Bernd Nennemann

Experimental Study and Numerical Simulation of the FLINDT Draft Tube Rotating Vortex

The dynamics of the rotating vortex taking place in the discharge ring of a Francis turbine for partial flow rate operating conditions and cavitation free conditions is studied by carrying out both experimental flow survey and numerical simulations. 2D laser Doppler velocimetry, 3D particle image velocimetry, and unsteady wall pressure measurements are performs to investigate thoroughly the velocity and pressure fields in the discharge ring and to give access to the vortex dynamics. Unsteady RANS simulation are performed and compared to the experimental results. The computing flow domain includes the rotating runner and the elbow draft tube. The mesh size of 500,000 nodes for the 17 flow passages of the runner and 420,000 nodes for the draft tube is optimized to achieve reasonable CPU time for a good representation of the studied phenomena. The comparisons between the detailed experimental flow field and the CFD solution yield to a very good validation of the modeling of the draft tube rotating vortex and, then, validate the presented approach for industrial purpose applications.

Steady and unsteady flow computation in an elbow dr aft tube with experimental validation

Steady state computations are routinely used by design engineers to evaluate and compare losses in hydraulic components. In the case of the draft tube diffuser, however, experiments have shown that while a significant number of operating conditions can adequately be evaluated using steady state computations, a few operating conditions require unsteady simulations to accurately evaluate losses. This paper presents a study that assesses the predictive capacity of a combination of steady and unsteady RANS numerical computations to predict draft tube losses over the complete range of operation of a Francis turbine. For the prediction of the draft tube performance using k-epsilon turbulence model, a methodology has been proposed to average global performance indicators of steady flow computation such as the pressure recovery factor over an adequate number of periods to obtain correct results. The methodology will be validated using two distinct flow solvers, CFX and OpenFOAM, and through a systematic comparison with experimental results obtained on the FLINDT model draft tube.

Steady and unsteady flow computation in an elbow draft tube with experimental validation

Steady state computations are routinely used by design engineers to evaluate and compare losses in hydraulic components. In the case of the draft tube diffuser, however, experiments have shown that while a significant number of operating conditions can adequately be evaluated using steady state computations, a few operating conditions require unsteady simulations to accurately evaluate losses. This paper presents a study that assesses the predictive capacity of a combination of steady and unsteady RANS numerical computations to predict draft tube losses over the complete range of operation of a Francis turbine. For the prediction of the draft tube performance using k- turbulence model, a methodology has been proposed to average global performance indicators of steady flow computations such as the pressure recovery factor over an adequate number of periods to obtain correct results. The methodology will be validated using two distinct flow solvers, CFX and OpenFOAM, and through a systematic comparison with experimental results obtained on the FLINDT model draft tube.

Comparison of steady and unsteady simulation methodologies for predicting no-load speed in Francis turbines

No-load speed is an important performance factor for the safe operation of hydropower systems. In turbine design, the manufacturers must conduct several model tests to calculate the accurate value of no-load speed for the complete range of operating conditions, which are expensive and time-consuming. The present study presents steady and unsteady methods for calculating no-load speed of a Francis turbine. The steady simulations are implemented using a commercial flow solver and an iterative algorithm that relies on a smooth relation between turbine torque and speed factor. The unsteady method uses unsteady RANS simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step and friction torque. The main goal of this research is to evaluate and compare the two methods by calculating turbine dynamic parameters for three test cases consisting of high and medium head Francis turbines. Overall, the numerical results agreed well with experimental data. The unsteady method provided more accurate results in the opening angle range from 20 to 26 degrees. Nevertheless, the steady results showed more consistency than unsteady results for the three different test cases at different operating conditions.

Numerical simulation of unsteady sheet/cloud cavitation

Unsteady Reynolds-averaged Navier-Stokes (URANS) coupling with mass transfer cavitation models was used to resolve the turbulent flow structure with cavitation. Kubota and Merkle cavitation models were tested. As part of the work, the Merkle model is implemented into CFX by User Fortran code because this model has shown good cavitation prediction capability according to the literature. The results will focus on the unsteady cavitation shedding dynamics around NACA66 hydrofoil. The predicted results compare well with the experimental measurements for unsteady sheet/cloud cavitating flows. Numerical visualizations of cloud cavity evolution and surface pressure signals show relatively good agreement with the experimental data.

Numerical Simulation of Cavitating Flow Around a Hydrofoil

The objective of this paper is to evaluate the applicability of different cavitation models and determine appropriate numerical parameters for cavitating flows around a hydrofoil. The simulations are performed for a NACA 66 foil at 6 degrees angle of attack, Reynolds number of 750 000 and for a cavitation number of 1.49 corresponding to the partial sheet cavitating regime. The incompressible, multiphase Reynolds-averaged Navier-Stokes (RANS) equations are solved by the CFD solver CFX with Kubota and Merkle cavitation models. As part of the work, the Merkle model is implemented into CFX by User Fortran code because this model has shown good cavitation prediction capability according to the literature. The effects of the k-e and SST turbulence models on the cavitating flow dynamics are compared. Also, an investigation on structured and hybrid meshes with different mesh sizes and concentrations is carried out in order to better understand the mesh influence for this cavitation simulation. The local compressibility effect is considered by correcting the turbulent eddy viscosity inside the mixture vapor/liquid zones. The numerical results are validated by experiments conducted in a cavitation tunnel at the French Naval Academy.© 2014 ASME

Numerical study of rotor-stator interactions in a hydraulic turbine with Foam-extend

In the development of high head hydraulic turbines, vibrations are one of the critical problems. In Francis turbines, pressure fluctuations occur at the interface between the blades of the runner and guide vanes. This rotor-stator interaction can be responsible for fatigue failures and cracks. Although the flow inside the turbomachinery is complex, and the unsteadiness makes it difficult to model, the choice of an appropriate setup enables the study of this phenomenon. This study validates a numerical setup of the Foam-extend open source software for rotor-stator simulations. Pressure fluctuations results show a good correspondence with data from experiments.