Centrifugal Compressor Design Research Articles

Centrifugal compressor design and cycle analysis of large-scale high temperature heat pumps using hydrocarbons

Using natural refrigerants in heat pumps, including hydrocarbons, is becoming attractive because of the tightening regulations concerning synthetic refrigerants. In the literature there is lack of studies investigating detailed component design, including compressor design when using hydrocarbons. This study investigates numerically centrifugal compressor design with hydrocarbons for large-scale high-temperature heat pumps. The compressor analysis was combined with a thermodynamic analysis of a two-stage heat pump. The effect of temperature lift and flash intercooler temperature on compressor sizing, rotational speed, efficiency, axial force, and Mach number were investigated. Generally, the use of hydrocarbons with five carbon atoms resulted in higher coefficient of performance and required lower compressor rotational speeds and larger compressor dimensions when compared to the hydrocarbons with four carbon atoms. The simulated rotational speeds were ranging from 8 krpm to 30 krpm, impeller diameters from 0.15 m to over 0.7 m and compressor efficiencies from 77% to over 85%. The use of cyclic molecules, namely cyclopentane and cyclobutene resulted in the highest heat pump coefficient of performance. Also, the flash intercooler temperature had a notable effect on the compressor design and axial force which has to be taken into account in heat pump cycle design.

Design and analysis of centrifugal compressor in carbon dioxide heat pump system

Based on the advantages of energy saving, environmental protection and high efficiency, carbon dioxide heat pump system has great application prospects. However, there are still many technical problems to be solved, especially the design and optimization of carbon dioxide centrifugal compressor. In this paper, a centrifugal compressor in carbon dioxide heat pump system is designed. The compressor is directly driven by a high-speed permanent magnet synchronous motor. Two-stage impellers are installed on both sides of the motor, and the bearings are active magnetic bearings. The influences of inlet pressure and temperature on compressor performance are analyzed. In the range of inlet temperature from 35 to 55 °C, with the decrease of inlet temperature, the compressor pressure ratio increases by 12–29.8%, the power increases by 2.7–8.6%. In the range of inlet pressure from 4 to 6 MPa, with the increase of inlet pressure, the compressor pressure ratio increases by 12.3–38.6%, and the power increases by 8.7–17.8%. In addition, the calculation method of compressor axial force is introduced, the axial force is calculated, analyzed and optimized. Furthermore, the rotor dynamics of compressor rotor and the influences of bearing stiffness and diameter of motor rotor on rotor dynamics are studied. With the increase of bearing stiffness, the first-order critical speed and maximum displacement of the rotor increase. The research provides a theoretical reference for the design and optimization of centrifugal compressor in carbon dioxide heat pump system.

An axisymmetric characteristic model for enhancing prediction of centrifugal compressor characteristics

Rapid and accurate determination of compressor characteristic maps is essential for the initial design of centrifugal compressors in aircraft power systems. The accuracy of existing methodologies, which rely on combinations of loss models, varies significantly depending on the compressor’s geometry and operational range. This variance necessitates substantial experimental or Computational Fluid Dynamics (CFD) data for coefficient calibration. To address this challenge, this study presents an axisymmetric characteristic model for compressor performance assessment. This model incorporates the factors of blade angle, meridional passage area, and the radial deflection angle of meridional streamlines of the compressor. These factors are derived from fundamental aerodynamic equations encompassing mass, momentum, and energy conservation of the compressor. In contrast to conventional one-dimensional approaches, the proposed method reduces the number of loss coefficients and more effectively accounts for the impact of geometric alterations on centrifugal compressor properties. Furthermore, the model reduces dependence on experimental and CFD data. Efficacy of the model is validated using experimental data from four distinct types of centrifugal compressors. Correlation analysis reveals that the model’s coefficients can be expressed as functions of the ratio of the Reynolds number to the impeller tip speed. This ratio serves as a characteristic parameter for the design and optimization of centrifugal compressors. Consequently, the proposed method offers an efficient and accurate means for the quick computation of centrifugal compressor characteristics. This is of great significance for improving the efficiency of centrifugal compressors and reducing energy consumption.

Centrifugal compressor design for high-temperature heat pumps

Heat pumps are identified as crucial technology for decarbonising the heat production. Especially for large-scale, high-temperature heat pumps the use of centrifugal compressors has been seen as an attractive technological option. However, there are very few detailed studies available discussing the compressor designs with different refrigerants for high-temperature heat pumps. This study combines the analyses of a 500 kW two-stage heat pump cycle and centrifugal compressor designs. The study qualitatively assesses the feasibility of compressor designs and provides information on how the dimensions, efficiency, and rotational speed are influenced by cycle conditions and the refrigerant. Temperature lift and evaporation temperature were varied with eight working fluids, including hydrocarbons and hydrofluoro-olefins. The estimated COPs were 2.0–2.55 with the highest values observed with pentane and isopentane. The compressor rotational speeds were between 20 – 50 krpm with hydrocarbons requiring higher values than fluids containing fluorine. The predicted compressor efficiencies were between 77% to 84%, with low pressure compressor reaching higher efficiencies than the high-pressure compressor. However, high compressor Mach numbers were observed, and the Mach number seems to be the main obstacle for the increase of the heat pump temperature lift when centrifugal compressors are used.

Design and development of a stable supercritical CO2 centrifugal compressor

Introducing supercritical carbon dioxide (s-CO2) as a working fluid can enhance the power generation of Brayton cycles. However, various challenges are involved in designing, and developing a stable s-CO2 centrifugal compressor on a reasonably sized s-CO2 Brayton cycle. This article presents a 1-D design and development procedure for s-CO2 centrifugal compressors suitable for Brayton power cycles. The 1-D compressor design approach employs the Widom region, AMC, and internal and external loss models to optimize the compressor design. The 1-D design code is validated with CFD simulation and experimental data from Sandia. Further, a new stable compressor for a 10 MWe s-CO2 Brayton cycle is designed, proposed, and discussed in this paper. Based on the 1-D design code and the CFD results, the newly designed compressor exhibits an improved operational range. Additionally, a loss analysis of the designed s-CO2 compressor revealed that internal losses accounted for 75.8% of the total enthalpy loss. The compressor's internal flow behavior at choke is investigated by utilizing a novel analogy between the impeller at choke and a converging–diverging nozzle. The CFD results showed that condensation is inevitable in an s-CO2 centrifugal compressor operation; however, condensation does not significantly affect compressor performance near the design point.

Investigation of turbulence models for aerodynamic analysis of a high-pressure-ratio centrifugal compressor

The aerodynamic design and optimization of modern centrifugal compressors are inseparable from computational fluid dynamics (CFD) tools. In a design environment, CFD simulations are expected to predict compressor performance characteristics with reasonable accuracy. Turbulence models and model simplification can play a significant role in the discrepancy between analysis and reality. To investigate the influence of turbulence models on aerodynamic analysis results of centrifugal compressors, the National Aeronautics and Space Administration's high efficiency centrifugal compressor stage is selected as the test vehicle. Six eddy-viscosity models, namely, Spalart–Allmaras (SA), standard k–epsilon (k−ε), renormalization group k–epsilon (RNG k−ε), Wilcox k–omega (k−ω), shear stress transport (SST), and SST-reattachment modification (SST-RM) are considered. In all numerical analyses, measured surface roughness is included with the aim of improving numerical solution accuracy. Although some discrepancies exist, the compressor performance characteristics predicted by the k−ω model are in satisfactory agreement with the test data over the whole speedline. The k−ε model overpredicts the isentropic efficiency than the other models, while the SST-RM model underestimates the total pressure ratio in the near-stall region. The RNG k−ε model matches high flow characteristics but overestimates the choked mass flow rate. The SST model obtains a similar operating range with the test data but underestimates the compressor's isentropic efficiency and total pressure ratio, while the SA model totally overestimates the compressor characteristics due to the wall function. Furthermore, variations caused by turbulence models are discussed together with the flow field analysis. Finally, to verify the impact of the impeller backplate bleed cavity, an additional simulation is conducted that specifically accounts for this feature.

High-performance aerodynamic design of supercritical CO2 centrifugal compressor for the Megawatt-class nuclear microreactor (MSC-GFR)

Design of the SCO2 centrifugal compressor is critical for the fuel economy and power output of the nuclear microreactors. In this study, we developed a high-performance aerodynamic design & analysis platform (XJHPCCD) for real-gas centrifugal compressors. Based on the single-zone method, the platform realizes efficient and fast solution to the direct and inverse problems of centrifugal stages in real gases. Using the XJHPCCD platform, we performed a principal component analysis on the performance prediction errors in different combinations of the multiply expressed loss mechanisms. On the results of composite analysis, an optimal loss model was proposed regarding the hundred-kW SCO2 compressor in Megawatt-class microreactors. By coupling the optimal loss model in performance prediction, the errors of pressure rises were reduced from 5–10% to 3% for the Sandia’s verified model, greatly promoting the reliability of aerodynamic analyses. To demonstrate the engineering values of the specifically optimized toolkit, we numerically conducted a design research on the main compressor of a 1 MWt SCO2-cooled fast breeder microreactor (MSC-GFR). Results showed the XJHPCCD platform (coupled with the optimal loss model) has well satisfied the technical demands with high-performance aerodynamic design and low prediction errors. Thereby, it is proved that we have successfully proposed a specifically optimized aerodynamic toolkit for the component design of Megawatt-class microreactors.

Aerodynamic design and experimental study of small flow rate centrifugal compressor

The aerodynamic design of a single-stage of a small flow rate centrifugal compressor is completed by theoretical analysis and CFD simulation, and the simulation results can meet the target performance. In addition, an equal proportion prototype performance test bench was designed and built, and the prototype performance test using air as working medium was completed. The results show that the overall trend of the measured flow-pressure ratio curve fits well with the simulation curve. Due to the error of simulation calculation, the measured mass flow rate is a little small and there is a certain difference with the simulation data. Based on the results, the structure of impeller and volute was optimized, and the performance was retested. The retest results show that the flow range of the measured pressure ratio and efficiency curve can basically cover the simulated curve, and the difference between the measured value and the simulated value of pressure ratio and efficiency decreases significantly. Using air medium, when the mass flow is 0.01kg/s, the pressure ratio is 1.17 and the isentropic efficiency is 85%. The results confirm the feasibility of this design method, which can provide important reference for the design of small-flow centrifugal compressor.

Centrifugal Compressors Gas-Dynamic Characteristics Influence on the Refrigerating Machines Efficiency

The article is devoted to the study R134a refrigerating machine efficiency and consisting of a centrifugal compressor, a condenser, a temperature-regulating valve and an evaporator. The main purpose of the work is to analyze the centrifugal compressor gas dynamic characteristics effect on the refrigeration machine vapor-compression cycle efficiency. This goal is achieved through the study by actual working process numerical experiment in the refrigeration machine centrifugal compressor with an idealized process for other elements. The object of the study are the refrigeration machine characteristics, expressed by the theoretical refrigeration coefficient COPRt. Single-stage centrifugal compressors with the design conditional flow coefficient in the range from 0.035 to 0.12 are considered. The design of centrifugal compressors was carried out according to a new calculation method to the flow part efficiency increase. The method comprehensively combines the inviscid and viscous flow calculations with the use of the single-criteria and multiparametric optimization. Previously, the method was tested and compared with experimental data. The most important result is the results of the refrigeration cycle efficiency evaluating through the centrifugal compressors highly efficient flow parts design methodology application. An increase in COPRt was obtained taking into account the centrifugal compressor actual process in the range from 2.6% to 7.2%. The significance of the results obtained lies in the possibility of using high-efficiency centrifugal compressors gas dynamic characteristics for the chillers refrigeration cycles analysis and calculation. The level of the compressors isentropic efficiency ranges from 0.80 to 0.85, depending on the design conditional flow coefficient.

Novel multidisciplinary design and multi-objective optimization of centrifugal compressor used for hydrogen fuel cells

Novel multidisciplinary design and multi-objective optimization of centrifugal compressor used for hydrogen fuel cells

Blade design and analysis of centrifugal compressors for the transcritical carbon dioxide refrigeration cycle

The compressor is one important component in the transcritical CO2 (carbon dioxide) refrigeration cycle. The CO2 compressor has a small volume at the same refrigeration capacity because its refrigeration capacity per unit volume is large. The radial compressor often used in the CO2 refrigeration cycle has complex internal flow and difficult blade modeling. The paper presented a new centrifugal compressor with radial flow, whose blades are straight and easy to process. Taking a single stage compressor blade for example, the effects of the velocity coefficient, energy head coefficient, rotational speed of the moving blade, blade height, inlet radius and radial distance of the blade on the efficiency and pressure ratio were studied. According to the conclusions, the paper designed and simulated the blades of two compressors of the transcritical CO2 two-stage refrigeration cycle. Simulation result showed that the efficiencies of the two compressors were respectively 84.97% and 87.23%.

Data-driven modeling of high-speed centrifugal compressors for aircraft Environmental Control Systems

The Environmental Control System (ECS) is the main consumer of non-propulsive power onboard aircraft. The use of an electrically-driven Vapor Compression Cycle (VCC) system, in place of the conventional air cycle machine, can lead to a substantial increase of the coefficient of performance. This work documents the development of an integrated design optimization method for VCC-based aircraft ECS, where the sizing of the system is performed along with the conceptual design of the compact heat exchangers and the high-speed centrifugal compressor. A data-driven model of the compressor has been developed to reduce the complexity of the VCC system model and the computational cost of the associated optimization problem. The model is based on artificial neural networks and has been trained on a synthetic dataset of 165k centrifugal compressor designs, generated with an in-house tool. The case study selected to demonstrate the capabilities of the proposed methodology is the multi-objective design optimization of an electrically-driven VCC system for the ECS of a single-aisle, short-haul aircraft, flying at cruise conditions. The results show that the number of function evaluations needed to identify the Pareto front reduces by a factor of three when using the data-driven model, in place of a meanline method. At the same time, the robustness of the numerical solver is improved, leading to the identification of optimal solutions covering a wider design space. Finally, the proposed methodology enables the analysis of the trends established between the system performance metrics and the design of the individual components.

Modification and Validation of 1D Loss Models for the Off-Design Performance Prediction of Centrifugal Compressors with Splitter Blades

One-dimensional (1D) aerodynamic performance predictions are very often conducted by researchers and designers during the preliminary design of centrifugal compressors. This paper focuses on a 1D prediction method for centrifugal compressors with splitter blades, which is rarely seen in the open literature. One-dimensional prediction of aerodynamic overall performance is made for centrifugal compressors with different technical design specifications. However, the aerodynamic overall prediction accuracy relies on the accuracy of the 1D-loss-models used. Therefore, an optimum combination of loss models is proposed by summarizing a variety of loss models presented in the public literature. In addition, an optimization method is utilized to optimize some coefficients involved in loss models in order to improve the generality of the combined model. The modified models obtained in this study are proved to have good predictive accuracy.

Physics-based surge point identification for unsupervised CFD-computation of centrifugal compressor speed lines

We present a speed line calculation model using physics-based surge point identification. When using surrogate model-based design of experiment (DOE) techniques, the large number of computations demands an automated, unsupervised process to calculate the speed line and to identify its significant points. A high-fidelity CAE workflow is used to calculate the machine design, create the CAD geometry, generate a mesh, and prepare a CFD setup. The choke point, the surge point and the peak efficiency point are sequentially identified using sophisticated identifiers and optimization algorithms. The model was used to calculate the speed line of 50 different centrifugal compressor designs with 50 geometry variations each. From these 2,500 designs, 2,449 speed lines were calculated successfully, without specific settings having been assigned to individual designs. This proves that the model enables the stable, reproducible, reliable and effective CFD calculation of the speed line of centrifugal compressors.

Performance Improvement of a Highly Loaded Transonic Centrifugal Compressor with Tandem Impeller and Freeform Blade Configuration

A highly loaded transonic centrifugal compressor is aerodynamically designed and numerically investigated. The objectives are to improve the compressor efficiency by using tandem impeller configuration and 3D free-formed blade design concepts. This approach has the potential to control both the transonic and distorted flows within impeller passages. The results suggest that employing the tandem impeller configuration can significantly improve the compressor efficiency by 1.4%. The efficiency gain is mainly contributed by the improved uniformity of the impeller discharge flow achieved when the newly generated inducer-shed vortices rearrange the secondary flow pattern. In addition, the location of the impeller passage shock moves downstream due to the potential effect within the tandem impeller and locally changes the back pressure of the inducer. These factors mitigate flow losses in the impeller and diffuser. Furthermore, the 3D design concepts of forward blade sweep and negative lean are employed in the tandem impeller configuration. The forward sweep design of the inducer weakens flow acceleration before the passage shock, and the negative lean design optimizes the secondary flow pattern, which yield an additional compressor efficiency improvement of 0.7%. The study conducted in this paper provides a valuable reference for future advanced transonic centrifugal compressor designs.

Preliminary Design of a Mini Gas Turbine via 1D Methodology

To address the increasing interest towards more environmentally friendly naval transportation and the introduction of IMO2020 restrictions on pollutant emissions onboard ships, the present work details the preliminary design of a mini gas turbine engine, i.e., a gas turbine engine with an output power up to 5 MW, for onboard energy generation. In comparison to conventional propulsion systems, gas turbine units benefit from known compactness, which can be further enhanced by employing single-stage uncooled radial machines, according to similar works in the field. As such, the present paper aims to set up a complete procedure that allows a reliable and fast (i.e., requiring a limited computational effort) preliminary design of one-stage centrifugal compressors and radial turbines operating at a high pressure ratio via the use of classical one-dimensional theory. The aerodynamic design outputs in terms of forces and torques are then used to perform a preliminary mechanical design of the shaft by means of a one-dimensional finite element model with commercial software to estimate the corresponding shaft line stress. Despite some necessary geometrical and modeling simplification of the design problem, which results in the unavailability of detailed information on individual components, the employed procedure nevertheless allows a comprehensive overview of the possibilities in terms of maximum machine performance achievable at an early design stage with the associated limited computational requirements. The design procedure and the geometry achieved for the application are presented along with aerodynamic and structural results.

A novel method for work capacity calculation of centrifugal compressor impellers in energy storage systems

As the basis for centrifugal compressor design, the traditional one-dimensional scheme assumes that there is no flow prewhirl at the impeller inlet (i.e., C1u=0) when inlet guide vanes (IGVs) are absent. However, various experiments have proved that a natural prewhirl does exist at the impeller inlet even there is no IGV. In this study, a novel method is proposed to calculate the Euler work of impellers considering the natural prewhirl, and firstly applied to centrifugal compressors without IGVs. Stodola’s approach dealing with the slip velocity at the impeller outlet is adopted to the flow at the impeller inlet. A new formula for the Euler work calculation is deduced to evaluate the effect of natural prewhirl on centrifugal impellers. Then, this method is applied in two different model stages of industrial centrifugal compressors to verify against the experimental and numerical data. The results indicate that, compared to the traditional scheme assuming C1u=0, the novel method with modified formula considering the influence of C1u≠ 0 is more feasible for high-speed centrifugal compressors. With the aid of the novel calculation method proposed in this paper, the one-dimensional design accuracies of centrifugal compressor stages are improved by 7% compared with the traditional method.

Optimizing the performance of a single-shaft micro gas turbine engine by modifying its centrifugal compressor design

The purpose of this paper is optimizing the performance parameters of a single-shaft micro gas turbine with a power of about 150 kW at the design point by modifying its centrifugal compressor geometry. According to the physics of this problem, a hybrid optimization approach (including modern and numerical optimizers) was employed. The optimization was based on the numerical simulation of each geometry using a three-dimensional Reynolds Averaged Navier-Stokes solver. The geometric parameters of the centrifugal compressor impeller constituted the design variables and the Latin hypercube was employed for sampling. To reduce the cost of the numerical calculation, the neural network was chosen for approximate modeling of the design space. The physics of the optimization problem ahead was identified and a state-of-the-art optimization process was presented accordingly. Furthermore, a detailed discussion was conducted on the design space transparency, convergence criteria, and optimum design selection, the disadvantages of current optimization processes and the advantages of the present one were explained. NASA CC3 compressor was scaled down for the baseline engine, which is a 150 kW single-shaft gas turbine. This engine was optimized using the proposed optimization process and a significant improvement was achieved, where efficiency increased by 13.93% and power by 11.07%, and the specific fuel consumption decreased by 12.15%. Furthermore, despite the constant impeller tip diameter and rotational speed, in the optimum compressor, the mass flow increased by 2.22% and the pressure ratio by 29.62% compared to the baseline compressor, while the isentropic efficiency not only did not decrease but also increased slightly (0.2%). However, the operating range decreased by 12.64%.

The Effect of Size and Working Fluid on the Multi-Objective Design of High-Speed Centrifugal Compressors

The impact of size and working fluid on the efficiency, operating range, and axial thrust on bearings is examined for high-speed, oil-free centrifugal compressors. First, the development and validation of a reduced-order model based on scaling principles is documented. Then, the validated compressor model is used to generate design maps for stages operating with arbitrary fluid molecules, and characterized by different size. The results show that compressors operating with fluids made by heavy and complex molecules provide lower efficiency over the entire design space, if compared to their simple-molecule counterparts. However, compressors for complex-molecule fluids require lower rotational speed, and generate lower axial thrust on bearings, thus making them particularly suitable for small-scale applications. Furthermore, a decreasing value of the size parameter has a detrimental effect on the stage efficiency, as a result of manufacturing constraints. The results computed by the compressor model suggest that the efficiency penalty is more sensitive to variations of clearance gap than to surface finishing. Lastly, the reduced-order model has been used to perform a design exercise, i.e., the multi-objective optimization of the first compressor stage of the heat pump test rig being realized at Delft University of Technology. The key characteristics of the optimal compressor design has been compared to those derived from the design maps, to corroborate their validity. The optimal design has been extensively characterized by means of CFD, providing further evidence that efficient mini-compressors operating with organic fluids, and featuring pressure ratios up to five at off-design, are feasible.

The flow mechanism of the shock structure and secondary flow control in transonic centrifugal compressors with leading edge sweep

Limitations still exist in the research on the effects of leading edge sweep on transonic centrifugal compressors. The lack of understanding of this mechanism also leads to confusion regarding the magnitudes of the performance changes that occur due to the use of leading edge sweep. According to a detailed numerical simulation verified by experiments on two different compressors, the effect and flow mechanism of leading edge sweep on transonic centrifugal compressors were investigated in detail to alleviate the confusion as clearly as possible. The conclusions show that leading edge sweep has different impacts on shock waves at different positions; a forward sweep leading edge can weaken the strength of the shock wave in front of the leading edge, while a backward sweep leading edge can weaken the strength of the passage shock wave that is swallowed in the blade passage. The improved compressor performance attributed to leading edge sweep is due to the weakening of the shock waves and the interaction between the shock waves and the secondary flow of the inducer tip. Furthermore, the intensity of the inducer tip secondary flow influences the potential for improved performance induced by using sweep leading edges in transonic centrifugal compressors. This study provides a valuable reference for applying leading edge sweep to existing transonic centrifugal compressors or future advanced centrifugal compressor designs.