Anne-Marie Giroux

Steady-state Capabilities for Hydroturbines with OpenFOAM

The availability of a high quality open source CFD simulation platform like OpenFOAM offers new R&D opportunities by providing direct access to models and solver implementation details. Efforts have been made by Hydro-Qubec to adapt OpenFOAM to hydroturbines for the development of steady-state capabilities. This paper describes the developments that have been made to implement new turbomachinery related capabilities: multiple frames of reference solver, domain coupling interfaces (GGI, cyclicGGI and mixing plane) and specialized boundary conditions. Practical use of the new turbomachinery capabilities are demonstrated for the analysis of a 195-MW Francis turbine.

Steady-state capabilities for hydroturbines with OpenFOAM

The availability of a high quality Open Source CFD simulation platform like OpenFOAM offers new R&D opportunities by providing direct access to models and solver implementation details. Efforts have been made by Hydro-Quebec to adapt OpenFOAM to hydroturbines for the development of steady-state capabilities. The paper describes the developments that have been made to implement new turbomachinery related capabilities: Multiple Frame of Reference solver, domain coupling interfaces (GGI, cyclicGGI and mixing plane) and specialized boundary conditions. Practical use of the new turbomachinery capabilities are demonstrated for the analysis of a 195-MW Francis hydroturbine.

Free vibration analysis of a large hydroelectric generator and computation of radial electromagnetic exciting forces

Major causes of vibration in electrical machines come from electromagnetic sources. A serious problem may arise when the frequencies of the periodic exciting force are identical with, or close to, one of the natural frequencies of the machine. This paper presents a 2D modal analysis which calculates the natural frequencies and mode shapes of the stator and rotor of a large hydroelectric generator. The structural model takes into account the shape of the stator teeth and rotor poles. Moreover, a 2D electromagnetic transient simulation is performed in order to predict the frequencies and mode shapes of the electromagnetic exciting force. This advanced electromagnetic numerical simulation takes into account the winding sequence and the geometry of the rotor and stator given by the manufacturer. The purpose of this work is to validate a multiphysics tool for the future and recognize its advantages over the classical method.

Study of the impact of eccentricity in large synchronous generator with finite elements

This paper presents a numerical model for computing electromagnetic forces in an existing large hydro generator at Hydro-Québec. Due to the high mass and low rotational speed, hydro generators present static, dynamic or combined eccentricities resulting in unbalanced magnetic pull in the air gap and unbalanced stator currents caused by the variation of the magnetic flux density distribution. The model uses advanced 2-D numerical finite-element analysis to compute the electromagnetic forces based on Maxwell stress tensor and the obtained results were used later as inputs to study the dynamic behavior of hydro power turbine shaft. The results obtained for electromagnetic forces, damper bar currents and circuit currents due to any magnetic unbalance are presented and discussed in detail.

Finite element simulation of hydro generators with rotor inter turn short circuit

This paper investigates the impact of rotor inter-turn short circuit on the radial magnetic flux density of a large hydro generator. A numerical model based on advanced 2D finite element analysis is used for computing the magnetic flux density with and without inter turn short circuit in rotor winding. At first, the simulated air gap flux density is compared to measurement in order to validate the FEA model. Then two methods based on magnetic flux density in the air gap analysis to detect this fault are presented. Simulation results performed on a 74 MVA industrial hydro generator are presented and discussed. The study showed that the magnetic flux density decreases in the defected pole with the increase of inter turn short circuits. Furthermore, the flux of the defected pole contributes less in the flux of the adjacent poles which facilitate the detection of the defected pole.

Computation of electromagnetic forces in a hydroelectric generator

Hydropower units are subject to several forces and couples, under certain conditions they can cause noise, vibration, and interaction problems with the electric network or even equipment failure. The objective is to develop numerical models of the hydroelectric generator taking into consideration the shaft interaction in order to study the behavior of the generator. An analytical method is implemented in order to study the interaction of the oscillation of the mechanical couple with the network, and its influence on the electromagnetic couple. A finite element model is developed in order to study the impact of eccentricity on the generator, which is the origin of unbalanced magnetic pulls. Depending on the degree of eccentricity, the resulting electromagnetic force can be significant. Finally, the aim of this work is to contribute to the study of the dynamic behavior of the shaft in hydropower units.

Modelling roughness and acceleration effects with application to the flow in a hydraulic turbine

This study reports the numerical predictions of flows over turbine blades, which include flow acceleration and deceleration. Two issues are addressed: (1) accurately predicting roughness effects, and (2) evaluating the performance of Reynolds-Averaged Navier-Stokes (RANS) simulations on moderately accelerating flows. For the present turbine surfaces, it is found that roughness correlations based on roughness surface slope better predict the roughness effects than both the correlations based on the moments of roughness height statistics and the IEC standard approach. It is shown that RANS simulations reproduce the flow evolution over rough-wall accelerating turbulent boundary layers, although, on a smooth wall, they fail to capture strong non-equilibrium flow behaviours. Finally, a hydraulic turbine simulation is performed to show the significant roughness impact on the total losses.

Challenges in assessing the grid sensitivity of hydro-turbine CFD simulations

Despite the great progress made over the last 20 years, making reliable predictions with CFD is still a challenge. In fact the hydraulic machinery community rarely relies entirely on numerical simulations to provide final designs. The core of the problem lies with the estimation of the solution accuracy. Grid convergence studies are a useful way to check for grid independence and can provide some insight into the actual solution accuracy. However, conducting such studies for complex, detached flows involves many challenges among which: creating progressively refined grids and comparing with measurement results. This paper investigates this problem through the practical example of the BulbT experimental setup for which multiple CFD runs were conducted.