Sebastian Muntean

• PhD.
• Senior Researcher at Romanian Academy

We perform a numerical analysis of the decelerated swirling flow into the discharge cone of a model Francis turbine operated at variable discharge and constant head, using an axisymmetric turbulent swirling flow model and a corresponding simplified computational domain. Inlet boundary conditions correspond to velocity and turbulent kinetic energy p...

Operating Francis turbines at partial discharge is often hindered by the development of the helical vortex (so-called vortex rope) downstream the runner, in the draft tube cone. The unsteady pressure field induced by the precessing vortex rope may also lead to hydro-acoustic resonance. We introduce in this paper a novel, simple and robust, method t...

The variable demand on the energy market requires great flexibility in operating hydraulic turbines. Therefore, turbines are frequently operated over an extended range of regimes. Francis turbines operating at partial load present pressure fluctuations due to the vortex rope in the draft tube cone. This phenomenon generates strong vibrations and no...

The paper reviews recent research and development activities in the field of hydropower technology. It covers emerging and advanced technologies to mitigate flow instabilities (active and passive approach) as well as emerging magneto-rheological control techniques. Recent research findings on flow instabilities are also presented , especially conce...

About 50% of all hydropower plants (HPPs) worldwide were originally commissioned more than 40 years ago, so that the advanced age of the fleet is a major concern across all continents, and especially in Europe. The modernization of HPPs can generate several benefits in terms of generation, flexibility, safety, operation, and may have neutral or eve...

This study examines recent energy transition policies and the conceptual framework of energy justice, emphasizing the role of hydropower, in ensuring the equitable distribution of energy services and burdens. The analysis focuses on the EU’s energy mix, highlighting the capacity of hydropower and its impact on energy justice, and examines the relat...

This chapter presents the evolution of research in Romania on the mechanics (the mechanics of rigid solids, deformable solids in elastic, elastoplastic or plastic behavior regimes, the theory of mechanical vibrations, fluid mechanics) but also closely related problems such as the theory of mechanisms, contact mechanics, construction of hydraulic eq...

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 ne...

There is a hidden energy potential in existing hydropower plants, and this was assessed for Europe. • Heat recovery from generators can reach 5 TWh/y, and can be used either for powerhouse heating or for district heating. • The harvesting of the tailrace water energy can increase European generation by 0.43 %, equivalent to thousands of micro hydro...

The present paper introduces the creation of an algorithm and the software used to determine the energetic performance and monitor the efficiency of hydraulic pumps working in various industrial applications, such as water supply systems, water treatment processes, and irrigation systems, particularly in the cases where there is no permanent monito...

Editorial of special issue in Sustainability journal

Water hammer shortens hydraulic passage lifespan and may cause sudden failure. The primary goal is to use a hierarchical approach to assess the main parameters associated with water hammer. This will help investigate their influence and assist in decision making. Analytical calculation results and a numerical model are compared against experimental...

Water treatment stations (WTSs) provide drinking water to the community and are critical infrastructures of any village or city. The main energy consumption of WTSs is associated with the operation of the pumping units installed in the water supply stations (WSSs). The parameters of the pumping units installed in the WSSs are continuously adjusted...

Water hammer influences the life cycle of hydraulic passages and may even cause catastrophic structural failures. Several catastrophic failures of hydraulic power units have been reported in the literature due to the effects of transient regimes. The objective of the study is to highlight the global trend in water hammer assessment and to quantify...

The paper focuses on design considerations and experimental investigations of a MRB with cylindrical configuration. The design considerations for MRB with cylindrical configuration are presented. The magneto-rheological torque contribution is estimated at the design stage based on geometric configuration and the proprieties of the magneto-rheologic...

The self-induced instabilities developed in decelerated swirling flows lead to pressure fluctuations. The swirling flows generated under variable speeds of the rotor of a swirl generator using a magneto-rheological brake are numerically and experimentally investigated. Three-dimensional turbulent flow computation is performed for several rotor spee...

The induced flow instabilities are developed in the draft tube cone of the hydraulic turbines leading to the pressure fluctuations with negative impact on the mechanical components lifetime. Usually, two components (rotating and plunging) are discriminated in the unsteady pressure field induced by the swirling flow in the draft tube cone. The rotat...

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...

The paper focuses on the influence of the reshaped elbow geometry of the draft tube on the unsteady pressure field under different operating conditions. The experimental investigations are conducted considering two simplified geometrical configurations of the draft tubes with sharp heel elbow and modified/reshaped elbow, respectively. The discrimin...

The paper presents O. Popa's analytical approach and the flow numerical methodology developed to determine the airfoil performances. Both methods are applied to determine the performances induced by flow over S shape airfoil for wide flow angles of attack (AoA). Also, the experimental investigations are conducted on S shape airfoil in the wind tunn...

In this paper, we introduce a novel passive control method to mitigate the unsteadiness effects associated to the swirling flows with self-induced instabilities. The control method involves a progressive throttling cross-section flow at the outlet of the conical diffuser. It adjusts the cross-section area with a diaphragm while maintaining all posi...

The pump inlet casing deflects the fluid flow from the inlet pipe, mainly arranged normally to the axis, into the axial direction. The pump inlet casing can take a large variety of geometrical shapes from curved pipes to three-dimensional elbows. The hydrodynamic field induced by symmetrical suction elbow (SSE) at the pump inlet is experimentally i...

In the last two decades the energy market policy was focused on development of renewable energy. The renewable energy (solar and wind power) induces a fluctuating component of the electrical grid. The solution to compensate fluctuating energy is provided by the hydraulic turbines in order to produce energy in a short time and the pumps units in ord...

Purpose The purpose of this paper is to focus on the structural integrity of the rainwater propeller pumps installed in the municipal wastewater treatment plant (WTP). Design/methodology/approach A numerical analysis is performed to determine the maximum shear stress on the fasten bolts. The rainwater propeller pump is examined in operation at n...

In hydropower systems among hydropower plants there are storage pump units. In order to ensure higher flow rate, the pumps have constructive differences besides regular. Consequently, the complex shape of the suction-elbow with symmetric inlet generates an unsteady flow which is ingested by impeller. These phenomena also generate stronger unsteady...

It is well known that the spiral vortex (or vortex rope) is developed in the conical diffuser of hydraulic turbines with fixed pitch blades (e.g. Francis and propeller) when operating conditions are far away from best efficiency point. The paper will present a new technique in order to mitigate, even eliminate the bad behave associated to this phen...

We introduce a new benchmark test case for assessing the robustness and accuracy of various numerical methodologies for simulating swirling flows in pipes and step diffusers. First we present the design and detailed geometry of the swirl generator and downstream test section. The swirl is generated with fixed plane blades, parallel to the symmetry...

Hydraulic turbines operating at off-design regimes inherently have a large residual swirl at runner outlet. This swirling flow is further ingested by the draft tube and its deceleration in the discharge cone results in self-induced instabilities (e.g. precessing vortex rope, low frequency pressure pulsations, power swing) that may hinder the turbin...

A strategy of urban sewage centralized management is implemented in the most cities (e.g. Timisoara). All wastewater is collected and conveyed to a central location for treatment or disposal. In the urban area, the rainwater is embedded in the wastewater management. Seven rainwater pumps are installed in a wastewater treatment plant to protect it a...

A magneto-rheological clutch was designed and built in order to modify the speed of an axial rotor mounted in front of a centrifugal pump. The main goal by modifying the speed of the axial rotor is to increase the operating regimes with less cavitation and to uniform the flow at the inlet of the pump impeller. The magneto-rheological clutch is test...

A magneto-rheological brake (MRB) is designed and embedded in a swirl generator apparatus in order to control the runner speed. Several swirling flow configurations are obtained slowing down the runner speed. The main challenge for MRB is associated with its operation under water conditions. As a result, two magneto-rheological fluids (a convention...

The paper presents the results of numerical investigation for stress distribution near runner blade-crown/band junctions. Starting with the geometrical investigation of the geometries for a real turbine, different junction geometries were identified. Eight different junction geometries were numerically investigated in order to determine the stress...

The rainwater axial pumps are specially designed to operate in municipal wastewater systems. The fibres and wastes are discharged during storms with this type of pumps. Several problems have occurred in service due to the fibres clogging up the impeller. Therefore, the chemical compositions of the waste, microscopic analyses, tensile and shearing t...

The paper focus on the numerical investigation of a pump impeller combined with an axial rotor for a double-flux pump storage. The axial rotor is investigated at variable speed between 2000 and 3000 rpm with a step size of 100 rpm, while the speed of the pump impeller is maintained constant at 2500 rpm. An improved cavitational behaviour is obtaine...

The purpose of this paper is to examine the failure problems encountered during the operation of the rainwater pump impellers installed in a wastewater station. The rainwater pumps are specially designed to operate in wastewater plants. The fibbers and wastes are discharged together with rainwater during storms with this type of pumps to avoid the...

The hydraulic fluctuations associated with partial load operating conditions of Francis turbines are often periodic and characterized by the presence of a vortex rope. Two types of pressure fluctuations associated with the draft tube surge are identified in the literature. The first is an asynchronous (rotating) pressure fluctuation due to the prec...

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...

The unsteady pressure at the wall of the conical diffuser without and with a 74° heel elbow is examined. The self-induced instability is experimentally investigated on the swirl generator test rig. As a result, the asynchronous (rotating) pressure pulsation associated with the vortex rope and it second harmonic are discriminated. The discriminated...

Currently there is a growing demand for renewable energy in the industrialized countries. However, these sources (e.g. solar, wind) introduce a fluctuating component within the electrical grid. Therefore, the hydraulic turbines have to operate far away from the best efficiency point (BEP) covering a wide range. As a result, the self-induced instabi...

An experimental model of a magneto-rheological brake (MRB) was designed, developed and built in order to analyze in detail different brake and clutch types models. Also the test rig serves to test different MR fluids available on the market as well as several MR fluids developed and characterized in our laboratory. The preliminary test rig consists...

In hydropower systems among hydropower plants there are integrated pumping stations (PS). In order to ensure higher flow rate, the pumps have constructive differences besides regular. Consequently, the complex shape of the suction-elbow with symmetric inlet generates an unsteady flow which is ingested by impeller. These phenomena's also generate st...

The fluid engineering problems require tailored numerical computation algorithms and deep mathematical insight. The computation of swirling flows has been an intense field of investigation during the last three decades. Swirling flows may develop stagnant regions separated from the main flow by an interface that is not a-priori known. The interface...

The paper investigates the hydrodynamic field generated by the symmetrical suction elbow at the pump impeller inlet. The full three-dimensional turbulent numerical investigation of the flow in the symmetrical suction elbow is performed using FLUENT then the flow non-uniformity generated by it is numerically computed. The numerical results on the an...

The hydraulic turbines operated at partial discharge (especially hydraulic turbines with fixed blades, i.e. Francis turbine), developing a swirling flow in the conical diffuser of draft tube. As a result, the helical vortex breakdown, also known in the literature as "precessing vortex rope" is developed. A passive method to mitigate the pressure pu...

The paper investigates an unexpected feature of the unsteady pressure field resulting from the self-induced instability of the decelerated swirling flow in a straight diffuser. Firstly, the self-induced instability is experimentally investigated on the swirl generator test rig. As a result, the asynchronous (rotating) pressure pulsation associated...

In hydropower systems among hydropower plants there are integrated pumping stations (PS). In order to ensure higher flow rate, the pumps have constructive differences besides regular. Consequently, the complex shape of the suction-elbow with symmetric inlet generates an unsteady flow which is ingested by impeller. These phenomena's also generate st...

Modern hydraulic turbines require optimized runners within a range of operating points with respect to minimum weighted average draft tube losses and/or flow instabilities. Tractable optimization methodologies must include realistic estimations of the swirling flow exiting the runner and further ingested by the draft tube, prior to runner design. T...

The flexible operation of the large hydraulic pumps requires to be run at variable discharges and speeds. The large pumps are equipped with a suction elbow which generates non-uniform flow at the impeller inlet leading to unsteady hydrodynamic phenomena. The experimental investigation presented in this paper is focused on unsteady pressure measurem...

Operating Francis turbine at part load is hindered by the flow instabilities developed downstream the runner, in the draft tube cone. The unsteady pressure field induced by the flow instabilities leads to vibrations. The paper presents the experimental investigations of the unsteady pressure field generated by the so-called vortex rope and its asso...

The flow unsteadiness generated in a swirl apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator which includes two blade rows, one stationary and one rotating, is designed such that the emanating flow at free runner rotational speed resembles that of a...

This paper introduces a numerical methodology based on a 3D turbulent flow computation developed to assess the aerodynamic performances of lumber drying kilns. The numerical results are validated against experimental data obtained by applying five different fan speeds and reversible airflow. The distributions of the energy loss coefficient and of t...

Operating hydraulic turbines over an extended range of discharge values requires an optimized runner that minimizes the weighted-average draft tube losses. It is shown that the draft tube losses can be minimized before actually designing the (new or refurbished) runner blades by optimizing the runner outlet flow. For this, a new runner surrogate mo...

The energy market imposes new requirements in hydraulic turbines operation. Usually, the old hydraulic turbines are not designed to meet these new requirements. Therefore, the refurbished solutions for hydraulic turbines are expected to be robust and flexible in operation in order to regulate the grid. A methodology is developed for a hydropower pl...

The paper focuses on numerical analysis of temperature field into a magneto-rheological break. The magneto-rheological fluid is used for a brake operated at different speeds. The magneto-rheological brake application was developed due to the magnetic control and fast response. The two dimensional axi-symmetric computational domain corresponds to th...

Decelerated swirling flows in conical diffusers can develop self-induced instabilities for some configurations of incoming flow. As a result, an upstream steady and axis-symmetrical flow becomes unsteady and three-dimensional with associated pressure fluctuations that hinder the operation of tubomachines. This paper introduces a new approach for mi...

Parallel computational fluid dynamics simulations produce vast amount of numerical result data. This paper introduces a method for reducing the size of the data by replaying the interprocessor traffic. The results are recovered only in certain regions of interest configured by the user. A known test case is used for several mesh partitioning scenar...

The paper presents an example of applying the theory for pipe swirling flow with stagnant region to the axial expansion turbines. It is shown that for most operating points examined the hub radius exceeds the stagnant region radius, thus the flow detachment from the hub is avoided. However, for the full-load operating points (large discharge and he...

The paper investigates numerically the flow non-uniformity generated by a symmetrical suction elbow at the large pumps inlet. The geometry of suction elbows, numerical setup and boundary conditions are presented. Two test cases available in storage power plants from Romania and Switzerland are considered. The numerical results for the first case (a...

The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming practically steady axi-symmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresse...

We introduce a variational formulation for computing swirling flow states with stagnant regions in a pipe. The new extended flow force functional is maximized with respect to the location of the stagnant region boundary, leading to a swirling flow state with minimum swirl number. By allowing a jump in velocity across the vortex sheet that bounds th...

The paper presents the hydrodynamic design of a new impeller using inverse method for one single stage double-flux pumped storage. The new impeller is designed for an existing hydraulic passage using inverse method. The old and new impellers are experimentally investigated without and with an inducer in order to assess the global performances. An i...

We introduce and validate a mathematical model, derived from the turbomachinery throughflow theory, for computing the swirling flow exiting a Francis turbine runner operated within a range of discharge values. This swirling flow, further ingested by the draft tube, is used as inlet condition for numerically assessing the draft tube performance for...

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...

Please note that terms and conditions apply. LDV measurements of the velocity field on the inlet section of a pumped storage equipped with a symmetrical suction elbow for variable discharge values Abstract. The storage pumps are equipped with various types of inlet casings. The flow non-uniformity is generated by the suction elbows being ingested b...

The decelerated swirling flow in the draft tube cone of hydraulic turbines (especially turbines with fixed blades) is responsible for self-induced instabilities which generates pressure pulsations that hinder the turbine operation. An experimental test rig was developed in order to investigate the flow instabilities. A new method was implemented to...

The decelerated swirling flow in the discharge cone of hydraulic turbine develops various self-induced instabilities and associated low frequency phenomena when the turbine is operated far from the best efficiency regime. In particular, the precessing helical vortex ("vortex rope") developed at part-load regimes is notoriously difficult and expensi...

We introduce a quasi two-dimensional (Q2D) methodology for assessing the swirling flow exiting the runner of hydraulic turbines at arbitrary operating points, within a wide operating range. The Q2D model does not need actual runner computations, and as a result it represents a surrogate runner model for a-priori assessment of the swirling flow inge...

The flow unsteadiness generated in the draft tube cone of hydraulic turbines affects the turbine operation. Therefore, several swirling flow configurations are investigated using a swirling apparatus in order to explore the unsteady phenomena. The swirl apparatus has two parts: the swirl generator and the test section. The swirl generator includes...

This paper aims to experimentally investigate the non-uniform velocity field on the suction elbow outlet of the large pumps. The suction elbow has a three-dimensional symmetrical shape that induces non-uniform hydrodynamic field at the impeller inlet. The velocity field is measured using LDV system in order to be quantified the non-uniformities gen...

The paper presents a methodology for reducing the hydraulic losses in the draft tube of hydraulic turbines within a wide operating range, from part-load to full-load. The velocity field at the draft tube inlet is parameterized by taking into account the runner blade geometry at the trailing edge via the so called swirl-free velocity. It is shown th...

The paper is focused on failure assessment of the shaft of a pumped storage unit. The shaft failure occurred during normal operation of the hydropower unit after 35 years of service. Three different methods are applied in order to assess its lifetime. Firstly, a finite element analysis was performed in order to obtain the static stress distribution...

The paper presents preliminary results of a work in progress addressing the hydrodynamic stability of swirling flows problems in ducts with variable radius which imply mathematical modeling, dynamic and stability investigations. The proposed quasi-analytical method aims to obtain the velocity profiles with a low order approximation method of which...

The paper presents the optimization procedure of a pump inducer taking into account the cavitation criterion. The pump inducer optimization procedure is based on inverse design method where the inflow and outflow conditions are known. Axial flow is considered as upstream condition while the downstream swirling flow is suited for the pump impeller i...

Numerical simulations produce large amounts of data. A key challenge is how to cope with the resulting bottlenecks. This paper introduces a highly efficient data handling method using parallel inter-processor traffic, called TRC. It is used to reconstruct the internal fields inside one or more regions of interest (ROIs) defined by the user. The met...

Operating Francis turbines over extended range is often hindered by the flow instabilities developed downstream the runner, in the draft tube cone. The unsteady pressure field induced by flow instabilities leads to pressure fluctuations. The paper presents the experimental investigations of the unsteady pressure field generated by flow instabilitie...

Developments of model reduction techniques for fluid dynamics applications have been widely analyzed, motivated mainly by the need to develop low costs effective computational models capturing as much of the flow physics as possible. In this paper, we investigate the efficiency of the proper orthogonal method's application in the reconstruction of...

The paper focuses on numerical analysis of hydrodynamic behavior in a discharge cone from a swirling flow generator when is operated at lower runner speeds. The swirl generator is used in laboratory instead of a turbine model in order to investigate different swirling flow configurations into a straight draft tube. The purpose for reducing the spee...

There are three resources currently used to mass-produce electricity: coal, nuclear and hydro. Hydroelectricity is a viable resource for production today. Being renewable, delivering system services and producing the cheapest electricity possible, these are facts that recommend hydroelectricity today. Often power plants work together with storage p...

We examine the application of a polynomials based spectral technique to approximate the ordinary differential equation with non-constant coefficients defining the eigenvalue problem governing the stability of the basic flow is a swirling flow problem with applications in turbomachinery. The method is based on Laguerre functions expansions on semi-i...