The asymmetric influence of the volute on the flow in a transonic, high-pressure ratio centrifugal compressor at off-design conditions was investigated. Fully three-dimensional vis- cous steady-state computational fluid dynamics (CFD) was applied to simulate the flow in a 4.2:1 design pressure ratio compressor for automotive application. Computed performance charac- teristics are presented for low- and high-pressure ratio operating conditions, with and without an overhung volute. The volute was found to severely harm aerodynamic stability of the investi- gated compressor when operating at lower than design mass flow. The relative narrowing effect of the volute on compressor map width increases with pressure ratio up to a 42 per cent drop in stable flow range at design speed. The inter-passage variations in performance quantities and the influence of the volute tongue region are discussed in detail. The circumferential variations of incidence angle correlate with rotational speed, which, in combination with the higher sensitivity to incidence angle at transonic inflow conditions, seems to deteriorates stability when transonic inflow conditions are reached.

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Influence of the volute on the flow in a centrifugal compressor of a high-pressure ratio turbocharger

Experimental investigations on a single stage centrifugal compressor showed that measured blade vibration amplitudes vary considerably along a constant speed line from choke to surge. The unsteady flow has been analyzed to obtain detailed insight into the excitation mechanism. Therefore, a turbocharger compressor stage impeller has been modeled and simulated by means of computational fluid dynamics (CFD). Two operating points at off-design conditions were analyzed. One was close to choke and the second one close to the surge line. Transient CFD was employed, since only then a meaningful prediction of the blade excitation, caused by the unsteady flow situation, can be expected. Actually, it was observed that close to surge a steady state solution could not be obtained; only transient CFD could deliver a converged solution. The CFD results show the effect of the interaction between the inducer casing bleed system and the main flow. Additionally, the effect of the nonaxisymmetric components, such as the suction elbow and the discharge volute, was analyzed. The volute geometry itself had not been modeled. It turned out to be sufficient to impose a circumferentially asymmetric pressure distribution at the exit of the vaned diffuser to simulate the volute. Volute and suction elbow impose a circumferentially asymmetric flow field, which induces blade excitation. To understand the excitation mechanism, which causes the measured vibration behavior of the impeller, the time dependent pressure distribution on the impeller blades was transformed into the frequency domain by Fourier decomposition. The complex modal pressure data were imposed on the structure that was modeled by finite element methods (FEM). Following state-of-the-art calculations to analyze the free vibration behavior of the impeller, forced response calculations were carried out. Comparisons with the experimental results demonstrate that this employed methodology is capable of predicting the impeller 's vibration behavior under real engine conditions. Integrating the procedure into the design of centrifugal compressors will enhance the quality of the design process.

Unsteady Flow in a Turbocharger Centrifugal Compressor: Three-Dimensional Computational Fluid Dynamics Simulation and Numerical and Experimental Analysis of Impeller Blade Vibration

This paper presents the experimental and numerical investigation of an outward volute of rectangular cross section. The investigation is carried out at the level of stage performance, volute performance, and detailed flow field study at selected peripheral positions for various operating points. The objective of the investigation was to gain further knowledge about the flow structure and loss mechanism in the volute. Simultaneously with the experimental investigation, a numerical simulation of the flow in the volute was carried out. A three-dimensional Euler code was used in which a wall friction term and a tuned artificial dissipation term account for viscous effects. A reasonable agreement between the experimental and numerical results is observed. As a result a good and detailed knowledge about the pressure recovery and loss mechanism in the volute is obtained.

Experimental and Numerical Investigation of the Flow in a Centrifugal Compressor Volute

A numerical simulation is performed on a single-stage centrifugal compressor using the commercially available CFD software, CFX-TASCflow. The steady flow is obtained by circumferentially averaging the exit fluxes of the impeller. Three runs are made at the design condition and off-design conditions. The predicted performance is in agreement with experimental data. The flow details inside the stationary components are investigated, resulting in a flow model describing the volute/diffuser interaction at design and off-design conditions

A Numerical Investigation on the Volute/Diffuser Interaction Due to the Axial Distortion at the Impeller Exit

The history of turbocharging is almost as old as that of the internal combustion engine. A turbocharger consists of a compressor and a turbine. The compressor is driven by the turbine extracting energy from exhaust gases. Compared to a naturally aspirated engine, the benefits of a turbocharged engine are increased power, lower fuel consumption, and reduced emissions [1,2].

High-pressure-ratio turbocharging technology is the developing trend of turbocharged internal combustion engines due to the following reasons: 1) significant downsizing to mitigate CO2 emission and reduce fuel consumption [3], 2) satisfying rigid future emission regulations, i.e., NOx treatment by engine control means high rates of exhaust gas recirculation (EGR) [3,4], and 3) the facilitation of high altitude operation [5]. However, a high pressure ratio causes the flow in the compressor to be transonic. Hence, the stable flow range is narrowed, since the stall incidence decreases with an increased relative Mach number at the inlet of the impeller [6]. Therefore, map width enhancement is a major issue for state-of-the-art high-pressure-ratio compressor design and development.

A turbocharger centrifugal compressor comprises an impeller, a diffuser, and a volute. While the former two are periodically symmetric in the circumferential direction, the volute is asymmetric due to its gas-collection function. It is usually designed as a spiral-collection overhung housing that collects the air from the diffuser and passes it to the pipe system. It has been recognized that the improvement of centrifugal compressor performance requires a good understanding of the flow mechanisms inside the volute [7–9]; especially the interaction among the volute-diffuser-impeller [10,11]. The volute is mostly designed in a way to shape a uniform circumferential static pressure distribution both in the volute and the diffuser. However, the volute acts as a diffuser at lower than the design flow rate and acts as a nozzle at higher than the design flow rate, respectively. A number of authors have researched this subject. It has already been confirmed that the asymmetrical configuration has a significant impact on the flow field in the diffuser and in the impeller [12,13]. This circumferential asymmetry has been recognized and intensive experimental investigations of the flow within the volute and the propagation of the distortion into upstream components were carried out for subsonic compressor units [14,15].

The work of Sorokes et al. confirmed that the pressure nonuniformity extended upstream of the impeller, implying that the impeller was subjected to varying inlet and exit conditions. The computational fluid dynamics (CFD) results further implied that the inlet flow distortion caused a large leading edge pressure differential along with a large negative incidence, which may induce a flow separation and thus a very disturbed flow field in the impeller [14]. A three-dimensional unsteady analysis of the flow in the impeller with circumferential distortion of the outlet static pressure was investigated using a numerical method by Fatsis et al. [16]. The perturbation was thus propagated upstream from the impeller outlet and influenced the incidence at the blade leading edges and other flow parameters. Gu et al. [10,11] found that the performance parameters of the single impeller passage differed because of the asymmetric flow at the outlet of the impeller. There was almost no phase shift between the distortion in the diffuser and impeller according to their results, and it was considered that the unsteady effects of the volute-impeller interaction can be neglected when the Strouhal number is small enough.

Little detailed measurement was carried out in the impeller to investigate the asymmetric flow. Furthermore, to the authors’ knowledge, very little research work has been focused on the impact of the volute on the flow field in a high-pressure-ratio turbocharger centrifugal compressor. The purpose of this two-part paper is first to understand the asymmetry of flow field due to the asymmetric geometry of the volute and, subsequently, to develop a novel asymmetric flow control method to widen the stable flow range of a turbocharger centrifugal compressor with a high-pressure-ratio, the narrowing flow range of which is of utmost importance for its application. In Part I, the nonaxisymmetrical flow characteristics in the high-pressure-ratio turbocharger centrifugal compressor are investigated by using experimental and numerical means, the results of which are the basis for the work presented in Part II.

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Stability Improvement of High-Pressure-Ratio Turbocharger Centrifugal Compressor by Asymmetric Flow Control-Part I: Non-Axisymmetrical Flow in Centrifugal Compressor.

This paper presents the results of performance testing of a single-stage centrifugal compressor operating under wet gas conditions. The test was performed at an oil and gas operator’s test facility and was executed at full-load and full-pressure conditions using a mixture of hydrocarbon gas and hydrocarbon condensate. The effect of liquid was investigated by changing the gas-volume fraction between 1.0 and 0.97, which covers the range encountered by the operator during regular gas/condensate field production in the North Sea. Other parameters that were evaluated include the 111 PERFORMANCE EVALUATION OF A CENTRIFUGAL COMPRESSOR OPERATING UNDER WET GAS CONDITIONS by Lars Brenne Staff Engineer Tor Bjorge Staff Engineer Statoil ASA Trondheim, Norway Jose L. Gilarranz Senior Aero/Thermodynamics Engineer Jay M. Koch Staff Engineer, Aero/Thermodynamics and Harry Miller Product Manager, Marketing Dresser-Rand Company Olean, New York compressor test speed, the suction pressure, and two different liquid injection patterns. During the tests, the machine flowrate was varied from near surge to choke conditions; hence, the evaluation covered the entire operating range of the machine. Although the test was primarily intended to evaluate the effects of the wet gas on the thermodynamic performance of the machine, the mechanical performance was also investigated by measuring the machine vibration levels and noise signature during the baseline dry gas tests as well as during the tests with liquid injection. INTRODUCTION Centrifugal compressor packages utilized for upstream gas processing often must operate under wet gas conditions in which the fluid handled by the compression package contains a mixture of liquid and gaseous phases. Typically, the liquid components of the mixture are separated from the gas stream before they enter the compressor by the use of scrubbers and separators located upstream of the compressor inlet. These devices are very large and heavy, requiring a large “footprint” (amount of floor space) as compared to the gas compression package. A compressor with the ability to directly handle wet gas without the need for separation equipment is very attractive from an economic standpoint, as it would drastically reduce the size, weight, and cost of the gas compression package. For the case of future subsea compression systems, this capability is even more attractive because of the high costs of deploying a compressor train and all of its associated equipment under water. Wet gas compression (WGC) technology represents new opportunities for enhanced, cost-effective production from existing and future gas/condensate fields. Many oil and gas operators face future challenges in tail-end production, unmanned operation, and improved recovery from topside and subsea wells. This emphasizes the need to develop more robust compression systems, which can be designed for remote operation in unmanned topside installations, or could be designed for subsea operation for reinjection and/or transport boosting. The use of this technology for subsea boosting represents a new and exciting application for rotating equipment, which will allow new gas/condensate field production opportunities as well as enhanced recovery of existing gas/condensate fields and cost-effective production from marginal gas fields. As mentioned above, these wet gas compression systems could be based on the use of a liquid tolerant dry gas compressor, which could boost a coarsely separated (via a scrubber) well-stream, however, an even more attractive solution would be the development of compression systems that can boost the well-stream directly. Many research projects and product qualification programs are currently underway to develop such a system either by modifying existing multiphase pump technology or by the adaptation of currently available gas compression technologies (Scott, 2004). Regardless of the choice of concept, the compressor solution should be able to tolerate liquid ingestion for an extended time without failure. For the case of subsea applications, the high cost associated with the retrieval of the compressor from the sea floor accentuates the importance of a reliable design. The work presented herein served as an initial test to verify the multiphase boosting capabilities of a centrifugal compressor as well as to provide an oil and gas operator with data to compare the performance of this technology with other available wet gas compression concepts. It is important to state that the test compressor used for this investigation was not originally designed for wet gas boosting, nonetheless it provided an economically viable test bed for centrifugal compressor technology. DESCRIPTION OF TEST VEHICLE The test vehicle used for this work was a barrel-type, singlestage compressor, manufactured by the coauthors’ company. Said compressor was equipped with a high-head impeller, with a diameter of 0.384 m (1.26 ft), and a design flow coefficient of 0.02380. The compressor was originally designed to handle an inlet flow of 4332 Kg/min [2167 Am3/hr (76,526.88 ft3/hr)] of dry hydrocarbon gas (molecular weight of 18.49), with an inlet pressure of 130.2 bar (1888.4 psi)and a discharge pressure of 161.8 bar (2346.7 psi). Figure 1 shows a cross-section of the test compressor; the inlet and discharge nozzles are located at a 45 degree angle with respect to the top dead center of the machine. The original design of this machine, which dates to 1986, was not intended for wet gas service, and hence the internal geometry was not optimal. Nevertheless, in order to increase the reliability of the machine, the original rotor design was modified to accommodate an electron-beam welded and vacuum furnace brazed impeller with a shrink fit to the shaft. The rest of the machine remained the same (i.e., casing and stationary components). This compressor was equipped with a vaneless diffuser configuration. Figure 1. Cross-Section of the Test Compressor. The compressor was driven by a 2.8 MW synchronous electric motor, through a speed increasing gearbox, with a gear ratio of 6.607. A variable speed drive permitted the operation of the compressor within its speed range of 6000 to 13,000 rpm. The test compressor is utilized in the coauthor’s closed loop test facility, and was equipped to simulate the conditions expected for a centrifugal compressor operating under wet gas conditions. Figure 2 shows a schematic diagram of the test loop that was used for the evaluations. The major components of the test loop included a scrubber, the test compressor, a pump, a cooler, and a liquid injection module (mixer). The scrubber, here called guard separator, was used to separate the dry gas (saturated hydrocarbon mixture) from the liquid (hydrocarbon condensate) in order to permit accurate measurement of the massflow of each stream (liquid and gas). The liquid stream was measured with a Coriolis flowmeter while the gas stream was measured with a calibrated orifice plate. Figure 2. Schematic Diagram of the Wet Gas Test Loop. PROCEEDINGS OF THE THIRTY-FOURTH TURBOMACHINERY SYMPOSIUM • 2005 112 Variable Speed Electric Motor (MW) Gas Flow 2 Phase Flow Condensate

Performance Evaluation Of A Centrifugal Compressor Operating Under Wet Gas Conditions.

The paper describes experimental and computational fluid dynamics analyses of the non-uniform static pressure distortion caused by the discharge volute in a high pressure, centrifugal compressor. The experiments described in this paper were done using a heavily instrumented gas re-injection compressor operating at over 6000 psia discharge. Instrumentation was installed to measure static, total, and dynamic pressure as well as impeller strain and mechanical vibrations. A brief description of the compressor and instrumentation are provided. Concurrent with the experimental work, CFD runs were completed to study the reasons for the pressure nonuniformity. The CFD pressure profile trends agreed well with the experimental results and provided analytical corroboration for the conclusions drawn from the test data. Conclusions are drawn regarding: a) the response of the non-uniformity to changing flow rates; b) the extent to which the non-uniformity can be detected upstream of the impeller; and c) the mechanical influences of the nonuniformity on the impellers.

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Investigation of the Circumferential Static Pressure Non-Uniformity Caused by a Centrifugal Compressor Discharge Volute

The performance of a centrifugal compressor stage can be seriously affected by inlet flow distortions due to an unsatisfactory inlet configuration and the resulting flow structure. In this study, two radial inlets were designed for a centrifugal compressor stage and investigated numerically using a commercially available 3D viscous Navier-Stokes code. The intent of the design was to minimize the total pressure loss across the inlet while distributing the flow as equally and uniformly as possible to the impeller inlet. For each inlet model, the aerodynamic performance was calculated from the simulation results and then the results from both models were evaluated and compared. The second radial inlet design outperformed the initial design in terms of total pressure loss, flow distortion and uniformity at the impeller inlet. Furthermore, the aerodynamic performance of the second radial inlet was insensitive to wide range of mass flow rates compared to the initial design due to the distinctive geometric features implemented for the second inlet

Design and Numerical Investigation of Advanced Radial Inlet for a Centrifugal Compressor Stage

This paper describes a full-scale, flexible test vehicle designed and built by the original equipment manufacturer (OEM) to validate the aerodynamic and mechanical performance of large compressors for a variety of applications. This paper provides a description of the test vehicle as well as mechanical and aerodynamic performance data gathered during testing of the vehicle.

Full-Scale Aerodynamic And Rotordynamic Testing For Large Centrifugal Compressors.

The paper is a sequel to an earlier work by Sorokes et al. 1998, “Investigation of the Circumferential Static Pressure Non-Uniformity Caused by a Centrifugal Compressor Discharge Volute.” The earlier work described experimental and computational fluid dynamics analyses of the non-uniform static pressure distortion caused by the discharge volute in a high pressure, centrifugal compressor with vaneless diffusers. This paper describes additional testing and analytical work done using low solidity vaned diffusers (LSD’s) in place of select vaneless diffusers to determine the alternate diffuser’s effectiveness in eliminating or reducing the magnitude of the non-uniform pressure field. As in the earlier studies, the experiments described in this paper were done using a heavily instrumented gas re-injection compressor operating at over 6000 psia discharge pressure. Instrumentation was installed to measure static, total, and dynamic pressure as well as impeller strain and mechanical vibrations. A brief description of the compressor and instrumentation are provided.Concurrent with the experimental work, CFD runs were completed to study the effect of the alternate vaned diffusers. The CFD pressure profile trends agreed well with the experimental results and provided analytical corroboration for the conclusions drawn from the test data.Conclusions are drawn regarding: a) the effectiveness of the LSD’s on the pressure non-uniformity; b) the associated effects on the measured dynamic strains in the impellers; and c) the usefulness of computational fluid dynamics (CFD) in assessing the aerodynamic forces associated with the non-uniformity.© 2000 ASME

Jay M. Koch

Papers

Performance Evaluation Of A Centrifugal Compressor Operating Under Wet Gas Conditions.

Investigation of the Circumferential Static Pressure Non-Uniformity Caused by a Centrifugal Compressor Discharge Volute

Design and Numerical Investigation of Advanced Radial Inlet for a Centrifugal Compressor Stage

Full-Scale Aerodynamic And Rotordynamic Testing For Large Centrifugal Compressors.

The Influence of Low Solidity Vaned Diffusers on the Static Pressure Non-Uniformity Caused by a Centrifugal Compressor Discharge Volute