Swirling flow in a conical diffuser generated with rotor-stator interaction

Application Challenge AC6-14 © copyright ERCOFTAC 2024

Introduction

Confined turbulent swirling flows are encountered in many industrial applications, such as hydraulic turbines, gas turbine combustors and internal combustion engines. The flow patterns associated with turbulent swirling flows are vortex-dominated and have long been of interest to scientists and engineers. The goal of the study on which this AC is based has been to understand and control the naturally occurring phenomena. The knowledge can be applied to increase the turbulent convection in heat and mass transfer applications [1], or to reduce unwanted pressure pulsations in e.g. hydro power systems [2]. The present test case primarily relates to the flow in water turbines. The numerical results reported here are based on the work published in [2].

Relevance to Industrial Sector

The flow in water turbines running at off-design conditions often has a strong swirl which may cause vortex breakdown and pressure pulsation in the draft tube. The pulsations may damage the structure and also produce significant electrical power swings. The occurrence of the pulsation and its impact on the efficiency of the draft tube depends mainly on the flow rate of the turbine, the local pressure level, and the velocity field downstream of the runner [3]. Under circumstances leading to the surge, the swirling flow tends to separate into two concentric flow regions. The axial flow basically occurs in the outer region, while the inner region may contain an on-axis recirculation region, also called stagnation region [1]. The breakdown of the swirling flow leads to a precessing helical vortex, also called vortex rope, which makes the recirculation region unsteady.

The swirling flow configuration of the present test case corresponds to part load operation of a Francis turbine. A swirl generator is designed to generate similar flow properties as the Francis turbine at the inlet to the draft tube, yielding a precessing vortex rope in the draft tube.

Design or Assessment Parameters

The dimensionless precession frequency of the vortex rope is expressed using the Strouhal number [3]. The swirl apparatus generates a vortex rope with Strouhal number equal to 0.39, which is quite close to the Strouhal number for a model Francis turbine ( ${\textit {St}}$ = 0.408). The shear layer between the two flow regions in the draft tube has two edges, one or both of which may give negative turbulence production [1]. The production of the turbulence structure depends mainly on the level of the swirl in the flow field [4]. The turbulence models have been assessed based how well they predict the on-axis recirculation region and the vortex breakdown.

Flow Domain Geometry

The study contains the Timişoara swirl generator, shown schematically in Fig. 1. This apparatus generates a highly turbulence swirling flow similar to the one encountered in a Francis turbine operating at 70% load [5, 6]. At this regime, the vortex rope is well developed and generates the largest pressure pulsations. The swirling flow apparatus is installed with two main parts, the swirl generator and the convergent-divergent test section. The swirl generator has an annular section with hub and shroud diameters of ${D_{hub}}$ = 0.09m and ${D_{shroud}}$ = 0.15m, respectively, and 13 guide vanes and a free runner with 10 blades for generating the swirling flow. The guide vanes create a tangential velocity component while keeping a constant pressure. The purpose of the runner is to redistribute the total pressure by inducing an excess in the axial velocity near the shroud and a corresponding deficit near the hub, like a Francis turbine operation at partial discharge. The runner blades thus act like a turbine near the hub and a pump near the shroud. This keeps the free runner at a constant rotational speed. The runner speed is 920rpm at a discharge of 30 l/s.

Flow Physics and Fluid Dynamics Data

The highly turbulent swirling flow with turbulent intensity of about 60% and the Reynolds number based on the throat diameter and bulk velocity of 3.81×10⁵; is a challenging task for CFD. The flow in the draft tube contains the wakes of the blades, the vortex rope, an on-axis recirculation region, as well as separations from both the hub and in the divergent walls of the draft tube.