Undesirable flow phenomena in Francis turbines are caused by pressure fluctuations induced under conditions of low flow rate; the resulting vortex ropes with precession in the draft tube (DT) can degrade performance and increase the instability of turbine operations. To suppress these DT flow instabilities, flow deflectors, grooves, or other structures are often added to the DT into which air or water is injected. This preliminary study investigates the effects of anti-cavity fins on the suppression of vortex ropes in DTs without air injection. Unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted using a scale-adaptive simulation shear stress transport turbulence model to observe the unsteady internal flow and pressure characteristics by applying anti-cavity fins in the DT of a Francis turbine model. A vortex rope with precession was observed in the DT under conditions of low flow rate, and the anti-cavity fins were confirmed to affect the mitigation of the vortex rope. Moreover, at the low flow rate conditions under which the vortex rope developed, the application of anti-cavity fins was confirmed to reduce the maximum unsteady pressure.

Most nuclear and nonnuclear power plants have been designed in the frequency range of 2 to 10 Hz, but now, the design guidelines for structural and nonstructural components such as electrical cabinets must be improved by including high frequency greater than 10 Hz for sustainable energy. The electrical cabinet is the essential piece of equipment for safety functions and the uncertainty of seismic capability in power plants. Consequently, the attention of this study focused on evaluating the seismic demands of the electrical cabinet under high frequency earthquakes and also, seismic qualification of the electrical cabinet using the identification of experimental tests and numerical models. An experimental test based on ICC-ES AC 156 and IEEE std.344 was conducted for seismic qualification of the cabinet and then, a high-fidelity finite element model to capture the significant deformation was developed in this study. It is observed that the fundamental frequencies were 16 and 24 Hz from the experimental tests, respectively. In order to verify the proposed high-fidelity simulation model, the target fundamental frequencies of the cabinet were evaluated in the ABAQUS platform. It was interesting to note that the reconciliation of experimental and analytical results was extremely identical. Furthermore, in order to evaluate seismic response characteristics of the cabinet subjected to high and low frequency earthquakes, time history analysis was conducted in this study, using the ABAQUS platform. As a result, the observation showed that the seismic response of the cabinet system under a high frequency earthquake was relatively higher than that of low frequency. It can be very important to note that the cabinet system was sensitive to high frequency vibration.

Francis turbines are often used for generating hydroelectric power, but their performance characteristics significantly depend on the operating conditions. In particular, interblade vortices in the passages between runner blades can occur at low flowrates, which can degrade performance, and increase vibrations and instability during operation. In a previous study, we showed that the hydraulic performance and flow characteristics depend on the flow passage area of runner blades under low-flowrate conditions. Under such operating conditions, the runner blade thickness can affect the interblade vortex characteristics, and in turn, affect the performance of the turbine. In this study, we investigated the effect of runner blade thicknesses in the presence of interblade vortices under low flowrates; steady- and unsteady-state Reynolds-averaged Navier–Stokes equations were solved using a shear stress transport as a turbulence model. The interblade vortices were described well at the near leading and trailing edges near the hub. These vortex regions showed flow separation and stagnation flow, and the interblade vortex characteristics were dependent on the high-magnitude unsteady pressures at the low-frequency region. For the same guide vane opening, at lower flowrates, higher blockage ratios reduced interblade vortex formation and unsteady pressure.

Autostrainer is used for the purpose of debris removal in order to increase the efficiency of the heat exchanger by taking the required raw water as a heat source for the pre-cooling hydrothermal system. During the operation of the autostrainer, a pressure drop occurs due to the blockage of the screen in the autostrainer. As a result, the resistance of the pipe network for the intake system is changed, and the operating efficiency point of the pump, valve, heat exchanger, etc., is altered. By calculating the system resistance taking into account the pressure drop caused by the blockage rate of the screen in the autostrainer, the optimum operating efficiency can be expected when the intake system such as a pump, valve or heat exchanger, etc. is constructed. In this study, Computational Fluid Dynamics (CFD) was used to construct a scenario in which screen blockage may occur, predicting pressure drop for the slot cross-section of the screen in the autostrainer to derive a resistance coefficient value. The resistance coefficient value was applied to the porous region corresponding to the screen in the autostrainer’s 3D shape and compared with the experimental value for the pressure drop and headloss coefficient. By predicting the pressure drop for the autostrainer’s screen blockage rate of 0% to 50%, the coefficient of headloss required for the design of the intake system was calculated. Additionally, in order to predict the debris removal rate, which is the original role of the autostrainer, the debris was assumed to be particles, and sedimentation rate was predicted according to the size and weight of the particles. Building on this, when introducing the autostrainer used in pre-cooling into the membrane filtration process, due to the pressure loss caused by the inflow of debris during the use of the autostrainer, this study aims to utilize Computational Fluid Dynamics (CFD) to derive the head loss coefficients according to the screen blockage rate, and use these coefficients to calculate the system’s resistance curve. Additionally, in this study, the term “autostrainer” is used instead of the term “membrane strainer” to align with more popular terminology.