Francis full-load surge mechanism identified by unsteady 2-phase CFD

Abstract

View the article online for updates and enhancements. Abstract. Francis turbines may produce spontaneous pulsations of pressure and output power when operating at very high discharge. In such cases there is a cavitating central vortex in the draft tube with variable cavity volume V c . Until today, researchers agree that the main destabilizing agent is the so-called mass flow gain factor, defined as the derivative of cavity volume by the local discharge. Recent studies about 1D high-load stability analysis assumed that the mass-flow gain factor obtained from steady-state vortex data acts on the transient discharge downstream of the cavity. There are however good reasons to question this assumption. Most strikingly, the direct cause of the mass flow gain effect is the increase of swirl produced at the runner exit and hence upstream, not downstream of the cavity. To enhance the reliability of full-load stability predictions, the authors directly investigated the vortex dynamics. The development of the transient cavitating flow in the draft tube was simulated by means of unsteady 2-phase CFD. CFD work started with 1-phase calculations as presented by other authors. This was then extended to a more realistic 2-phase calculation. To contain the computing time within acceptable limits, given the very fine mesh and short time step required, the simulation domain was restricted to the draft tube and, at the same time, the problem was reduced to a basically 2-dimensional rotationally symmetric case. The response of the cavitating draft tube flow to a time-dependent inflow and time-dependent pressure at the draft tube exit was simulated. The results were input to a statistical identification procedure to check possible 1D transient models and find representative parameter values in the sense of a best fit between 1D model and CFD result. As we had suspected, the conventional vortex model with mass flow gain controlled by downstream discharge is not compatible with direct simulation and needs to be modified. The CFD results correspond to a model with the mass flow gain depending almost entirely on the runner exit discharge, delayed by a small dead time.

Cavitation Surge in a Small Model Test Facility simulating a Hydraulic Power Plant

“…Fig.13(c) shows that the cavity volume fluctuation occurs with a phase delay of about /2  behind this relation. The importance of the phase delay has already been pointed out by Doerfler et al [7]. The phase delay makes the swirl flow effects at part load to be neutrally stable.…”

Transient two-phase CFD simulation of overload pressure pulsation in a prototype sized Francis turbine considering the waterway dynamics

“…Because of high wave speeds at low flow velocities, convective terms can be neglected with respect to propagative terms, which leads to eq (1).…”

“…The corresponding pressure pulsation frequency depends on load and tail water level. Typical frequencies are about 20%-100% of the runner rotation frequency [1]. The state of the art approach is to investigate the dynamic behaviour for the whole hydraulic circuit in a one-dimensional (1D) hydroacoustic manner.…”

“…However, the biggest drawback of this approach, beside some uncertainties in the definition of the lumped parameters, are missing insights in complex three-dimensional (3D) flow features. Doerfler et al [1] used ANSYS CFX to investigate the transient behaviour depending on different numerical variations and boundary conditions, whereas Chirkov et al [7] or Cherny et al [8] coupled a 1D approach representing the waterway with a three-dimensional in-house CFD code. The present work combines the advantages of a well validated, efficient and maintained commercial code to simulate distributor, turbine and draft tube in an unsteady 3D manner and attaches a 1D model to represent the penstock and spiral case.…”

U-RANS Simulations and PIV Measurements of a Self-excited Cavitation Vortex Rope in a Francis Turbine

“…Several efforts have been undertaken to simulate full-load pressure surge analytically and numerically. Dörfler et al [6] examined the stability criteria in the draft tube cone flow by unsteady 2-phase Computation Fluid Dynamic (CFD). The cavitation vortex rope volume fluctuation seems accurately captured, however the computational domain does not include the runner.…”