Investigation on Surge in Centrifugal Compressors

A test bench for conducting compressor surge experiments is set up, and different system configurations formed by changing the length of compressor outlet pipeline are tested for surge. Dynamic pressure signals relating to surges are acquired at different locations of the configurations using unsteady measurement ＆ data acquisition system. The sliding window method is adopted to set up quantitative criterion on the surge. Parameters included in the criterion, such as location of data collection, size and step of sliding window, a mathematical quantity surge-judging and its threshold, etc., are given. Flow chart of surge evaluation is shown, and surge frequency was evaluated based on system configurations. With all these, the problem of judging the existence of surge by human experiences in compressor performance experiments can be solved. Hence this new approach may help to achieve intelligent operations on automatic compressor performance testrig.

Comparative Analysis of Diagonal and Centrifugal Compressors with Synergy Theory in Compressed Air Energy Storage System

Energy storage technology is an essential part of the efficient energy system.Compressed air energy storage(CAES)is considered to be one of the most promising large-scale physical energy storage technologies.It is favored because of its low-cost,long-life,environmentally friendly and low-carbon characteristics.The compressor is the core component of CAES,and the performance is critical to the overall system efficiency.That importance is not only reflected in the design point,but also in the continuous efficient operation under variable working conditions.The diagonal compressor is currently the focus of the developing large-scale CAES because of its stronger flow capacity compared with traditional centrifugal compressors.And the diagonal compressor has the higher single stage pressure ratio compared with axial compressors.In this paper,the full three dimensional numerical simulation technologies with synergy theory are used to compare and analyze the internal flow characteristics.The performance of the centrifugal and diagonal impellers that are optimized under the same requirements for large-scale CAES has been analyzed.The relationship between the internal flow characteristics and performance of the centrifugal and diagonal impellers with the change of mass flow rates and total inlet temperature is given qualitatively and quantitatively.Where the cosine value of the synergy angle is high,the local flow loss is large.The smaller proportion of the positive area is the pursuit of design.Through comparative analysis,it is concluded that the internal flow and performance changes of centrifugal and diagonal impellers are different.The results confirm the superiority and feasibility of the off-design performance of the diagonal compressor applied to the developing large-scale CAES.

Real-Time Instability Detection of Centrifugal Compressors Based on Motor Speed Measurements

Flow instability in the centrifugal compressor should be detected and avoided for stable and safe operation.Due to the popularity of electric centrifugal compressors,instability detection could be achieved by measuring motor signals instead of traditional aerodynamic signals.In this paper,the feasibility of instability detection by motor signals(i.e.rotating speed and phase current)was studied experimentally.The physical structure and control method of the electric centrifugal compressor were discussed to reveal the potential of instability detection by motor signals.Dynamic pressure signals and motor signals measured during unsteady experiments were analyzed in the time domain and frequency domain.Characteristics of these signals were then compared under different operating conditions to indicate the feasibility of instability detection by motor signals.Finally,the ability of Short-Time Fourier Transform(STFT)of rotating speed signals in real-time instability detection was discussed.Results showed that the rotating speed signal is a good alternate for instability detection in spite of signal distortion,while the phase current signal can only detect surge due to the low resolution of the controller.Based on the variations of the amplitude and frequency of rotating speed signals,the real-time instability can be captured accurately by STFT with a window size of 0.5 s.Besides,the interference caused by the controller can be removed by STFT.

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.

Multi-point Optimization of Recirculation Flow Type Casing Treatment in Centrifugal Compressors

High-pressure ratio and wide operating range are highly required for a turbocharger in diesel engines. A recirculation flow type casing treatment is effective for flow range enhancement of centrifugal compressors. Two ring grooves on a suction pipe and a shroud casing wall are connected by means of an annular passage and stable recirculation flow is formed at small flow rates from the downstream groove toward the upstream groove through the annular bypass. The shape of baseline recirculation flow type casing is modified and optimized by using a multi-point optimization code with a metamodel assisted evolutionary algorithm embedding a commercial CFD code CFX from ANSYS. The numerical optimization results give the optimized design of casing with improving adiabatic efficiency in wide operating flow rate range. Sensitivity analysis of design parameters as a function of efficiency has been performed. R is found that the optimized casing design provides optimized recirculation flow rate, in which an increment of entropy rise is minimized at grooves and passages of the rotating impeller.

Investigation on One-Dimensional Loss Models for Predicting Performance of Multistage Centrifugal Compressors in Supercritical CO Brayton Cycle

The main compressor in a supercritical carbon dioxide(SCO2)Brayton cycle works near the critical point where the physical properties of CO_(2)are far away from the ideal gas.To investigate the effectiveness of the conventional one-dimensional(1D)loss models for predicting the performance of compressors working in such nontraditional conditions,detailed comparisons of 1D predicted performance,experimental data and threedimensional CFD results are made.A 1D analysis method with enthalpy and total pressure based loss system is developed for multistage SCO2 centrifugal compressors,and it is firstly validated against the experimental results of a single stage SCO2 centrifugal compressor from the Sandia National Laboratory.A good agreement of pressure ratios with experiments can be achieved by the 1D method.But the efficiency deviations reveal the potential deficiencies of the parasitic loss models.On the basis of the validation,a two-stage SCO2 centrifugal compressor is employed to do the evaluation.Three-dimensional CFD simulations are performed.Detailed comparisons are made between the CFD and the 1D results at different stations located in the compressor.The features of the deviations are analyzed in detail,as well as the reasons that might cause these deviations.

Two 3-D multi-cascade co-design inverse methods based on streamline curvature approach and Clebsch formulation for single-stage centrifugal compressors

Two strategies extended the single-cascade methods from a compressible three-dimensional inverse method for radial and mixed flow turbomachines to two three-dimensional multi-cascade co-design methods for single-stage centrifugal compressors.These two three-dimensional methods and a typical quasi-threedimensional streamline curvature through-flow inverse method were employed to design the same subsonic high-speed single-stage centrifugal compressors.The compressor performances were simulated by a commercial Reynolds averaged Navier-Stokes(RANS) equations solver.The studies show that two three-dimensional codesign methods are reasonable and feasible.It was found that : firstly the blade camber angle designed by the three-dimensional methods was larger than that designed by the quasi-three-dimensional method;and secondly with regard to two three-dimensional methods with different boundary conditions,the co-design result differences between the diffusers were small,but those between the deswirlers were relatively large.

Effect of bent inlet pipe on the flow instability behavior of centrifugal compressors

Bent inlet pipes are often used in centrifugal compressors due to limited installation space,and an understanding of the effect on compressor stability is essential for safety and durability.This paper firstly investigates flow instability behaviors in two compressors,one with a straight inlet pipe and the other with an S-shaped bent pipe.In detail,it analyzes the resulting flow fields,instability evolution paths and surge boundaries.The results show that the S-shaped pipe obviously affects the flow field at high mass flow rates,while reverse flow mainly influences the flow field at low mass flow rates.Reverse flow first occurs at certain flow passages with a high pressure difference that is predominantly decided by the volute rather than the S-shaped bent pipe.In addition,centrifugal compressors can tolerate reverse flow to some extent so that surge would not occur immediately if reverse flow occurs unless the reverse flow region extends circumferentially and radially to a sufficiently large size.Since the S-shaped pipe is not dominant in the creation and extension of reverse flow,it does not exacerbate the stability of the central compressor to a great extent.Last but not least,the S-shaped pipe is noted to delay the occurrence of surge at 90%rotating speed,which suggests the possibility of improving compressor stability with bent inlet pipes.This result differs from the conventional understanding that inlet distortion usually deteriorates compressor stability and emphasizes the particularity of centrifugal compressors.

Review on Stress Corrosion and Corrosion Fatigue Failure of Centrifugal Compressor Impeller

Corrosion failure,especially stress corrosion cracking and corrosion fatigue,is the main cause of centrifugal compressor impeller failure.And it is concealed and destructive.This paper summarizes the main theories of stress corrosion cracking and corrosion fatigue and its latest developments,and it also points out that existing stress corrosion cracking theories can be reduced to the anodic dissolution（AD）,the hydrogen-induced cracking（HIC）,and the combined AD and HIC mechanisms.The corrosion behavior and the mechanism of corrosion fatigue in the crack propagation stage are similar to stress corrosion cracking.The effects of stress ratio,loading frequency,and corrosive medium on the corrosion fatigue crack propagation rate are analyzed and summarized.The corrosion behavior and the mechanism of stress corrosion cracking and corrosion fatigue in corrosive environments,which contain sulfide,chlorides,and carbonate,are analyzed.The working environments of the centrifugal compressor impeller show the behavior and the mechanism of stress corrosion cracking and corrosion fatigue in different corrosive environments.The current research methods for centrifugal compressor impeller corrosion failure are analyzed.Physical analysis,numerical simulation,and the fluid-structure interaction method play an increasingly important role in the research on impeller deformation and stress distribution caused by the joint action of aerodynamic load and centrifugal load.

Fluid-structure Interaction Analysis and Lifetime Estimation of a Natural Gas Pipeline Centrifugal Compressor under Near-choke and Near-surge Conditions

Up to present, there have been no studies concerning the application of fluid-structure interaction（FSI） analysis to the lifetime estimation of multi-stage centrifugal compressors under dangerous unsteady aerodynamic excitations. In this paper, computational fluid dynamics（CFD） simulations of a three-stage natural gas pipeline centrifugal compressor are performed under near-choke and near-surge conditions, and the unsteady aerodynamic pressure acting on impeller blades are obtained. Then computational structural dynamics（CSD） analysis is conducted through a one-way coupling FSI model to predict alternating stresses in impeller blades. Finally, the compressor lifetime is estimated using the nominal stress approach. The FSI results show that the impellers of latter stages suffer larger fluctuation stresses but smaller mean stresses than those at preceding stages under near-choke and near-surge conditions. The most dangerous position in the compressor is found to be located near the leading edge of the last-stage impeller blade. Compressor lifetime estimation shows that the investigated compressor can run up to 102.7 h under the near-choke condition and 200.2 h under the near-surge condition. This study is expected to provide a scientific guidance for the operation safety of natural gas pipeline centrifugal compressors.

Inlet Recirculation Influence to the Flow Structure of Centrifugal Impeller

Inlet recirculation is proved as an effective way for centrifugal compressor surge margin extension,and is successively used in some engineering applications.Unfortunately its working mechanism is still not being well understood,which leads to redesigning of inlet recirculation mostly by experience.Also,most study about inlet recirculation is steady to date.It is necessary to study surge margin extension mechanism about inlet recirculation.To expose the mechanism in detail,steady and unsteady numerical simulations were performed on a centrifugal compressor with and without inlet recirculation.The results showed that,with inlet recirculation,the inlet axial velocity is augmented,relative Mach number around blade tip leading edge area is significantly reduced and so is the flow angle.As the flow angle decreased,the incidence angle reduced which greatly improves the flow field inside the impeller.Moreover,inlet recirculation changes the blade loading around blade tip and restrains the flow separation on the blade suction side at the leading edge area.The unsteady results of static pressure around blade surface,entropy at inlet crossflow section and vorticity distributions at near tip span surface indicated that,at near stall condition,strong fluctuation exists in the vicinity of tip area due to the interaction between tip leakage flow and core flow.By inlet recirculation these strong flow fluctuations are eliminated so the flow stability is greatly enhanced.All these improvements mentioned above are the reason for inlet recirculation delays compressor stall.This research reveals the surge margin extension reason of inlet recirculation from an unsteady flow viewpoint and provides important reference for inlet recirculation structure design.

Simulation of volute tongue of small-size centrifugal compressor

A high speed and small mass-flow-rate centrifugal compressor with original and modified volute tongue shape was simulated by 3D viscous Navier-Stokes equations.A sharp and a round tongue of volute were modeled to compare their pressure ratios and efficiency characteristics.The flow fields around volute tongues were investigated;the velocity and pressure distributions of volute inlet were studied by unsteady simulation.Static pressure fluctuation near volute tongue was monitored and transformed into amplitude spectrum to identify blade passing frequency influence.The results show that the tongue simplification can cause certain difference on pressure ratio and efficiency.The pressure and velocity distribution of volute inlet indicate obvious circumferential distortion due to volute tongue especially at low mass flow rate.In addition,the static pressure pulsation of volute inlet and the noise level in diffuser and volute increase significantly under low mass flow operating condition.

Numerical assessment on improving multistage centrifugal impeller performance by changing inlet skew angle at impeller inlet

Multistage centrifugal impellers with four different skew angles were investigated by using computational fluid dynamics.The purpose of this work is to investigate the influences of lean angle at the blade tip of the impeller inlet.Four variations of lean angles,that is,8°,10°,15° and 20°,were made at first stage impeller.Reynolds Average Navier Stokes equation was used in simulation together with a shear?stress transport(SST) k-w turbulence model and mixing-plane approach,respectively.Three dimensional fluid flows were simplified using periodic model to reduce the computational cost and time required.A good performance was expected that the secondary flow can be effectively reduced in the flow passage of the impeller without excessive increase in manufacturing cost caused by the secondary flow.The results show that secondary flow affects the main flow intricately to form vortices or having non-uniform velocity in the flow passage,which in turn results in substantial fluid energy loss not only in the impeller but also in the guide vane downstream of impeller.The numerical solutions were performed and allowed the optimum design and operating conditions to be obtained.

Investigation into the Interaction of Centrifugal Compressor Impeller and Vaneless Diffuser

Centrifugal compressors with parallel-wall and contracting wall vaneless diffuser are designed by using centrifugal compressor computer-aided integrated design system. The internal flow fields of the compressor are calculated by solving three-dimensional Navier-Stokes equation. Four aspects are investigated and calculation results show that the total efficiencies and total pressure ratios of the compressor with contracting wall vandess diffuser is higher than that of the compressor with parallel-wall. The jet and wake don＇t mix rapidly inside vandess diffuser. The outlet blade lean angle doesn＇t affect the compressor performance. The greater the mass flow rate through impeller, the more uneven the velocity distribution at impeller outlet is.

Design and Key Technology of Oil-Free Centrifugal Air Compressor for Hydrogen Fuel Cell

For a 120 kW hydrogen fuel cell system,a centrifugal air compressor with fixed power of 22 kW fuel cell is designed.Firstly,the theoretical calculation is carried out for the aerodynamic characteristics of a ultra-high-speed permanent magnet synchronous motor,an air compressor,and an aerodynamic foil bearing.Then,a prototype is trial-produced and a related test bench is built for test verification.Finally,both the simulation and test results indicate that the designed centrifugal air compressor meets the overall requirements of the hydrogen fuel cell system,and the relevant conclusions provide both theoretical and experimental references for the subsequent series development and design of the centrifugal air compressor.

NUMERICAL AND EXPERIMENTAL SURVEY OF CENTRIFUGAL MACHINE COMPONENTS

Although centrifugal compressors have been in use for over a century, much empiricism is still present in the compressor(or pump) design process. Therefore, a better understanding is needed of the flow process inside their components(impeller, diffuser including vaneless and vaned, and volute casing). The article reviewed process for the experimental and numerical published researches concerning this issue and calculated the 3 D flow pattern in Krain’s centrifugal impeller using CFX Tascflow code with CFX Turbogrid for grid generation. Higher number of grid points is used and the tip clearance is simulated using five grid points. The results show good agreement with that measured by Krain. The CFX Tascflow code can be applied to the analsis of the complicated flow structures in centrifugal impeller.

Experimental and Computational Analysis of the Unstable Flow Structure in a Centrifugal Compressor with a Vaneless Diffuse

The unstable flow phenomena in compressors, such as stall and surge, are closely related to the e ciency and the operating region. It is indispensable to capture the unstable flow structure in compressors and understand the mechanism of flow instability at low flow rates. Cooperated with the manufacturer, an industrial centrifugal compressor with a vaneless di user is tested by its performance test rig and our multi-phase dynamic measurement system. Many dynamic pressure transducers are circumferentially mounted on the casing surface at seven radial locations, spanning the impeller region and the di user inlet region. The pressure fields from the design condition to surge are measured in details. Based on the multi-phase dynamic signals, the original location of stall occurring can be determined. Meanwhile, the information of the unstable flow structure is obtained, such as the circumferential mode and the propagating speed of stall cells. To get more details of the vortex structure, an unsteady simulation of this tested compressor is carried out. The computational result is well matched with the experimental result and further illustrates how the unstable flow structure in the impeller region gradually a ects the stability of the total machine at low flow rates. The dynamic mode decomposition(DMD) method is applied to get the specific flow pattern corresponding to the stall frequency. Both experimental and computational analysis show that the flow structure at a particular radial location in the impeller region has a great impact on the stall and surge. Some di erences between the computational and experimental result are also discussed. Through these two main analytical methods, an insight into the unstable flow structure in an industrial compressor is gained. The result also plays a crucial role in the guidance of the compressor stabilization techniques.

EXPERIMENTAL INVESTIGATION OF ROTATING STALL FOR A LOW-SPEED CENTRIFUGAL COMPRESSOR

Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed. Secondly, the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire, which was compared with SENOO＇s analysis results. At last, the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry （PIV） system at different small flow fluxes. The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions. The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor.

Dynamic Analysis of a Centrifugal Compressor by Finite Element Method

This paper mainly deals with dynamic analysis of rotor-bearing system in a centrifugal compressor. A finite element model of the rotor-bearing system has been developed. The considered factors of the model include the rotary inertia of solid elements, stiffness and damping of hydrodynamic bearing. In the calculating, ANSYS software was used. Both calculated and measured results are in good agreement.

Study on seal improvement and rotor thrust control of centrifugal compressor

Fluid pressure variations due to process fluctuations or balance drum seal degradation can result in rotor thrust increasing that may jeopardize thrust bearing and compressor’s reliability. Also, the leakage flow through balance drum seal can seriously affect the efficiency of compressor. A method that can improve both the efficiency and reliability of centrifugal compressor is presented. The method focused on rotor thrust control and balance drum seal upgrading. The low leakage feature of Dry-Gas-Seal（DGS）, high reliability of labyrinth, and the feasibility of upgrading existing structure are taken into account at the same time to design a combined labyrinth-dry gas seal system on the balancing drum. Based on the combined seal system, a Fault Self-Recovering（FSR） system for the fault of rotor shaft displacement is introduced to assure the safety and reliability of centrifugal compressor. The modern Computational Fluid Dynamics（CFD） is used to validate this envision. The numerical result and relevant information indicate that the combined sealing system could improve the efficiency of the centrifugal compressor by about 4%.