streamwise vortices, as is consistent with the experimental results. The pressure fluctuations of different frequencies are found to be induced by the interblade vortices on incipient and developed interblade vortex lines, respectively, on the Hill diagram of the model runner’s parameters. From the centrifugal Rayleigh instability criterion, it follows that the columnar interblade vortices are stable and the streamwise interblade vortices are unstable in the model Francis turbine.

Numerical Simulation of Hydraulic Turbine Based on Fluid-Structure Coupling

By adopting the arbitrary Lagrange-Euler (ALE) method of software ADINA, fluid-structure coupling (FSC) calculation of a Francis turbine is conducted. The vibration frequency and mode of the runner in the air and water are obtained. The calculation results show that runner frequency of the turbine is reduced in certain degree under the effect of water pressure and viscous force, and the mode is changed, too. By using dynamic fracture mechanics, the possible cracking damage of the runner is predicated.

Numerical Predictions of the Incipient and Developed Interblade Vortex Lines of a Model Francis Turbine by Cavitation Calculations

The existence of runner interblade vortices can cause instability problems in Francis turbines, for example, pressure fluctuations, vibrations of runners, noise, and so forth. It is favorable in engineering practice to have the knowledge of the appearance of the incipient and developed inter blade vortex lines on the Hill diagram in unit parameters. Most numerical research on the inter blade vortices has been focused on the study of the characteristics of pressure fluctuations by single-phase flow calculations. However, since the two vortex lines are distinguished by observations of visible vortices, which contain cavitating flows, it is clear that cavitation calculations are needed for their predictions. A method by solving RANS equations with RNG k – ∊ turbulence model and ZGB cavitation model was proposed for the predictions in a Francis model turbine. Modifications of the turbulence viscosity were made for better simulation results. Vorticity criterion was chosen to identify the vortices. The fact that the results of cavitation calculations have a better agreement with experimental observations than single-phase calculations proves the validity of this prediction method.

Analysis on Cavitation Characteristics of Flow in a Francis Turbine with Different Content of Non-Condensable Gas

Abstract Cavitation causes serious damages to Francis turbine, e.g., noise and vibration. Its mechanism is complex and may be affected by many factors. The present paper compares cavitation behavior of flow in a Francis turbine with different content of non-condensable gas (NCG) concluded from experiment and numerical simulations. The experimental results show small difference in characteristics of cavitation with different content of non-condensable gas, while numerical simulation shows larger difference. It thus can be concluded that present simulation over-predict this difference.

NUMERICAL ANALYSIS OF HYDROFOIL NACA0015'S CAVITATION CHARACTERISTICS BASED ON THE THERMODYNAMIC EFFECTS

The present paper studies the cavitation characteristics in thermodynamic condition. The present work modifies thermodynamic cavitation mass transfer expression based on the Rayleigh-Plesset equation. The pressure difference, surface tension and thermodynamic effects are considered in new mass transfer expression on the basics of the evaporation and condensation mechanics according to the micro-kinetic theory. The hydrofoil NACA0015s thermal cavitation characteristic is calculated at 25°C, 50°C, 70°C with the improved model. The shear stress transport model is adopted as turbulence kinetic energy transport equation. The pressure coefficient is compared with experiment data at different temperatures to validate the model. The temperature difference between local temperature and ambient temperature is obtained and the cavitation volume fraction is calculated.

Numerical Investigation on Channel Vortex in a Francis Turbine

When a Francis hydraulic turbine operates under different working heads at small flow condition, the fluid in the flow passage will generate vortex shedding near the blade leading-edge and form the channel vortex in the blade passages due to the mismatch between the outlet angle of guide vane and the inlet angle of runner blade. The severity of channel vortex will trigger high-frequency vibration or generate unit resonant vibration, affecting the operational stability of the turbines. In this paper some typical operation points were chosen out for the steady simulation of a model turbine according to a unit hill-chart. The computational domain was chosen as the whole flow passage from the inlet of the volute to the outlet of the draft tube. Based on RNG k–e turbulence model, the internal flows was simulated, and the occurrence of vortex between the turbine runner blades was discussed. The numerical results show that the vortex motion near the development-line (IVDL) is stronger than that near the channel vortex inception-line (IVIL) in channel vortex zone marked in the hill-chart. The velocity triangle is used to explain the reasons that channel vortex occur in the suction side at high working head while in the pressure side at the low working head, and two different forms and formation mechanism of the channel vortex were analyzed.Copyright

Numerical Simulation of Cavitation Characteristic Around Hydrofoil NACA0015

The applicability of numerical prediction method for cavitation around hydrofoil NACA0015 was studied in this paper. For the present study, the mixture fluid methods was adopted for mass transfer between phases. For validation of this approach, simulations for the following problems were carried out: (1) leading edge cavitation on a hydrofoil; and (2)Cavitation performance and flow field analysis for a hydrofoil NACA0015. A full discussion of the results is presented below. This paper modified cavitation mass transfer equation based on the Rayleigh-Plesset equation. The pressure difference, surface tension and the turbulence effects were considered in the new mass transfer equation on the basic of the evaporation and condensation mechanics in the micro-kinetic theory. According to the governing equations for mass, momentum, volume conservation, the hydrofoil NACA0015’s cavitation characteristic was calculated by the new model. The cell-central difference finite element method was employed to discretize the governing equations. The pressure coefficient was contrasted with experiment data to validate the model. The calculation data is identical to the experiment data. As the result, it’s shown that this method can be used for the prediction of the behavior of sheet cavitation of the hydrofoil NACA0015.© 2010 ASME

Numerical Analysis of Airfoil NACA0015 and Centrifugal Pump’s Cavitation Characteristic Based on the Thermodynamic Effects

Cavitation is not only driven by the pressure difference, but also affected by the temperature difference. In high temperature water or cryogenic fluids, temperature decline of liquids is caused by latent heat of vaporization. The cavitation characteristics in this conditions are different from that of room temperature. The thermodynamic effects of cavitation have very important application in the fluid machine, so high temperature and low temperature cavitation are comprehensively applied at the astronautics. The presented paper researched thermodynamics cavitation based on the Rayleigh-Plesset equation and deduced a new thermodynamics cavitation model with fully considering thermodynamic effects on the basic transport equation. The airfoil NACA0015 was calculated by the new model and the thermal cavitation characteristic was calculated at different temperature, that is, 25°C, 50°C and 100°C. Besides, the pressure coefficient was contrasted with experiment data at different temperature. The centrifugal pump’s suction performance curve was calculated at 25°C and 100°C respectively, and the main conclusion is that the suction performance of the pump at the high temperature is better than that at the normal temperature. The thermodynamic effects of cavitation model are more accurate at predicated centrifugal pump’s suction performance, which can provide beneficial referenced indicator for energy conservation.Copyright