Oscillation of Cavitating Vortices in Draft Tubes of a Simplified Model Turbine and a Model Pump–Turbine

Abstract

The self-oscillation of the cavitating vortices is one of the dangerous phenomena of hydraulic turbine operation near full-load conditions. This work is an attempt to generalize data and expand insight on the phenomenon of self-excited oscillations by comparing the experimental results obtained on a simplified turbine and scaled-down pump–turbine models. In both cases, a series of high-speed imaging was carried out, which made it possible to study these phenomena with high temporal resolution. The high-speed imaging data was subjected to additional processing such as binarization, cropping, and scaling. For a simplified turbine model, the volume of the vapor cavity was calculated based on the assumption of the axial symmetry of the cavity, after which fast Fourier transform (FFT) analysis was carried out. A proper orthogonal decomposition (POD) analysis was also performed to examine individual modes in the original digital imaging data. For the pump–turbine, visualization data on the cavitation cavity oscillations were supplemented by pressure measurements in the draft tube cone to determine the frequency characteristics. Based on obtained experimental data, an improved one-dimensional model describing the oscillations of the cavitation cavity arising behind the hydraulic turbine runner is proposed.

Influence of Runner Downstream Structure on the Flow Field in the Draft Tube of a Small-Sized Water Turbine

“…In summary, although many researchers have conducted sufficient research on draft tube flow after changing the downstream flow structure of the runner, most of them have mainly focused on the simplified inherent hydraulic components of the draft tube and their structural changes in the draft tube on the flow state, pressure fluctuation law, and generation mechanism in the draft tube. After simplifying the draft tube, bolts, nuts, and other components were ignored [30][31][32]. However, the water turbine in this study belongs to small-size structures, and the influence of the nut downstream of the runner on the flow pattern of the draft tube cannot be ignored.…”

Data acquisition in a simplified turbine model for prediction of unsteady vortex phenomena

“…The threshold value of S > 0.6 is a rough criterion for identifying the formation of a PVC and central recirculation zone, but it does not provide information about frequency characteristics. Various studies have proposed more advanced approaches for describing vortex rope or PVC dynamics, such as semiempirical models predicting precession frequency, one-dimensional draft tube models, analytical representations of swirl flow, and interpretations of the vortex rope as a global mode based on linear global stability analysis [13][14][15]. These include semiempirical models predicting precession frequency [14], one-dimensional draft tube models [13] , analytical representations of swirl flow, and interpretations of the vortex rope as a global mode based on linear global stability analysis [16].…”

Influence of Runner Downstream Structure on the Flow Field in the Runner of Small-Sized Water Turbine

“…In addition, the simulation calculations of existing hydraulic turbines are simplified and ignore the structures, such as bolts and nuts [27][28][29]. For the small-sized water turbine studied in this paper, the influence of structures such as bolts and nuts on the flow field is not negligible, and the role of the locking nut downstream of the runner blade is similar to Machines 2024, 12, 392 3 of 22 that of the runner cone.…”