Centrifugal Compressor Rotor Research Articles

Study on Dynamic Characteristics of Single-Span Rotor Under Cross-Coupling Stiffness Control on Non-drive End

Abstract The sealing-induced cross-coupling stiffness (CCS) often decreases the rotordynamic stability of turbomachinery. In our previous study, an active negative CCS control strategy applied between two bearings in the middle of the rotor was validated to enhance the stability of the rotor system. In this paper, a different approach was taken by applying both positive and negative CCS control strategies to the non-drive end, aiming to investigate their effects on rotordynamic stability using a rotordynamic model. The model represented a single-span centrifugal compressor rotor with CCS at the seal node. The logarithmic decrements and unbalance response of the rotor system were compared under different CCS applications at the non-drive end and in the span. The results demonstrate that applying both positive and negative CCS at the non-drive end can improve system stability, but the positive CCS strategy has limitations, and its applicability is not as extensive as Negative CCS. A “critical stiffness” phenomenon emerges when the rotor system is actively stabilized at the non-drive end, resembling critical resonance. Furthermore, when components like seals introduce cross-coupling stiffness out of the span, they contribute to system stabilization. These findings provide valuable insights into actively enhancing the stability of rotor systems.

Machine learning algorithm functional on environmental sustainability assessment in turbomachinery sector: Application on centrifugal compressors

The current government directives have focused industries' attention on environmental sustainability issues in products and processes. There is indeed a growing demand from customers to conduct environmental impact assessments of the products they purchase. This work presents the implementation of a predictive model developed in an industrial context to evaluate the environmental sustainability of a centrifugal compressor rotor assembly. The development of the predictive model arises from the objective of overcoming the limitations of the traditional Life Cycle Assessment approach, which is based on a retrospective evaluation of the product life cycle. The functionality of predictive models is to estimate product environmental sustainability to meet customer demands and guide them toward choices that aim for carbon neutrality. The implementation of the model has been conducted in parallel with a tailored measurement campaign of the primary inventory flows involved in various manufacturing operations. The article details the methodological approach that led to the development of the predictive models and their respective functionality in supporting the design engineer in evaluating the eco-profile of the assembly. In addition to the methodological aspect, the work also includes a case study through which the functionality of the models can be illustrated.

Numerical study of the coherent characteristics of the blade tip of a micro centrifugal compressor and its application in a new unsteady casing-treatment experiment

The tip-leakage vortex, as the dominant coherent structure of the blade tip of a compressor, is an important source of losses in centrifugal compressor rotors; it limits the stall margin and can even cause compressor surge. The miniaturization of mechanical components has attracted significant attention; however, the miniaturization of compressors—particularly centrifugal compressors—further increases the adverse effects of the tip-leakage vortex. This study sought to establish a new passive unsteady flow-control method for a micro centrifugal compressor using equal-circumferential-spacing through-holes on its casing. This approach fully exploits the unsteady characteristics of the flow field. A numerical study of the coherent characteristics of the blade tip was carried out using dynamic mode decomposition. The numerical conclusions were then applied to a physical system. The results of experimental tests of this system indicate that the improved flow-control method can increase the maximum efficiency and stall margin of the compressor by 2.5% and 9.0%, respectively, and it can also increase the maximum pressure ratio.

The Characteristics of Lateral Critical Speeds of Flexible Rotors with Rigid Supports.(Dept.M)

A simple method of calculation of lateral critical speeds of flexible rotors with rigid supports is presents is presented. The method is mainly based on the former work of Myklestad and Prohl and considers the shaft to be divided into a number of finite elements connected by massless elastic sections. A digital computer program is then prepared to solve the finite equations resulting from the analysis. By means of this program, the critical speeds of flexible rotors can be predicted at different stiffnesses of the rigid supports and mode shape of the whirling rotor is predicted. An actual cases of a centrifugal compressor rotor having different characteristics is considered.

A new unsteady casing treatment for micro centrifugal compressors to enlarge stall margin

Tip clearance flow, particularly tip leakage vortex, is an important source of losses in centrifugal compressor rotors, which limits the stall margin and even causes compressor surge. To address this issue, a passive unsteady flow control method is proposed with equal-circumferential-spacing through-holes on the casing of a micro centrifugal compressor. The numerical simulation shows that the unsteady excitation near the generating position of the leakage vortex can weaken the fluctuation caused by the leakage vortex. The stall margin increases by 19.3% and the efficiency decreases by 0.8% when the mass flow rate of the excitation flow is only 0.12% of the main flow. Furthermore, a proof experiment on a micro centrifugal compressor shows that the stall margin increases by 13.3% considering a 100% design rotation speed and 15.4% considering an 80% design rotation speed, while the efficiency and pressure ratio decrease slightly. Therefore, both the simulation and experiment prove that the flow control method is effective and can easily be used in practice.

A Study for Rotordynamic and Leakage Characteristics of a Long-Honeycomb Seal With Two-Phase, Mainly Air Mixtures

Abstract This paper investigates the impact of the oil (silicone oil PSF-5cSt) presence in the air on the leakage and rotordynamic characteristics of a long-honeycomb seal with length-to-diameter ratio L/D = 0.748 and diameter D = 114.656 mm. Tests are carried out with inlet pressure Pi = 70.7 bars, pressure ratio (PR) = 0.35 and 0.25, inlet liquid volume fraction (LVF) = 0%, 3.5%, and 7%, and shaft speed ω = 10, 15, and 20 krpm. During the tests, the seal is centered. Test results show that leakage mass flow rate m˙ increases (as expected) as inlet LVF increases. Increasing inlet LVF makes direct stiffness K increase more rapidly with increasing excitation frequency Ω. Increasing inlet LVF has a negligible effect on K at low Ω values, but increases K at high Ω values. The value of effective damping Ceff at about 0.5ω is an indicator to the system stability since an unstable centrifugal compressor rotor can precess at about 0.5ω. Increasing inlet LVF increases the value of Ceff at about 0.5ω, reducing the possibility of subsynchronous vibrations (SSVs) at about 0.5ω. San Andrés's model is used to produce predictions. The model assumes that the test fluid in the seal clearance is an isothermal-homogenous mixture. The model adequately predicts m˙, K, and the value of Ceff at about 0.5ω.

Synergy Methodology for Internal Flow of Turbomachinery

The complex curvature of turbomachinery rotor blade channels combined with strong rotational effect and clearance leakage brings on intricate internal flow phenomenon. It is necessary to study the internal flow and energy loss mechanism to reveal the influence law of the key parameters and to achieve its optimal design. Considering features of flow and temperature fields in rotor passage, the concept of synergy analysis derived from equation of energy conservation was put forward. Typical NASA low-speed centrifugal compressor (LSCC) rotor was chosen for analysis using CFD. Numerical results showed remarkable agreement with experiment datum in both the tendency of the performance characteristics and quantitative pressure values. Under different flow rates and inlet total temperatures conditions, thermal-fluid interaction effect and losses were studied by synergy analysis. Results showed that peak synergy positive value zones located around blade leading edge, across the shroud wall and hub wall, and at the position where tip-leakage flow was mixing with the bulk flow and high entropy zones existed. Increasing flow rate from design condition, positive and negative synergy areas both changed tiny around leading edge and trailing edge. Reducing flow rate, positive synergy areas tended to increase and negative areas decreased at same positions. The relationship between flow separation, heat transfer and losses in turbomachinery rotor can be revealed based on synergy analyses.

Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

This paper experimentally studies the effects of changing radial clearance Cr on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with Cr = 0.188, 0.163, and 0.140 mm, inlet pressure Pi = 62.1 bars, exit pressure Pe = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients Hij are measured. The real parts of Hij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct KΩ and cross-coupled kΩ stiffness coefficients are used. The imaginary parts of direct Hij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing Cr decreases (as expected) the leakage mass flow rate m˙. Under mainly-air conditions, decreasing Cr decreases KΩ. This outcome is contrary to the test results at pure-air conditions, where KΩ increases as Cr decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping Ceff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing Cr increases Ceff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. m˙ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing Cr on KΩ and the Ceff value near 0.5ω. After the oil is added, as Cr decreases, predicted KΩ increases while measured KΩ decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing Cr does not discernibly change predicted Ceff but increases the measured value.

Some problem of centrifugal compressors upgrading

The results of computational investigation of the influence of a gas labyrinth seals, oil end seals with floating rings and oil journal bearings lubricating layer stiffness coefficient on centrifugal compressor rotor critical speed are presented and analysed. The main principles of a method for axial forces acting on centrifugal compressor rotor calculation are listed. The method has been developed at the compressor, vacuum and refrigeration technologies department of SPbSTU. In addition, the estimation of method applicability in engineering calculations is given.

Исследование критических частот ротора центробежного компрессора с учётом жёсткости опор и уплотнений

Исследование критических частот ротора центробежного компрессора с учётом жёсткости опор и уплотнений

Numerical modeling of aeroelastic behavior of GTU centrifugal compressor rotor

This paper deals with simulating oscillatory modes for the centrifugal compressor rotor of the gas transmittal unit by means of computational experiments taking into account rigid type supports and aeroelastic processes in the wheels. Validation of the aeroelastic approach was carried out on the model problem of interaction between a supersonic flow and a deformable plate in the shock tube. For numerical simulation the ANSYS engineering analysis system was used in this work. The research methods are the finite volume method for gas dynamics and the finite element method for estimating components of the stress-strain state. Dependences of pressure and stress-strain state components on geometric, physical-mechanical, kinematic parameters of the rotor and characteristics of the working fluid in compressors of gas transmittal units are obtained.

Investigation of deformation at a centrifugal compressor rotor in process of interference on shaft

In this paper, according to the finite element method, we had implemented “master- slave” method of contact interaction in elastic deformable bodies, with consider of the friction in the contact zone. We had compiled the orientation of solving extremum problems with inequality restrictions, projection algorithm, which called “the closest point projection algorithm”. Finally, an example, had brought to show the calculation of the rotor nozzle centrifugal compressor on the shaft with interference.

Rotordynamic Optimization of Fixed Pad Journal Bearings Using Response Surface Design of Experiments

The design process of journal bearings of turbomachines is complex and time-consuming due to the many geometric and physical variables involved. This paper reports on the design of experiments (DOE) and the response surface design of experiments (RSDOE) methods employed on the design of the drive-end and free-end three-lobe journal bearings supporting a centrifugal compressor rotor. The suitability of each technique is discussed. The bearing design variables employed are bearing slenderness ratio, radial clearance, preload, and lubricant inlet temperature. The rotordynamic response variables selected were the critical speed location, the vibrations at critical speed and operating speed for both bearings, and the threshold speed of instability. The use of a nonlinear (quadratic) RSDOE model is justified. An optimization approach combining an SRDOE and rotordynamic finite element modeling is presented. This method leads to arrive to a multivariate model for multi-objective optimization with very few computations. Identification of the dominant design variables and their effects on several response variables allows establishing engineering feasible solutions with focus on manufacturing versus operating conditions tradeoff.

Flow Stability Model of Centrifugal Compressors Based on Eigenvalue Approach

A stall inception model of centrifugal compressors is developed on the basis of the global stability analysis in the present paper. This model takes specific flow patterns and impeller geometry into account, which can be used to analyze the effect of blade design parameters on the stall limit of centrifugal compressors. In particular, the body force model and the solution of the stability eigenvalue equation are two important problems to be addressed in this frame. The spectral method and singular value decomposition are used to discrete stability equations to obtain the eigenvalues. By applying conservation laws, a perturbation-response form of the body force, specifically for centrifugal compressor, is also established to simulate blade behavior. Two different forms (the meridional model and the normal expansion model) are derived to make a comparison of grid independence and computation time. The capacity of these models is assessed against the experimental data of a NASA low-speed centrifugal compressor rotor. The results show that these models could be used to predict stall onset with acceptable accuracy and low computational cost.

English

This paper continues the recent research of the author, with application to 3D computational fluid dynamics multicriterial optimization of turbomachinery parts. Computational Fluid Dynamics has been an ubicuous tool for compressor design for decades, helping the designers to test the aerodynamic parameters of their machines with great accuracy. Due to advances of multigrid methods and the improved robustness of structured solvers, CFD can nowadays be part of an optimization loop with artificial neural networks or evolutive algorithms. This paper presents a case study of an air centrifugal compressor rotor optimized using Numeca's Design 3D CFD suite. The turbulence model used for the database generation and the optimization stage is Spalart Allmaras. Results indicate a fairly quick convergence time per individual as well as a good convergence of the artificial neural network optimizer.

Steady Air Injection Flow Control in Centrifugal Compressor

In the present work steady air injection upstream of the leading edge was used in a centrifugal compressor, Whose preliminary design of compressor injection systems can be modeled by a geometrical relationship between user-specified yaw angle and resulting blade incidence angle based on simple velocity triangles, the error between the best yaw angle obtained from this relationship and that obtained from numerical simulation is less than 3%. To reveal the mechanism, steady numerical simulations were performed on high pressure ratio centrifugal compressor rotor operated with a rotor tip speed of 586 m/s. Parametric studies of the injection yaw angle was performed to determine the configuration that provide the best steady results for the compression systems studied in this work. The injectors were placed at short distance (ten percent of the inlet tip radius upstream of the compressor face) the objective of this was to achieve maximum control over the leading edge flow by varying individual injection parameters. The injection angle, α, was fifteen while the yaw angle, β, was parametrically varied. The results show that at design speed (n= 50 000 r/min) with injection flow rate equal to 3% of the main flow rate and 25 degree air injection yaw angle can lower the mass flow rate at stall for approximately 7.5%.

Flow Study of a Redesigned High-Pressure-Ratio Centrifugal Compressor

A high specific speed, 6:1 total pressure ratio centrifugal compressor rotor has been improved with the knowledge gained from previous investigations on similar types of compressors and with the help of advanced 3D-design calculations. The new rotor has the same main dimensions as its predecessor and was designed for the same operating point. It was manufactured and was tested at DLR with conventional and optical measurement techniques. Performance and optical measurements were carried out up to tip speeds of 586 m/s. The investigations were supported by 3D stage calculations which delivered detailed insight into the impeller and diffuser flow development.

A Fully Three-Dimensional Inverse Method for Turbomachinery Blading in Transonic Flows

This paper presents a procedure to extend a recently developed fully three-dimensional inverse method for highly loaded turbomachine blades into the transonic-flow regime. In this inverse method, the required three-dimensional blade profile to produce a prescribed swirl schedule is determined iteratively using the blade boundary conditions. In the present implementation, the flow is assumed to be inviscid and the blades are assumed to be infinitely thin. The relevant equations are solved in the conservative forms and are discretized in all three directions using a finite-volume technique. Calculations are carried out for the design of high-pressure axial- and centrifugal-compressor rotors. These examples include prescribed swirl schedules corresponding to blade designs that are shock-free and blade designs that have rapid compression regions in the blade passage.

A Three-Dimensional Euler Solver for Turbomachinery Blade Rows

This paper describes a three-dimensional Euler solver for turbomachinery blade rows. The algorithm used is based on the explicit, four-step, finite volume method advocated by Jameson. Some of the issues addressed include the spatial interpolation scheme compatible with the wall boundary condition; the appropriate interpolation scheme for correctly interpolating a uniform absolute flow in a rotating coordinate system; smoothing techniques that assure global conservation; and code vectorization. The Euler solver has been tested on turbine nozzles, turbine rotors, centrifugal compressor rotors, transonic fans, and propellers, in all cases without any modification to the code or any parameter adjustment.

Damage to Axial Compressors

A centrifugal compressor rotor is quite robust, but the blading of an axial compressor looks horrifyingly fragile and very susceptible to damage by objects sucked into the engine. The introduction of axial compressor engines into airline service brings this again to the attention of designers and service engineers. The object of this paper is to outline the problem, and to describe one experimental approach to assessing the relative merits of different blade materials from the damage point of view.