Doosan (Czechia)

ABSTRACT While tomography-based non-destructive testing (NDT) techniques have advanced in identifying inclusions in reinforced concrete (RC) structures, no existing method fully visualises sub-rebar objects with high accuracy. This study introduces the null subtraction imaging (NSI) technique, a novel nonlinear ultrasound beamforming algorithm, to identify sub-rebar objects located beneath rebar configurations. To evaluate the effectiveness of NSI, ultrasonic imaging was performed on an RC block that simulates a radioactive waste storage unit in a nuclear power plant (NPP). Compared to the traditional total focusing method (TFM), NSI improved the contrast-to-noise ratio (CNR) by approximately 48%, significantly suppressing artefact and background noise in the lateral direction. This capability enabled the clear identification of sub-rebar objects (drums) and precise differentiation of the interface between rebar layers and drums. The proposed NSI method also demonstrated high compatibility with other imaging modalities, such as ground penetrating radar (GPR), providing significantly enhanced image contrast and resolution of the internal structure of the RC block. These results highlight the feasibility of the NSI technique for assessing the health conditions of RC structures, particularly in managing critical components like hazardous chemical or radioactive waste storage units in NPPs.

Flow inside the axial test turbine VT-400 is measured by using a standard methodology of Particle Image Velocimetry (PIV). The studied area lies between the stator and rotor wheel in meridional plane. This area covers both: the regular flow near hub and the endwall secondary structures as well. Regular structure of wakes and jets past stator blades is observed in terms of mean velocities and turbulence intensity. The fluctuations are close to isotropy and the energy distribution across length-scales exhibits almost Kolmogorov scaling. Near the hub, we observe secondary vortices. Despite the positive radial ejections in that regions, these secondary vortices drift towards the hub, they dissipate energy and circulation and their core grows in size.

The feasibility study of Particle Image Velocimetry (PIV) measurements inside a test turbine at the University of West Bohemia. The current VT-400 turbine is not prepared for optical measurement with seeding particles, thus several technical issues had to be addressed until low-quality data were obtained only at low speed of 2000 RPM (rounds per minute). Even the low quality data are able to show the fluctuation anisotropy or the size of fluctuation structures, which are quantities not measurable by classical pressure methods.

Abstract A nuclear power plant also includes a secondary cycle, which main component is a steam turbine. The steam turbine processes the thermal and pressure energy of steam and converts it into mechanical energy. Reliable and safe operation of the steam turbine, thus of the entire block, is ensured by valves. The flow in a pair of control valves, where the steam flows through the valve chamber past the first valve to the second valve, is investigated in this paper. The experimentally determined flow characteristics of both valves were evaluated and results were generalized. The experiments were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences in Novy Knin where a wind tunnel for high speeds could be used. Data of the distribution of pressures in selected places were evaluated. The energy loss in the valves was compared with the loss in a separate diffuser with varying degrees of expansion. The influence of a valve strainer, which is a part situated upstream of the control valve cone, was also investigated. It was found out in which case and where there is a significant disturbance of the velocity flow field. The first valve in the chamber is more sensitive to pressure losses in contrast to the second valve which is comparable to an ideal design of the diffuser, which indicates that this part of the valve assembly is optimal. Regarding the strainers, it was found out that the strainer helps to balance the velocity ratios at the inlet to the valve which has a positive effect on reducing the total energy pressure loss. As a result, the loss in the first valve can be significantly reduced which helps to increase the efficiency of the whole turbine.

The paper deals with a flow in control valves of steam turbines and in classical diffusers. The effect of the cone lift and the change of the flow cross-section on the energy losses is investigated. Total loss coefficients for a valve with a strainer and a valve without a strainer are evaluated along with the losses for the operating characteristics of the turbine. The effect of a flow dissipation and a disordered unsteady velocity flow field in the valve on the increase in the loss coefficient for a given angle of diffuser opening is considered.

Abstract This paper is part of a two-part publication that aims to experimentally and numerically evaluate the aerodynamic and mechanical damping of a last stage ST blade at low load operation. A three-stage downscaled steam turbine with a snubbered last stage moving blade LSMB has been tested in the T10MW test facility of Doosan Skoda Power R&D Department in the context of the FLEXTURBINE European project (Flexible Fossil Power Plants for the Future Energy Market through new and advanced Turbine Technologies). Aerodynamic and flutter simulations of different low load conditions have been performed. The acquired data are used to validate the unsteady CFD approach for the prediction of the aerodynamic damping in terms of logarithmic decrement. Numerical results have been achieved through an upgraded version of the URANS CFD solver, selecting appropriate and robust numerical setups for the simulation of very low load conditions, such as increased condenser pressure at the exhaust hood outlet. The numerical methods for blade aerodamping estimation are based on the computation of the unsteady pressure response caused by the row vibration. They are usually classified in time-linearized, harmonic balance and non-linear approaches both in frequency and time domain. The validation of all these methods historically started in the field of aeronautical low-pressure turbines and has been gradually extended to compressor blades and steam turbine rows. For the analysis of a steam turbine last rotor blade operating at strong part load conditions, non-linear methods are recommended as these approaches are able to deal with strong nonlinear phenomena such as shock waves and massive flow separations inside the domain. Experimental data have been used to separate the contributions of mechanical and aerodynamic damping, extrapolating to zero mass flow the total measured damping. Finally, the comparisons between the aerodynamic damping coming from measurements and CFD results have been reported in order to highlight the capability to properly predict the last stage blade flutter stability at low load conditions. Such comparisons confirms the flutter free design of the new snubbered LSMB blade.

Rotor vibration attenuation is achieved with damping devices which work on different, often mutually coupled, physical principles. Squeeze film dampers are damping devices that have been widely used in rotordynamic applications. A new concept of a 5-segmented integral squeeze film damper, in which a flexure pivot tilting pad journal bearing is integrated, was investigated. The damper is studied for the eccentric position between the outer and inner ring of the squeeze film land. The ANSYS CFX software was used for solving the pressure and velocity distribution. The development of the complex three-dimensional computational fluid dynamics model of the squeeze film damper, learning more about the effect of the forces in the damper, and the knowledge about the behaviour of the flow are the principal contributions of this article.

The increasing demands on the flexibility of steam turbines due to the use of renewable energy sources substantially alters the fatigue strength requirements of components of these devices. This paper presents Thermo-Mechanical Fatigue (TMF) design calculations for the steam turbine shaft. The steam turbine shaft is exposed to complex thermo-mechanical loading conditions during the operating cycle of the turbine. An elastic-plastic structural Finite Element Analysis (FEA) of the turbine shaft is performed for the turbine operating cycle on the basis of calculated temperature fields obtained in a previous transient thermal FEA. The temperature dependent material parameters, which are used in the elastic-plastic FEA, are obtained from the uniaxial tests. Consequently, the TMF is predicted for the steam turbine shaft. Several fatigue criteria are used for the identifications of the critical domain and for the TMF damage assessment of the turbine shaft.