Role of unsteady tip leakage flow in acoustic resonance inception of a multistage compressor

In previous studies,a theoretical model was developed after Acoustic Resonance(AR)was experimentally detected in a four-stage compressor,and AR inception was proposed to be triggered by an unknown sound source,which is a pressure perturbation of a specific frequency with a suitable circumferential propagation speed.The present paper,which is not dedicated to the simulation of acoustic field,aims to identify the specific sound source generated by the unsteady tip leakage flow using the unsteady Computational Fluid Mechanics(CFD)approach.After a comprehensive analysis of an Unsteady Reynolds Averaged Navier-Stokes(URANS)simulation,a pressure perturbation of non-integer multiple of rotor frequency is found at the blade tip.Since the essence of the tip leakage flow is a jet flow driven by the pressure difference between two sides of blade,a simplified tip leakage flow model is adopted using Large Eddy Simulation(LES)in order to simulate the jet flow through a tip clearance.It is found that the convection velocity of shedding vortices fits the expected propagation speed of the sound source,the frequency is also close to one of the dominating frequencies in the URANS simulation,and the resultant combination frequency coincides with the experimentally measured AR frequency.Since such a simplified model successfully captures the key physical mechanisms,it is concluded that this paper provides a piece of unambiguous evidence on the role of unsteady tip leakage vortex in triggering the AR inception of the multistage compressor.

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.

Numerical and experimental study of bleed impact in multistage axial compressors

In this study,the influence of inter-stage bleeding on the compressor performance and inter-stage flow field of a multistage axial compressor is investigated by both experimental and numerical methods.The experiment is conducted on a four-stage low-speed axial compressor,and a specific computational model is built to simulate the experiment environment accurately.To illuminate the fluid mechanisms of bleeding effect in detail,both the experiment and the simulation are carried out twice,i.e.,in the first time,the mass flow rate upstream the bleed location is constant under different bleed rate conditions;while in the second time,the mass flow rate downstream the bleed location is constant under different bleed rate conditions.The results demonstrate that inter-stage bleeding has little influence on upstream compressor characteristics,and affects the upstream flow field only in the rear half of the stator.The bleed effect on the downstream flow field is embodied in the variation of an incoming flow profile,an increase as the compressor inlet flow coefficient decreases.Therefore,such an effect is only significant on compressor characteristics at small flow coefficient conditions.In multistage compressors,the variation of compressor characteristics and flow field caused by inter-stage bleeding is the comprehensive result of the bleeding and the variation of the upstream working condition.In addition,the comparison between numerical and experimental results shows that the flow moves towards top half of span through the downstream rotor passage in the numerical simulation,whereas the trend of flow field variation with different bleed rates at the outlet of the downstream rotor and stator is the same with that at the inlet of the downstream rotor in the experiment,which means that the numerical method has overestimated the radial mixing intensity of the flow.

Experimental study of rotor blades vibration and noise in multistage high pressure compressor and their relevance

Rotor blades fault of aeroengine compressor is mostly caused by mechanical and aerodynamic excitation.And the excitation factor of high intensity sound wave to rotor blades should not be ignored.Experimental researches are conducted on a multistage high pressure compressor.When high level vibration occurs on the first stage of rotor blades, the noise spectrum presents typical characteristic for discrete multi-tone in the compressor.The amplitude of blade vibration displacement and the sound pressure level of characteristic frequency noise increases and decreases simultaneously and reaches the maximum value at the same time.This frequency merely occur on a certain speed range and is locked in a specific range which presents no variation with the rotating speed.When high level vibration occurs on the first stage of rotor blades, the noise spectrum presents a sharp peak and the propagation state of the characteristic frequency is a helix structure in the compressor.It can be confirmed that acoustic resonance occurs in the multistage compressor.The acoustic resonance frequency and its side band frequencies are generated by modulation of a rotating noise source at the rotor speed which is the excitation source of the rotor blades vibration.

Stator re-stagger optimization in multistage axial compressor

As a widely applied technique in multistage axial compressors,variable stator vanes(VSV)can flexibly rematch the blade rows to fulfil a variety of aerodynamic performance requirements,such as high efficiency and wide surge margin.The purpose of this paper is to develop an optimization method to quickly determine VSV settings during the preliminary design phase.A mean-line method with a model calibration procedure is adopted to evaluate compressor performance,and the NSGA-II algorithm is employed for automatic optimization.The developed optimization system is then employed to determined re-stagger arrangement in a multistage compressor.A single-speed optimization with performance constraints of specific operating point and a multi-speed optimization with different control laws are conducted.Results are compared with available experimental re-stagger scheme,which verifies the effectiveness of the re-stagger optimization method.Moreover,method is proposed to determine operating parameters of a working point with a user-defined pressure ratio or mass flow rate after variable geometry.

A Simple and Unified Model for Spanwise Mixing in Multistage Axial Flow Compressors

A basic equation system for meridional throughflow fields in multistage axial flow compressors has been deduced, containing many unknown correlation terms,which describe different kinds of spanwise mixing mechanism in a unified form.The equation system shows that spanwise mixing of meridional flows in compressors is attributed to three kinds of mechanism:molecular motion,turbulent diffusion,and circumferential non-uniformities,the last of which includes secondary flow effects and others.Therefore the equation system unifies the two models for spanwise mixing analyses by Adkins & Smith (1981) and Gallimore & Cumpsty (1986).With three kinds of apparent mixing coefficients defined and introduced into the basic equation system,a novel,much simpler equation system,without additional unknown correlation terms included,has been obtained.This novel equa- tion system makes throughflow computations including mixing far easier for multistage compressors.It has been rigorously shown that these apparent mixing coefficients contain full information of all the three kinds of mixing mechanism,so that the simpler equation system can also be taken as a unified model for meridional flows with all the kinds of the mixing effects.Calculations of the flow through multistage machines have been made by incorporating the new model into a streamline curvature throughflow calculation method and the improved agreement with experimental data has been obtained.It is believed that the simpler equation system can be applied to the flows not only in subsonic but in transonic and supersonic compressors if an appropriate model is proposed for the apparent mixing coefficients.

Search for Compromise Solution of the MultistageAxial Compressor's Stochastic Optimization Problem

The aim of this paper is to discuss a method of the compromise region determination for the multistage axial flow compressor stochastic optimization problems. This method is based on the 2-D axisynunetrical mathematical model of the compressor and on the new multicriteria optimization procedure.A specific feature of the multicriteria optimization procedure is a possibility to obtain a set of the Edgeworth-Pareto optimal solutions within the frame of single optimization task. The paper presents some examples of the compressor’s geometrical parameters multicriteria optimization.

Numerical Investigation of Base-Setting of Stator's Stagger Angles for a 15-stage Axial-Flow Compressor

A 15-stage axial-flow compressor utilized in steel industry was studied in this paper. All the stator's stagger angles of the compressor are variable to ensure the multistage compressor operate effectively within a wide range of flow rate and meanwhile satisfy the demand for sufficient pressure ratio, adiabatic efficiency and stall margin. Three in all different base-settings of stator's stagger angles were presented and commercial CFD software was applied to obtain the overall performance characteristics. The results showed that both of the optimized base-settings improved the performances both in summer and winter conditions, although the adiabatic efficiency was somewhat decreased. Taking incidence angle and stage loading into consideration, differences among the three cases were analyzed in detail. On the basis of numerical computations, the performance could be effectively improved through adjusting the base-setting of stator's stagger angles.

Modal Decomposition for the Analysis of the Rotor-stator Interactions in Multi-stage Compressors

A modal analysis method of the rotor-stator interactions in multistage compressors has been developed by LMFA. This method, based on a double modal decomposition of the flow over space and time, has been applied to nu- merical and experimental results of the high-speed 3Y2-stage compressor CREATE based at LMFA, Lyon-France. It reveals the presence of a very strong rotor-stator interaction which completely drives the flow at casing behind all the rotors. This modal analysis method applied to an unsteady RANS simulation permits to calculate the en- ergy of the rotor-stator interactions and to plot energetic meridian maps to explain experimental results and to analyze the interaction in the whole machine.

Physical model for acoustic resonance in annular cavity structure

A physical model for acoustic resonance in the annular cavity structure is developed to represent the typical characteristic when acoustic resonance occurs.Firstly,the measurement of sound pressure in the casing and rotor blades vibration is operated in a multistage high pressure compressor.The sharp peak frequency and discrete multi-tone occur in the frequency spectrum of sound pressure in the compressor,and the vibration of the first stage of rotor blades synchronously presents the high amplitude.The frequencies associated with rotor blades vibration can be calculated with rotating sound source theory.It is also confirmed that acoustic resonance occurs in the multistage compressor.With acoustic similarity principle,an annular cavity model is established to simulate the typical characteristics of acoustic resonance in the compressor based on Large Eddy Simulation(LES)and Lighthill acoustic analogy.The coupling relationship between cavity acoustic mode and disc vibration mode shape is expounded when acoustic resonance occurs in the model.And acoustic resonance will be locked in the certain flow rate range.All these characteristics match well with those occur in the multistage high pressure compressor.

A minimalist approach for detecting sensor abnormality in oil and gas platforms

Compressors play an important role in day-to-day operation in most oil and gas platforms,especially in the case for maintaining gas pressure in transportation pipe.Its complex problem to detect the sensors health and abnormality as the sensor reading would reflect the various states of the compressor.In ideal situation,sensor readings offer vast amounts of information on compressor health and could possibly indicate early fault of machines.Furthermore,due to harsh site and process operating conditions,sensors are often found to have drifted or failed,and there is no standard methodology to predict abnormality apart from applying emerging industrial smart sensor technologies.In this paper,we investigate a minimalist approach for detecting abnormality of compressor's shaft's RPM sensor.As the sensors in the compressor are correlated,we first use the outputs of other sensors to predict the shaft's RPM using regression-based models(neural networks and multiple linear regression).Second,we calculate the histogram of residuals by taking the difference between the predicted sensor value and the actual sensor value plus the abnormality in terms of bias/miscalibration and noise.The histogram of residuals can be used for sensor abnormality monitoring.In general,sensor states can be monitored by observing the shifting of the mean in the histogram of residuals.The sensor readings contaminated with noise can be seen by a shifted mean whose value is between the normal condition mean and the biased condition mean.This method is compact and would be relevant to monitor irregularity of the sensors.