Draft tube discharge fluctuation during self-sustained pressure surge: fluorescent particle image velocimetry in two-phase flow

Abstract

Hydraulic machines play an increasingly important role in providing a secondary energy reserve for the integration of renewable energy sources in the existing power grid. This requires a significant extension of their usual operating range, involving the presence of cavitating flow regimes in the draft tube. At overload conditions, the self-sustained oscillation of a large cavity at the runner outlet, called vortex rope, generates violent periodic pressure pulsations. In an effort to better understand the nature of this unstable behavior and its interaction with the surrounding hydraulic and mechanical system, the flow leaving the runner is investigated by means of particle image velocimetry. The measurements are performed in the draft tube cone of a reduced scale model of a Francis turbine. A cost-effective method for the in-house production of fluorescent seeding material is developed and described, based on off-the-shelf polyamide particles and Rhodamine B dye. Velocity profiles are obtained at three streamwise positions in the draft tube cone, and the corresponding discharge variation in presence of the vortex rope is calculated. The results suggest that 5-10 % of the discharge in the draft tube cone is passing inside the vortex rope.

Experimental method for the evaluation of the dynamic transfer matrix using pressure transducers

“…In the context of hydraulic turbines, the mass flow gain factor and cavitation compliance are used for the stability analyses by Koutnik & Pulpitel (1996), Chen et al (2008) and Alligné, Nicolet, Tsujimoto, & Avellan (2014). The corresponding instabilities involve complex unsteady flow fields in the draft tube with significant discharge fluctuations, as recently reported by Müller, Dreyer, Andreini, & Avellan (2013). Thus, for the discussion of the hydraulic system stability, it is essential to establish a reliable method to measure the transfer matrix.…”

Detection and analysis of part load and full load instabilities in a real Francis turbine prototype

“…For the part load instability the peak of 1.5 Hz dominates instead of the peak of 0.625Hz. According to the part load [5] and full load analyses [6,8] on the reduced scale model, the first unstable point is excited by a vortex rope with a precession motion while for the full load instability, the excitation source is an axial vortex rope. This precession produces differences in the frequency content of the pressure field on the rotating and stationary frame.…”

“…Inside the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit has been investigated by means of numerical models [1,2] and experimentally on both the reduced scale model of this unit [3][4][5][6] and the real prototype [7].The real unit has a rated power of 444MW and it is located on the British Columbia in Canada. The tests on the reduced scale model (complete model with scale 1/16) were performed in the Laboratory of Hydraulic Machines (EPFL) in Switzerland.…”

“…Nevertheless, some operating points can be unstable, limiting the effective operating range of the unit. For this reason, it is of paramount importance to accurately predict (during the design phase of the unit) and to determine (during the operation of the installed unit) these particular points.Inside the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit has been investigated by means of numerical models [1, 2] and experimentally on both the reduced scale model of this unit [3][4][5][6] and the real prototype [7].The real unit has a rated power of 444MW and it is located on the British Columbia in Canada. The tests on the reduced scale model (complete model with scale 1/16) were performed in the Laboratory of Hydraulic Machines (EPFL) in Switzerland.…”

RANS computations for identification of 1-D cavitation model parameters: application to full load cavitation vortex rope

“…In the case of Francis turbines, the turbine off design operating conditions yield to the development of flow instabilities. One of them, that develops at full load, is the cavitation vortex rope leading to self-excited pressure and discharge oscillations [1]. One-dimensional (1-D) unsteady cavitation models are used to investigate such instabilities [2].…”