The transition process from synchronous condenser pump (SCP) mode to pumping mode determines the response rapidity of the startup procedure and operational stability of pump-turbines; however, the complex gas–liquid interaction and transient hydraulic characteristics during this process remain insufficiently understood. To address this, this study develops a numerical framework for the SCP-to-pumping transition process, incorporating the full-passage system, a multiscale mesh strategy for coupling mainstream and clearance flow regions, and a gas–liquid two-phase flow model based on the Volume of Fluid (VOF) method. The reliability of the numerical model is verified through comparison with model experiments, demonstrating good agreement between simulations and experimental data. Based on the validated model, the transient evolution of hydraulic forces, pressure pulsations, and internal flow structures is systematically analyzed. Axial force analysis reveals a significant internal equilibrium; the crown bears a maximum instantaneous fluctuation of approximately 2800 kN. Conversely, the radial force is primarily dominated by blade hydraulic thrust (1294 kN), showing distinct anisotropic behavior. The runner blade channels and the upper draft tube region are identified as critical areas with intense pressure fluctuations, with peak-to-peak pressure amplitudes reaching 45~48 m and 54 m head, respectively. Furthermore, reducing the duration of the exhaust process constitutes the main strategy for accelerating the transition and mitigating prolonged high-amplitude force and pressure fluctuations. The findings provide new insights into the transient hydraulic mechanisms of SCP-to-pumping transitions and offer guidance for optimizing transition control strategies in pumped-storage units.
During load rejection in pumped-storage power stations, the rotational speed of the pump-turbine increases abruptly. The consequent structural deterioration of the internal flow induces high-amplitude hydraulic excitations, posing a serious threat to the operational stability of the unit. This study investigates a Francis pump-turbine to elucidate its flow evolution and instability mechanisms during load rejection. The fluid is modeled as weakly compressible water to capture finite pressure wave propagation. Dynamic mesh simulates guide vane closure, while vortex identification and short-time Fourier transform analyze transient pressure pulsations. The results indicate that the transient process can be sequentially divided into four typical stages—turbine mode, turbine-braking mode, reverse-pump mode, and return-to-turbine mode—to account for the most critical periods during the load rejection transient. The unit exhibits the poorest stability near the maximum rotational speed (443.34 r/min), where flow reversal and the full development of vortex structures significantly amplify fluctuations in hydraulic thrust. The vaneless space is identified as the primary source of pressure pulsations, whose characteristics are dominated by rotor–stator interaction mechanisms, and such disturbances decay rapidly in the downstream direction. Under turbine-braking and reverse-pump conditions, vortex rings, backflow, and asymmetric vortex structures generated within the spiral casing collectively contribute to the severe deterioration of the internal flow field quality.
Lei Deng, Wenfu Han, Yuhao Yan et al.· Water· 0 citations
Siphon vertical axial-flow pump systems are widely used in large-scale low-head pumping stations, irrigation and drainage projects, urban flood control, and water diversion engineering, where safe and stable operation is essential for hydraulic system reliability. Compared with steady operating conditions, the startup process involves rapid variations in impeller speed, flow rate, pressure distribution and hydraulic energy transfer, resulting in highly transient hydraulic behavior and obvious instability in the internal flow field. To clarify the transient hydraulic response of a siphon vertical axial-flow pump system during startup, an unsteady numerical model was established by coupling the impeller speed control equation with a dynamic boundary condition implemented through a user-defined function. The time-dependent evolution of flow rate, head, blade loading, pressure distribution, internal flow pattern and energy conversion characteristics was then investigated during the whole startup process. The results show that the pump system undergoes a distinct transition from an initially unstable flow state to a quasi-steady operating condition. At the early startup stage, the flow passage is not fully established, and the pump operates under a low-flow transient condition, leading to strong flow disorder, uneven blade loading and a sharp fluctuation in head. With the increase in rotational speed and flow rate, the internal flow gradually becomes organized, the pressure distribution on the blade surface tends to be more uniform, and the hydraulic energy transfer process becomes progressively stable. The pump head first exhibits an abnormal transient peak, then decreases rapidly, and finally recovers to a stable value as the flow field is fully established. The study reveals the transient evolution mechanism of hydraulic performance and energy conversion during startup, providing a theoretical basis for improving startup control strategy and operational stability of siphon vertical axial-flow pump systems.
Yadong Zhu, Hui Wang, Zhuangzhuang Sun et al.· Water· 1 citation
To improve the energy conversion performance and long-term structural stability of stay vane mixed-flow chemical pumps used for industrial residual pressure recovery, this paper establishes a coupled numerical framework of computational fluid dynamics (CFD) and finite element structural analysis (FEA). The internal flow evolution, radial hydraulic excitation, transient pressure oscillation and impeller mechanical bearing capacity are systematically investigated under three typical flow states: partial load 0.7 Qd, design condition 1.0 Qd and overload 1.2 Qd. The results show that the flow inside the pump is smooth and there is no obvious backflow or separation under the rated working condition, and the energy conversion efficiency is the best. When operating under partial discharge, boundary layer separation and recirculating secondary vortices easily emerge inside the pump passage, which drastically elevates hydraulic energy dissipation. Meanwhile, operating load exerts a remarkable influence on the impeller’s radial hydraulic load and transient pressure oscillation intensity. The radial force and the pressure pulsation amplitude at the impeller outlet are the largest under the small flow condition, and the force is the most stable under the rated working condition. Blade passing frequency dominates the frequency components of transient pressure fluctuations. The maximum von-Mises stress on the impeller concentrates at the filet where blade roots connect with the hub, and this peak value hits 86.3 MPa under partial-load low-flow operating status. Calculated stress values for all three flow rates satisfy the structural safety criteria. The outcomes of this numerical investigation can offer reliable technical support for hydraulic performance optimization and structural dimension design of this type of mixed-flow chemical pump.
Jiahao Lu, Baiyang Xiao, Shaobin Li et al.· Energies· 0 citations
Centrifugal pumps serve as key equipment in water conveyance systems. Different tee inlet structures distort the internal inflow field, induce extra hydraulic losses and reduce system energy efficiency, since pump power consumption and operational stability are highly sensitive to tee pipeline layouts. Pressure pulsation directly reflects internal flow disorder and reveals the root causes of hydraulic performance attenuation, so it is adopted as the core analytical tool in this work. Using the Shear Stress Transport (SST) k-ω turbulence model, numerical simulations are carried out on an 80 mm centrifugal pump with four inlet structures: straight pipe (SP), reducing tee (RT), reducing wye (RW), and asymmetric reducing wye (ARW). Combined with pressure pulsation signals, this study reveals the propagation rules of flow disturbances induced by tee inlet structures and their inherent energy loss mechanisms. The results show that tee inlet structures barely affect overall pump performance, except RT, which reduces the hydraulic head by 0.44 m and efficiency by 1.47%, accompanied by higher power consumption. Impeller pulsation intensity increases from the leading edge to the trailing edge, with the mid-passage leading edge being the most sensitive region. Impeller spectra are dominated by low-order shaft frequency harmonics, while volute signals are dominated by blade frequency (BF) harmonics. Different tee inlet structures have little impact on circumferential volute pulsation but significantly alter flow characteristics at the volute tongue and outlet diffuser, where 2BF becomes the primary dominant frequency. Disturbance propagation laws differ greatly between the impeller and downstream volute. Centered on low-energy pump system design, this study provides theoretical support for inlet pipeline optimization and energy-saving operation of municipal, industrial, marine and water conservancy pumping facilities.
Zheng Wu, Hanqiao Han, Y. Long et al.· Energies· 0 citations
To investigate the dynamic transmission of unsteady hydraulic excitation within pumped-storage units, this study presents a numerical investigation on the three-dimensional shafting vibrations of a pump-turbine based on a one-way transient fluid-structure interaction approach. To resolve the flow field, we adopt the shear stress transport (SST) k-ω turbulence model in conjunction with the Zwart-Gerber-Belamri (ZGB) cavitation model, and the combined numerical approach is first calibrated against experimental measurements. The results indicate that under the 70% partial load, an eccentric helical vortex rope develops inside the draft tube, generating intense low-frequency pressure pulsations that induce chaotic shaft orbits and distinct orbital drift at the turbine guide bearing. Conversely, near the optimal efficiency point (90% load), the vortex rope transitions into a slender, straight conical core, yielding minimum vibration magnitude and exceptional operational stability. At the 100% rated load, the vortex rope expands into a robust straight conical structure extending continuously into the elbow section. Stress analysis reveals that while equivalent stress concentrations consistently occur at the blade root regions, a reduction in the cavitation number at both 90% and 100% loads leads to a counterintuitive decline in blade surface peak stress values. Additionally, stiffness sensitivity analysis demonstrates that the relative change rates of the shaft runout are highly sensitive to the stiffness variations of the turbine guide bearing, where a stiffness reduction triggers a substantial runout growth of approximately 100% along the X- and Y-directions, whereas the variations in the upper and lower guide bearings exert extremely weak impacts.
Yanhao Li, Lei Chen, Li-hua Ding et al.· Water· 0 citations
Hydraulic short-circuit (HSC) operation is an important approach to enhancing the operational flexibility of pumped-storage power plants (PSPPs). However, under this new operating mode, the flow characteristics in the bifurcated pipe deteriorate significantly, posing a threat to the efficiency of the piping system and potentially affecting the inflow conditions for the turbine. In this study, six improved bifurcated pipe models were designed, and their internal flows under pumping, generating, and HSC modes were numerically simulated. Entropy production theory and vortex identification method were employed for flow field analysis. The results show that local modifications confined to the bifurcation are insufficient to simultaneously improve energy characteristics across different modes. In contrast, the bypass pipe enables early flow diversion, weakening the original high-dissipation regions while introducing controllable additional losses. M6 achieves an average energy loss reduction of 47.85% in the mid-to-high flow split ratio range (FSR > 0.3). A strong correlation is observed between vortex suppression and energy loss reduction: the bypass pipe substantially shortens the main vortex length at the inlet section of the generating branch, while simultaneously inducing new shear vortices at the junction; adjustment of its installation position is expected to further shorten their extension, thereby ensuring the normal operation of the turbine. This study provides a new technical pathway for extending the operating range of HSC operation and contributes to enhancing the grid-regulation capability of PSPPs.
Shan Zhu, M. Xia, Shizhe Liu et al.· Machines· 0 citations
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