High-pressure Centrifugal Compressors Research Articles

Cooling System of A High-Pressure Centrifugal Compressor: Development and Impact on Wheel Temperatures (GT2024-121464)

Abstract High power density, low fuel consumption, and a compact design with competitive total costs of ownership are key requirements for internal combustion engines for conventional fuels and carbon-free or carbon-neutral future fuels. These requirements influence the development targets of charging systems and call for single-stage charging systems with increased charge air pressures. However, high compressor pressure ratios of the turbocharger lead to challenging material temperatures in aluminum alloy wheels. Consequently, creep becomes the fundamental lifetime-limiting damage mechanism for these wheels. In order to meet customer requirements regarding total costs of ownership, the lifetime is of major importance for the development of aluminum compressor wheels for turbochargers with pressure ratios beyond 6. This paper focuses on a high-pressure compressor design with a water-cooling system for a new turbocharger generation. The development of its components and their validation based on numerically predicted and measured wheel temperatures are discussed. After introducing the need for high-pressure ratios and the accompanying lifetime challenges, the paper presents the design strategy and methods of the development process. Finally, the paper summarizes the achievements obtained by this procedure. Detailed and rarely before documented temperature measurements from the rotating compressor wheel are published. As will be shown, the temperature measurements confirm the predictions from the pre-development phase. A comparison of the temperature measurements from the recently developed wheel with measurements carried out for a state-of-the-art wheel reveals the improvements achieved by this new generation of high-pressure compressors.

VALIDATION OF THE HYDRAULIC TURBINE OPENFOAM LIBRARY FOR A HIGH-PRESSURE CENTRIFUGAL COMPRESSOR

A special program module was added to OpenFoam more than 10 years ago, and a large number of studies have been devoted to its use for solving turbine engineering problems. Modeling inaccuracies can occur when the velocity is increased to supersonic flow. The aim of this work is to validate the OpenFoam open library of hydraulic turbines for the a high-pressure centrifugal compressor flow calculation and finding the parameters and calculation model that are optimal in terms of efficiency and total/total pressure ratio. For the numerical calculation, we used Reynolds averaging of the Navier-Stokes equations, which are used in the CFD library OpenFoam v. 2212. In this work, the two most commonly used turbulence models for such problems are used to close the equations of the mathematical model: the k-e model and the Shear stress transport (SST) model. The simulation was performed using a modified version of the sonicFoam solver, which is available in OpenFoam. The maximum error in calculating the total pressure ratio in the working zone for all models was 3 %. All models behave poorly in the low mass flow rate zone. The slope of all curves corresponds to the experimental characteristic. The calculation using the models with very fine meshes on average determines a 2 % increase in the total-to-total pressure ratio. Using OpenFoam, the integrated parameters of a high-pressure centrifugal compressor can be predicted accurately (maximum error does not exceed 5 %). The most accurate efficiency is predicted by the SST model using meshes of more than 2.56 million elements, but this number of elements is not enough to predict the compressor's performance at low flow rate zone. In further research, it is rational to use grids consisting of at least 5 million elements and use the SST turbulence model.

Effect of Different Types of External Guide Vanes on the Performance of High-Pressure Centrifugal Compressor

In order to reduce exit swirl and obtain the desired Mach number, axial exit guide vanes (EGV) are often employed in a centrifugal compressor. NASA CC3 compressor, with wedge vane diffuser and without EGV, is considered as the base model for the analysis and validation. An axial flow domain with exit guide vane is added to this base model after the diffuser outlet to study the effect on the compressor performance. The performance of exit guide vane with different profiles: flat plate, symmetric wedge, circular arc, and airfoil vane profiles by maintaining the same chord, number of vanes, and flow angle of the vanes are studied. Numerical simulations are carried out with 60 number of exit guide vanes for all four types of vanes. Among several combinations, when the centrifugal compressor is equipped with 60 circular arc vanes as EGV, the efficiency and pressure recovery values at the design point have increased by 6.5% and 8.9%, respectively.

IMPROVING CHARACTERISTICS OF THE IMPELLER OF A HIGH-PRESSURE CENTRIFUGAL COMPRESSOR BY DESIGNING WITH THE HELP OF CAD

High-pressure compressors are among the most common pneumatic machines in industry and transport. Classical methods of designing such machines are based on flow modeling methods in two-dimensional and one-dimensional settings. The specified design methods lead to the creation of sufficiently perfect constructions, but their characteristics can still be improved. To simulate the flow in a high-pressure compressor, it becomes relevant to study the possibilities of optimizing the design using the Ansys Vista CAD system, which can significantly increase the efficiency of the compressor. The goal of the work is to improve the characteristics of the high-pressure centrifugal compressor impeller by designing with the help of CAD. The study was carried out on the basis of the design of the flow part of the impeller of a high-pressure centrifugal compressor using the Ansys VistaCCD automated design system based on the operation parameters of a mass-produced compressor. It was found that the efficiency maxima location of the designed and serial compressor impellers are approximately the same, while the polytropic efficiency of the designed impeller is 6 % more. At the same time, the ratio of total pressures decreases by 18 %. On the other hand, the high-efficiency zone of the impeller is significantly expanded for the designed impeller. This range has increased more than three times. At the same time, in the zone of optimal efficiency, the power consumed for the designed impeller decreases by 20 % to 240 kW. The designed impeller has a more perfect distribution of pressures for the splitter with approximately the same characteristics for the main blades of the impellers. Analyzing the velocity vectors and distributions, it can be derived that the serial compressor has two separation zones: in the blade bending region and at the outlet, unlike the designed one.

A Novel Vane-twisted Conformal Diffuser for Compact Centrifugal Compressors

Compact aero-engines that use centrifugal compressors are in high demand due to their small size and cost-effectiveness. However, limited improvements in the aerodynamic performance of centrifugal impellers have led to a greater focus on improving the performance of diffusers. This paper introduces a novel vane conformal diffuser designed to match an impeller with a high pressure ratio. The diffuser utilizes a unique design method that creates transitions from a two-dimensional meridian to a three-dimensional configuration, to achieve a twisted design for the vane and hub. Eventually an integrated vane configuration is formed from the radial section to the bend and axial sections. The novel diffuser significantly reduces the radial size of the entire compressor compared with a conventional vaned diffuser. Different cross-section area distributions are studied to explore the reasonable static pressure recovery ability of the diffuser. To validate the new concept, the diffusers of two existing high-pressure centrifugal compressors are redesigned using the novel conformal diffuser configuration. The numerical results show that the aerodynamic performances of the two redesigned centrifugal compressors are improved in terms of both the total pressure ratio and the isentropic efficiency compared with their counterparts. These results demonstrate the effectiveness and applicability of the developed design method for the novel conformal diffuser.

PATTERNS OF GAS FLOW IN A HIGH-PRESSURE CENTRIFUGAL COMPRESSOR

The development of three-dimensional modeling and analysis of the flow based on the solution of Reynolds-averaged Navier–Stokes equations allows to better determine the characteristics and flow parameters, but requires significantly more time spent on simulation and proof or verification of mathematical models in order to obtain minimal calculation errors. The aim of the work is to determine gas flow patterns in a high-pressure centrifugal compressor based on numerical methods for calculating the flow and to compare the integral characteristics with experimental data. The adequacy of the mathematical model with a displacement of the mass flow rate by 0.3 kg/s was confirmed. Nevertheless, the maximum mismatch of the determination of the total pressure ratio is equal to 8% in the zone of optimal values with flow displacement. The maximum error of determining efficiency is 9%. In the zone of optimal values, if the flow rate characteristic is shifted, the error of efficiency calculating does not exceed 2%. The pressure distribution along the impeller blades and the diffuser blades showed ways to optimize the blade shape to increase the compressor efficiency: there is an intersection of the lines for the optimal mode of the rotor blade; the splitter blade works in normal mode, but the distribution can be improved by changing the blade shape in the initial section; the intersection of the pressure lines for the optimal mode indicates the insufficient quality of the diffuser, which can be improved by changing the shape of the blade. For a more qualitative mathematical description of the flow patterns in the compressor, it is advisable to use the SST turbulence model, with a larger number of elements. The compressor characteristics were obtained numerically and the integral parameters of operation were determined. Keywords: high-pressure centrifugal compressor, numerical simulation, mathematical modeling, performances, adequacy

Experimental and numerical investigation on off-design performance of a high-pressure centrifugal compressor in compressed air energy storage system

In order to explore the off-design performance of a high-pressure centrifugal compressor (HPCC) applied in the compressed air energy storage (CAES) system, the author successfully built a high-pressure centrifugal compressor test rig for CAES, whose designed inlet pressure can reach 5.5 MPa, and carried out some experiments on adjustment of inlet guide vanes (AIGVs), adjustment of rotational speed (ARS) and adjustment of inlet pressure (AIP). Firstly, by analyzing the experimental data with the angle of AIGVs varying from −20° to +50°, it can be concluded that the mass flow range is widened by 41.18 %, the maximum efficiency is increased by 3.28 %, and the maximum pressure ratio can realize a slight increase. Then, the comprehensive performance map of the HPCC with AIGVs is obtained through interpolating the existing experimental data. Subsequently, the comparison between experimental and CFD results has been analyzed. Since the losses in the inlet expansion joint and outlet expansion joint, the aerodynamic performance obtained by experiments is slightly lower than CFD. Afterwards, the experiments on the ARS are carried out by three different speeds, and the change regulation of the performance by ARS is obtained. Lastly, the experiments on the AIP are carried out by seven different inlet pressure, when the inlet pressure of the HPCC is >50 % of the designed pressure, the results after simulated modeling are very close to the experiment results of the designed pressure. But once the inlet pressure is lower than 50 % of the designed pressure, the further the inlet pressure deviates from the designed pressure 5.5 MPa, the further the performance curve obtained by simulated modeling deviates from the design working condition. In this research, the experimental performance of a high-pressure centrifugal compressor applied in CAES under off-design conditions was obtained for the first time, which is of great significance to the research and development for the CAES system in the future.

Internal inflow study on a high-pressure centrifugal compressor with shroud and backside cavity in a compressed air energy storage system

The internal flow field and loss distributions are quite complicated in the high-pressure compressor with shroud and backside cavity applied in the compressed air energy storage (CAES) system. It’s necessary to develop physical understandings on the internal flow and losses inside the impeller, shroud cavity and backside cavity by the physical synergy relationship between some key quantities, which is innovatively applied to the internal flow of a high-pressure compressor used in CAES. First, the author successfully built a high-pressure centrifugal compressor test rig for CAES, and carried out the high pressure performance experiment for the first time. The aerodynamic performance of the high-pressure centrifugal compressor is compared by experiment and calculation. Then, the main flow characteristics and loss generation inside the impeller has been studied by analyzing the synergy between velocity and temperature gradient, and some other important quantities. And, the leakage flow characteristics inside the shroud cavity and backside cavity has been discussed by analyzing the synergy between velocity and pressure gradient, and some other key quantities. It is found that the regions where the high energy losses and entropy generation locate correspond to the relatively high synergy angle. At last, the interaction between leakage flow and main flow has been researched through clarifying the loss of temperature and radial velocity. It is found that there exit two boundary lines for leakage flow injecting into main flow.

Numerical study on the influence of shroud cavity in the high-pressure centrifugal compressor for compressed air energy storage system

As a key energy equipment of the compressed air energy storage (CAES) system, the centrifugal compressor with shroud cavity is employed to avoid the leakage flow from the rotor, especially in the high-pressure stages. In order to clarify the leakage characteristics and its influences, the high-pressure centrifugal compressor with shroud cavity is taken as the research object in the present work. First, the leakage flow across the shroud cavity has been studied numerically, it’s found that there are negative effects on the high-pressure compressor’s aerodynamic performance by the shroud cavity. And then, the flow characteristics inside the shroud cavity are showed by finding several secondary flow regions and cavity vortices. In addition, through the entropy contours and streamlines, the influence on the main flow by the leakage are analysed in detail. By analysing the influence of the shroud cavity on the high-pressure compressor, this paper can provide a practical guidance for the design of the impeller in the CAES system.

Experimental Investigation of Centrifugal Flow in Rotor–Stator Cavities at High Reynolds Numbers >108

The designers of radial turbomachinery need detailed information on the impact of the side chamber flow on axial thrust and torque. A previous paper investigated centripetal flow through narrow rotor–stator cavities and compared axial thrust, rotor torque and radial pressure distribution to the case without through-flow. Consequently, this paper extends the investigated range to centrifugal through-flow as it may occur in the hub side chamber of radial turbomachinery. The chosen operating conditions are representative of high-pressure centrifugal compressors used in, for example, carbon capture and storage applications as well as hydrogen compression. To date, only the Reynolds number range up to Re=2·107 has been investigated for centrifugal through-flow. This paper extends the range to Reynolds numbers of Re=2·108 and reports results of experimental and numerical investigations. It focuses on the radial pressure distribution in the rotor–stator cavity and shows the influence of the Reynolds number, cavity width and centrifugal mass flow rate. It therefore extends the range of available valid data that can be used to design radial turbomachinery. Additionally, this analysis compares the results to data and models from scientific literature, showing that in the higher Reynolds number range, a new correlation is required. Finally, the analysis of velocity profiles and wall shear delineates the switch from purely radial outflow in the cavity to outflow on the rotor and inflow on the stator at high Reynolds numbers in comparison to the results reported by others for Reynolds numbers up to Re=2·107.

Numerical Investigation of Rotating Stall Characteristics in a Full-Annulus Transonic Centrifugal Compressor

The present paper numerically investigates the rotating stall mechanism in the NASA CC3 transonic centrifugal compressor with wedge-type vaned diffuser. The three-dimensional compressible, unsteady Navier–Stokes equations have been applied to a full-annulus high-speed centrifugal compressor comprising an inlet duct, impeller, and vaned diffuser. The scale-adaptive simulation model is selected as the turbulent closure model. The aim of this paper is to predict and capture the key features of rotating stall in a transonic high-pressure centrifugal compressor based on the scale-adaptive simulation approach. First, the steady numerical model was validated by the experiment for the aerodynamic compressor performance at different operating conditions from choke to surge. Then, unsteady simulations were conducted near the surge point to propose a detailed analysis of stall characteristics within the compressor diffuser passages. Comparison of the unsteady pressure signals with those of the measurement data revealed that the numerical results matched well with the experimental data to capture the rotating speed of the stall cells. Time/space analysis of transient entropy production distribution was performed within the diffuser passages to show the stall behavior and to confirm the existence and development of rotating stall cells during the impeller revolutions. Evaluation of the spectral analysis of the computed pressure fluctuation by the current numerical approach showed reasonable agreement with the experimental data for blade passing and rotating stall frequencies. The present paper demonstrates that the current numerical model is a beneficial engineering approach to provide accurate solutions compared to experimental data for prediction of the rotating stall as well as to resolve the unsteady turbulence structures.

Generalized Model for the Approximation of Coupled Acousto-Mechanical Natural Frequencies in High-Pressure Centrifugal Compressors

Abstract The oil and gas, chemical, and process industries employ centrifugal compressors for a wide range of applications. Due to this, the conditions under which centrifugal compressors have to operate vary significantly from case to case. Gas pipeline compressors, for example, may feature discharge pressures well over 100 bar. In other fields of application, like gas injection, which is used to enhance oil recovery, this quantity can reach considerably higher values. Here, discharge pressures over 600 bar and gas densities over 300 kg/m3 are not uncommon. During the last several decades, comprehensive research was conducted on the impact of high-pressure operating conditions on the vibrational behavior of centrifugal compressor wheels. Nowadays, it is well-known that an increase in gas pressure levels leads to a more pronounced interaction between the side cavities and the impeller, which results in a frequency shift of the acoustic and structural modes. For the safe operation of compressors, it is necessary to predict these coupled natural frequencies accurately. The state-of-the-art approach to achieve this objective is the finite element method. While this technique provides high-quality results, the simulation of acousto-mechanical systems is still a time-consuming process that incurs high computational costs. Therefore, finite element models are, in this case, not suitable for probabilistic studies, sensitivity analyses, and comprehensive simulations of the full operating range of the compressor. In 2013, Magara proposed a simplified model based on an annular plate between two cylindrical cavities to solve this problem. While this method reduces the required computational effort significantly, its use is limited to platelike impellers. The authors of this paper propose a more generalized method to overcome the challenges mentioned earlier. It uses the uncoupled structural and acoustic modes of the actual impeller and side cavities in a modal superposition to approximate the natural frequencies of the coupled acousto-mechanical system. In this way, the intended design geometries of the impeller and side cavities are considered while maintaining the advantages of Magara's model regarding the computational effort. In a numerical study, Magara's method and the generalized model are applied to different systems of increasing complexity. The investigation starts with a simple annular plate in a cylindrical cavity and ends with an actual compressor impeller. At every complexity level, the results of both approaches are compared to a finite element analysis. Moreover, measurement data of a simplified rotor in a cylindrical cavity are used to validate the numerical models. Finally, this paper concludes with a discussion of the limitations and benefits of all employed numerical methods.

К определению циркуляционной газодинамической силы, действующей на ротор центробежного компрессора с оппозитными рабочими колесами

This paper considers the main factors affecting the occurrence of a circulating gas-dynamic force in the labyrinth seal of a two-section high-pressure centrifugal compressor acting on the rotor. A method for calculating the circulating force is presented on the example of a real centrifugal compressor.

Detection of vaneless diffuser rotating stall by means of dynamic pressure sensors and acoustic measurements

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors. Several techniques have been proposed to detect the stall onset; inter alia, few dynamic pressure probes have been shown to not only properly detect the phenomenon, but also reconstruct the stall characteristics after an ensemble averaging approach. The massive use of this technique in the field is, however, not a common practice yet. In the present study, the use of dynamic pressure probes has been combined with that of an environmental microphone to evaluate the prospects of this latter for a possible stall onset detection. To this end, experimental tests have been carried out in the experimental test rig of the Department of Industrial Engineering (DIEF) of Università degli Studi di Firenze. Results show that the microphone was able to distinguish the onset of rotating stall accurately and promptly, even though – differently from dynamic pressure sensors - it does not provide sufficient information to determine the characteristics of the stall pattern. On this basis, the use of acoustic measurements could find room for automatic detection of rotating stall onset.

Numerical research on sealing performance of high speed and high pressure centrifugal compressor seal

In this paper, two seal schemes of stepped labyrinth seal and straight through-hole seal are selected for the shaft end seal of S-CO2 centrifugal compressor with the output of 100kW. The full three-dimensional CFD numerical simulation method is adopted to analyze and compare the sealing characteristics of two S-CO2 seals in S-CO2 centrifugal compressor under different outlet back pressures and sealing gaps. The results show that the leakage flow rate of S-CO2 stepped labyrinth seal and through-hole seal increases with the decrease of back pressure. The leakage flow rate of the stepped labyrinth seal and the through-hole seal reaches the corresponding critical value when the back pressure is 1.0 MPa and 3.0 MPa respectively. The critical leakage flow rate of the stepped labyrinth seal and the through-hole seal under the condition of 0.1 mm sealing gap is 0.158 kg/s and 0.128 kg/s respectively. Compared to the through-hole seal, the stepped labyrinth seal can withstand greater pressure differentials at the same sealing gap and has superior S-CO2 sealing performance.

Simple Contact Stiffness Model Validation for Tie Bolt Rotor Design With Butt Joints and Pilot Fits

Abstract Tie bolt rotors for centrifugal compressors comprise multiple shaft components that are held together by a single tie bolt. The axial connections of these rotors—including butt joints, Hirth couplings, and Curvic couplings—exhibit a contact stiffness effect, which tends to lower the shaft bending frequencies compared to geometrically identical monolithic shafts. If not accounted for in the design stage, shaft bending critical speed margins can be compromised after a rotor is built. A previous paper had investigated the effect of tie bolt force on the bending stiffness of stacked rotor assemblies with butt joint interfaces, both with and without pilot fits. This previous work derived an empirical contact stiffness model and developed a practical finite element modeling approach for simulating the axial contact surfaces, which was validated by predicting natural frequencies for several test rotor configurations. The present work built on these previous results by implementing the same contact stiffness modeling approach on a real tie bolt rotor system designed for a high pressure centrifugal compressor application. Each joint location included two axial contact faces, with contact pressures up to five times higher than previously modeled, and a locating pilot fit. The free-free natural frequencies for different amounts of tie bolt preload force were measured, and the frequencies exhibited the expected stiffening behavior with increasing preload. However, a discontinuity in the data trend indicated a step-change increase in the contact stiffness. It was shown that this was likely due to one or more of the contact faces becoming fully engaged only after sufficient tie bolt force was applied. Finally, a design calculation was presented that can be used to estimate whether contact stiffness effects may be ignored, which could simplify rotor analyses if adequate contact pressure is used.

A Novel Bulk-Flow Model for Pocket Damper Seals

Abstract In this paper, a novel bulk-flow model for pocket damper seals (PDS) is introduced. The model is based on two control volumes (CVs) for each circumferential pocket of the seal. The continuity, circumferential momentum, and energy equations are considered for each control volume. The circumferential recirculating flow within the pocket is modeled for the first time. The boundary layer theory is used to estimate the recirculating flow area, and the Swamee–Jain friction factor correlation allows for defining the dissipation of the circumferential velocity. The perturbation method is used to solve the partial derivative governing equations in the zeroth- and first-order system of equations. The rotordynamic coefficients are evaluated by integrating the dynamic pressure and rotor shear stresses along the circumferential direction. The predictions are compared to the experimental data, which refer to test conditions representative of high-pressure centrifugal compressors. Numerical predictions are accurate for both high positive–negative inlet preswirl ratios. Leakage predictions are also aligned with measurements. Finally, sealing selection approach is introduced in the paper for comparing the dynamic behavior of two different sealing technologies and identifying stable regions as a function of the rotor natural frequency and preswirl ratio.

Некоторые особенности реализации расчетной модели высоконапорной центробежной компрессорной ступени с входным направляющим аппаратом

Некоторые особенности реализации расчетной модели высоконапорной центробежной компрессорной ступени с входным направляющим аппаратом

Development of a Criterion for a Robust Identification of Diffuser Rotating Stall Onset in Industrial Centrifugal Compressors

Recent studies showed that a prompt detection of the stall inception, connected with a specific model to predict its associated aerodynamic force, could provide room for an extension of the left margin of the operating curve of high-pressure centrifugal compressors. In industrial machines working in the field, however, robust procedures to detect and identify the phenomenon are still missing, i.e., the operating curve is almost ever cut preliminarily by the manufacturer by a proper safety margin; moreover, no agreement is found in the literature about a well-defined threshold to define the onset of the stall. In particular, in some cases, the intensity of the arising subsynchronous frequency is compared to the revolution frequency, while in many other ones it is compared to the blade passage frequency. A large experience in experimental stall analyses collected by the authors revealed that in some cases unexpected spikes could make this direct comparison not reliable for a robust automatic detection. To this end, a new criterion was developed based on an integral analysis of the area subtended to the entire subsynchronous spectrum of the dynamic pressure signal of probes positioned just outside the impeller exit. A dimensionless parameter was then defined to account for the spectrum area increase in proximity to stall inception. This new parameter enabled the definition of a reference threshold to highlight the arising of stall conditions, whose validity and increased robustness was here verified based on a set of experimental analyses of different types of full-stage test cases of industrial centrifugal compressors at the test rig.

A Three-Dimensional Time-Accurate Computational Fluid Dynamics Simulation of the Flow Field Inside a Vaneless Diffuser During Rotating Stall Conditions

An accurate characterization of rotating stall in terms of inception modality, flow structures, and stabilizing force is one of the key goals for high-pressure centrifugal compressors. The unbalanced pressure field that is generated within the diffuser can be in fact connected to a non-negligible aerodynamic force and then to the onset of detrimental subsynchronous vibrations, which can prevent the machine from operating beyond this limit. An inner comprehension on how the induced flow pattern in these conditions affects the performance of the impeller and its mechanical stability can therefore lead to the development of a more effective regulation system able to mitigate the effects of the phenomenon and extend the left-side margin of the operating curve. In the present study, a 3D-unsteady computational fluid dynamics (CFD) approach was applied to the simulation of a radial stage model including the impeller, the vaneless diffuser, and the return channel. Simulations were carried out with the TRAF code of the University of Florence. The tested rotor was an industrial impeller operating at high peripheral Mach number, for which unique experimental pressure measurements, including the spatial reconstruction of the pressure field at the diffuser inlet, were available. The comparison between experiments and simulations showed a good matching and corroborated the CFD capabilities in correctly describing also some of the complex unsteady phenomena taking place in proximity of the left margin of the operating curve.