Gabriel Dan Ciocan

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.

Analysis of the swirling flow downstream a Francis turbine runner

An experimental and theoretical investigation of the flow at the outlet of a Francis turbine runner is carried out in order to elucidate the causes of a sudden drop in the draft tube pressure recovery coefficient at a discharge near the best efficiency operating point. Laser Doppler anemometry velocity measurements were performed for both axial and circumferential velocity components at the runner outlet. A suitable analytical representation of the swirling flow has been developed taking the discharge coefficient as independent variable. It is found that the investigated mean swirling flow can be accurately represented as a superposition of three distinct vortices. An eigenvalue analysis of the linearized equation for steady, axisymmetric, and inviscid swirling flow reveals that the swirl reaches a critical state precisely (within 1.3%) at the discharge where the sudden variation in draft tube pressure recovery is observed. This is very useful for turbine design and optimization, where a suitable runner geometry should avoid such critical swirl configuration within the normal operating range.

Analysis of the Cavitating Draft Tube Vortex in a Francis Turbine Using Particle Image Velocimetry Measurements in Two-Phase Flow

Partial flow rate operation of hydroturbines with constant pitch blades causes complex unstable cavitating flow in the diffuser cone. A particle image velocimetry (PIV) system allows investigating the flow velocity field in the case of a developing cavitation vortex, the so-called vortex rope, at the outlet of a Francis turbine runner. The synchronization of the PIV flow survey with the rope precession allows applying the ensemble averaging by phase technique to extract both the periodic velocity components and the rope shape. The influence of the turbine setting evel on the volume of the cavity rope and its centerline is investigated, providing a physical knowledge about the hydrodynamic complex phenomena involved in the development of the cavitation rope in Francis turbine operating regimes.

3D PIV AND LDV MEASUREMENTS AT THE OUTLET OF A FRANCIS TURBINE DRAFT TUBE

For certain geometries of elbow draft tubes of a hydraulic turbine, a drop in the pressure recovery coefficient is observed for a small variation of the flow rate. In order to determine the possible causes of this characteristics shape, the flow field analysis for 4 nearby operating points have been investigated. For velocity and turbulence fields investigation in the outlet section of the studied draft tube, LDV measurements were performed in a transversal section and the 3D-PIV system was qualified for global velocity measurements in longitudinal sections, with an accuracy of less than 3%. By correlating the LDV and PIV results, the quantification of the flow rate through each channel, related to the operating points, and the description of the secondary flow in the outlet zone are possible.Copyright

WALL FRICTION MEASUREMENTS : APPLICATION IN A FRANCIS TURBINE CONE

The boundary layers at a turbine runner outlet and their evolution with the operating points are unknown and represent an important factor for the pressure recovery of the elbow diffuser. To characterize the boundary layer, a hot film probe is used. The measurement accuracy is an important aspect of this study and the measurement procedure is exposed. An experimental campaign is realized in a turbine cone in order to characterize the wall friction evolution for 4 operating points. Both steady and transient analysis of the experimental results are presented.Copyright

PIV Measurements Applied to Hydraulic Machinery: Cavitating and Cavitation-Free Flows

Hydraulic machinery is an ideal field of application for Particle Image Velocimetry in terms of scientific interest, due to the complexity of the flow behaviour and to the need of detailed unsteady experimental data simultaneously recorded over large sections of the flow field. Within the same machine, a whole range of phenomena are encountered in the different components: wake patterns, separation, rotating vortex structures, vortex breakdown, etc. The unsteady interactions between the stationary and rotating frames, both upstream and downstream the runner, contribute to the efficiency loss. The generated quasi-periodic fluctuations overlay onto the average flow field, which may be symmetrical or not, issuing an unpredictable dynamic behaviour with respect to the operating regime. Whilst the intrinsic parameters of the local phenomena (e.g. sheared flow mixing length) vary, the flow topology may be modified drastically for conditions situated relatively close to one another in terms of head, flow rate and efficiency. The mapping of the unsteady velocity fields and corresponding turbulence levels is thus an essential tool in the analysis of these complex phenomena. The PIV technique opens large perspectives in the analysis of internal flows in hydraulic machinery, providing valuable insight towards an extensive understanding of the underlying physical mechanisms. Nevertheless, the use of PIV systems in this context is one of the most challenging applications, due to the structural constrains related to the optical access to the measurement areas, to the spatial and temporal scales of the phenomena that are to be investigated, two phase flow structure in cavitating regime and also due to the industrial aspects of the application.

Two Phase PIV Measurements at the Runner Outlet in a Francis Turbine

Part load operation of hydro turbines with fixed pitch blades causes complex instable cavitation flow in the diffuser cone. Application of PIV systems provides the opportunity to investigate the flow velocity and turbulent fields in the case of development of cavitation vortex, the so-called turbine rope, at the outlet of a Francis turbine runner. The synchronization of the PIV flow survey with the rope precession allows to apply phase averaging techniques in order to extract both the periodic velocity components and the rope layout. The influence of the turbine setting level on the volume of the cavity rope and its center is investigated, providing a physical insight on the hydrodynamic complex phenomena involved in the development of the cavitation rope at Francis turbine operating regimes.Copyright

Tip Clearance Flow in Turbomachines — Experimental Flow Analyses

To analyse the tip leakage flow, it is considered as necessary the set up of an experimental database. For that, a test’s water tunnel, without rotation, that represents the blade-to-blade canal of a rocket inducer, has been constructed and qualified at Centre de Recherches et d’Essais de Machines Hydrauliques de Grenoble (CREMHyG — France). For these configurations the flow evolution in the water tunnel has been analysed, and specially the vortex of the leading edge by the 3D velocity measurements, performed by LDV. The tip clearance flow analysis has been carried out by velocity measurements: to the inlet and the exit of the tip clearance by LDV and into the tip clearance by PTV. The different levels of the turbulence kinetic energy has been measured at the different positions. The analysis of the phenomenon has been thus achieved.