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Zoltán S. Spakovszky - One of the best experts on this subject based on the ideXlab platform.
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Spike and Modal Stall Inception in an Advanced Turbocharger Centrifugal Compressor
2020Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:ABSTRACT In turbocharger applications bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bearings. There is experimental evidence that the performance and operability of highly loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a pre-production, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Compressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without bleed flow, the pre-stall behavior is dominated by short wavelength disturbances, or so called 'spikes', in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highlyloaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long wavelength, modal pre-stall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured, in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained, endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to bleed flow and yielding a 1 point increase in adiabiatic compressor efficiency. INTRODUCTION AND BACKGROUND Besides meeting the pressure ratio requirements of the application, one of the major challenges in turbocharger compressor design is to achieve both a broad compressor map and high efficiency levels throughout the entire operating envelope. Over many years, the design trend has been towards highly loaded advanced compressor designs as for example reported by Rodgers The phenomena limiting the stable operation of centrifugal compression systems are rotating stall and surge. Although the manifestation of the full scale instabilities are similar to the ones observed in axial machines, the path into instability and pre-stall behavior in centrifugal compressors is less well understood. The pioneering work of Emmons et al. A major challenge in developing a generalized description or theory for rotating stall and surge inception is that the overall compression system, the compressor geometry, and Impeller speed cover a very large range. However, there is evidence that a number of compressors with vaned diffusers seem to exhibit long wave-length perturbations prior to the onset of full scale instability. For example, Lawless and Fleete
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Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO2
Volume 6C: Turbomachinery, 2013Co-Authors: Claudio Lettieri, N. Baltadjiev, Michael Casey, Zoltán S. SpakovszkyAbstract:This paper presents a design strategy for very low flow coefficient multi-stage compressors operating with supercritical CO2 for Carbon Capture and Sequestration (CCS) and Enhanced Oil Recovery (EOR). At flow coefficients less than 0.01 the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance.The Impeller Exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale re-injection compressor currently in service. Three-dimensional, steady, supercritical CO2 CFD simulations of the full stage with leakage flows are carried out with the NIST real gas model. The design study suggests that a non-dimensional Impeller Exit width parameter b2* = (b2/R)ϕ of 6 yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the non-dimensional Impeller Exit width parameter b2* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at Impeller Exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO2, the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.Copyright © 2013 by ASME
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Using Isolated Diffuser Simulations
2010Co-Authors: Zoltán S. Spakovszky, Jonathan EverittAbstract:In compression systems the range of stable operating is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to these phenomena: the development of long-wavelength modal waves or a short-wavelength, three-dimensional flow breakdown which is typically known as a "spike". The cause of the latter is not well under-stood; in axial machines it has been suggested that over-tip spillage flow has a significant role, but spikes can also occur in shrouded vaned diffusers of centrifugal compressors, where these flows are not present, suggesting an alternative mechanism may be at play. Unsteady Reynold's Averaged Navier-Stokes simulations are performed for an isolated vaned radial diffuser from a highly loaded centrifugal compressor. Key to their success is the definition of pitchwise "mixed out " averaged inlet conditions derived from the Impeller Exit flow field from separate single passage stage calculations. This guarantees the relevan
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Spike and Modal Stall Inception in an Advanced Turbocharger Centrifugal Compressor
Journal of Turbomachinery, 2009Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:In turbocharger applications, bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bear- ings. There is experimental evidence that the performance and operability of highly- loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a preproduction, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Com- pressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without the bleed flow, the prestall behavior is dominated by short-wavelength disturbances, or so called “spikes,” in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at the Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highly-loaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long- wavelength modal prestall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to the bleed flow and yielding a one point increase in adiabatic compressor efficiency
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spike and modal stall inception in an advanced turbocharger centrifugal compressor
ASME Turbo Expo 2007: Power for Land Sea and Air, 2007Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:In turbocharger applications bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bearings. There is experimental evidence that the performance and operability of highly loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a pre-production, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Compressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without bleed flow, the pre-stall behavior is dominated by short wavelength disturbances, or so called ‘spikes’, in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highly-loaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long wavelength, modal pre-stall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured, in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained, endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to bleed flow and yielding a 1 point increase in adiabiatic compressor efficiency.Copyright © 2007 by ASME
Christian H. Roduner - One of the best experts on this subject based on the ideXlab platform.
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Spike and Modal Stall Inception in an Advanced Turbocharger Centrifugal Compressor
2020Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:ABSTRACT In turbocharger applications bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bearings. There is experimental evidence that the performance and operability of highly loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a pre-production, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Compressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without bleed flow, the pre-stall behavior is dominated by short wavelength disturbances, or so called 'spikes', in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highlyloaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long wavelength, modal pre-stall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured, in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained, endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to bleed flow and yielding a 1 point increase in adiabiatic compressor efficiency. INTRODUCTION AND BACKGROUND Besides meeting the pressure ratio requirements of the application, one of the major challenges in turbocharger compressor design is to achieve both a broad compressor map and high efficiency levels throughout the entire operating envelope. Over many years, the design trend has been towards highly loaded advanced compressor designs as for example reported by Rodgers The phenomena limiting the stable operation of centrifugal compression systems are rotating stall and surge. Although the manifestation of the full scale instabilities are similar to the ones observed in axial machines, the path into instability and pre-stall behavior in centrifugal compressors is less well understood. The pioneering work of Emmons et al. A major challenge in developing a generalized description or theory for rotating stall and surge inception is that the overall compression system, the compressor geometry, and Impeller speed cover a very large range. However, there is evidence that a number of compressors with vaned diffusers seem to exhibit long wave-length perturbations prior to the onset of full scale instability. For example, Lawless and Fleete
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Spike and Modal Stall Inception in an Advanced Turbocharger Centrifugal Compressor
Journal of Turbomachinery, 2009Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:In turbocharger applications, bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bear- ings. There is experimental evidence that the performance and operability of highly- loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a preproduction, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Com- pressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without the bleed flow, the prestall behavior is dominated by short-wavelength disturbances, or so called “spikes,” in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at the Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highly-loaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long- wavelength modal prestall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to the bleed flow and yielding a one point increase in adiabatic compressor efficiency
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spike and modal stall inception in an advanced turbocharger centrifugal compressor
ASME Turbo Expo 2007: Power for Land Sea and Air, 2007Co-Authors: Zoltán S. Spakovszky, Christian H. RodunerAbstract:In turbocharger applications bleed air near the Impeller Exit is often used for secondary flow systems to seal bearing compartments and to balance the thrust load on the bearings. There is experimental evidence that the performance and operability of highly loaded centrifugal compressor designs can be sensitive to the amount of bleed air. To investigate the underlying mechanisms and to assess the impact of bleed air on the compressor dynamic behavior, a research program was carried out on a pre-production, 5.0 pressure ratio, high-speed centrifugal compressor stage of advanced design. The investigations showed that bleed air can significantly reduce the stable flow range. Compressor rig experiments, using an array of unsteady pressure sensors and a bleed valve to simulate a typical turbocharger environment, suggest that the path into compression system instability is altered by the bleed flow. Without bleed flow, the pre-stall behavior is dominated by short wavelength disturbances, or so called ‘spikes’, in the vaneless space between the Impeller and the vaned diffuser. Introducing bleed flow at Impeller Exit reduces endwall blockage in the vaneless space and destabilizes the highly-loaded vaned diffuser. The impact is a 50% reduction in stable operating range. The altered diffuser characteristic reduces the compression system damping responsible for long wavelength, modal pre-stall behavior. A four-lobed backward traveling rotating stall wave is experimentally measured, in agreement with calculations obtained from a previously developed dynamic compressor model. In addition, a self-contained, endwall blockage control strategy was employed, successfully recovering 75% of the loss in surge-margin due to bleed flow and yielding a 1 point increase in adiabiatic compressor efficiency.Copyright © 2007 by ASME
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investigation of an inversely designed centrifugal compressor stage part ii experimental investigations
Journal of Turbomachinery-transactions of The Asme, 2004Co-Authors: Matthias Schleer, Christian H. Roduner, S. S. Hong, Mehrdad Zangeneh, B. Ribi, F Ploger, Reza S. AbhariAbstract:This paper presents an experimental investigation of two centrifugal compressor stage configurations. The baseline configuration has been designed using conventional design engineering tools. The second configuration was designed using advanced inverse design rules as described in Part 1. It is designed to match the choke, flow as well as the best point of the conventionally designed stage. The experimental investigation is conducted in the industry-scale centrifugal compressor facility at the Turbomachinery Laboratory of the Swiss Federal Institute of Technology. Performance maps for both configurations at several speed lines are presented. These plots show the overall behavior of the stages designed using the different design approaches and their operating range. Time-resolved measurements show details of the unsteady flow field within the diffuser close to the Impeller Exit. The time-resolved data have been analyzed to assist the explanation of changes in the characteristics and associated efficiency penalties and gains. The processed data show the benefits of the new inverse design method with respect to an improvement of the compressor efficiency and the operating range. It is seen that the application of an inverse design method results in a more uniform flow into the diffuser.
Shinhyoung Kang - One of the best experts on this subject based on the ideXlab platform.
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steady and unsteady flow phenomena in a channel diffuser of a centrifugal compressor
Jsme International Journal Series B-fluids and Thermal Engineering, 2004Co-Authors: Jeongseek Kang, Shinhyoung KangAbstract:The aim of this paper is to understand the time averaged pressure distributions and unsteady pressure patterns in a channel diffuser of a centrifugal compressor. Pressure distributions from the Impeller Exit to the channel diffuser Exit are measured and discussed for various flow conditions. And unsteady pressure signals from six fast-response sensors in the channel diffuser are analyzed by decomposition method. The strong non-uniformity in the pressure distribution is obtained over the diffuser shroud wall caused by the Impeller-diffuser interaction. As the flow rate increases, flow separation near the throat, due to large incidence angle, increases aerodynamic blockage and reduces the aerodynamic flow area downstream. Thus the minimum pressure location occurs downstream of the geometric throat, and it is named as the aerodynamic throat. And at choke condition, normal shock occurs downstream of this aerodynamic throat. The variation in the location of the aerodynamic throat is discussed
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flow at the centrifugal pump Impeller Exit with circumferential distortion of the outlet static pressure
Journal of Fluids Engineering-transactions of The Asme, 2004Co-Authors: Soonsam Hong, Shinhyoung KangAbstract:The effects of circumferential outlet distortion of a centrifugal pump diffuser on the Impeller Exit flow were investigated. A fence with sinusoidal width variation was installed at the vaneless diffuser Exit. The flow field was measured at the Impeller Exit with and without the fence, using a hot film probe and an unsteady pressure sensor. Flow parameters varied with the circumferential position and the mean flow parameters plotted against the local flow rate at each circumferential position showed loops along the quasi-steady curves, which were obtained from the result without the fence. Simple theoretical calculations were used to predict the velocity components at the Impeller Exit with the relative flow angle or total pressure assumed. Good result was obtained when the relative flow angle was assumed to vary quasi-steadily, not constant with the local flow rate. The radial velocity was also reasonably predicted when the total pressure was assumed to vary quasi-steadily. A simple method is proposed to predict the Impeller Exit flow with downstream blockage in two-step sequence
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Exit flow measurements of a centrifugal pump Impeller
KSME International Journal, 2002Co-Authors: Soonsam Hong, Shinhyoung KangAbstract:Discharge flows from a centrifugal pump Impeller with a specific speed of 150 [rpm, m^3/min, m] were experimentally investigated. A large axisymmetric collector instead of a volute casing was installed to obtain circumferentially uniform flow, i.e. without interaction of the Impeller and the volute. The unsteady flow was measured at the Impeller Exit and vaneless diffuser using a hot film probe and a pressure transducer. The flow at Impeller Exit showed pronounced jet-wake flow patterns. The wake, which was on the suction/hub side at high flow rate, became enlarged pitchwisely on both the hub and the shroud side as the flow rate decreases. The pitchwise non-uniformity of the flow rapidly decreased along the downstream and the nonuniformity almost disappeared at radius ratio of 1.18 for medium flow rate. The mean vaneless diffuser flow was reasonably predicted using a one dimensional analysis when an empirical constant was used to specify the skin friction coefficient. The data can be used for a centrifugal pump Impeller design and validation of CFD codes and flow modeling.
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unsteady flow phenomena in a centrifugal compressor channel diffuser
Volume 1: Aircraft Engine; Marine; Turbomachinery; Microturbines and Small Turbomachinery, 2000Co-Authors: Jeongseek Kang, Sungkook Cho, Shinhyoung KangAbstract:The aim of this paper is to understand the time averaged pressure field and unsteady pressure patterns in a high speed centrifugal compressor channel diffuser. Pressure distributions from the Impeller Exit to the channel diffuser Exit are measured and discussed for various flow conditions. Unsteady pressure signals from six fast-response sensors in the channel diffuser are analyzed by decomposition method and wavelet transform. Measured results are shown for various operating condition from choke to surge that the effect of operating condition is well discussed.The strong non-uniformity in the pressure distribution is obtained over the diffuser shroud wall caused by the Impeller-diffuser interaction. As the flow rate increases, flow separation near the throat, due to large incidence angle, increases aerodynamic blockage and reduces the aerodynamic flow area downstream. Thus the minimum pressure location occurs downstream of the geometric throat, and it is named as the aerodynamic throat. And at choke condition, normal shock occurs downstream of this aerodynamic throat. The variation in the location of the aerodynamic throat is discussed.The pressure ratio waveforms by blade passing show regular oscillation not only for the normal but also for the surge conditions and the high frequency fluctuations are superposed on the oscillating pressure waveform as the flow rate increases. Periodic unsteadiness by blade passing does not decay in the diffuser channel. It depends on the operating point and is generally larger in the channel than in the vaneless space. Aperiodic unsteadiness rapidly decrease downstream of diffuser channel.At surge, the spectrum becomes broad banded with peaks at the surge frequency as well as blade passage frequency and the Impeller rotating frequency. The surge signal was analyzed using wavelet transform and it is found that surge signal is composed of not only surge scale and blade scale but also multi-scale aperiodic waves. The broadband spectrum in surge condition is due to this multi-scale aperiodic waves.© 2000 ASME
Spakovszky, Zoltan S - One of the best experts on this subject based on the ideXlab platform.
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Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO₂
'ASME International', 2018Co-Authors: Casey M., Lettieri Claudio, Baltadjiev, Nikola Dimitrov, Spakovszky, Zoltan SAbstract:This paper presents a design strategy for very low flow coefficient multi-stage compressors operating with supercritical CO 2 for Carbon Capture and Sequestration (CCS) and Enhanced Oil Recovery (EOR). At flow coefficients less than 0.01 the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance. The Impeller Exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale re-injection compressor currently in service. Three-dimensional, steady, supercritical CO 2 CFD simulations of the full stage with leakage flows are carried out with the NIST real gas model. The design study suggests that a non-dimensional Impeller Exit width parameter b 2 * =(b2/R) φ of 6 yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the non-dimensional Impeller Exit width parameter b 2 * can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at Impeller Exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO 2 , the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.Mitsubishi Jūkōgyō Kabushiki Kaisha. Takasago R&D Cente
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The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers
'ASME International', 2018Co-Authors: Rusch Daniel, Schiffmann Jürg, Everitt, Jonathan Neil, Spakovszky, Zoltan SAbstract:Highly loaded Impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is nonuniform across the span and pitch and swirling at angles approaching tangential. With the exception of the flow angle, conflicting data exist regarding whether these attributes have substantial influence on the performance of the downstream diffuser. This paper quantifies the relative importance of the flow angle, Mach number, nonuniformity, and unsteadiness on diffuser performance, through diffuser experiments in a compressor stage and in a rotating swirling flow test rig. This is combined with steady and unsteady Reynolds-averaged Navier-Stokes (RANS) computations. The test article is a pressure ratio 5 turbocharger compressor with an airfoil vaned diffuser. The swirling flow rig is able to generate rotor outflow conditions representative of the compressor except for the periodic pitchwise unsteadiness and fits a 0.86 scale diffuser and volute. In both rigs, the time-mean Impeller outflow is mapped across a diffuser pitch using miniaturized traversing probes developed for the purpose. Across approximately two-thirds of the stage operating range, diffuser performance is well correlated to the average Impeller outflow angle when the metric used is effectiveness, which describes the pressure recovery obtained relative to the maximum possible given the average inflow angle and Mach number and the vane Exit metal angle. Utilizing effectiveness captures density changes through the diffuser at higher Mach numbers; a 10% increase in pressure recovery is observed as the inlet Mach number is increased from 0.5 to 1. Further, effectiveness is shown to be largely independent of the time-averaged spanwise and unsteady pitchwise nonuniformity from the rotor this independence is reflective of the strong mixing processes that occur in the diffuser inlet region. The observed exception is for operating points with high time-averaged vane incidence. Here, it is hypothesized that temporary excursions into high-loss flow regimes cause a nonlinear increase in loss as large unsteady angle variations pass by from the rotor. Given that straight-channel diffuser design charts typically used in preliminary radial vaned diffuser design capture neither streamtube area changes from Impeller Exit to the diffuser throat nor vane incidence effects, their utility is limited. An alternative approach, utilizing effectiveness and vane leading edge incidence, is proposed.ABB Turbo Systems A
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An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers
'ASME International', 2018Co-Authors: Everitt, Jonathan Neil, Spakovszky, Zoltan SAbstract:In compression systems the stable operating range is limited by rotating stall and/or surge. Two distinct types of stall precursors can be observed prior to full scale instability: the development of long-wavelength modal waves or a shortwavelength, three-dimensional flow breakdown (so-called "spike" stall inception). The cause of the latter is not well understood; in axial machines it has been suggested that rotor blade-tip leakage flow plays an important role, but spikes have recently been observed in shrouded vaned diffusers of centrifugal compressors where these leakage flows are not present, suggesting an alternative mechanism may be at play. This paper investigates the onset of instability in a shrouded vaned diffuser from a highly loaded turbocharger centrifugal compressor and discusses the mechanisms thought to be responsible for the development of short-wavelength stall precursors. The approach combines unsteady 3D RANS simulations of an isolated vaned diffuser with previously obtained experimental results. The unsteady flow field simulation begins at the Impeller Exit radius, where flow is specified by a spanwise profile of flow angle and stagnation properties, derived from single-passage stage calculations but with flow pitchwise mixed. Through comparison with performance data from previous experiments and unsteady fullwheel simulations, it is shown that the diffuser is accurately matched to the Impeller and the relevant flow features are well captured. Numerical forced response experiments are carried out to determine the diffuser dynamic behavior and point of instability onset. The unsteady simulations demonstrate the growth of short-wavelength precursors; the flow coefficient at which these occur, the rotation rate and circumferential extent agree with experimental measurements. Although the computational setup and domain limitations do not allow simulation of the fully developed spike nor fullscale instability, the model is sufficient to capture the onset of instability and allows the postulation of the following necessary conditions: (i) flow separation at the diffuser vane leading edge near the shroud endwall; (ii) radially reversed flow allowing vorticity shed from the leading edge to convect back into the vaneless space; and (iii) recirculation and accumulation of low stagnation pressure fluid in the vaneless space, increasing diffuser inlet blockage and leading to instability. Similarity exists with axial machines, where blade-tip leakage sets up endwall flow in the circumferential direction leading to flow breakdown and the inception of rotating stall. Rather than the tip leakage flows, the cause for circumferential endwall flow in the vaned diffuser is the combination of high swirl and the highly non-uniform spanwise flow profile at the Impeller Exit.ABB Turbo Systems A
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The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers
'ASME International', 2017Co-Authors: Everitt, Jonathan N., Spakovszky, Zoltan S, Rusch Daniel, Schiffmann, Jürg AlexanderAbstract:Highly loaded Impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is nonuniform across the span and pitch and swirling at angles approaching tangential. With the exception of the flow angle, conflicting data exist regarding whether these attributes have substantial influence on the performance of the downstream diffuser. This paper quantifies the relative importance of the flow angle, Mach number, nonuniformity, and unsteadiness on diffuser performance, through diffuser experiments in a compressor stage and in a rotating swirling flow test rig. This is combined with steady and unsteady Reynolds-averaged Navier–Stokes (RANS) computations. The test article is a pressure ratio 5 turbocharger compressor with an airfoil vaned diffuser. The swirling flow rig is able to generate rotor outflow conditions representative of the compressor except for the periodic pitchwise unsteadiness and fits a 0.86 scale diffuser and volute. In both rigs, the time-mean Impeller outflow is mapped across a diffuser pitch using miniaturized traversing probes developed for the purpose. Across approximately two-thirds of the stage operating range, diffuser performance is well correlated to the average Impeller outflow angle when the metric used is effectiveness, which describes the pressure recovery obtained relative to the maximum possible given the average inflow angle and Mach number and the vane Exit metal angle. Utilizing effectiveness captures density changes through the diffuser at higher Mach numbers; a 10% increase in pressure recovery is observed as the inlet Mach number is increased from 0.5 to 1. Further, effectiveness is shown to be largely independent of the time-averaged spanwise and unsteady pitchwise nonuniformity from the rotor; this independence is reflective of the strong mixing processes that occur in the diffuser inlet region. The observed exception is for operating points with high time-averaged vane incidence. Here, it is hypothesized that temporary excursions into high-loss flow regimes cause a nonlinear increase in loss as large unsteady angle variations pass by from the rotor. Given that straight-channel diffuser design charts typically used in preliminary radial vaned diffuser design capture neither streamtube area changes from Impeller Exit to the diffuser throat nor vane incidence effects, their utility is limited. An alternative approach, utilizing effectiveness and vane leading edge incidence, is proposed
Reza S. Abhari - One of the best experts on this subject based on the ideXlab platform.
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vaned diffuser induced Impeller blade vibrations in a high speed centrifugal compressor
Journal of Turbomachinery-transactions of The Asme, 2013Co-Authors: Armin Zemp, Reza S. AbhariAbstract:Blade failure in turbomachinery is frequently caused by an excessive resonant response. Forced response of the blades typically originates from unsteady fluid structure interactions. This paper presents the experimental and computational results of a research effort focusing on the blade forced response in a high-speed centrifugal compressor caused by the downstream vaned diffuser. The potential field from the downstream vaned diffuser acts as an unsteady Impeller relative circumferentially nonuniform disturbance. In this work the effect of varying the radial gap between Impeller Exit and diffuser vane leading edges was examined. Dynamic strain gauges, which were installed on the blade surfaces, were used to measure the forced response levels of the blades and to estimate the damping properties for different compressor operating conditions and vaneless gap dimensions. Unsteady fluid flow simulations were used to quantify the forcing function acting on the compressor blades due to Impeller-diffuser interaction. The time-resolved blade pressure distribution showed the temporal evolution of the dynamic load on the blade surface caused by the diffuser's potential field. The magnitude of the vibratory stress levels was found to depend on the radial gap size, the blade damping properties, and on the compressor operating point. The variation of the radial gap size resulted in a shift of the Impeller-diffuser interaction zone towards the main blade leading edge by up to 5% of the streamwise location.
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clearance effects on the evolution of the flow in the vaneless diffuser of a centrifugal compressor at part load condition
Journal of Turbomachinery-transactions of The Asme, 2008Co-Authors: Matthias Schleer, Reza S. AbhariAbstract:This work reports on flow measurements taken within the vaneless diffuser of a scaled-up model of a small-scale, highly loaded unshrouded compressor with large relative tip clearance. The aims are to describe and to analyze the influence of the clearance flow on the flow structure at the Impeller Exit in part load operation. The kind of compressor described herein is widely used in distributed power applications and automotive turbo-charging. It demands further enhancement of the operation range, as well as a high head rise and an improved efficiency. Therefore, the understanding of flow features and their interaction is crucial. The interaction and mixing of the flow pattern downstream of the Impeller are shown using spatially and temporally resolved 3D-velocity data. The measurements have been obtained by using a 3D laser Doppler anemometry system throughout the vaneless parallel wall diffuser. This unique data set provides insight into the development of the flow within the diffuser and allows conclusions on the mixing and migration of the three-dimensional pattern. The flow structure in part load condition is strongly affected by the flow across the large relative tip gap. Due to the large relative tip clearance, a low momentum zone is formed as an additional pattern at the shroud. This clearance flow is highly vortical and interacts with the channel wake structure but remains stable throughout the vaneless diffuser. At the pressure side hub corner, a jet structure is formed, which interacts rapidly with the blade wake. This flow behavior does not comply with the classical jet-wake pattern. It is proposed that in a centrifugal compressor with large relative tip clearance, a modified flow model that includes tip leakage is more appropriate to describe the flow structure at part load condition.
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clearance effects on the evolution of the flow in the vaneless diffuser of a centrifugal compressor at part load condition
ASME Turbo Expo 2006: Power for Land Sea and Air, 2006Co-Authors: Matthias Schleer, Reza S. AbhariAbstract:This work reports on flow measurements taken within the vaneless diffuser of an scaled-up model of a small-scale, highly loaded unshrouded compressor with large relative tip clearance. The aims are to describe and to analyze the influence of the clearance flow on the flow structure at Impeller Exit in part load operation. The kind of compressor described herein are widely used in distributed power applications and automotive turbo-charging. They demand further enhancement of the operation range as well as a high head rise and an improved efficiency. Therefore, the understanding of flow features and their interaction is crucial. The interaction and mixing of the flow pattern flow downstream of the Impeller is shown using spatially- and temporally resolved 3D-velocity data. The measurements have been obtained by using a 3D Laser Doppler Anemometry system (LDA) throughout the vaneless parallel wall diffuser. This unique data set provides insight into the development of the flow within the diffuser and allows conclusions on the mixing and migration of the 3-dimensional pattern. The flow structure in part load condition is strongly affected by the flow across the large relative tip gap. Due to the large relative tip clearance a low momentum zone is formed as an additional pattern at the shroud. This clearance flow is highly vortical and interacts with the channel wake structure but remains stable throughout the vaneless diffuser. At the pressure side hub corner a jet structure is formed which interacts rapidly with the blade wake. This flow behavior doesn’t comply with the classical jet wake pattern. It is proposed that in centrifugal compressor with large relative tip clearance a modified flow model which includes tip leakage is more appropriate to describe the flow structure at part load condition.Copyright © 2006 by ASME
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investigation of an inversely designed centrifugal compressor stage part ii experimental investigations
Journal of Turbomachinery-transactions of The Asme, 2004Co-Authors: Matthias Schleer, Christian H. Roduner, S. S. Hong, Mehrdad Zangeneh, B. Ribi, F Ploger, Reza S. AbhariAbstract:This paper presents an experimental investigation of two centrifugal compressor stage configurations. The baseline configuration has been designed using conventional design engineering tools. The second configuration was designed using advanced inverse design rules as described in Part 1. It is designed to match the choke, flow as well as the best point of the conventionally designed stage. The experimental investigation is conducted in the industry-scale centrifugal compressor facility at the Turbomachinery Laboratory of the Swiss Federal Institute of Technology. Performance maps for both configurations at several speed lines are presented. These plots show the overall behavior of the stages designed using the different design approaches and their operating range. Time-resolved measurements show details of the unsteady flow field within the diffuser close to the Impeller Exit. The time-resolved data have been analyzed to assist the explanation of changes in the characteristics and associated efficiency penalties and gains. The processed data show the benefits of the new inverse design method with respect to an improvement of the compressor efficiency and the operating range. It is seen that the application of an inverse design method results in a more uniform flow into the diffuser.
