Romeo Susan-Resiga

In industrialized countries, existing regulations generally require the use of renewable energy to the greatest feasible extent. A major difficulty with renewable sources is the inherent fluctuation in their power output due to the main source character. By now, one of the best technologies capable of providing rapid compensation for these fluctuations is hydroelectric power. Hydropower plants, those equipped with hydraulic turbines with fixed blades (e.g., Francis, propeller) are typically designed to operate close to their best efficiency point (BEP) with acceptable load limits in the vicinity due to vibrations and pressure pulsations. Usually, the swirling flow exiting the runner is tailored for peak overall efficiency, which minimizes energy losses in the draft tube cone. When operating away from the design point, draft tube cone losses increase abruptly, and pronounce flow instabilities arise (e.g., vortex rope). This study proposes a new method to control such instabilities that inject a radial-axial water jet into the draft tube cone. Compared with conventional axial water jet injection, the radial-axial jet requires a lower additional flow rate while still effectively suppressing hydraulic instabilities in the draft tube cone. The carried-out analysis was done numerically by using Ansys Fluent 2023 R2. The performed 3D unsteady numerical simulations were carried out to examine the internal flow behavior and evaluate the effect of the radial-axial water jet injection on the unsteady behavior of the flow unsteadiness. Finally, the paper quantifies the relationship between the draft tube pressure fluctuation amplitude and the auxiliary flow rate needed to mitigate these instabilities.

The Francis runner hydrodynamic performance significantly deteriorates when operating far from the best efficiency point, at deep part load in particular. We show that by introducing a splitter band both the runner efficiency and the flow stability are improved. Essentially, by introducing a splitter band, shaped as an axisymmetric stream-surface, the classical Francis runner is split into an inner runner and an outer runner, respectively, operating in parallel with a common shaft. The numerical setup used for the present runner flow computation accounts for the fact that the flow provided by the turbine distributor, and further ingested by the inner and outer runners, has a practically constant total pressure while the static pressure at the runner outlet is continuous across the splitter band and satisfies the radial equilibrium condition. Numerical investigations are presented for discharge ranging from deep part load to nominal at constant runner head, quantifying the efficiency gain due to the splitter band. A detailed flow field analysis reveals the mechanisms for runner flow improvement when inserting the splitter band.

Optimizing the linear blade cascade geometry aims at achieving a prescribed flow deviation with minimum hydraulic losses. However, when using the simplified model of an inviscid incompressible fluid in steady irrotational flow one needs a different objective function to be minimized. In this paper we introduce an inviscid optimization method for two-dimensional blade cascades whereby the integral magnitude of the strain rate is minimized. We show that, within the framework of streamfunction formulation and finite element solver, this objective function is evaluated with negligible aditional computational costs. For given flow direction upstream and downstream, a cascade pitch and a blade thickness distribution we optimize the blade camberline and simultaneously determine its stagger angle. Numerical examples show that such inviscid optimal blade geometries also achieve lower hydraulic losses when assessed in viscous turbulent flow. Being numerically tractable, this method can be employed for multi-point optimization which is essential in design hydraulic turbines for a wide operating range.

We develop a novel methodology for investigating numerically the complex hydrodynamics near the tip of Kaplan runner blades, aiming at developing techniques for mitigating cavitation development at full load operation regime. First, a two-dimensional swirling flow analysis in the meridian half plane of the flow passage is used to define a streamtube bounded on one side by the guide vanes lower ring and the runner chamber and on the other side by an axisymmetric streamsurface. Second, this restricted three-dimensional computational domain allows us to focus on flow details corresponding to guide vanes overhanging at full opening, variable tip gap of the runner blades, as well as on the strong interactions between guide vanes wakes and runner blades. We present an in-depth analysis of the flow field and conclude that re-shaping the lower ring of the guide vanes and the runner chamber wall within the framework of Minimum Gap Runner (MGR) technology should be considered for large Kaplan turbines.

The current requirements of industrialized countries require the use of as much renewable energy as possible. One significant problem with renewable energy is that the produced power fluctuates. Currently, the only method available for energy compensation in the shortest time is given by hydroelectric power plants. Instead, hydroelectric power plants (especially the plants equipped with hydraulic turbines with fixed blades) are designed to operate in the vicinity of the optimal operating point with a maximum ±10% deviation. The energy market requires that hydraulic turbines operate in an increasingly wide area between −35% to 20% from the optimum operating point. Operation of hydraulic turbines far from the optimum operating point involves the appearance downstream of the turbine of a decelerated swirling flow with hydraulic instabilities (known in the literature as the vortex rope). The main purpose of this paper is to investigate numerically a new concept by using a free runner downstream on the main hydraulic runner turbine more precisely in the draft tube cone. The free runner concept requires rotations at the runaway speed with vanishing mechanical torque. The main purpose is to redistribute the total pressure and the moment between the shaft and the periphery. In addition, the free runner does not modify the operating point of the main hydraulic turbine runner.

This paper represents a welcome synthesis of the results obtained by the authors over more than a decade. The reason why such an approach is perfectly justified is found in the novelty of the control techniques of decelerated swirling flows from the conical diffuser of hydraulic turbines. The results presented in this paper refer strictly to the new passive control techniques of the swirling flows instabilities from the conical diffuser of hydraulic turbines. Although the results of these new techniques have been disseminated in various papers, it is difficult to outline an overview from a collection of articles. In addition, a lot of valuable information about modern experimental and numerical investigations is not found in articles that usually distill only the most significant results. Therefore, the present paper achieves a welcome unitary synthesis, useful to specialists in the field of turbomachine hydrodynamics. The reluctance of the turbine manufacturers on active control techniques that use external/additional energy sources led us to the choice of passive control techniques review, especially the ones developed in the last years. The first part of the paper analyzes the specialized literature that includes a variety of passive solutions for mitigating self-induced instabilities of decelerated swirling flow downstream of hydraulic turbines. Such inherent instabilities manifest intensely at far from optimal operating regimes and represent one of the challenges of modern hydraulic turbines. The mitigation of these instabilities is an open problem, so far there are no unanimously accepted technical solutions implemented on prototype turbines. The second part of the paper includes detailed investigations on axial water injection with flow-feedback, but also more recent approaches using adjustable diaphragm in the conical diffuser.

Within the framework of the classical turbine design setup, we present a numerical investigation of unsteady cavitating flow in tandem cascades of guide vanes and rotor blades. The practical relevance of the present investigation is ensured by choosing real blade profiles and domain geometry for the large Kaplan turbines (9.5 m rotor diameter) from the Iron Gates I hydropower plant on the Danube River. The 2D tandem cascade setup is obtained with a conformal mapping of the circular guide vane cascade into a straight cascade, while the rotor blade cascade is by default a straight one for axial turbines. Both single-phase and two-phase cavitating flows results are presented and analysed. The single-phase liquid flow displays rather modest pressure and tangential force fluctuations, with the characteristic blade passing frequency, due to the interaction of the rotor blades with the wakes of guide vanes. The two-phase cavitating flow displays a completely different dynamics in comparison to the liquid flow, with periodic cavitation growth and collapse at a specific frequency lower than the blade-passing one. Moreover, the tangential force on the rotor blade displays severe intermittency and the force fluctuations are almost one order of magnitude larger than in the single-phase flow.

The present study examines the possibility of using an industrial stirred chemical reactor, originally employed for liquid–liquid mixtures, for operating with two-phase liquid–solid suspensions. It is critical when obtaining a high-quality chemical product that the solid phase remains suspended in the liquid phase long enough that the chemical reaction takes place. The impeller was designed for the preparation of a chemical product with a prescribed composition. The present study aims at finding, using a numerical simulation analysis, if the performance of the original impeller is suitable for obtaining a new chemical product with a different composition. The Eulerian multiphase model was employed along with the renormalization (RNG) k-ε turbulence model to simulate liquid–solid flow with a free surface in a stirred tank. A sliding-mesh approach was used to model the impeller rotation with the commercial CFD code, FLUENT. The results obtained underline that 25% to 40% of the solid phase is sedimented on the lower part of the reactor, depending on the initial conditions. It results that the impeller does not perform as needed; hence, the suspension time of the solid phase is not long enough for the chemical reaction to be properly completed.

The paper explores a tandem configuration of three-blade impellers in a stirred reactor. The working fluid is a liquid-solid mixture and the stirring mechanism fitted with the two impellers must prevent the sedimentation of solid particles while homogenously dispersing them in the bulk liquid. The present numerical investigation, performed with the expert software Ansys^® Fluent, Release 16, employs the Eulerian multiphase model along with the RNG k–ε turbulence model to simulate the free-surface liquid–solid flows in the baffled stirred reactor. A sliding mesh approach is used to model the impellers rotation. The tandem configuration is clearly superior to a single impeller, while the existing electrical motor that drives the stirring mechanism still provides the necessary power.

The decelerated swirling flow in the discharge cone of Francis turbines operated at partial discharge, far from the best efficiency point, develops self-induced instabilities featuring a precessing helical vortex (so-called vortex rope) which hinders the stable and safe turbine operation. This is an intrinsic characteristic of the swirl exiting the Francis runner, which at part load has a relatively large residual flux of moment of momentum as well as an imbalanced specific hydraulic energy with an excess near the band. We address in this paper the question of how one should alter this swirling flow in order to mitigate further instabilities in the discharge cone. In doing so, we consider an actuator disk located in the upstream part of the discharge cone that model a runaway runner which alter the hub-to-shroud angular momentum and specific head distributions without altering the turbine operating point. However, such a runaway runner provides a swirl stabilisation further downstream thereby effectively mitigating the vortex rope.