Low-frequency Pressure Pulsation Research Articles

Effectiveness of Air Admission through Short Pipes in Reducing Pressure Fluctuation of Propeller Hydro-turbine Flow Channel

Abstract When the hydro-turbine operates under part-load conditions, a large-scale vortex flow will form in the draft tube. This vortex can cause low-frequency pressure pulsations that endanger the safe operation of the hydro-turbine. By injecting air into the draft tube, the structure of the vortex can be effectively disrupted, reducing the amplitude of the pressure pulsations. In this paper, a propeller hydro-turbine was studied, and the SST k-ω turbulence model based on the Reynolds averaged Navier-Stokes equations (RANS) was utilized to perform a full-channel unsteady numerical simulation on a part-load condition with the guide vane opening (GVO) of 35°. Then, the fast Fourier transform (FFT) was applied to analyze the pressure fluctuations at monitoring points on the draft tube wall, the head cover, and the nose of the spiral casing. The results indicated that injecting air with an appropriate flow rate into the draft tube through short pipes can reduce the pressure fluctuations on the draft tube wall by 51.4% to 72.5% and decrease the pressure fluctuations on the head cover and the nose of the spiral casing by approximately 40% to 42%. This study proposes a new air admission structure that can enhance the operating stability of the propeller hydro-turbine under part-load conditions.

Study on the flow characteristic of pump turbine at partial load

Abstract Pump turbine adopts the same runner to realize forward pumping for pumping condition and reverse power generation for turbine condition, which is a widely used power equipment in pumped storage power station. Compared with conventional pump units or turbine units, the pump turbine has a complex internal flow, especially under part load conditions, and the vibration and noise problems of the structure are prominent. In this paper, the internal flow characteristics of the pump turbine at 60% load are studied by numerical simulation. The low-frequency pressure pulsation 3.5fn and 5.5fn that occur under this operating condition are analyzed. The results show that under partial load conditions, the opening of the guide vane decreases, the angle of attack of the water entering the runner increases, and the rotating stall phenomenon occurs in the pump turbine. The stall vortices show periodic variations on the blade inlet pressure surface and undergo the processes of vortex reduction, fusion, splitting and increase, respectively. The frequency of change is 0.5 times the runner rotation frequency (0.5fn ). The frequency of the corresponding monitoring points in the 7 blade rotor is 7 times (1fn -0.5fn ), which is 3.5fn .

Inner Flow Analysis of Kaplan Turbine under Off-Cam Conditions

Kaplan turbines are widely utilized in low-head and large flow power stations. This paper employs Computational Fluid Dynamics (CFD) to complete numerical calculations of the full flow channel under different blade angles and various guide vane openings, based on 25 off-cam experimental working conditions. The internal flow characteristics of the runner blade and draft tube are analyzed, and a discriminant number for quantitatively assessing the flow uniformity of the draft tube is proposed. The results indicate that low-frequency and high-amplitude pressure pulsations occur on the high- and low-pressure edge of the blade when the opening is small, with pulsations decreasing as the opening increases. The inner flow line of the draft tube is disturbed when both the blade angle and opening are small. Additionally, the secondary frequency of the draft tube inlet is double that of the vane passing frequency. The discriminant number of the flow inhomogeneity approaches 0 under optimal flow conditions. The number increases continuously with the decrease in efficiency, and the flow in the three piers of draft tube becomes more nonuniform. The research results provide a reference for enhancing performance and ensuring the operational stability of Kaplan turbines.

Analysis of Cavitation-Induced Unsteady Flow Conditions in Francis Turbines under High-Load Conditions

Hydraulic vibrations in Francis turbines caused by cavitation profoundly impact the overall hydraulic performance and operational stability. Therefore, to investigate the influence of cavitation phenomena under high-load conditions, a three-dimensional unsteady numerical simulation is carried out for a Francis turbine with different head operating conditions, which is combined with the SST k-w turbulence model and two-phase flow cavitation model to capture the evolution of cavitation under high-load conditions. Additionally, utilizing entropy production theory, the hydraulic losses of the Francis turbine during cavitation development are assessed. Contrary to the pressure-drop method, the entropy production theory can quantitatively reflect the characteristics of the local hydraulic loss distribution, with a calculated error coefficient τ not exceeding 2%. The specific findings include: the primary sources of energy loss inside the turbine are the airfoil cavitation and cavitation vortex rope, constituting 26% and 71% of the total hydraulic losses, respectively. According to the comparison with model tests, the vapor volume fraction (VVF) inside the draft tube fluctuates periodically under high-load conditions, causing low-frequency pressure pulsation in the turbine’s power, flow rate, and other external characteristic parameters at 0.37 Hz, and the runner radial force fluctuates at a frequency of 1.85 Hz.

Effect of wave disturbance on the fluid dynamics of the pump turbine in a seawater pumped storage hydroplant

Compared with conventional pumped-storage hydroplants, seawater pumped-storage hydroplants are constructed in a complex marine environment. The wave disturbance may affect the safety operation of the pump turbine in the seawater pumped hydroplant via affecting the level of the free water surface. To explore the effect of wave disturbance on the operation of the pump turbine in a seawater pumped-storage hydroplant, assuming that the wave disturbance follows a sine function, on the basis of Airy’s micro-amplitude wave theory and the comprehensive characteristic curve of a pump turbine, we use sliding grid technology and the SST k–ω turbulence model to simulate the dynamics of fluid in a full-flow channel. In this study, the unsteady flow field, Q-criterion vorticity, relative pressure pulsation amplitude, and frequency of pressure pulsation are analyzed to determine the internal flow field and pressure pulsation under different wave disturbance conditions and operating loads. The results show that both the pressure pulsation in the pump turbine and its frequency domain are affected by the wave disturbance. In addition, the turbulent flow with a large flow rate tends to produce higher relative pressure pulsation under wave disturbance. Due to the large flow rate and strong static and dynamic interference in the vaneless area, the relative amplitude of the pressure pulsation is significantly affected by wave disturbance. Furthermore, the low-frequency pressure pulsation of the draft tube is superimposed with the wave disturbance frequency.

Evolution mechanism of water column separation in pump turbine: Model experiment and occurrence criterion

Whether the superposition of pressure pulsation and water hammer pressure caused by the fast closing of guide vanes causes water column separation in draft tube inlets is a hot topic in engineering and academic fields. It is also not known whether the bridging of water column separation endangers the operation of the units. This study uses a model pumped storage power station platform with two units to perform water column separation. Data on the pressure pulsation at different measurement points, discharges, and speeds are collected by an acquisition system, whereas the development process of water column separation is characterized by a high-speed camera. The measured pressure is decomposed into the water hammer pressure, which migrates with the unit operation point and pressure pulsation. The test results reveal that the pressure in the inlet section of the draft tube below the vaporization pressure that lasts for several seconds is a sufficient and necessary condition for water column separation to occur. The water hammer pressure has a significant influence on the water column separation, while low-frequency pressure pulsation has less influence. Water column separation is divided into four stages: starting, improvement, separation, and bridging. The main frequency of the pressure pulsation in the water column separation process is the frequency of the water hammer caused by load rejection and has a value of 0.47f/fn. Cavity bridging generates a frequency of 0.30f/fn. The flow-speed curve remains in the braking condition of turbine for 1.25 s. The separation and part of the bridging stage occur under the braking condition of turbine.

EXPERIMENTAL AND CALCULATION STUDY OF PRESSURE PULSATIONS IN THE RADIALAXIAL HYDROTURBINE

The force interaction of the vortex swirls in the draft tube of the hydraulic turbine RO with the elements of the flow path can lead to serious accidents. Reducing low-frequency pressure pulsations is an urgent task, as it helps to increase the reliability and increase the power of hydraulic units. For this, special devices are sometimes used. The aim this of the work was to compare modern methods for calculating low-frequency bundle pressure pulsations in a draft tube with experiment. Experimental studies of pressure pulsations in the flow part and the impeller blade of the RO115 hydraulic turbine were carried out in a wide range of operating modes. Numerous experiments were carried out to determine the bundle pressure fluctuations using a spatial mathematical model according to the method of NTU ''KhPI''. In addition, calculations of pressure fluctuations were performed based on a modern hydrodynamic software package that solves problems of continuum mechanics and uses the Reynolds equation. The process of solving problems in this case is carried out by means of the CFD application package, which includes the following steps: creating a three-dimensional model of the object under consideration using the CAD system; construction of the computational grid; choice of mathematical model of turbulence; problems of limiting conditions. A comparison of the experimental and calculated results indicates their good agreement. Prediction of pressure fluctuations using the above methods when designing hydraulic turbines makes it possible in the future to choose the best flow path with lower pressure fluctuations (with lower losses) and with higher energy-cavitation performance. In the first case, the task requires insignificant resources of computer time, and the error in calculating the amplitudes of pressure fluctuations is up to 15 % compared with the experimental data, in the second case it is 10 % with a much longer calculation time. In the future, the results of calculations of bundled pressure pulsations are used in strength calculations of elements of the flow path of a hydraulic turbine with large margins in terms of strength factors, so it is possible to use mathematical models of varying complexity in predictive calculations of bundled pressure fluctuations in a draft tube.

The low frequency pressure pulsation and control of the open-jet wind tunnel

An open jet wind tunnel has low-frequency pressure pulsation in common wind speed range due to its unique structural form, which seriously damages the quality of flow field in the test section. The low-frequency pressure fluctuation performance and control mechanism of Jilin University open jet and return flow wind tunnel are investigated by experiments and numerical simulation. The results show that the low-frequency pressure fluctuation is a narrow pulse phenomenon that only occurs in certain intervals, and several velocity intervals may be found in the same wind tunnel. The reliability of the numerical simulation is verified by comparing the peak frequency and amplitude of pressure fluctuation in numerical simulation and wind tunnel tests. A simplified model similar to and amplifying the phenomenon is established. The flow structure and vortex evolution are analyzed via detached eddy simulation. In the test section, large-scale shedding vortices are formed at the nozzle exit, introducing periodic pulsating instantaneous velocity and acting with the collector to form an edge-feedback. This acoustic feedback forms resonance with the pipeline circuit, resulting in poor flow field quality. In accordance with the mechanism of nozzle jet, two methods of controlling pulsation are proposed: spoiler and flow-follow device. The study shows that the effects of two methods are abrupt, and the frequency of pressure pulsation is changed. The spoiler destroys the complete structure of vortex ring in free jet and develops into a complementary double vortex ring structure, which is highly sensitive to size factors. The flow-follow device supplements the velocity loss of the free jet at the nozzle and develops into a double vortex ring with master–slave structure in the middle of the test section. Its vibration reduction effect is greatly affected by the flow velocity. It takes effect in an appropriate range where the flow velocity is higher than the nozzle velocity. If the follow velocity is extremely low, the flow-follow device cannot change the original jet structure. If the follow velocity is extremely high, the momentum of the fan will be greatly reduced, the flow field will be unstable, and another order of pulsation may be induced. This work lays a solid foundation for further understanding the aerodynamic characteristics and optimization mechanism of open jet wind tunnel.

Research on pressure pulsation characteristics of a reactor coolant pump in hump region

The reactor coolant pump (RCP) is the machinery used in nuclear power facilities to move coolant. So RCP operation must be safe in order for nuclear power plants to operate reliably. It was discovered throughout the experiment that the flow rate of RCP was unsteady between 0.875 Q0 and 0.85 Q0 and the pressure pulsation increased dramatically. As the flow drops, the head no longer increases, but instead declines. The condition of pump is in the hump region. This research conducts tests to determine the characteristics of pressure pulsation and uses numerical simulation to investigate the causes of increased pressure pulsation. According to the study, when the pump was working in hump region, the amplitude of low-frequency (<fr) pressure pulsation increase dramatically. The impeller's flow separation created vortices, which propagated downstream and caused stalls in the diffuser. The A_RMS of the low-frequency signal increases as a result of this.

Influence of Inlet Groove on Flow Characteristics in Stall Condition of Full-Tubular Pump

The full-tubular pump is a new type of pump with a narrow range of stable operation. In order to improve the internal flow characteristics of the full-tubular pump under small flow conditions and improve the safe and stable operating range of the pump, this paper conducts numerical simulation of the full-tubular pump model based on the Reynolds time-averaged N-S equation and the SST k-ω turbulence model. The improvement mechanism of the parameters of the inlet grooves on the stall area of the full-tubular pump is studied, and the reliability of the numerical simulation of the full-tubular pump is verified by model tests. The research results show that the inlet groove can improve the head and efficiency of the full-tubular pump in the small flow area, and the head at the deep stall condition is increased by nearly 1.61 m. The inlet groove increases the pressure difference of the impeller, which increases the head and improves the hump. At the same time, the increase in the pressure difference of the impeller increases the backflow flow in the gap between the stator and the rotor. The groove can reduce the vortex strength and backflow range at the inlet pipe wall near the stall operating, and also improve the flow field at the impeller inlet. In terms of pressure pulsation, the groove can effectively suppress the low-frequency pressure pulsation at the inlet of the impeller of the full-tubular pump under stall conditions, and effectively reduce the amplitude of the main frequency pressure pulsation and improve the internal flow. The research in this paper can provide a reference for improving the flow characteristics in the stall condition of the full-tubular pump.

Standing waves in the plenum of an open jet wind tunnel: Resonance and self-excited oscillation

The low-frequency pressure pulsations in a large low-speed open jet wind tunnel were studied using a 1:20 scaled model wind tunnel. It was found that plane pressure standing waves existed in all three directions of the wind tunnel’s plenum. It was confirmed that the large pressure pulsations in the transverse direction of the plenum were caused by the resonance of the plane pressure standing waves in that direction with the edgetone feedback (vortex-sound feedback between the nozzle and the collector). The analysis of the standing waves in the vertical direction of the plenum suggested that they might come from self-excited oscillations. For the model wind tunnel with a 1:20 strictly scaled collector, the standing waves in the transverse direction of the plenum were much stronger than those in the other two directions. By increasing the width of the collector, the intensity of the standing waves in the transverse direction was significantly reduced to a level similar to that of the standing waves in the vertical direction. It was also found that the standing waves in the transverse direction were significantly enhanced after a floor was added to the test section.

On the generation mechanisms of low-frequency synchronous pressure pulsations in a simplified draft-tube cone

This paper focuses on the generation mechanisms of synchronous pressure pulsations featuring a lower frequency than the rotating frequency of a vortex core which are observed in a simplified draft-tube cone through analyzing the results of experiments along with unsteady-state numerical simulations. A Venturi-tube is selected as a simplified geometry of hydro-turbine draft-tube; and a stationary-blades swirl generator is installed upstream to generate a uniform swirling flow in the Venturi cone (diffuser part). The experiments, including the dynamic pressure measurements, are conducted for several flowrates to determine the characteristics of pressure fluctuations in the Venturi cone. Two types of pressure fluctuations are identified, namely convective and synchronous. Both cover wide ranges of different frequencies, revealing that the phenomena responsible for generation of these pressure fluctuations feature a stochastic component. The synchronous pressure pulsations, which feature lower frequencies than the convective fluctuations, are identified as the product of self-excited oscillations of vortex breakdown. High-speed visualization of the vortex shape reveals that the vortex features three states: an axisymmetric vortex, a breakdown, and appearance of several helical vortices generated downstream of the breakdown location; while the location of the vortex breakdown fluctuates in time. By using the results of unsteady CFD simulation, it is confirmed that the location of the vortex breakdown fluctuates with the same frequency (Strouhal numbers 0 to 1) as the low-frequency synchronous pressure pulsations observed in the Venturi cone. Moreover, it is observed that the volume of the stagnation region produced due to the counterflow downstream of the breakdown location fluctuates with the same low frequency. Further analysis of the CFD results showed that the flow field at the location of the vortex breakdown experiences a severe shear layer formation around the center of the Venturi. This shear layer stretches the vortex center spatially on the plane perpendicular to the vortex axis. It is also observed that several intermittent helical vortices are generated downstream the breakdown location and rotate periodically in the Venturi while featuring a stochastic nature, as well.

Hydrodynamic improvement by adding inlet baffles on centrifugal pump for reducing cavitation instabilities

Cavitation instability is a common phenomenon that causes vibration and noise of turbomachinery. In this study, an attempt is made to suppress the cavitation instability. A high-speed centrifugal pump with inducer is taken as the research objective. Four baffles are evenly arranged at the inlet of the inducer as a hydrodynamic improvement. The energy characteristics of the pump are measured on a closed hydraulic test rig. The pressure, vibration, and noise under different flow rates and different cavitation number are acquired for comparative analyses. Experimental results show that the energy characteristics changed after hydrodynamic improvement. The original pump is mainly affected by y-direction vibration and is clearly suppressed in the new pump. The low-frequency pressure pulsation under partial flow rate condition can be effectively suppressed. The baffles can also reduce the broadband center frequency at the pump outlet and change the relationship between center frequency and cavitation number. These results show that the hydrodynamic improvement at the inlet helps the suppression of cavitation instability of the high-speed centrifugal pump.

Entropy production analysis for vortex rope of a Francis turbine using hybrid RANS/LES method

The precessing vortex core in the draft tube of Francis turbine is investigated using entropy production method. The high-fidelity Delayed Detached Eddy Simulation (DDES) method with SST k-ω turbulence model is used to acquire the entropy dissipative components of the flow, and the Q-criterion is selected to evaluate the vortex cores inside the draft tube under different operating conditions, separately. The results show that the numerical simulation is in good agreement with the experimental data obtained from the model test. It is found that the vortex motion, separation flow and shock phenomenon are considered to be hydrodynamic factors for the formation of entropy. Furthermore, draft tube accounts for the greatest proportion of entropy production, which is ascribed to the vortex rope. Vortex rope will rotate in the same direction as the runner, but the rotational speed is lower, resulting in low frequency pressure pulsation with large amplitude in the draft tube. It is concluded that the key reason why great energy dissipation is caused in the draft tube is that the vortex rope is transporting continuously towards the downstream, especially when the turbine deviating from optimum operating point.

Suppression of low-frequency pressure pulsations in an open jet wind tunnel by corner vortex generators

Suppression of low-frequency pressure pulsations using vortex generators at the jet nozzle is investigated for a large low-speed open jet wind tunnel. The pressure pulsations are measured for different arrangements of the vortex generators at the jet nozzle. It is found that only generators installed at the corners of the nozzle can reduce low-frequency pressure pulsations and generators installed in the middle parts of the four inner walls of the nozzle not only fail to decrease the pulsations but also excite them outside the jet even more strongly. On the basis of these results, corner vortex generators (CVGs) are adopted for control of low-frequency pressure pulsations in the open jet wind tunnel. After installation of CVGs, both the amplitude of pressure pulsations in the plenum and the turbulence intensity at the center of the test section are significantly reduced.

Numerical Study on Hydrodynamic Characteristics in a Centrifugal Pump at off Design Conditions Based on a Cycling-System Model

In this paper, the unsteady hydrodynamic characteristics of a centrifugal pump handled at low-flow-rate conditions are numerically investigated. A cycling-system model is established to capture the influence of off-design operation on the internal flow field and the pressure pulsation. The preliminary results show that the cycling-system model can predict the flow field of the pump more conforming to the practical situation than that of the pump alone; besides, the generation of stall clusters in the impeller will induce low-frequency pressure pulsation.

Standing wave and its impact on the low-frequency pressure fluctuation in an open jet wind tunnel

Experiments and numerical studies of the low-frequency pressure pulsation in an open jet wind tunnel were conducted. Measuring the pressure pulsation in the wind tunnel’s plenum, demonstrated that the low-frequency pressure pulsation was a standing wave along plenum’s horizontal direction. By numerically simulating plenums with different widths, we found that the pressure fluctuation reached the maximum when standing wave’s frequency approached edge-tone feedback’s frequency. The organized vortex structures in the two shear layers around the jet in the horizontal direction were antisymmetrically arranged. We propose that the low-frequency fluctuation in the plenum originated from the resonance between the edge-tone feedback (generated by the vortex-acoustic feedback between the nozzle and the collector) and the standing wave. The mechanism of resonance was analyzed and possible remedies are suggested in this paper.

Low-Frequency Pressure Pulsations in the Penstocks of Hydroelectric Power Plants and Swing of the Generator Rotor

Full-scale data on the pulsations of pressure and the oscillations of the real power in units of hydraulic power plants are analyzed. The relationships among the low-frequency pressure pulsations, oscillations in the turbine torque, and oscillations in the real power of the generator are established.

Numerical analysis of the formation mechanism and suppression method of the reverse flow in a semi-open centrifugal pump

Reverse flow has a detrimental effect on the stable and safe operation of centrifugal pumps. To study the formation mechanism and suppression of the reverse flow, a semi-open centrifugal pump with circumferential groove in the shroud was simulated. Then, the flow field and pressure fluctuation were analysed. The absolute flow angle at the blade inlet nearing the shroud was close to 180° because of the joint action of the leakage flow and blade inlet impact under low flow rate. This phenomenon resulted in the formation of a low-speed region and the reverse flow and low-frequency pressure fluctuation. The circumferential groove provided a channel for the leakage flow, which could quickly pass through the groove, and reduced the absolute flow angle at the blade inlet nearing the shroud and weakened the trend of the tip leakage flow to upstream. The low-frequency pressure pulsation was eliminated, and the amplitude of the blade passing frequency was reduced under 0.7 Qd (Qd is the design flow rate). The reverse flow thickness coefficient became zero with the circumferential groove. The proportion of the reverse flow volume to the volume of inlet pipe decreased from 14.7 % to 2.2 % under 0.4 Qd. This research indicated that the circumferential groove arranged in the shroud could effectively suppress or eliminate reverse flow.

Research on Groove Method to Suppress Stall in Pump Turbine

The pump turbine is prone to stall when running at part-load operation. Stalls would cause a hump-like head characteristic curve, low-frequency high-amplitude pressure pulsation, and surge or resonance in the system. There is a lack of efficient methods for pump turbine stall suppression. The traditional blade hydrodynamic optimization method has limited effect and would influence the other characteristics. As the essence of stall is flow separation, forming a severe backflow vortex, a “Groove Method” is put forward and employed to suppress stall in a pump turbine with the full consideration of the mechanical structure, flow field, and pressure field. Both experiments and calculations are carried out to test the effectiveness of this new method. Furthermore, its deep mechanisms are revealed. This method can weaken the head hump to a certain extent and reduce the pressure pulse amplitude induced by stall. Meanwhile, the performance at the design operating point is not disturbed much.