Impeller-diffuser Interaction Research Articles

Dynamic mode characteristics of flow instabilities in a centrifugal compressor impeller

Centrifugal compressors are key parts of medium and small aircraft engines, the flow instabilities of centrifugal compressors have become the bottleneck of the developments of engines. It is vital to investigate the dynamic mode characteristics of flow instabilities in centrifugal compressors for solving the issues. In this study, unsteady full annular simulations of the centrifugal compressor flow are firstly implemented. It is found that the flow instability of the compressor primarily arises in the impeller. To capture the dominant transient modes in process of flow instabilities and accurately describe their temporal evolutions, a modified dynamic mode decomposition method (DMD-TC) is adopted to analyze the dynamic mode characteristics, which was proposed in our previous study. The results show that perturbations in the impeller can be mainly classified into three categories: impeller-diffuser interactions, self-excited perturbations, and vortex-induced perturbations. In detail, the modes induced by the impeller-diffuser interactions and self-excited perturbations are intrinsic modes, and they reflect the inherent flow unsteadiness in the impeller. On the one hand, the vortex-induced perturbations couple with the self-excited ones, which affect the spatial structures of modes induced by self-excited perturbations. The coupling effects enhance with the reduced mass flow rate, and the “resonance” effect might be excited. As a result, the mode of 4 RF (rotor frequency) grows during rotating stall process (RSP), which is a typical characteristic of rotating stall in the impeller. On the other hand, the vortex-induced perturbations emerge as different characteristic modes, due to the variant vortex structures with the mass flow rate. During the RSP, the growth of low-frequency characteristic modes is another typical feature of rotating stall in the impeller.

Numerical Examination of the Dynamic Evolution of Fluctuations in Cavitation and Pressure in a Centrifugal Pump during Startup

Rapid changes in the performance-related parameters of a pump during its startup operation lead to large pressure fluctuations and structural vibrations in it. In view of these problems, this study investigates the evolution of cavitation and the characteristics of fluctuations under pressure in a centrifugal pump during startup. We use the finite volume method to simulate this dynamic process. To characterize the acceleration, we assume that the rotational speed and rate of flow of the pump are not constant but vary with time. A steady-state numerical simulation is performed to examine the external characteristics of the pump to verify the accuracy of the numerical procedure. The evolution of fluctuations in the cavity and pressure over time is then analyzed in detail. We use the short-term Fourier transform for post-processing in light of its advantage in treating non-stationary signals. The results indicate that both the frequency and the amplitude of the fluctuations in pressure increase with the speed of the impeller. The transient operation causes the average pressure at the inlet of the impeller to decrease to the evaporation pressure, and this leads to an increase in the volume fraction of vapor. Moreover, both the impeller–tongue interaction and the impeller–diffuser interaction influence the fluctuations in cavitation.

Influence of flow coefficient on unsteady impeller loading induced by impeller-diffuser interaction

Influence of flow coefficient on unsteady impeller loading induced by impeller-diffuser interaction

Centrifugal Compressor and Bell mouth Air Intake Preliminary Design for Micro Turbojet Engine

This paper presents a design of centrifugal compressor and air intake for micro turbojet engine of 200 N thrust. Because of the small size of the micro turbine engines the air intake is designed in the form of bellmouth air intake in order to capture the needed amount of mass flow rate (0.45 kg/s). The function of the air intake is to deliver the air to the entry of the centrifugal compressor. The air intake has been drawn with start-up motor to take in the consideration the effect. The centrifugal compressor was designed to deliver 3.8 pressure ratio with efficiency about 77%. The compressor impeller was built with 7 main blades and 7 splitters in order to reduce the effect of slip at the exit of the impeller due to pressure difference between pressure and suction side of the main blade. Diffuser of 13 vanes also was designed to increase the static pressure, direct the flow to the combustion chamber and decrease the flow velocity before to enhance the combustion process. The structural integrity for impeller was validated using ANSYS software for aluminum alloy 7075-T6; 7075651. In addition, the aerodynamic analysis was investigated using NUMECA software. The most important findings of the extensive analysis were; the impact of air intake on compressor performance is very crucial. It add an increment in efficiency and pressure ratio, it should be taken in consideration in early design stage of the compressor. The highest performance characteristics of compressor were achieved by accurate design optimization of impeller-diffuser interaction.

Investigation on the impeller-diffuser interaction on the unstable flow in a mixed-flow pump using a modified partially averaged Navier-Stokes model

In this paper, the effect of the impeller-diffuser interaction on the unstable flow in a mixed-flow pump operating under one part-load condition is investigated based on the local energy loss analysis using a modified SST k-ω partially averaged Navier-Stokes (MSST PANS) model. The comparison on the pump performance shows a good agreement between the experiment and simulation. The characteristic curve of the test pump presents a positive slope region, where hydraulic loss increases 90.75% in the impeller and 16.75% in the diffuser. The internal flow analysis depicts that the rotating stall evolution captured in the impeller passages involves larger energy transfer while the separating flow in the diffuser passage involves lower energy transfer. As the impeller blade tailing edge (TE) begins to interact with the diffuser blade leading edge (LE), the vortex attached to the impeller blade TE is cut off into two parts, one of which keeps propagating circumferentially and forms the rotating stall in the impeller, and the other is regarded as the shedding vortex, moving into the diffuser. Further analysis by the Fast Fourier Transform and proper orthogonal decomposition reveals the frequency of rotating stall is 1.49fn, and the frequency of the shedding vortex is 0.53fn.

Assessment of single passage simulations for the aeroelastic stability of a multistage centrifugal compressor

In this paper, the aeroelastic flutter stability of an impeller is investigated numerically with single passage and full annulus URANS simulations carried out on one side on a single stage configuration including a strut and the impeller and on the other side on a multistage configuration in which two additional diffusers are considered downstream. The robustness of single passage simulations relying on the use of multiple frequency phase-lagged boundary conditions is put to the test especially in the case of the multistage configuration for which both blade passage and aeroelastic effects have to be taken into account. Comparisons between single passage simulations on the single and multistage configurations indicate that the impeller-diffuser interaction acts locally on the impeller fluid-structure interface and has only little effect on the generalized aerodynamic forces. Full annulus simulations have also been carried out to assess the accuracy and efficiency of single passage simulations which introduce additional hypotheses of periodicity in the flow. Full annulus simulations are competitive in terms of wall clock since the convergence to the periodic state is faster than with single passage simulations involving the Fourier approximation of the flow field.

Effects of unsteady interaction on the performance of an ultra-high-pressure-ratio centrifugal compressor

A numerical investigation of an ultra-high-pressure-ratio centrifugal compressor is presented, mainly focusing on the unsteady impeller-diffuser interaction. The compressor is characterized by a high total pressure ratio of about 11.5 and a small radial gap between the impeller and diffuser, whose radius ratio is 1.067. The unsteady effect is stringently assessed by comparing the results of steady and unsteady Reynolds-averaged Navier–Stokes simulations. Within the impeller, the unsteady perturbation, dominated by the blade passing frequency of the diffuser, can only propagate to 20% of the normalized streamwise length from the impeller outlet. The velocity component distribution at the impeller outlet is affected by the unsteadiness, resulting in a variation of the impeller efficiency. In contrast with the impeller, the unsteady perturbation is present inside the whole diffuser passage. The circumferential flow angle variation at the diffuser inlet induces the unsteady formation and shedding of the diffuser leading edge vortex, increasing the flow loss correspondingly. However, the smaller incidence angle at the diffuser inlet makes for better diffusion inside the diffuser passage. The positive and negative effects of the unsteadiness counteract each other. Therefore, both the stage and component performances change slightly, although the flow field changes significantly due to the unsteadiness.

Dynamic mode decomposition analysis of the flow characteristics in a centrifugal compressor with vaned diffuser

To understand the flow dynamic characteristics of a centrifugal compressor, the dynamic mode decomposition (DMD) method is introduced to decompose the complex three-dimensional flow field. Three operating conditions, peak efficiency (OP1), peak pressure ratio (OP2), and small mass flow rate (near stall, OP3) conditions, are analyzed. First, the physical interpretations of main dynamic modes at OP1 are identified. As a result, the dynamic structures captured by DMD method are closely associated with the flow characteristics. In detail, the BPF/2BPF (blade passing frequency) corresponds to the impeller–diffuser interaction, the rotor frequency (RF) represents the tip leakage flow (TLF) from leading edge, and the 4RF is related to the interaction among the downstream TLF, the secondary flow, and the wake vortex. Then, the evolution of the dynamic structures is discussed when the compressor mass flow rate consistently declines. In the impeller, the tip leakage vortex near leading edge gradually breaks down due to the high backpressure, resulting in multi-frequency vortices. The broken vortices further propagate downstream along streamwise direction and then interact with the flow structures of 4RF. As a result, the 8RF mode can be observed in the whole impeller, this mode is transformed from upstream RF and 4RF modes, respectively. On the other hand, the broken vortices show broadband peak spectrum, which is correlated to the stall inception. Therefore, the sudden boost of energy ratio of 14RF mode could be regarded as a type of earlier signal for compressor instability. In the diffuser, the flow structures are affected by the perturbation from the impeller. However, the flow in diffuser is more stable than that in impeller at OP1–OP3, since the leading modes are stable patterns of BPF/2BPF.

Investigation of the Influence of Splitter Blades on the Resonance Conditions of Impellers

The conventional resonance conditions are derived based on the conventionally designed impellers without splitter blades. This paper proposes the resonance conditions for impellers under the excitation from the impeller–diffuser interaction with attention paid on the influence of splitter blades. A lumped parameter model is established and the modal analysis is carried out. The blade-based representative modal vector (RMV) is defined. The influence of splitter blades on the impeller’s traits of modes is investigated by analyzing the spatial harmonic contents of the RMV. Then, given the specific form of the diffuser-induced engine order excitation acting on the main and splitter blades, the resonance conditions are derived. Tuned and mistuned cases are provided for a practical impeller. The resonance conditions are verified by harmonic response calculations. The applications of the proposed resonance conditions in resonance identification and hazard evaluation of different excitations are given. The differences between the proposed resonance conditions and the conventional ones are discussed. The research indicates that even the RMV of the tuned impeller contains two harmonic components due to the existence of splitter blades. When the excitation frequency equals the natural frequency of the impeller and the excitation order matches with either harmonic index of the two harmonics, the resonance occurs. The results of case studies show that the harmfulness of various engine orders of excitation can be exactly evaluated by the joint use of the spatial harmonic contents analysis result and the proposed resonance conditions; however, analyzing based on the conventional resonance conditions may lead to the misjudgment of the harmfulness of the excitations.

Numerical Investigation of Impeller-Vaned Diffuser Interaction in a Centrifugal Compressor

The work presents the results of a CFD campaign to investigate the impeller–diffuser interaction in a centrifugal compressor, taking advantage of experimental data from the open literature. Previous studies on the same turbomachine focused on an experimental investigation to understand the flow interaction between the impeller and the vaned diffuser. These experimental data have been used to validate the simulation approach and discuss its results. Several CFD models with increasing complexity have been developed to take into account different aspects. The steady analysis has been performed to highlight the potentials and limitations of such models and to carry out a first study of the flow. In order to analyze the impeller–diffuser interaction, a further model for the unsteady analysis has been set up. Two different operating points have been investigated: one on the surge limit and another in a more stable working zone. A good agreement with the experimental reference data has been obtained with the unsteady analysis and some insights in the complex flow field are deduced.

Automatic optimization of a centrifugal pump based on impeller–diffuser interaction

In this study, a centrifugal pump has been optimized using the genetic algorithm coupled with computational fluid dynamics considering the flow physics for various impeller–diffuser configurations. During the automatic optimization process, the population was selected from a pool of pump geometries generated by four design variables; namely the relative diffuser vane angle, number of diffuser vanes, number of impeller blades, and the impeller wrap angle. The genetic algorithm was combined with a flow solver and a computer aided design software which was used also to create the mesh for the generated geometry. Two objective functions were adopted for the optimization: maximum pressure increase and minimum relative flow angle, which is an indication of reverse flow in the impeller. The iteration history of the optimization for the design (2.4 kg/s) and off-design (3.6 kg/s) flow rate showed that the optimization has been converged to an impeller–diffuser configuration within approximately 250 computational fluid dynamics analyses. Three geometries from each optimization with the highest pressure increase were studied for various mass flow rates and the results were compared with those of the original pump. The results show that the first optimization indicates a significant improvement of pressure increase at design flow rate (15.5%) but decrease at larger flow rates. The second optimization which was required after the results of the first optimization enhanced the head for the entire mass flow rates with an average increase of 25.74%.

Effects of Airflow Deflection Angle in Diffuser on Forced Response Caused by Impeller-diffuser Interaction in Centrifugal Compressors

Aeroelasticity has always been a significant issue of compressors. Impeller-diffuser interaction (IDI) produces periodic aerodynamic excitation which forces the blades to vibrate in centrifugal compressors, and eventually causes the damage of blades. Therefore, it is of great importance to investigate the IDI in centrifugal compressors and the effects on the vibration of blades. In this paper, it is discussed in detail that the deflection angle of the airflow in diffuser affects the IDI of the centrifugal compressor and the vibration of blades. The results show that if deflection angle is positive, the interaction of the diffuser on impeller and the vibration of main blades are decreased. However, on condition that the deflection angle is excessive, the interaction of the diffuser on the flow field of impeller will be intensified, and the vibration of the main blades will be stronger. Therefore, it is necessary to reasonably control the range of the deflection angle so that centrifugal compressors are able to work in a safer environment.

Investigation of transient flow characteristics inside a centrifugal compressor for design and off-design conditions

As the flow rate decreases from the design to the rotating stall condition, the enhanced impeller–diffuser interaction considerably deteriorates the flow condition at the diffuser inlet, which may trigger stall and blade vibration problems. In order to reveal the underlying mechanism and estimate the impact of the interaction within the impeller and diffuser passages, measurements using the wireless acquisition technique and high-frequency response system have been conducted on a 1.5 stage centrifugal compressor with the vaned diffuser. The details of its transient flow characteristics suggest that the effects of the impeller sweep extensively propagate in the diffuser passage. Distinctions of the shroud reversed vortex effect exist between the main and splitter blades at the impeller outlet, which initiates the predominant passage passing frequency at the diffuser inlet under small flow rate condition. In addition, the present study explains why the dominant disturbance shifts back and force between the passage passing frequency and blade passing frequency for different positions of the diffuser and flow rates. Through flow throttling, the diffuser reaction toward the impeller passage considerably strengthens due to the growing pressure potential near the convex surface of the diffuser vane, which is associated with the reversed flow.

Experimental investigation on clocking effect of vaned diffuser on performance characteristics and pressure pulsations in a centrifugal pump

Clocking effects can generally have a great influence on the operational state of centrifugal pumps. In order to analyze the effects of the positions of the vaned diffuser relative to the annular volute on the performance characteristics and the pressure pulsations of a centrifugal pump, experimental tests were carried out at three different relative positions. Frequency domain and time-frequency domain of the pressure pulsations were obtained using the Welch’s power spectral density estimate and the short-time Fourier transform (STFT) methods, respectively. The diameter mode analysis was applied to explain the dominant frequency induced by rotor-stator interaction. The results showed that the centrifugal pump can obtain the highest efficiency when the diffuser was fixed at position A, where the diffuser was rotated 90 degrees in counterclockwise direction from position B. The maximum difference in the efficiency between 0.9Qd and 1.1Qd was 1.5% under three different diffuser positions. The pressure pulsations in the volute were determined not only by the impeller-diffuser interaction but also by the diffuser-volute interaction. Furthermore, nonlinear frequency existed in the frequency domain of the pressure pulsations at the volute outlet. The relative positions of the diffuser had great effect on the dominant frequency and the amplitudes of the decomposed frequencies of the pressure pulsations. A comparison of the amplitudes of the main frequencies of the pressure pulsations showed that the transient pressure on the volute had the lowest pulsations when the trailing edge of the diffuser reached the left corner of the volute outlet (position B).

Numerical evaluation of transient flow characteristics in a transonic centrifugal compressor with vaned diffuser

Three-dimensional, compressible, unsteady Navier–Stokes equations are solved to investigate the flow field of a transonic centrifugal compressor with high compression ratio. The computational domain is consisted of an inlet bell mouth and an impeller with splitter blades, followed by a two-dimensional wedge vaned diffuser. The numerical method was validated by comparing the results with those of experiments in terms of aerodynamic compressor performance and flow field within the compressor passages. A detailed analysis of instantaneous and time-averaged flow field was conducted in the impeller and diffuser passages. The present study focuses on the pressure fluctuations and entropy production within the impeller and diffuser passages at the compressor design point. It is shown that the interaction between the impeller and diffuser blades leads to unsteadiness at the interface region and a pulsating behavior within the diffuser passages. Pressure waves with different convective velocities, generated by the impeller–diffuser interaction and pseudo-periodic unsteady separation bubbles, are captured in the time/space domain along the diffuser blade surfaces. The pressure fluctuation spectra were evaluated to analyze the noise characteristics of the centrifugal compressor as the main source of blade passing frequency noise. It is expected that the current unsteady Reynolds-Averaged Navier–Stokes (URANS) approach can be used as a tool for the prediction of unsteady flow and compressor noise characteristics with a proper turbulence model and well-resolved grids.

Forced Response of a Centrifugal Compressor Stage Due to the Impeller–Diffuser Interaction

The impact mode coupling between impeller blades and the disk backwall has on the forced response amplitude of impeller blades is assessed. The assessments focus on the forced response of two splitter blade modes to a variety of representative boundary conditions and unsteady loadings. The forcing function is the synchronous unsteady loading generated by the impeller–diffuser interaction at resonance. The results indicate that modal coupling of blade- and disk-dominant modes renders the forced response highly sensitive to small variations in airfoil and disk backwall thickness. As a complement, a reduced-order model based on the forced response of a two mass–spring system is used to elucidate the physical interaction of modal coupling. The practical implication of this finding is that a forced response issue with an impeller blade cannot be addressed adequately by stiffening the structure, such as thickening the blade or disk. Thus, appropriate measures need to be taken to avoid potential blade–disk mode couplings within the manufacturing tolerances of the part.

The effect of impeller–diffuser interactions on diffuser performance in a centrifugal compressor

ABSTRACTThe unsteady phenomenon abounds in centrifugal compressors and significantly affects the compressor performance. In this paper, unsteady simulations are carried out to investigate the aerodynamic performance of a process-unshrouded centrifugal compressor and the unsteady mechanism in the vaned diffuser. The predicted stage performance and pressure fluctuations at some locations are in good agreement with experimental data. The predicted main pressure fluctuation frequency spectrums at the diffuser inlet and outlet are consistent with the measured results. The results indicate that at the inlet of the diffuser there are two pressure peaks in a passage cycle. The higher pressure peak relates to the impeller wake and the lower peak is connected with the vortex generated at the diffuser’s leading edge. With a decrease in the mass flow coefficient, the vortex core region becomes larger and the lower pressure peak becomes more pronounced. The change in circumferential flow angle at the diffuser inlet is mainly responsible for the unsteadiness in the diffuser flow field, which in turn affects the inlet incidence of the diffuser vane and the vane loading distributions.

Experimental investigation of velocity fluctuations in a radial diffuser pump

The strong interaction in a radial pump due to the relative movement between the impeller and the diffuser may excite not only strong pressure fluctuations but also velocity fluctuations. In this paper, the laser Doppler velocimetry (LDV) technique is successfully applied to measure the periodic flow field in a radial diffuser pump with low-specific speed, in order to investigate the velocity fluctuations caused by the impeller-diffuser interactions both in the impeller and diffuser regions. The velocity fluctuations in the impeller region are quantitatively examined at different radial positions, and the flow structure at the radial gap between two flow components is analyzed at different relative positions. In addition, the downstream effect on the diffuser flow is quantitatively and qualitatively assessed and compared with the turbulence effect.

Investigation of model development for deterministic correlations associated with impeller-diffuser interactions in centrifugal compressors

The average-passage equation system (APES) provides a rigorous framework to account for the deterministic unsteady effects by the so-called deterministic correlations (DC), which include both deterministic stress correlations (DCS) and deterministic total enthalpy correlations (DCH). These correlations should be modeled to close the system of equations. In this paper, the distribution of DC in a transonic centrifugal compressor is presented, and its relative importance is revealed. The assumption made by Adamczyk that the pure unsteady fluctuation is significantly smaller than the spatial fluctuation is verified at the impeller-diffuser interface. The decomposition of DCH is also discussed to determine its two different physical mechanisms. Finally, the transport equations in terms of DCS in cylindrical coordinates are derived, and the terms are evaluated to determine the ones that are necessary to model. All these analyses significantly contribute to our model development for DC in centrifugal compressors.

Influence of Unsteadiness on the Control of a Hub-Corner Separation Within a Radial Vaned Diffuser

The present work aims at evaluating the effect of the impeller–diffuser interaction on the control of a hub corner separation, which develops in the radial vaned diffuser of a centrifugal compressor designed and built by Turbomeca, Safran group. Unsteady numerical simulations of the flow in the aspirated centrifugal compressor are then performed. Numerical results are validated by comparison with the available experimental results. The analysis of the numerical flow field shows that the hub-corner separation is not completely removed by the suction, on the contrary to the steady-state results that were obtained in previous work. The boundary layer separation is only translated downstream. Its location is explained by the scrolling of the pressure waves generated by the impeller–diffuser interaction, which strengthen when crossing the diffuser throat. This result highlights the major role played by the impeller–diffuser interaction, which should be taken into account for developing control strategies in radial vaned diffusers, and stresses the shortcoming of the steady-state numerical model when suction is applied.