David Stefan

Brno University of Technology · Energy Institute

The simulations of high head Francis turbine model (Tokke) are performed for three operating conditions -Part Load, Best Efficiency Point (BEP) and Full Load using software Ansys Fluent R15 and alternatively OpenFOAM 2.2.2. For both solvers the simulations employ Realizable k-ɛ turbulence model. The unsteady pressure pulsations of pressure signal f...

Swirling flow with compact coherent structures is very good candidate for proper orthogonal decomposition (POD), i.e. for decomposition into eigenmodes, which are the cornerstones of the flow field. Present paper focuses on POD of steady flows, which correspond to different operating points of Francis turbine draft tube flow. Set of eigenmodes is b...

The decay of the jet produced by the Pelton turbine nozzle is analyzed using the proper orthogonal decomposition of image ensemble obtained from a high speed camera. The ensembles of images were recorded with frequency of 1000Hz and frame size 800x600 pixels, where only the part with a boundary between jet and air is considered. The spatio-temporal...

Cavitation phenomena occuring in converging-diverging nozzle (Venturi tube) are described in the paper. A closed test circuit with possibility to control both flow rate and static pressure level were used. Loss coefficient was evaluated for different sigma numbers resulting in full "static" characterization of the nozzle. Visualizations of the cavi...

Practical application of proper orthogonal decomposition (POD) is presented. Spatio-temporal behaviour of the coherent vortical structures in the draft tube of hydraulic turbine is studied for two partial load operating points. POD enables to identify the eigen modes, which compose the flow field and rank the modes according to their energy. Swirli...

The electric-pump-fed system of the rocket engine represents an important innovation in the field of rocket propulsion. While the general concept itself is already several decades old, the technology was not still adopted in the commercial applications on a large scale. The exception being the Rutherford engine developed by RocketLab. Nonetheless,...

The swirling flow in a diffuser has been studied using the stereo PIV technique. The DANTEC hardware and software were used for the experiment. The water pipe flow in the diffuser inlet has been set spinning by introducing tangential jet flow. Both piping and diffuser were of circular cross-section, however diffuser casing had rectangular (square)...

The very low specific speed pump (nq = 8.9) operated in turbine mode was analyzed. The experimental and numerical studies were carried out in order to show effect of different blade layout on pump-as-turbine (PaT) performance. In total three different impellers were analyzed. One impeller consisting of four main blades and two impellers consisting...

Editorial of special issue in Sustainability journal

The operation of any hydraulic power plant is accompanied by pressure pulsations that are caused by vortex rope under the runner, rotor–stator interaction and various transitions during changes in operating conditions or start-ups and shut-downs. Water in the conduit undergoes volumetric changes due to these pulsations. Compression and expansion of...

The usage of splitter blades to enhance the performances of low specific speed pumps is common practice. Based on experimental and numerical studies, the influence of the addition of one and two splitter blades is investigated on a very low specific speed pump to assess their impact not only on the performance characteristics but also on the losses...

The hydraulic turbines are recently forced to operate far away from the optimal conditions in order to balance fluctuations in electricity generation. In case of Francis, pump and propeller turbines, using only single control component of guide vanes, it means that in regions where the high residual swirl enters the draft tube, the flow is decelera...

Low specific speed pumps find applications in a broad range of domains, but suffer from a low efficiency and a risk of head instability close to shut-off. The numerical computations on these pumps performed in the last years have shown discrepancies with experimental results. Recent studies suggest that the use of wall-functions underpredicts the l...

This work presents a detailed analysis of the three-dimensional flows in a Pump-as-Turbine (PaT). The flow field is studied through URANS simulations and a velocity vector analysis. Moreover, a prediction model is used to forecast the performance of the PaT. The results of these methodologies were validated using experimental results. Flow rate and...

The unsteady shape variation of a cavitating spiral vortex is experimentally studied pointing out the repetitive creation of vortex ring (or vortex loop). To generate the spiral vortex structure, a newly designed vortex generator is employed. This generator is used to study the decelerated swirling flow leading to the spiral vortex formation as a c...

The necessity of high pressures within the combustion chamber leads to the requirement of pressure systems in order to provide the correct operability of the thruster unit. Pressure-fed or turbopump-fed propulsion systems are commonly employed. The former type is simple and reliable but is limited by the maximum pressure available in the tank and h...

This paper analyses an energy recovery solution based on the Pump-as-Turbine (PaT) technology applied in an oil refinery. Since under-ground water might be contaminated by the chemical processes, an intervention named “Pump & Treat” was adopted in the studied oil refinery to reclaim the installation site and to safeguard the zones nearby. After the...

The added effects from the fluid on a structure submerged in water significantly affect its dynamic response. Since the hydraulic turbine runner is geometrically complex and involves complicated flow phenomena, the research on simple hydrofoil offers a unique opportunity to investigate added effects and mutual interaction of the elastic structure a...

This paper presents a comparison of a decomposition of unsteady vector fields, based on the principle of Shannon-Kotelnik theorem and singular decomposition methods. The article points out the relation between the discrete Fourier transform based on Kotelnik series and methods of singular Orthogonal Decomposition. It is used with success in the ima...

Vortex rope in Francis turbine draft tubes poses serious problems. Emphasis on operation of the turbines in wide operating range, quite far from the design point, is a challenge for both the turbine designers and power plant owners. While flow control becomes popular in aircraft and automotive industry, it is still feared in turbomachinery. Present...

Design of a pump turbine (ns = 135) is carried out using a very fast quasi-potential flow approach for the preliminary design. Theory behind this novel method is presented and its advantages discussed. Further finetuning is performed by coupling of CFD (ANSYS CFX) and particle swarm optimization method. The final design features suppression of the...

It is known that the cause of the instability of stationary vortex structures are the convective terms of the Navier-Stokes equations. Here, the formation of vortex structures is fundamentally influenced by the vector product of the velocity curl and the velocity vector. Spiral vortices are contingent on the zero value of this product and are one o...

It is a well-known fact and a much studied problematic that the performance of low-head hydraulic turbines is highly dependent on the runner-draft tube coupling. Around the optimal operating conditions, the efficiency of the turbine follows closely the performance of the draft tube that in turn depends on the velocity field exiting the runner. Henc...

This paper analyses an energy recovery solution for the application in an oil refinery, which has undergone significant changes in the last decades due to the construction of different plants, services, storage and logistics systems. Nowadays, with the aim to reduce the water contamination close to the refinery, the local regulations imposed to int...

Hydrodynamic cavitation represents complex physical phenomenon undesirably affecting operation as well as lifespan of many hydraulic machines from small valves to the large hydro power plants. On the other hand, the same phenomenon and its concomitants such as pressure pulsations can be exploited in many positive ways. One of them which seems to be...

Vortex rope, which induces substantial pressure pulsations, arises in the draft tube (diffuser) of Francis turbine for off-design operating conditions. Present paper focuses on mitigation of those pulsations using active water jet injection control. Several modifications of the original Susan-Resiga's idea were proposed. All modifications are drive...

Vortex rope, which induces substantial pressure pulsations, arises in the draft tube (diffuser) of Francis turbine for off-design operating conditions. Present paper focuses on mitigation of those pulsations using active water jet injection control. Several modifications of the original Susan-Resiga’s idea were proposed. All modifications are drive...

The swirling flow exiting the runner of a hydraulic turbine is further decelerated in the discharge cone of the draft tube to convert the excess of dynamic pressure into static pressure. When the turbine is operated far from the best efficiency regime, particularly at part load, the decelerated swirling flow develops a self-induced instability with...

Cavitation is usually considered as undesirable phenomena. On the other hand, it can be utilized in many applications. One of the technical applications is using cavitation in water treatment, where hydrodynamic cavitation seems to be effective way how to reduce cyanobacteria within large bulks of water. The main scope of this paper is investigatio...

This article deals with design and measurement of hydraulic recuperation unit. Recuperation unit consist of radial turbine and axial pump, which are coupled on the same shaft. Speed of shaft with impellers are 6000 1/min. For economic reasons, is design of recuperation unit performed using commercially manufactured propellers.

Spatio-temporal description of the cavitating flow around hydrofoil with 8 degrees incidence using proper orthogonal decomposition (POD) is presented. POD is a suitable tool, which provides information not only about the flow dynamics, but also about relevance of different flow structures. POD also enables to track energy transport within the domai...

The decelerated swirling flow often breaks down into helical structure which is unstable and causes unsteady velocity and pressure fields. The numerical and experimental investigation of this flow pattern is carried out on the experimental apparatus consisting of the swirl generator (source of strong swirling flow) and the conical diffuser (equippe...

Cavitating flow in converging-diverging nozzle (axisymmetric Venturi tube) is investigated. Characterization of the cavitating flow was done with high speed camera and the basic flow patterns were determined: incipient cavitation, partial cavitation, fully developed cavitation and supercavitation. Pressure transducers of tenzometric and piezoelectr...

Highly swirling flow leaves runner of the hydraulic turbine, which is operated off the best efficiency point. Instability of the swirling flow results in so called vortex rope. Rotating vortex rope is a source of pressure pulsations, causes structural loads, vibrations and noise. Proper orthogonal decomposition (POD) provides a spatio-temporal desc...

This paper discusses the comparison of the flow fields downstream of two different swirl generators. Both swirl generators are used to imitate the flow at the exit of the hydraulic turbine runner and study spatio-temporal behaviour of the swirling flow in the draft tube (i.e. outlet diffuser part of the hydraulic turbine), which undergoes breakdown...

The decelerated swirling flow often breaks down into helical structure which is unstable and causes unsteady velocity and pressure fields. The numerical and experimental investigation of this flow pattern is carried out on the experimental apparatus consisting of the swirl generator (producing a strong swirling flow) and the transparent conical dif...

Paper presents a computational study of energy transformation in two geometrical configurations of Kaplan turbine elbow draft tube. Pressure recovery, hydraulic efficiency and loss coefficient are evaluated for a series of flow rates and swirl numbers corresponding to operating regimes of the turbine. These integral characteristics are then correla...

Swirling flows are very susceptible to instabilities. Very often they are present in draft tubes of hydraulic turbines as so called vortex ropes resulting from spiral vortex breakdown. Moreover the situation can be complicated by cavitation within the core of the vortex. Paper presents various approaches, which were applied to enhance understanding...

Paper presents results of the experimental research and computational modeling aimed on cavitating vortical structures induced by swirling flow. An experimental circuit was built with swirl generator and converging-diverging nozzle made of plexiglass. Cavitating zone develops along the nozzle axis due to balance of pressure and centrifugal forces....

Vapor fraction appears in the vortex core, when the static pressure drops below saturated vapor pressure. Experimental circuit was built with swirl generator and plexiglass converging-diverging (CD) nozzle, which enables to study development of the cavitated vortical structures along the nozzle axis and in the separated boundary layers on the walls...