High-speed Centrifugal Compressor Research Articles

A Review of Numerical Methodologies for Predicting Rotating Stall and Surge in High-Speed Centrifugal Compressors

Abstract High-speed supersonic radial compressors are a critical enabling technology for meeting the requirements of future aviation-propulsion and thermal-management systems. These turbomachines must be designed to be both efficient and robust on the widest possible operating range. Flow instabilities in the form of rotating stall and surge are therefore phenomena that must be accurately predicted early in the design process. Unsteady full-annulus computational fluid dynamics (CFD) can be used to get accurate information about the onset of instabilities, but at the expense of costly simulations. As a result, the design of new compressors continues to rely on existing correlations for the prediction of the critical mass flowrate. This approach, however, leads to suboptimal compressor designs. This article provides a review of the numerical methodologies that can be used for the accurate prediction of the critical mass flowrate in high-speed centrifugal compressors. Methods of different fidelity level and computational cost are described. Two particularly promising models, namely, those proposed by Spakovszky and Sun, are subsequently examined in more detail. Exemplary applications of these two models are finally discussed.

Compressor Surge Identification in Innovative Heat-Pump Systems Equipped by Vaned Diffuser Centrifugal Compressor

Abstract In the present energy scenario, heat-pumps play an important role to improve energy efficiency in reversible cooling systems. In case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they work in variable operation and therefore they must withstand off-design conditions. Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor to study unstable operation of refrigerant closed loop systems. To this aim, vibro-acoustic signature analysis of such closed loop rig is performed mainly focusing on compressor mechanical response. This experimental characterization is firstly conducted in system stable operation. Afterwards, surge transients are obtained by acting on plant feeding lines valves; meanwhile, experimental signals are acquired at relevant plant locations to investigate system response just before instability onset. Indeed, system dynamics is affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. Therefore, system dynamics is analysed both in sub-synchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions towards surge. By doing so, suitable surge precursors can be defined by relying only on vibro-acoustic signals both in low and high frequency ranges to perform early surge detection in such chiller systems. The proposed approach main advantage is to exploit non-intrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid.

Viability assessment of large-scale Claude cycle hydrogen liquefaction: A study on technical and economic perspective

The competitiveness of hydrogen as a sustainable energy carrier depends greatly on its transportation and storage costs. Liquefying hydrogen offers advantages such as enhanced purity, versatility, and higher density, yet current industrial liquefaction processes face efficiency and cost challenges. Although various large-scale and efficient liquefaction concepts exist in the literature, they often overlook the economic and technical viability of such plants. Here, we addresses this issue by establishing a framework for modeling a large-scale hydrogen liquefaction concept and conducting both technical and economic assessments, with a specific focus on 125 tonnes per day (TPD) high-pressure hydrogen Claude-cycle concept. The technical analysis involves preliminary designs of key process components, while the economic assessment utilizes Aspen Process Economic Analyzer. Our findings indicate that at an electricity price of €0.1/kWh, the Claude-cycle liquefier concept yields a specific liquefaction cost (SLC) of €1.55/kgLH2. A sensitivity analysis was performed, which shows that electricity price has a significant influence on the economics. Further investigation on the compressors design shows that incorporating high-speed centrifugal compressors could reduce the SLC by 5.42% and potentially more. Scaling up to 250 and 500 TPD reveals further cost improvements, while cost projections indicate substantial declines as the technology matures. Ultimately, this paper presents novel cost-scaling and experience curves of hydrogen liquefaction technology, demonstrating the compelling economic viability of integrating large-scale hydrogen liquefaction into sustainable energy infrastructure.

A numerical investigation of the effect of vaned diffuser water cooling on the internal flow field of a centrifugal compressor

Due to their high pressure rises, the gas temperature of high-speed centrifugal compressors can be very high to the detriment of compressor aerodynamic performance. A water-cooled vaned diffuser technique is proposed to alleviate this problem. Compressor housing integrated water-cooling passages are employed to introduce cooling water to the vanes through holes near the vane centre. The technique was applied to a 180 mm turbocharger compressor running at the tip speed of 452 m/s, and numerical studies with conjugate heat transfer and fluid-structure thermal interaction were carried out to find the effects of the cooling to compressor performance and the mechanical stress of diffuser vanes. The results show that compressor efficiency is improved by the cooling, and the improvement increases as compressor mass flow reduces. At modest pressure ratios around 3.3, the maximum improvement of 1.09 percentage points is achieved using 30°C inlet water temperature. The cooling is found to significantly reduce the air temperature near diffuser passage solid walls, thus lead to more uniform air temperatures at compressor exit. The reduction of 1.33°C and 2.21°C in the diffuser outlet temperature have been found at 1.68 kg/s air mass flow with 70°C and 30°C water inlet temperature respectively. Moreover, the thermal stress in diffuser vanes is shown to be lower with the cooling.

Integrated design optimization method for novel vapour-compression-cycle-based environmental control systems

The aircraft Environmental Control System (ECS) is the primary consumer of non-propulsive power at cruise conditions, hence, its performance optimization is crucial for the reduction of specific fuel consumption. A novel integrated system design optimization method is presented: thermodynamic cycle, component sizing and working fluid are taken into account simultaneously. This method was applied to the ECS of large rotorcraft based on a Vapour Compression Cycle system electrically driven by a high-speed centrifugal compressor. Steady-state and lumped parameter system component models have been developed using the Modelica acausal modelling language. The optimization design framework consists of an in-house code, featuring a Python-Modelica interface. The study case refers to a critical operating condition: the helicopter is on the ground during a hot and humid day. The working fluid is R-134a. The multi-objective optimization targets the maximization of the system efficiency and the minimization of system weight. The results show that more efficient systems can be designed only with heavier components. The design feasibility of high-speed centrifugal compressors is demonstrated. The advantage of an integrated system design optimization framework for complex energy systems is proved, allowing for the analysis of the impact of both component design and working fluid on system performance.

Technical benefits of the subcritical inlet condition for high-speed CO2 centrifugal compressor in the advanced power-generation cycle

In the CO2 power-generation cycles, the compressor inlet condition has significant impacts on the performances of turbomachinery as well as the economies of cycle systems. In this article, based on numerical simulation and system parametric analysis, we compared the subcritical and supercritical inlet conditions of the CO2 compressor, which is from a prototype in a MWt simple recuperated cycle (SRC). As the result, the advantages and particularities of the subcritical inlet condition for the CO2 compressor are discussed and confirmed from the aero/thermo-dynamics and thermal/technical economies. In the study, we found the efficiency and pressure ratios of the CO2 compressor were significantly improved as the inlet pressure reduced below the critical point. And the operation of transcritical compression would not be obviously impacted by phase changes at the far-blockage conditions. In the system parametric analysis, we found the thermal and technical economies of the SRC system obtained significant rises at the subcritical compressor inlet condition. The increase of system efficiency reached 2–3%, while the financial cost reduced by 7.7%. The structural and mechanical safety of key components was also improved. Thereby, the present study provides different views to the benefits of subcritical compressor inlet conditions in CO2 cycles.

Numerical model of predicting surge boundaries in high-speed centrifugal compressors

A numerical model of the whole compression system is established in this paper. The introduction of the Laval nozzle and opening boundary methods adaptively simulate the physical fluctuating boundary conditions during the surge without any artificial hypothesis. Based on the model, the steady simulation method has the capacity to well predict the low-order systematical oscillations during the surge by monitoring flow parameters on several representative surfaces. The surge boundaries of a high-speed centrifugal compressor, including the mild surge and the deep surge, are predicted based on the current numerical model in different flow regimes. The discrepancies of the mild surge boundary between experimental data and numerical predictions are within 5% in the transonic regime, but within 0.5% in the subsonic and supersonic regimes. The sensitivities of the mild surge to the throttle opening degree differ much in different flow regimes: the highest is in the transonic regime and the lowest is in the supersonic regime. In the supersonic regime, there exist various mild surges in a wide range of throttle opening degrees and an unstable equilibrium point (UEP) which is the most typical characteristic of the two-regime surge. To the best knowledge of the authors, it is the first time that all the oscillation characteristics of various mild surges and the existence of UEP in the supersonic regime are well predicted by one numerical method.

Numerical investigation on the combined effects of pinching and rotation on the performance of a high-speed centrifugal compressor with a vaneless diffuser

Computational investigations are carried out on a high-speed centrifugal compressor with the vaneless diffuser to study the combined pinch and rotation effect. The axial inlet width of the rotating vaneless diffuser is reduced at diffuser inlet, either at the hub wall or at the shroud wall, or both the walls. The reduction is varied from 0% (base case) to 20% at an interval of 5%. Numerical simulations are conducted for different speed range by varying the rotating diffuser diameter ratio from 1.25–1.75. Among all these cases, the best performance of 25% increase in pressure recovery with uniform diffusion and 2.3% increase in isentropic efficiency is obtained with 15% shroud pinch at a diameter ratio of 1.375 at the design mass flow rate.

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.

Early surge detection on a turbocharger used to pressurize a SOFC plant emulator

High-speed centrifugal compressors are commonly exploited to pressurize fuel cell-based hybrid energy systems. In such complex plants, because of significant interposed volumes due to fuel cells, dynamic compressor response can induce severe vibrations caused by low mass flow rates instability. In particular, surge strongly limits centrifugal compressors stable working region when moving towards low mass flow rate due to a change in system operating point. Consequently, a complete system identification is performed in order to adequately characterize compressor dynamic response for early surge detection. To this goal, a tailored experimental activity has been carried out at the Thermochemical Power Group of the University of Genoa on a vaneless diffuser compressor turbocharger used for the pressurization of an innovative solid oxide fuel cell (SOFC) emulator plant. Several post-processing methods have been performed on system vibro-acoustic responses to better predict and classify compressor status as stable or unstable. The obtained results provide original diagnostic insights for monitoring systems capable of preventing surge and other low mass flow unstable phenomena, such as rotating stall cells inception. Low mass flow rate fluid-dynamic instabilities prevention can extend compressor operating range, performance, and reliability to allow better integration with other plant components.

Experimental investigation on the vibrational and fluid dynamics behaviour of a turbocharger compressor in the transition to surge operation

High-speed dynamic centrifugal compressors are widely used both in the modern internal combustion engine design and in advanced pressurized cycles and innovative plant layouts as fuel cell systems. Surge strongly limits the stable operating region of centrifugal compressors in low mass flow rate conditions especially during fast transients. Therefore, it is of great importance to investigate transient system dynamic response of compressor surge evolution and early detection of incipient compressor surge. A specific experimental investigation on compressor surge was carried out at the University of Genoa turbocharger test facility and results are presented and analysed in this paper. The activity consists of steady state and transient measurements used to characterize and identify compressor behaviour in correspondence of surge inception conditions. The frequency and time frequency data analysis have been applied on inflow pressure, anemometric and vibrational signals to identify their contents and so to be able to classify compressor operation as stable or unstable. Synchronous averages performed in the time domain have been identified as a suitable algorithmic tool to detect incipient surge conditions. Anemometric signal analysis allowed to identify intermitting phenomena in deep surge conditions may be related to the rise of a rotation stall condition. The obtained results provide original diagnostic and predictive methods to be integrated in a monitoring system capable of preventing surge and extending compressor operating range, performance and reliability to allow the integration with the other plant components.

An Investigation of Non-Linear Surge Characteristic in a High-Speed Centrifugal Compressor

Abstract In centrifugal compressors, it is common to observe a rapid reduction in surge margin toward high rotational speed, which shows a non-linear surge characteristic against the change of rotational speed. This paper presents a comprehensive experimental and numerical study to understand the flow mechanisms leading to the non-linear surge behavior in a high-speed centrifugal compressor. It shows that for the studied compressor, there are two critical flow coefficients (ϕ = 0.255 and 0.136) where the stability of the compressor stage is significantly weakened. At a higher speed, the impeller rotating stall happens at the first critical point where the diffuser instability is also enhanced. This is caused by the increase of the flow non-uniformity at the impeller exit as well as increased diffuser inflow angle due to the impeller compressibility effect. Therefore, both the impeller and diffuser’s instability are matched and trigger the surge at a high flow coefficient. In contrast, at a lower speed, the diffuser instability is not enhanced by the impeller rotating stall, this mismatch of the two component’s instability allows the compressor to pass through the first critical point and extend the surge limit to the second critical point where the diffuser reaches the stability limit and causes the rotating stall spontaneously. By these different behaviors at each speed, the non-linearity of the surge characteristic is established.

Study on the coupled characteristics of high-speed centrifugal compressor and turboexpander of a reverse Brayton air refrigerator

Reverse Brayton air refrigerators are environmentally friendly and have broad application prospects, which can work under various conditions. In this paper, the characteristics of a open-loop air cycle for room temperature applications were investigated. The compressor and turboexpander in the cycle were modeled by numerical simulation. Based on coupled characteristics of the compressor and expander, a system model was established. The effects of the first stage compression ratio and the expander inlet temperature on the refrigerator performance were studied by experiments. The accuracy of the system model is verified by comparing with the experiments, and the maximum deviations of expander rotating speed and supplied mass flow are 3.1% and 4.83%, respectively. With the increase of the first-stage compression ratio and the decrease of expander inlet temperature, the mass flow rate of supplied air and cooling capacity of the cycle increase. The expander efficiency can be maintained near the optimal value. At the expander inlet temperature of 313.2 K, COP drops by 31.7 % when the cooling capacity is reduced from 3.34 kW to 0.74 kW.

Investigation of Surge in a Transonic Centrifugal Compressor With Vaned Diffuser: Part II—Correlation With Subcomponent Characteristics

AbstractIn the first part of the paper, surge signatures and the underlying mechanisms thereof for a high-speed centrifugal compressor were investigated using signals from fast-response transducers installed along the flow path. The compressor surge signature is observed to vary as the impeller inlet tip flow transitions from subsonic to supersonic conditions. Spike-type deep surge is observed at subsonic and supersonic impeller inlet tip conditions while modal-type mild surge occurs at 90% speed with transonic inlet tip conditions. In Part II of the paper, a detailed analysis of the static pressure rise characteristics at the stage, component, and subcomponent levels is conducted. Additionally, the influence of the impeller inlet conditions on the instability is also considered. A connection between the surge mechanism and component static pressure rise characteristics is shown which exhibits the potential for prediction of the stall inception mechanism including identification of the destabilizing component. Furthermore, the transition from subsonic to transonic impeller inlet conditions is shown to be the cause of the unique mild surge instability at 90% speed. Lastly, both experimental and computational results are utilized to investigate the development of a shock wave at the impeller leading edge. The shock is considered to be critical to stage stability at speeds where the compressor experiences transonic inlet conditions.

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.

Experimentally Validated Design of an Anode Recirculation Blower for Pem Fuel Cells Under Variable Fluid Composition

Abstract This paper focuses on the design, construction and the experimental testing of an anode recirculation blower (ARB) based on a turbo-compressor for PEM-FCs. The blower presented consists of a high-speed centrifugal compressor, driven by a so-called media-gap motor. Additionally it includes a droplet separator to eliminate liquid water from the anode loop. The aerodynamic design is based on 1D-correlations and 3D-numerical CFD simulations. Different gas compositions of hydrogen, nitrogen and water vapor are considered in the design, depending on the operating points of a PEM-FC. The gas composition has a significant influence on the achievable pressure ratio at constant compressor speeds, depending mainly on the specific heat capacity of the gas. Performance maps for different gas compositions are calculated using CFD for the designed ARB. A prototype is built and tested. The results validate the CFD predictions and show that the operating range of a PEM fuel cell can be covered with the present blower. In addition, it could be shown that an enthalpy-based approach using common characteristic numbers of turbomachines, allows converting performance maps - acquired with different gas compositions - into one another. This allows the performance map prediction for different gas compositions based on existing performance maps. This approach is illustrated by numerical and experimental results.

Rotor Stability Effects of Tilting Pad Journal Bearings With Assembled Clearance Asymmetry

Abstract This paper presents component and system-level analyses to understand the stiffness, damping, and rotor stability effects of tilting pad journal bearings (TPJB) with assembled clearance variation across pads. This variation results from a normal manufacturing process of the bearing components, i.e., shaft, bearing shell, pad radius, and pad thickness, whose machining tolerances add in a closed-loop producing a different assembled clearance per pad. The results are compared to the standard analysis practice of uniformly setting all pads assembled clearance to its maximum and minimum stack-up values to find the worst case for stability. The component-level analysis follows a rigid rotor approach. While in the system-level analysis, a full rotor-bearings-seals model of a lightweight, high-speed centrifugal compressor is employed. A four-pad TPJB with a load between pads (LBP) configuration is considered. Large differences in stiffness and log dec are encountered, while almost negligible differences are found in damping. Also, some interesting observations related to rotor-bearing planar mode shapes are reported when the bearing pads have dissimilar assembled clearances. These modes disappear once the labyrinth seal force coefficients are incorporated into the full model.

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.

Comprehensive vibro-acoustic characteristics and mathematical modeling of electric high-speed centrifugal compressor surge for fuel cell vehicles at various compressor speeds

The electric high-speed centrifugal compressor is the mainstream air compressor in present fuel cell vehicles. Surge strongly limits the stable operating range of centrifugal compressors. Therefore, it is of tremendous significance to study transient vibro-acoustic characteristics of the compressor surge evolution and early warnings of compressor surge. In this paper, comprehensive vibro-acoustic experiments of the compressor surge evolution are first conducted, and time-varying frequency characteristics are visualized through the short-time Fourier transform. The results show that the increase and fluctuation of the total sound pressure level at deep surge are mainly caused by the pressure pulsation at the centrifugal compressor outlet. Moreover, a rotating stall onset criterion based on acoustic signals is proposed, and the occurrence of severe rotating stall can be predicted by the precursor about 1.5–3 s in advance. Then, a mathematical model of the centrifugal compressor surge is established considering the variation of sound speed and introducing an additional volume, which unifies centrifugal compressor surge models at different compressor speeds. The maximum prediction error of the surge frequency is 1.95%. The model eliminates the pain point of the traditional Moore-Greitzer model which fails to predict the compressor surge characteristics at variable compressor speeds accurately. Finally, the impact of compression system dimensions on centrifugal compressor surge characteristics is quantitatively analyzed, and corresponding verification experiments are conducted. The proposed surge model can be used not only for quantifying the dynamic characteristics of compressor surge at transient speeds but also for designing active surge control strategies for variable vehicle operating conditions.

Experimental and Numerical Investigation of Clearance Effects in a Transonic Centrifugal Compressor

Extensive research studying tip clearance effects on compressor performance has been conducted during the past few decades with significant progress achieved. However, most of the previous investigations focus on stage-level analysis at the design condition, with very limited studies at the component level or part speed conditions. To fill this gap, this paper investigates the effects of impeller tip clearance on the performance of a high-speed centrifugal compressor, at both stage and component levels, covering a variety of operating speeds resulting in subsonic to transonic inlet conditions. Experiments were conducted on the Single Stage Centrifugal Compressor facility at two different running tip clearances. With increased clearance, the stage and impeller pressure ratio, isentropic efficiency, and work factor drop, but the static pressure recovery coefficient in the diffuser increases. As the compressor transitions from subsonic to transonic operation from 60 to 85% corrected speed, the sensitivities of stage and impeller pressure ratio and efficiency reach their peak values. Lastly, numerical simulations at design speed showed that the computational fluid dynamics (CFD) model sufficiently predicts the sensitivities due to tip clearance for most of the parameters at the stage and component levels, indicating the potential of using CFD to investigate the flow physics.