Search

A reactor coolant pump (RCP) is essential for transporting coolant in the primary loop of pressurized water reactors. In the advanced passive reactor, the absence of a long pipeline between the steam generator and RCP serves as a transition section, resulting in a non-uniform flow field at the pump inlet. Therefore, the characteristics of the pump should be investigated under non-uniform flow to determine its influence on the pump. In this study, the pressure pulsation characteristics were examined in the time and frequency domains, and the sources of low-frequency and high-amplitude signals were analyzed using wavelet coherence analysis and numerical simulation. From computational fluid dynamics (CFD) results, non-uniform inflow has a great effect on the flow structures in the pump's inlet. The pressure pulsation in the pump at the rated flow increased by 78–128.7% under the non-uniform inflow condition in comparison with that observed under the uniform inflow condition. Furthermore, a low-frequency signal with a high amplitude was observed, whose energy increased significantly under non-uniform flow. The wavelet coherence and CFD analysis verified that the source of this signal was the low-frequency pulsating vortex under the steam generator.

Venturi tube is based on turbulent flow, whereby the microbubbles can be generated by the turbulent fragmentation. This phenomenon is common in several venturi bubblers used by the nuclear, aerospace and chemical industries. The first objective of this paper is to study the liquid-phase velocity field experimentally and develop correlations for the turbulent quantities. The second objective is to research velocity field characteristics theoretically. Stereoscopic PIV measurements for the velocity field have been analyzed and utilized to develop the turbulent kinetic energy in the venturi tube. The tracking properties of the tracer particles have been verified enough for us to analyze the turbulence field. The turbulence kinetic energy has a bimodal distribution trend. Also, the results of turbulence intensity along the horizontal direction is gradually uniform along the downstream. Both the mean velocity and the fluctuation velocity are proportional to the Reynolds number. Besides, the distribution trend of the mean velocity and the velocity fluctuation can be determined by the geometric parameters of the venturi tube. An analytical function model for the flow field has been developed to obtain the approximate analytical solutions. Good agreement is observed between the model predictions and experimental data.

AbstractThe gas-liquid separator is a key component in the gas removal system of the Thorium Molten Salt Reactor. Phase separation is driven by a swirling flow, and the fundamental principle is that dispersed bubbles are accumulated and coalesced into an air core to realize separation from the liquid phase. In this paper, simultaneous particle image velocimetry (PIV) and pulsed shadowgraphy techniques are applied to characterize the two-phase-flow patterns in the evolutionary process of the air core. The PIV technique utilizes fluorescent particles as tracers in the liquid flow field, and a charge coupled device (CCD) camera records the planar laser-induced fluorescence signal of the particles. Another camera simultaneously detects the shadow and motion of the air core via backlighting from an array of infrared light-emitting diodes. The signals originating from the different phases are separated by a beam splitter with a dichroic filter and optical filters, and only undisturbed signals from the shadow of the air core and fluorescence tracer particles of the fluid are effectively captured by the two CCD cameras, respectively. Experimental data are carried out for three Reynolds numbers Re for a range of outlet pressures Pout. The morphology of the air core tail periodically transforms from a linear type to a single-helix type to a double-helix type before reaching a stable state at the critical outlet pressures Pcout. The analysis of gas-liquid flow patterns indeed indicates that axial velocity has a strong influence on the air core evolution. The periodic fluctuation results from the magnitude and direction of axial velocity.

AbstractThe Venturi-type bubble generator proposed by Oak Ridge National Laboratory is used to produce micro-sized bubbles. In this paper, a numerical simulation is carried out to study the process of the bubble formation and detachment from a wall orifice of a Venturi-type bubble generator in a cross-flowing liquid. The Volume-of-Fluid (VOF) method is applied to track the two-phase interface and study the evolution of the bubble formation. The result of the computation provides a visual three-dimensional bubble and shows the process of bubble formation and detachment. Three stages are identified during bubble formation (the expansion stage, the rising stage, and the collapse stage). Because of the compressibility of the gas, a fluctuation of the pressure and the mass flow rate in the gas chamber is observed, which is considered a significant effect on the bubble formation and detachment. The mechanism of the bubble detachment is clarified with the help of the mass flow rate fluctuation. The equivalent diameter is compared with that predicted by a previous model. A Coupled Level Set Volume-of-Fluid (CLSVOF) simulation is compared with the VOF simulation; the three stages and the fluctuation are also observed in the CLSVOF simulation.

In this paper, the numerical simulation for the gas–liquid flow in a separator applied in the fission gas removal system for thorium molten salt reactor was investigated. The numerical model was established in the frame of Eulerian–Eulerian approach, in which the modeling of the forces acting on the bubbles was introduced. Based on the model, numerical simulations with three flow rates were carried out. Three key parameters (the pressure loss, the separation length, the liquid entrainment ratio) concerned with the separation performance were compared between the numerical results and the experimental data, the results indicate that the calculated results agree well with the experimental data. Hence, the numerical approach shows a promising tool for the performance prediction and the optimization of the gas–liquid separator.

Instability of pump turbine with S-shaped curve is characterized by large fluctuations of rotational speed during the transient processes. For investigating this phenomenon, a numerical model based on the dynamic sliding mesh method (DSSM) is presented and used to numerically solve the 3D transient flow which is characterized by the variable rotation speed of runner. The method is validated by comparison with measured data for a load rejection process in a prototype pump turbine. The results show that the calculated rotation speed agrees well with the experimental data. Based on the validated model, simulations were performed for the runaway process using an artificially assumed operating condition under which the unstable rotation speed is expected to appear. The results confirm that the instability of runner rotational speed can be effectively captured with the proposed method. Presented results include the time history profiles of unit flow rate and unit rotating speed. The internal flow characteristics in a typical unstable period are discussed in detail and the mechanism of the unstable hydraulic phenomenon is explained. Overall, the results suggest that the method presented here can be a viable alternative to predict the dynamic characteristics of pump turbines during transient processes.

The 3D inverse design method, which methodology is far superior to the conventional design method that based on geometrical description, is gradually applied in pump blade design. However, no complete description about the method is outlined. Also, there are no general rules available to set the two important input parameters, blade loading distribution and stacking condition. In this sense, the basic theory and the mechanism why the design method can suppress the formation of secondary flow are summarized. And also, several typical pump design cases with different specific speeds ranging from centrifugal pump to axial pump are surveyed. The results indicates that, for centrifugal pump and mixed pump or turbine, the ratio of blade loading on the hub to that on the shroud is more than unit in the fore part of the blade, whereas in the aft part, the ratio is decreased to satisfy the same wrap angle for hub and shroud. And the choice of blade loading type depends on the balancing of efficiency and cavitation. If the cavitation is more weighted, the better choice is aft-loaded, otherwise, the fore-loaded or mid-loaded is preferable to improve the efficiency. The stacking condition, which is an auxiliary to suppress the secondary flow, can have great effect on the jet-wake outflow and the operation range for pump. Ultimately, how to link the design method to modern optimization techniques is illustrated. With the know-how design methodology and the know-how systematic optimization approach, the application of optimization design is promising for engineering. This paper summarizes the 3D inverse design method systematically.

Abstract: The performance of a reversible pump turbine with S-shaped characteristics is of great importance to the transition processes such as start-up and load rejection. In order to predict the S-shaped curve accurately and develop a reliable tool for design improvement, a shear stress transport model (SST) with various numerical schemes for pressure term in the governing equation was investigated in a whole pump turbine including spiral casing, stay vanes, guide vanes, runner and draft tube. Through the computation, it was shown that different zones in the curve should employ different schemes to get the solution converged. Comparison of discharge-speed performance showed that good correspondence is got between experimental data and CFD results. Based on this, internal flow analysis was carried out at three typical operating points representing turbine mode, shut-off mode and reversible pump mode, respectively. According to the flow field concerned, the mechanism for the speed-no-load instability was explained, which provides good guidelines to take countermeasures in future design.

Abstract: Vortex diode, as an important component in power fluidics, has been used in nuclear reprocessing engineering where it is desirable to avoid pumps with moving parts. The performance of the fluidic system depends on the matched design parameters of vortex diodes and the supporting facilities. However, no clear guidelines for design and optimization of vortex diodes system are available. Therefore, we carried out detailed experimental study on hydraulic characteristics of the vortex diodes system with changeable parameters to evolve such guidelines. The study covered a wide range of a vortex diode and vortex diodes system. The variation of averaged discharge and volumetric efficiency in respect to air supply pressure, liquid level of water tank and liquid level stroke of gas-liquid energy converter was studies. The experimental results were analyzed to develop design guidelines. The guidelines are shown to be useful for obtaining the design parameters that would yield the desired hydraulic performance for the required operating conditions.

Abstract: The main goal of this work is to investigate the possible different flow patterns existing in pump turbine under off-design conditions in pump mode. Numerical simulations by solving the Navier-Stokes equation, coupled with the “SST k-ω” turbulence model, were carried out. Flow characteristics were assumed to be stalled in the appropriate region of flow rate levels of Q/Q D=0.15–0.61. The simulation result was compared with experimental data and they showed good agreement. Consequently, velocity fields in three axial locations in stay vanes and guide vanes were analysed in details. It was shown that “jet-wake” flow pattern exists near the band, which changes little in the whole shape with flow rate increasing; to the middle location of vanes, reverse flow begins to appear on the interface between the runner and guide vanes, which will disappear gradually as the flow rate increases; massive reverse flow is captured near the crown, whose intensity will be weakened as the flow rate increases. Ultimately, it was found that the special head-flow profile can be ascribed to the special hydraulic loss characteristics of the stay vanes and guide vanes.