Vortex ropes in draft tube of a laboratory Kaplan hydro turbine at low load

Application Area 6: Turbomachinery Internal Flow

Application Challenge AC6-15

Description

Introduction

Computer simulations have been seen as a potential tool for capturing the subtleties of the development of instabilities and associated vortical and turbulence structures. Such information can be obtained by direct and large-eddy simulations (DNS, LES) over a range of important scales, but the accuracy of DNS and LES depends much on the numerical resolution. For complex configurations the Reynolds-averaged Navier-Stokes (RANS) approach, being computationally less demanding, has been considered as a more rational option for industrial purposes.

However, because of its empirical nature, the RANS approach in general has not been regarded as a trustful research tool, recognizing that its credibility depends much on the choice of the turbulence model to close the time- or ensemble-averaged conservation equations. Hybrid LES/RANS schemes (including detached eddy-simulations, DES), where LES is used in the flow bulk and a RANS model for the wall-adjacent, or complete wall-attached flow regions, are becoming popular and, by some in the community, are seen as the future industrial standard (Slotnick et al., 2014).

Using the here reported experimental data and a fine-grid LES as the reference, we analyzed the performance of two levels of RANS models representing the LEVM and RSM families, the basic DES and LES with WALE sub-grid scale viscosity in reproducing not only the time-averaged basic flow and turbulence properties in the turbine draft tube, but also the flow patterns, vortical structures and their relation with the, industrially most critical, frequencies and amplitudes of pressure pulsations, all in the range of off-design conditions. The material presented here is based on the work published in Minakov et al. (2017).

Relevance to Industrial Sector

Common hydraulic turbomachinery has long reached a mature stage of development, and at design conditions usually performs with high efficiency and reliability. However, the ever-increasing share of intermittent and unpredictable wind and solar power in electricity generation requires higher flexibility of operation and fast load adjustment of the base power plants (including hydro) over a wide range of operating conditions. At part loads and in transient regimes the stability of the hydropower system can seriously be impaired leading to a decrease in efficiency, mechanical damage, fatigue and system failure. The intrinsic unsteadiness at suboptimal conditions leads often to vortex breakdown and precessing helical vortices in form of a single or twin rope behind the runner, which cause intense flow pulsations and vibrations of the turbine structure that pose a serious threat to the system reliability and safety. These issues have long been in the focus of experimental research complemented by some simplified analytics, but a full account of the three-dimensional time dynamics has remained to a large extent beyond the reach of even the most advanced laser-based measurements and diagnostics techniques.

Design or Assessment Parameters

The design and assessment parameters for the test case considered here can be grouped into two categories, one characterizing the experimental facility and its operation, and the other being relevant for assessing the numerical modeling.

The first group includes the Reynolds number, , the geometric parameters of the facility (the runner diameter D, geometry of the guide vane and runner blades, geometry of the draft tube), rotating speed of the runner, volume flowrate through the unit, specific speed the runner and the swirl parameter

The second group includes parameters that allow evaluating the reliability of the calculated data. In the present case, it is the vertical velocity component field in the central section, the vortex structure behind the impeller, the velocity components and their pulsations in different sections behind the impeller, pressure pulsations and their spectra in the diffuser of the draft tube.

Contributed by: A. Minakov [1,2], D. Platonov [1,2], I. Litvinov [2], S. Shtork [2], K. Hanjalić [3] —

[1] Institute of Thermophysics SB RAS, Novosibirsk, Russia,

[2] Siberian Federal University, Krasnoyarsk, Russia,

[3] Delft University of Technology, Chem. Eng. Dept., Holland.