Xiao-Bin Li

Assessment of Les Performance in Simulating Complex 3D Flows in Turbo-Machines

Abstract This paper aims at making an assessment of the performance of the large eddy simulation (LES) method in simulating the complex vortical flow in a Francis hydraulic turbine. Firstly the principle of the LES method is presented. Then, three-dimensional turbulent flow of the Francis turbine under selected condition is simulated using the LES method with the Smagorinsky-Lilly model. In the hydraulic turbine, pressure distribution, vortex structures and other flow information are captured, from which the low pressure area beside the suction side of the blade and vortex structure in the draft tube can be seen clearly. Compared with the results from both experiments and simulations by the Reynolds average Navier-Stokes (RANS) method with the k-E turbulent model, the LES results are proven to be more accurate in predicting the overall performance and interior flow characteristics of Francis hydraulic turbine. This work attempts to apply the LES method to simulate the flow field in the flow passages of the whole runner, cone, elbow and diffuser, thereby obtaining the instantaneous characteristics of hydro-turbine, including the pressure distribution, vorticity field and streamlines.

Experimental Investigation on the Characteristics of Hydrodynamic Stabilities in Francis Hydroturbine Models

This paper presents an experimental investigation of flow phenomena related to the characteristic frequencies of pressure fluctuation in Francis hydroturbine models. The experiments were carried out on two test rigs with two model runners having hydraulic similarities. Flow field around the guide vanes was measured with a particle image velocimetry (PIV) on the first PIV test rig. Flow structures at the inlet region of runner and in draft tube at different operating conditions were visualized on another hydrodynamic test rig. Analyses of dominant frequency of unsteady hydraulic behaviors in the tested hydroturbines were performed. It was observed that the main frequency of flow over the guide vanes and the dominant frequency of channel vortex equal the blade passing frequency; the dominant frequency of flow separation at the suction side of blade inlet equals the vane passing frequency; the vortex rope in the draft tube displays a low-frequency nature. The flow instabilities and fluctuations directly influence the running of hydroturbine, thus these experimental results could provide important evidence for the stability study of a real hydroturbine.

Comparisons of LES and RANS Computations with PIV Experiments on a Cylindrical Cavity Flow

A comparison study on the numerical computations by large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) methods with experiment on a cylindrical cavity flow was conducted in this paper. Numerical simulations and particle image velocimetry (PIV) measurement were performed for two Reynolds numbers of the flow at a constant aspect ratio of H/R = 2.4 (R is the radius of the cylindrical cavity, and H is liquid level). The three components of velocity were extracted from 100 sequential PIV measured velocity frames with averaging, in order to illustrate the axial jet flow evolution and circulation distribution in the radial direction. The results show that LES can reproduce well the fine structure inside the swirling motions in both the meridional and the horizontal planes, as well as the distributions of velocity components and the circulation, in good agreement with experimental results, while the RANS method only provided a rough trend of inside vortex structure. Based on the analysis of velocity profiles at various locations, it indicates that LES is more suitable for predicting the complex flow characteristics inside complicated three-dimensional geometries.

Effect of blade perforation on Francis hydro-turbine cavitation characteristics

A new approach, blade perforation, is presented to prevent flow cavitation on the blades’ suction side in Francis hydro-turbines. The blades are perforated at the sites of cavitation so that the pressure and suction sides are connected by the through-hole to increase the pressure at the suction side. The detailed characteristics of flow, performance, cavitation in the runner and vortex rope, and pressure fluctuations in the draft tube of the perforated and unperforated turbines are investigated numerically and experimentally. Under the conditions of this study, the blade perforations do not change the energy extraction performance and the critical cavitation number, but do influence the flow characteristics and ameliorate the turbine instability problems: change the shape of vortex rope; eliminate high-frequency bubbles and reduce the amplitude of pressure fluctuations in the draft tube. This indicates that blade perforation can be used as a novel method to improve the stability of hydro-turbines.

On the Flow Instabilities and Turbulent Kinetic Energy of Large-Scale Francis Hydroturbine Model at Low Flow Rate Conditions

This paper is to make a better understanding of the flow instabilities and turbulent kinetic energy (TKE) features in a large-scale Francis hydroturbine model. The flow instability with aspect of pressure oscillation and pressure-velocity correlation was investigated using large eddy simulation (LES) method along with two-phase cavitation model. The numerical simulation procedures were validated by the existing experimental result, and further the TKE evolution was analyzed in a curvilinear coordinates. By monitoring the fluctuating pressure and velocities in the vanes’ wake region, the local pressure and velocity variations were proven to have a phase difference approaching π/2, with a reasonable cross-correlation coefficient. Also the simultaneous evolution of pressure fluctuations at the opposite locations possessed a clear phase difference of π, indicating the stresses variations on the runner induced by pressure oscillation were in an odd number of nodal diameter. Considering the TKE generation, the streamwise velocity component u s ′2 contributed the most to the TKE, and thus the normal stress production term and shear stress production term imparted more instability to the flow than other production terms.

Large eddy simulation of pressure fluctuations at off-design condition in a Francis turbine based on cavitation model

To study the instability of a Francis turbine at off-design operating condition, a hydraulic model was established and the flow characteristics at the off-design point were studied based on large eddy simulation (LES). The simulation was conducted for both single phase model and cavitation model. The results were compared with the experimental data. Results show that the simulation based on cavitation model can capture more channel vortex structures than single phase calculation. The result of vortex rope by cavitation model is similar to the experimental result. The dominant frequency can be obtained by these two methods, while the result based on cavitation model can capture the high frequency component at the inlet of draft tube. Great difference can be seen from the internal flow of the two simulation results. These conclusions can provide a basis for the study of instability of Francis turbine.