Rotor-stator Interaction Research Articles

Investigation into Dynamic Pressure Pulsation Characteristics in a Centrifugal Pump with Staggered Impeller

For the centrifugal pump, the rotor–stator interaction (RSI) induces high-energy pressure pulsation, which directly affects the stability of systems and equipment. Therefore, this work proposes a new staggered impeller structure to suppress high-energy pressure pulsation in centrifugal pumps. The original impeller blade is divided into two layers and is staggered at 10°, 20° and 30° to form a staggered impeller. The dynamic pressure pulsation characteristics of both the original impeller and the staggered impeller are predicted using large eddy simulation (LES). The results indicate that the uniform staggered arrangement of blades can significantly reduce the pressure pulsation energy in the pump by 54.69% under the design conditions, while also achieving the best performance. Even under off-design conditions, the pressure pulsation energy can still be effectively suppressed by the staggered blades. The study of the time–frequency domain of the monitoring points near the tongue found that the phase difference in the pressure fluctuation caused by the RSI between the staggered impeller and the tongue prevents the superposition of pressure pulsation energy and efficiently suppresses it in the pump. The results can provide a reference for optimizing low-vibration-noise pump impellers in engineering applications.

Forced Response Analysis of an Embedded Compressor Rotor Induced by Stator Disturbances and Rotor–Stator Interactions

Accurate predictions of the blade response in a multi-row compressor is one of the most important tasks within the design process of compressor blades. Some recent studies have shown that the decoupled method considering only the stator disturbances cannot obtain accurate results for cases with strong rotor–stator interactions, especially for the interaction between the rotor and downstream stator, and the coupled method with multi-row configurations is necessary. Factors determine what computational domains to model need to be clarified to find a balance between accuracy requirements and computational costs. To this end, this study conducted full-annulus unsteady calculations with decoupled and coupled configurations to investigate the forced response of an embedded compressor rotor induced by upstream and downstream stator disturbances and rotor–stator interactions, respectively. The results show that the upstream IGV disturbances were dominated by the wake, and the IGV and S1 potential fields had little effect on the R1 response. Meanwhile, the IGV–R1 interactions and S1–R1 interactions were dominated by one cut-on mode, respectively. The comparisons of the blade vibration amplitude and the unsteady pressure field calculated by decoupled and coupled methods revealed the mechanism of the forced response, namely, for the R1 response induced by upstream aerodynamic disturbances, the dominant excitation source was the IGV wake, and the blade vibration amplitude can be predicted by the decoupled method. In terms of the response induced by downstream disturbances, the cut-on S1-R1-interaction mode was dominant and the use of the decoupled method without considering its influence will lead to an inaccurate prediction. This study concluded that the formation process of rotor–stator interactions was the key factor that determines whether the decoupled method or coupled method should be used, and analogized a process independent of the downstream stator disturbance. The results can provide a preliminary configuration for accurate and efficient blade response predictions and explain the reason why including downstream stator vanes is very important.

Transient characteristics of PAT in micro pumped hydro energy storage during abnormal shutdown process

Pumps as turbines play an important role in micro pumped hydro energy storage (PHES) systems, which are widely applied in remote areas, and their operational safety can be significantly affected by the transient power-off process. The aim is to analyze the unsteady internal flow characteristics and time–frequency characteristics of the pressure fluctuations of the pump as turbine (PAT) after power-off process by using finite volume method (ANSYS CFX) and bidirectional test rig. During the transient process of power failure, internal flow instabilities were revealed by analyzing the flow field and turbulence kinetic energy. The results indicated that the flow field between the blades was extremely unstable during the brake mode. Additionally, the absolute values of the rotational velocity and flow rate were 1.21 and 0.93 times higher than the initial values, respectively, during the runaway mode. The pressure fluctuations were analyzed in the time-frequency domains using the continuous wavelet transform. The main frequency of pressure pulsation at the volute was related to the rotating speed owing to the rotor-stator interaction. This study revealed the transient flow features during the process of mixed-flow pump as turbine failure, which provides a fundamental theory for the safe transient operations of pump as turbine.

Experimental Investigation on Velocity Fluctuation in a Vaned Diffuser Centrifugal Pump Measured by Laser Doppler Anemometry

Turbulent flow, mainly originating from the rotor-stator interaction (RSI), is closely associated with the normal and safe operation of the centrifugal pump. In the current research, to clarify turbulent flow in the centrifugal pump with a vaned diffuser, the non-intrusive LDA (Laser Doppler Anemometry) system is applied to measure velocity pulsation signals at different regions when the pump operates at various flow rates. Time and frequency domain analysis methods are combined to investigate the velocity signals, and the velocity distribution around the volute tongue region is reconstructed from twenty measuring points. Results show that the velocity spectrum is characterized by the discrete components at the blade passing frequency and its higher harmonics, and it is caused by the RSI between the impeller and the diffuser. For the points in the volute spiral and diffusion sections, due to the significantly reduced RSI effect, the velocity spectrum shows an evident difference from comparison with the points between the impeller and diffuser, and the blade passing frequency is not always the dominant frequency. The comparison of velocity amplitudes and RMS* (root mean square of velocity) values at different points proves that the measuring position and flow rate affect velocity pulsations. As observed from velocity distribution reconstructed by LDA signals, high velocity regions are developed downstream of the diffuser channel for all the measured flow rates.

Research of the vibration induced by cavitation in a centrifugal pump under part load condition

Cavitation and the corresponding induced vibration characteristics in a centrifugal pump under part load (0.4Qd, Qd is rated flow) conditions were investigated by combining numerical simulation and experimental analysis to determine the internal flow characteristics and vibration signals under different cavitation conditions. Under part load conditions, cavitation is initiated at the inlet of blades close to the tongue. Expansion of a low-pressure region increases the size of vortex in the flow passages, which increases the severity of the flow instability in the pump during severe cavitation. As cavitation develops, the random generation and rapid collapse of cavitation bubbles produce unstable vibration of the pump system. The high-frequency and broadband pulsation of vibrations distributed in the frequency band of 4–10 kHz can be regarded as typical signals induced by cavitation in the pump at 0.4Qd. The net positive suction head available of the cavitation incipient point is determined to be 9 m at 0.4Qd. The rotor–stator interaction between the impeller and volute affects the vibration signals at individual measurement points, which exhibit clear shaft-frequency characteristics in the frequency band of 0–600 Hz.

An Affordable Acoustic Measurement Campaign for Early Prototyping Applied to Electric Ducted Fan Units

New innovative green concepts in electrified vertical take-off and landing vehicles are currently emerging as a revolution in urban mobility going into the third dimension (vertically). The high population density of cities makes the market share highly attractive while posing an extraordinary challenge in terms of community acceptance due to the increasing and possibly noisier commuter traffic. In addition to passenger transport, package deliveries to customers by drones may enter the market. The new challenges associated with this increasing transportation need in urban, rural, and populated areas pose challenges for established companies and startups to deliver low-noise emission products. The article’s objective is to revisit the benefits and drawbacks of an affordable acoustic measurement campaign focused on early prototyping. In the very early phase of product development, available resources are often considerably limited. With this in mind, this article discusses the sound power results using the enveloping surface method in a typically available low-reflection room with a reflecting floor according to DIN EN ISO 3744:2011-02. The method is applied to a subsonic electric ducted fan (EDF) unit of a 1:2 scaled electrified vertical take-off and landing vehicle. The results show that considerable information at low costs can be gained for the early prototyping stage, despite this easy-to-use, easy-to-realize, and non-fine-tuned measurement setup. Furthermore, the limitations and improvements to a possible experimental setup are presented to discuss a potentially more ideal measurement environment. Measurements at discrete operating points and transient measurements across the total operating range were conducted to provide complete information on the EDF’s acoustic behavior. The rotor-self noise and the rotor–stator interaction were identified as primary tonal sound sources, along with the highest broadband noise sources located on the rotor. Based on engineering experience, a first acoustic improvement treatment was also quantified with a sound power level reduction of 4 dB(A). In conclusion, the presented method is a beneficial first measurement campaign to quantify the acoustic properties of an electric ducted fan unit under minimal resources in a reasonable time of several weeks when starting from scratch.

Effects of tip clearance on energy performance of three-stage electrical submersible pump

In order to investigate the influence of tip clearance on the energy performance of electrical submersible pump (ESP), numerical simulation and experimental study were carried out. The three-dimensional transient Reynolds-Averaged Navier–Stokes (RANS) equation is solved by using the ANSYS CFX software. The experimental results were used to verify the reliability of the numerical method. The results shown that the increase of tip clearance leads to the decrease of energy performance. The increase of tip clearance leads to a proportional drop in energy performance under part-load flow rate conditions, while the increase of tip clearance leads to a significant loss under large flow rate conditions due to increased leakage. The rotor-stator interaction between the impeller and the diffuser is the main factor for the enhanced turbulence at part-load flow rate conditions, resulting in energy losses. The maximum turbulence intensity along the axial direction in the tip clearance is 4.4 times and 4.7 times of the minimum turbulence intensity at rated flow rate 1.0Qdes and large flow rate 1.4 Qdes operating conditions, respectively. The results could be referred to predict the internal flow loss and optimize the design of ESPs.

Simulation of Particle Trajectories in Gas Turbine Components and Assessment of Unsteady Effects Using an Efficient Eulerian-Lagrangian Technique

In recent years, CFD has proven to be a very useful asset to help with predicting complex flows in a wide range of situations, including multiphase and gas-particle flows. On this track, numerical modelling of particle-laden flows in multistage turbomachinery has become an important step in helping to analyse the behaviour of a discrete phase in gas turbines. Furthermore, unsteady effects due, for example, to rotor–stator interaction may have an effect on trajectories and capture efficiencies of the discrete phase. Unfortunately, computational times for transient simulations can be exceedingly high, especially if a discrete-phase needs also to be simulated. For this reason, this work reports a new method for the efficient and accurate simulation of particle-laden flows in gas turbine engines components. The Harmonic Balance Method is exploited to gain orders of magnitude speedup exploiting the idea that once the flow field has been embedded in the spectral basis, it can be reconstructed at any desired time. In this way, not only can the computational time needed to reach convergence of the flow field be dramatically reduced, but there is also no need to keep simulating the flow field during particle tracking. On the contrary, the continuous phase field can be retrieved at any desired time through flow reconstruction. This technique is conceptually simple, but, to the authors’ knowledge, has never been applied so far in particle-laden flow simulations and represents a novelty in the field. First, the implementation of the method is described, and details are given on how phase-lagged boundary conditions can be applied to flow and particles to further speed up the calculation. Then, some relevant case studies are presented to highlight the performance of the method.

CFD Analysis of a Transient Rotor-Stator Axial Steam Turbine Stage

An axial steam turbine stage has both stationary and rotating components, which are coupled together in a CFD model by multiple frames of reference. There are three types of interface techniques available to exchange the information between the different frames of reference namely frozen rotor, stage, transient stator rotor. Both frozen rotor and stage interfaces are used in steady state analysis. The operation of axial turbine is inherently an unsteady process. The aerodynamic interaction between the rotating part and the stationary parts are the important contributor to the unsteadiness of the flow present in the turbine. Neither of the two interfaces implemented in the steady state CFD analysis is capable of predicting the unsteady effects resulting from the rotor-stator interaction due to their relative position change. The third type of interface, the transient stator-rotor interface, is available to simulate the fluid motion caused by the relative movement between a rotor and stationary components in axial turbine. Although enormous computer resources are needed for this modelling, it simulates real flow physics best of all, while steady or quasi-steady numerical approaches only approximate the real flow, because they neglect important effects of blade row interactions. The paper presents the CFD analysis carried out for an axial steam turbine stage with three different interface techniques and compares the results.

Dynamic characteristics of a running away pump-turbine with large head variation: 1D + 3D coupled simulation

The dynamic characteristics of running away pump turbines (PTs) with a large head variable amplitude have not been understood thus far, primarily because of two difficulties in simulation and analysis. The first is how to provide accurate time-varying boundary conditions for transient simulation of the turbine runaway process (TRP). The other is how to determine the specific appearance time of each frequency component of the complex pressure fluctuations. This study presented a one- and three-dimensional (1D-3D) coupled approach considering waterway dynamics to provide accurate unsteady boundary conditions for the transient flow simulation of a PT with a large head variable amplitude. The short-time Fourier transformation (STFT) approach was adopted to analyse the time-frequency characteristics of the transient pressures and impeller forces. The study found that the fluctuations of pressures and impeller forces during the TRP of the PT with a large head variable amplitude contained two exclusive fluctuation frequency components. The former was approximately three times the rated rotational frequency of the impeller. The later was a series of integer fold transient rotational frequencies of the impeller, which was irrelevant to the rotor-stator interactions. The findings have important value for controlling the pressure fluctuations during the TRP of PTs.

Research on wake and potential flow effects of rotor–stator interaction in a centrifugal pump with guided vanes

In this paper, the wake and potential flow effects of the rotor–stator interaction in a centrifugal pump with guide vanes are investigated from the view of the separation of turbulent and acoustic pressure fluctuations. The highest vibration levels in pumps are, in general, originated in the potential flow and wake effects. However, it is challenging to distinguish their effects on flow evolution. The pellicular mode decomposition method is applied to innovatively separate potential flow and wake disturbances in a centrifugal pump. By pellicular, we mean an infinitely thin layer of air located on the monitoring surface. The pellicular modes are a set of acoustic modes, with which a set of normalized orthogonal basis can be constructed. The impacts of potential flow and wake disturbances are visualized and evaluated quantitatively. The results show that only a very limited region is where the potential flow disturbance works. The higher the harmonics, the smaller the disturbance range. The wake disturbance is responsible for the modal pressure field. Modal pressure fields with low diametrical nodes decay more slowly than those with low harmonics. In addition, special attention is paid to the impact of the geometric asymmetry of the volute on the impeller force. The circular volute with a two-stage pressure drop improves the radial force of the impeller. More deep understandings on the mechanism of the rotor–stator interaction are reached by decoupling the potential flow and wake disturbances. This work serves as a guide for further research in fault diagnosis and vibration control of centrifugal pumps.

Numerical investigation on hydrodynamic performance of a pre-swirl stator pump-jet propulsor with special emphasis on energy loss mechanism

The objective of this paper is to evaluate the hydrodynamic performance of the newly developed pre-swirl stator pump-jet propulsor (PJP) and to investigate its underlying flow characteristics and energy loss mechanisms. Numerical simulations are carried out for the scaling model of the PJP, the experimental tests are also performed to verify the numerical simulation results. The energy balance equation is loaded to investigate the energy loss combining with vortex structures. The numerical results show a reasonable agreement with the available experiments. The stator performs well for generating pre-swirl flow with tangential velocity, which could be effectively absorbed by the rotating rotor to varying levels under different operating conditions. The stator-rotor interaction flow is complicated and causes obvious non-uniformity for the flow field due to the generation of trailing vortices. The energy losses within the PJP are well quantified and visualized by different loss terms in energy balance equation. The production of turbulent kinetic energy and then dissipates due to viscous effects is the leading cause for energy loss, which originates from vortex evolutions and impact loss without considering the friction loss. The generation of tip leakage vortex dominates the energy loss in the rotor domain and wake field of PJP.

Investigation of Unsteady Rotor–Stator Interaction and Deterministic Correlation Analysis in a Transonic Compressor Stage

Abstract Unsteady blade row interaction (UBRI) has a large impact on performance in high-speed axial compressors. Thus, the influence of UBRI should be accurately predicted in compressor routine design. In this study, the unsteady rotor–stator interaction in a transonic single-stage compressor, NASA Stage 35, is studied by steady and unsteady Reynolds-Averaged Navier–Stokes methods. First, the UBRI is analyzed. Periodic separation bubbles on the stator suction surface induced by the rotor wake are observed. Comparisons between steady and unsteady results show that the radial distribution of flow angle near the hub and tip region of the stator is strongly influenced by UBRI. Second, deterministic correlations are analyzed based on the average-passage equation system. The distribution of deterministic stress and the relationship between spatial correlations and deterministic correlations are analyzed. Results show that a strong nonlinear interaction caused by the rotor wake and the stator potential field is found in the rotor–stator gap, which is responsible for the generation of spatial–time correlation. Then, the anisotropy of deterministic correlations is analyzed using the Lumley triangle. Results show that the deterministic stress is highly anisotropic in the stator passage, which can be considered in the modeling using a similar strategy of turbulence modeling. At last, the relationships between the harmonics and decay rate of deterministic correlations are analyzed. The decay rate of deterministic correlation is highly related to the corresponding harmonics, and the higher-order harmonic exhibits a higher decay rate. A modified exponential decay model is proposed for deterministic correlation based on harmonics, which provides improved performance.

Transition of amplitude–frequency characteristic in rotor–stator interaction of a pump-turbine with splitter blades

With the increase in the designed net head and output capacity, the operational stability of the pump turbine, which is the core component of a pumped storage power station, requires further investigation. The originality of the current research lies in the analysis of the pressure pulsation characteristics in the vaneless space of a pump-turbine runner designed with splitter blades. Under the designed net head and high-flow-rate conditions, the dominant frequency of the pressure pulsation in the vaneless space corresponds to half the total number of runner blades, 4fn. As the flow rate decreases, a blade-passing frequency of 8fn corresponding to the total number of runner blades was observed. Subsequently, the amplitude of 8fn increased, which gradually became the dominant frequency, whereas that of 4fn diminished. The contribution of current research is that the transition of the amplitude-frequency characteristic of the rotor-stator interaction of pump-turbine runners with splitter blades is revealed, which is significantly different from the analysis of equal-blade pump-turbines in the existing literature. For pumped storage power stations equipped with pump-turbine runners with splitter blades, it is necessary to consider the dominant frequency in the vaneless space to avoid the adverse effect of the transition of the dominant frequency of the rotor–stator interaction on the structural response of the station.

Design space analysis for supercritical CO2 radial inflow turbine stators

Radial inflow turbines(RITs) play a vital role in power generation due to their small size, compact construction and large power density, especially when applying to supercritical CO2(sCO2) power cycles. Correct preliminary design of the RITs is important for enhancing the efficiency of cycles. Most preliminary design studies for RITs simply impose empirically developed angle constraints for the rotor inlet conditions instead of giving much attention to the stators. In addition, the previous design procedure can only calculate one geometry at a time, which does not allow the selection of the most suitable geometry that fits the cycle parameters and constraints. Hence, this paper presents a workflow to correctly size a stator for achieving a desired rotor inflow conditions. Then a stator design-space analysis with utilising an existing 500 kW sCO2 rotor is conducted. Based on a number of given stator radius ratio(r3/r4), blade number(Zs) and blade trailing edge thickness(t3), the obtained design map contains a total number of Nr3/r4×NZs×Nt3 preliminary design cases. New insights are discussed, including that the workflow is functional; the blade angles need to adjust between no-loss and loss cases; the rotor–stator interaction loss will increase the stator inlet/outlet operational parameter when guaranteeing the target rotor inlet conditions; and including of the stator losses in turbine design is significant.

Numerical analysis of bubble size effect in a gas-liquid two-phase rotodynamic pump by using a bubble coalescence and collapse model

The inlet bubble sizes and their coalescence and collapse behavior are the key factors affecting the performance and transportability of the multiphase pump. In this paper, a bubble coalescence and collapse model is applied for the simulation of a gas-liquid two-phase rotodynamic pump. The external characteristic, gas-liquid flow, phase interaction, and pressure fluctuation are analyzed using computational fluid dynamics (CFD) methods as the inlet bubble diameter d0 varies from 0.1 mm to 2.2 mm. The CFD results show that the pump’s efficiency and pressure rise decrease sharply at 0.1 mm ≤ d0 ≤ 0.4 mm, slow down at 0.4 mm < d0 ≤ 1.3 mm, and become stable at 1.3 mm < d0 ≤ 2.2 mm. The effects of inlet bubble size on flow characteristics in the impeller, such as gas-liquid separation, flow separation, vorticity change rate, and interphase force, are larger than that in the guide vane. The drag is the most significant interphase force at d0 = 0.1 mm, while the lift force plays a dominant role at d0 = 0.7 mm and 1.9 mm. The effect of the turbulent dispersion force is weakest at d0 = 0.1 mm, 0.7 mm, and 1.9 mm. Rotor-stator interaction is the key driver of the pressure fluctuation in the gas-liquid two-phase pump but is restrained by serious phase separation, flow separation, and interphase interaction at the impeller outlet. The most obvious fluctuation occurs near the guide vane inlet at d0 = 0.1 mm, 0.7 mm, and 1.9 mm. The maximum fluctuation at d0 = 0.7 mm and 1.9 mm are 5.98 times and 7.17 times that at d0 = 0.1 mm.

Cooling fan aerodynamic noise reduction with short inlet duct and its applicability

This paper presents experimental and numerical analyses of the noise-reduction method of applying short inlet ducts to axial-flow cooling fans. Far-field noise tests of three different-sized cooling fans with tip Mach numbers of 0.143, 0.156, and 0.174 demonstrated that tonal noise was the primary source of aerodynamic noise. Through the installation of short inlet ducts on these fans, the upstream propagation of tonal noise was reduced. The average total sound pressure levels were decreased by up to 3.5, 4.8, and 7.1 dBA, respectively. At the blade passing frequency, the dominant upstream-propagating azimuthal acoustic modes were suppressed. Full-passage unsteady computational fluid dynamics calculations revealed that the duct transforms multiple suction vortices into a single annular vortex structure and weakens the interaction with the rotor blades. In addition, the acoustic analogy was used to compute the reduction of far-field noise and the variations in the major acoustic modes. This revealed a noise-reduction trend similar to that seen in the experiments. Essentially, the duct restrains the non-uniform inlet flow to influence the various inlet–rotor–stator interaction modes. The application of this approach to various fan configurations confirmed that a duct with a parameter of L/D (duct length/duct inner diameter) equal to 0.08 can be recommended as a compact and efficient way to reduce noise.

Numerical investigation of unsteady pressure pulsation characteristics in an ultra-low specific-speed centrifugal pump as a turbine

The unsteady flow characteristics of pump as hydraulic turbine play a vital role in its safe and stable operation, while the ultra-low specific-speed centrifugal pump may face more stability problems due to the limitations of its flow conditions under the turbine working condition. Therefore, in this study, the unsteady characteristics of an ultra-low specific-speed centrifugal pump under turbine conditions are studied using a numerical simulation method, and the numerical simulation is verified using an experimental method. Based on the hydraulic losses of each flow passage component, the energy characteristics of pump as turbine (PAT) are established, and the distribution pattern of total pressure fluctuation in the turbine is studied. The results show that the rotor–stator interaction between the impeller and the tongue makes the hydraulic performance and the internal flow field change periodically. The pressure fluctuation intensities at the tongue, blade inlet edge, and balance hole are large, and the total pressure fluctuation in the three areas is intense in space and time. The internal flow characteristics at typical blade positions show that the secondary flow phenomena such as separation flow and wake flow near the tongue make the pressure gradient larger, which is an important influence mode of the rotor–stator interaction. This study provides a reference and guidance for the unsteady study of low specific-speed PAT.

Modulation effect on rotor-stator interaction subjected to fluctuating rotation speed in a centrifugal pump

In the paper, the modulation effect of rotor-stator interaction subjected to fluctuating rotation speed is investigated numerically. A quasi-bivariate variable mode decomposition method for analyzing non-stationary flow fields is proposed for the first time. Different from the previous studies, this paper emphasizes the local features of the transient flow field at fluctuating rotation speed. The results show that the impact of fluctuating rotation speed on the impeller force is limited, but it significantly impacts the pressure pulsation at fluctuating frequency. The correlation between amplitude and fluctuation ratio is positive. Meanwhile, the fluctuating rotation speed also induces sidebands centered on the blade passing frequency and its harmonics. The periodic changes in rotational speed lead to frequency and amplitude modulation of pressure pulsations. The magnitude of the instantaneous frequency and amplitude increases as the fluctuation ratio grows. The change in pressure field is well synchronized with the rotational speed, while the change in velocity field lags behind the rotational speed. The variation of pressure field with rotational speed is mainly attributed to two aspects. One is the flow structure at the fluctuating frequency, and the other is the instantaneous component of the flow structure at blade passing frequency.

Study on the Unsteady Characteristics and Radial Force of a Single-Channel Centrifugal Pump.

To research the internal flow characteristics of a single-channel centrifugal pump, the computational fluid dynamics method is used in this study. Several monitoring points are set on the volute to analyze the change of pressure pulsation and radial force when the impeller rotates at different angles. The results show that the pressure pulsation in the single-channel pump volute is induced by the rotor-stator interaction and its harmonics. The monitoring points close to the separation tongue along the rotation direction of the impeller are more affected by the flow rate.