Experimental and Numerical Analysis of the Cavitating Part Load Vortex Dynamics of Low-Head Hydraulic Turbines

Abstract

The draft tube flow is a two-sided challenge for the operation of a hydraulic turbine. On one side, it is an important component for the performance of low to medium head turbines, where it can provide up to 40% of the extracted energy from the flow. On the other side, being a diffuser with a complex vorticity distribution at the inlet, vortex breakdown instability can occur at part load and generate a corkscrewed precessing vortex that can be associated with cavitation. The cavitating vortex rope, may generate undesired power output fluctuation and/or structural vibration. Therefore, draft tubes are much studied components but hard to tackle both numerically and experimentally. Within the framework of the AxialT project, the flow in the draft tube of a propeller turbine model operating at part load was studied using a combination of two-phase Particle Image Velocimetry (PIV) measurements and Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations. The paper main focus is on the experimental methodology and results. It explains how Particle Image Velocimetry measurements were implemented, validated and post-treated to provide flow measurements in the draft tube cone at part load in the cavitating and non-cavitating regimes. It also describes various image processing techniques used to extract the velocity field around the cavitating vortex rope and to estimate the location of the water-vapour interface of the cavitating region. In the spirit of feeding experimental data to numerical simulations, an analysis of measured velocity profiles just under the runner is presented. Comparison between PIV measurements and preliminary URANS simulations is also illustrated.

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

“…The flow field measurement inside the guide vanes and runner of a hydroturbine is scarce because of the local complicated geometries of flow passage. Houde et al [10] studied the characteristics of the cavitating and noncavitating part-load vortices in the draft tube of a propeller turbine by means of PIV/LIF and CFD. Ciocan et al [11] presented a CFD methodology to study the unsteady rotating vortex in the Flindt draft tube and also reported an associated experimental study of the flow phenomena.…”

Hydraulic Turbine Vortex Detection and Visualization Using Strain Gauge Sensor

“…In (4), the unity circle is considered. Further, we will study the response of different types of input signals, when we will apply the proposed operator X.…”

“…In today practice, for hydraulic system monitoring, intrusive pressure sensors are used in order to detect pressure fluctuations. Mathematical simulations and reduce scale tests are also performed for flow prediction and vortex identification [4]- [9]. Due to flow instabilities and turbulences, these methods cannot be generalized, despite the fact that they may provide satisfying results [10]- [12].…”

Numerical simulation of a cavitating draft tube vortex rope in a Francis turbine at part load conditions for different σ-levels

“…In experiments the appearance of cavitation allows to visualize the shape of the vortex rope (e.g. [2,3]), which can be used for comparison with simulation results. Typically the frequency of the vortex rope rotation is between 0.2 and 0.4 times the runner frequency [4].…”