scholarly journals The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Jonathan N. Everitt ◽  
Zoltán S. Spakovszky ◽  
Daniel Rusch ◽  
Jürg Schiffmann
Keyword(s):  
Mach Number ◽  
Swirling Flow ◽  
Pressure Ratio ◽  
Leading Edge ◽  
Pressure Recovery ◽  
Strong Mixing ◽  
Mixing Processes ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
High Loss

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.

scholarly journals The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers

2016 ◽  
Author(s):  
Jonathan Everitt ◽  
Zoltán Spakovszky ◽  
Daniel Rusch ◽  
Jürg Schiffman
Keyword(s):  
Mach Number ◽  
Swirling Flow ◽  
Pressure Ratio ◽  
Leading Edge ◽  
Pressure Recovery ◽  
Strong Mixing ◽  
Mixing Processes ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
High Loss

Highly-loaded impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is non-uniform 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, non-uniformity 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 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 non-uniformity 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.

Some Studies on Low Solidity Vaned Diffusers of a Centrifugal Compressor Stage

2005 ◽  
Author(s):  
T. Ch. Siva Reddy ◽  
G. V. Ramana Murty ◽  
Prasad Mukkavilli ◽  
D. N. Reddy
Keyword(s):  
Centrifugal Compressor ◽  
Static Pressure ◽  
Pressure Ratio ◽  
Pressure Recovery ◽  
Compressor Stage ◽  
Recovery Coefficient ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
Centrifugal Compressor Stage ◽  
Pressure Recovery Coefficient

Numerical simulation of impeller and low solidity vaned diffuser (LSD) of a centrifugal compressor stage is performed individually using CFX- BladeGen and BladeGenPlus codes. The tip mach number for the chosen study was 0.35. The same configuration was used for experimental investigation for a comparative study. The LSD vane is formed using standard NACA profile with marginal modification at trailing edge. The performance parameters obtained form numerical studies at the exit of impeller and the diffuser have been compared with the corresponding experimental data. These parameters are pressure ratio, polytropic efficiency and flow angle at the impeller exit where as the parameters those have been compared at the exit of diffuser are the static pressure recovery coefficient and the exit flow angle. In addition, the numerical prediction of the blade loading in terms of blade surface pressure distribution on LSD vane has been compared with the corresponding experimental results. Static pressure recovery coefficient and flow angle at diffuser exit is seen to match closely at higher flows. The difference at lower flows could be due to the effect of interaction between impeller and diffuser combinations, as the numerical analysis was done separately for impeller and diffuser and the effect of impeller diffuser interaction was not considered.

Stator Performance and Unsteady Loading in Transonic Compressor Stages

1998 ◽  
Vol 120 (2) ◽  
pp. 224-232 ◽  
Author(s):  
M. Durali ◽  
J. L. Kerrebrock
Keyword(s):  
Mach Number ◽  
Pressure Distribution ◽  
Pressure Ratio ◽  
Pressure Fluctuations ◽  
Leading Edge ◽  
Large Degree ◽  
Angle Of Incidence ◽  
Mean Flow ◽  
Flow Angle ◽  
Transonic Compressor

The structure and behavior of wakes from a transonic compressor rotor and their effect on the loading and performance of the downstream stator have been investigated experimentally. The rotor was 23.25 inches in diameter with a measured tip Mach number of 1.23 and a pressure ratio of 1.66. Time and space-resolved measurements have been completed of the rotor and stator outflow, as well of the pressure distribution on the surface of the stator blades. It was found that the wakes from this rotor have large flow angle and flow Mach number variations from the mean flow, significant pressure fluctuations, and a large degree of variation from hub to tip. There was a significant total pressure defect and practically no static pressure variation associated with the stator wakes. Wakes from the rotor exist nearly undiminished in the exit flow of the stator and decay in the annular duct behind the stator. The pressure at all points along the chord over each of the stator blades’ surfaces fluctuated nearly in phase in response to the rotor wakes, that is the unsteady chordwise pressure distribution is determined mainly by the change in angle of incidence to the blade and not by the local velocity fluctuations within the passage. The unsteady forces on the stator blades, induced by the rotor wakes, were as high as 25 percent of the steady forces, and lagged the incidence of the wakes on the leading edge by approximately 180 deg at most radii.

Numerical Investigation of the Inclined Leading Edge Diffuser Vane Effects on the Flow Unsteadiness and Noise Characteristics in a Transonic Centrifugal Compressor

2017 ◽  
Author(s):  
Ali Zamiri ◽  
Byung Ju Lee ◽  
Jin Taek Chung
Keyword(s):  
Centrifugal Compressor ◽  
Pressure Ratio ◽  
Pressure Fluctuations ◽  
Leading Edge ◽  
Pressure Recovery ◽  
Vaned Diffuser ◽  
Transonic Centrifugal Compressor ◽  
Transient Simulations ◽  
Leading Edges ◽  
Compressor Efficiency

The three-dimensional, compressible, unsteady Navier-Stokes equations are solved to investigate the influence of the inclined leading edge diffuser vanes on the flow field and radiated noise from a transonic centrifugal compressor with high compression ratio. The computational domain is consisted of an inlet duct and a rotating impeller with splitter blades followed by a two-dimensional wedge vaned diffuser. The numerical method was validated by comparing the steady computational results with those of experiments in terms of pressure ratio and compressor efficiency at different operating points for the original diffuser. The transient simulations were verified by comparison of the velocity distribution with PIV data in normal flow condition before the onset of surge. In the case of steady simulations, seven types of diffuser vane with various inclination angles of leading edge were numerically modeled to investigate the effects of inclined leading edge on the diffuser pressure recovery and total pressure loss characteristics. The vaned diffuser with inclined leading edge reduces the interaction between the impeller discharge flow and diffuser leading edge which leads to improve the pressure recovery characteristics within the diffuser passage. Detailed flow analysis inside the diffuser passage showed the pressure ratio and compressor efficiency have been improved by the inclined leading edges. The maximum diffuser pressure recovery coefficient, 0.7185, and compressor efficiency, 84.80%, were observed in the case of 30 degree inclination angle from hub-to-shroud. In the case of transient simulations, five different inclined leading edge diffuser vanes were numerically conducted. The present study focuses on the unsteady pressure fluctuations and noise prediction within the impeller and diffuser passages at the compressor design point. The influences of inclination angle of diffuser vane leading edges on the pressure waves with different convective velocities, generated by the impeller-diffuser interaction and pseudo-periodic unsteady separation bubbles, were captured in the time/space domain along the diffuser blade surfaces. Since it is important to understand that the far-field acoustics are dominated by the internal pressure fluctuations inside the passages, the near-field pressure fluctuation spectra captured at the impeller-diffuser interface are evaluated to analyze the tonal BPF noise as the main noise source in the centrifugal compressors. It is shown that the inclined leading edges are very useful not only for improvement of the pressure recovery characteristics within the diffuser but also for the reduction of the interaction tonal BPF noise (around 7.6 dB SPL reduction). Furthermore, it was found that by using the inclined leading edge, the vortical structures and separations within the diffuser passages were reduced which may cause the attenuation of the broadband noise components and the overall sound pressure level.

Aerodynamic Design to Increase Pressure Ratio of Centrifugal Compressors for Turbochargers

2009 ◽  
Author(s):  
Hideaki Tamaki ◽  
Masaru Unno ◽  
Tomoki Kawakubo ◽  
Yutaka Hirata
Keyword(s):  
Mach Number ◽  
Rotational Speed ◽  
Pressure Ratio ◽  
Pressure Coefficient ◽  
Leading Edge ◽  
Centrifugal Compressors ◽  
Additional Increase ◽  
Vaned Diffuser ◽  
Operating Range ◽  
Flow Features

This paper describes the ways to increase pressure ratio of centrifugal compressors on condition of fixed impeller diameter. The compressors discussed in the paper were designed for turbochargers applied to 500kW class Diesel engines and their impeller diameters were about 140mm. Pressure ratio was increased from 3.6 to 4.8 at first and then increased from 4.8 to 5.7. The pressure was raised by means of increase in rotational speed together with additional increase in pressure coefficient. The pressure coefficient was increased by means of increase in work coefficient. The increases in rotational speed and work coefficient lead to increase in relative Mach number at inducer. However, the relative Mach number at inducer of the compressor with a pressure ratio of 3.6 already exceeded 1.3. Therefore, it was expected that complicated flow features related to the occurrence of shock wave would deteriorate the new compressors’ efficiency and operating range. In order to avoid this problem, following new measures were taken with conventional design methods. (1) Reduction of a blade loading — an increase in number of blade by means of double splitter (two adjoining splitters). (2) Introduction of technologies to enhance an operating range — recirculation device. (3) Selection of a proper diffuser to convert kinetic energy at an impeller exit to static pressure — vaned diffuser with elliptic leading edge. Newly designed compressors realized expected performance, stage pressure ratio and efficiency. Suitability of these measures was confirmed.

scholarly journals Interpreting Aerodynamics of a Transonic Impeller from Static Pressure Measurements

2018 ◽  
Vol 2018 ◽  
pp. 1-9
Author(s):  
Fangyuan Lou ◽  
John Charles Fabian ◽  
Nicole Leanne Key
Keyword(s):  
Flow Field ◽  
High Speed ◽  
Static Pressure ◽  
Pressure Ratio ◽  
Leading Edge ◽  
Pressure Measurements ◽  
High Loss ◽  
Supersonic Inlet ◽  
Mach Numbers ◽  
Inlet Conditions

This paper investigates the aerodynamics of a transonic impeller using static pressure measurements. The impeller is a high-speed, high-pressure-ratio wheel used in small gas turbine engines. The experiment was conducted on the single stage centrifugal compressor facility in the compressor research laboratory at Purdue University. Data were acquired from choke to near-surge at four different corrected speeds (Nc) from 80% to 100% design speed, which covers both subsonic and supersonic inlet conditions. Details of the impeller flow field are discussed using data acquired from both steady and time-resolved static pressure measurements along the impeller shroud. The flow field is compared at different loading conditions, from subsonic to supersonic inlet conditions. The impeller performance was strongly dependent on the inducer, where the majority of relative diffusion occurs. The inducer diffuses flow more efficiently for inlet tip relative Mach numbers close to unity, and the performance diminishes at other Mach numbers. Shock waves emerging upstream of the impeller leading edge were observed from 90% to 100% corrected speed, and they move towards the impeller trailing edge as the inlet tip relative Mach number increases. There is no shock wave present in the inducer at 80% corrected speed. However, a high-loss region near the inducer throat was observed at 80% corrected speed resulting in a lower impeller efficiency at subsonic inlet conditions.

Steady and Transient Computations of Interaction Effects in a Centrifugal Compressor With Different Types of Diffusers

2010 ◽  
Author(s):  
S. Anish ◽  
N. Sitaram
Keyword(s):  
Centrifugal Compressor ◽  
Static Pressure ◽  
Leading Edge ◽  
Design Flow ◽  
Flow Coefficient ◽  
Pressure Recovery ◽  
Maximum Efficiency ◽  
Vaned Diffuser ◽  
Lower Flow ◽  
Transient Simulations

A computational study has been conducted to analyze the performance of a centrifugal compressor under various levels of impeller-diffuser interactions. The study has been conducted using a low solidity vaned diffuser (LSVD), a conventional vaned diffuser (VD) and a vaneless diffuser (VLD). The study is carried out using Reynolds-Averaged Navier-Stokes simulations. A commercial software ANSYS CFX is used for this purpose. The intensity of interaction is varied by keeping the diffuser vane leading edge at three different radial locations. Frozen rotor and transient simulations are carried out at four different flow coefficients. At design flow coefficient maximum efficiency occurs when the leading edge is at R3 (ratio of radius of the diffuser leading edge to the impeller tip radius) = 1.10. At lower flow coefficient higher stage efficiency occurs when the diffuser vanes are kept at R3 = 1.15 and at higher flow coefficient R3 = 1.05 gives better efficiency. It is observed that at lower flow coefficients positive incidence causes separation of flow at the suction side of the diffuser vane. When the flow rate is above design point there is a negative incidence at the leading edge of the diffuser vane which causes separation of flow from the pressure side of the diffuser vane. Compressor stage performance as well as performance of individual components is calculated at different time steps. Large variations in the stage performances at off-design flow coefficients are observed. The static pressure recovery coefficient (Cp) value is found to be varying with the relative position of impeller and diffuser. It is observed that maximum Cp value occurred at time step where Ψloss value is lowest. From the transient simulations it has been found that the strength and location of impeller exit wake affect the diffuser vane loading which in turn influences the diffuser static pressure recovery.

Effects of Inlet Flow Field Conditions on the Performance of Centrifugal Compressor Diffusers: Part 2 — Straight-Channel Diffuser

1998 ◽  
Author(s):  
Sabri Deniz ◽  
Edward M. Greitzer ◽  
Nicholas A. Cumpsty
Keyword(s):  
Mach Number ◽  
Flow Field ◽  
Centrifugal Compressor ◽  
Axial Flow ◽  
Rotating Stall ◽  
Pressure Recovery ◽  
Straight Channel ◽  
Inlet Flow ◽  
Flow Angle ◽  
Operating Range

This is Part 2 of an examination of influence of inlet flow conditions on the performance and operating range of centrifugal compressor vaned diffusers. The paper describes tests of straight-channel type diffuser, sometimes called a wedge-vane diffuser, and compares the results with those from the discrete-passage diffusers described in Part 1. Effects of diffuser inlet Mach number, flow angle, blockage, and axial flow non-uniformity on diffuser pressure recovery and operating range are addressed. The straight-channel diffuser investigated has 30 vanes and was designed for the same aerodynamic duty as the discrete-passage diffuser described in Part 1. The ranges of the overall pressure recovery coefficients were 0.65–0.78 for the straight-channel diffuser and 0.60–0.70 for the discrete-passage diffuser; the pressure recovery of the straight-channel diffuser was roughly 10% higher than that of the discrete-passage diffuser. Both types of the diffusers showed similar behavior regarding the dependence on diffuser inlet flow angle and the insensitivity of the performance to inlet flow field axial distortion and Mach number. The operating range of the straight-channel diffuser, as for the discrete-passage diffusers was limited by the onset of rotating stall at a fixed momentum-averaged flow angle into the diffuser, which was for the straight-channel diffuser, αcrit = 70° ±0.5°. The background, nomenclature and description of the facility and method are all given in Part 1.

Mixed Flow Turbines: Inlet and Exit Flow Under Steady and Pulsating Conditions

2000 ◽  
Author(s):  
N. Karamanis ◽  
R. F. Martinez-Botas ◽  
C. C. Su
Keyword(s):  
High Speed ◽  
Pulse Propagation ◽  
Combustion Engine ◽  
Pressure Ratio ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Mixed Flow ◽  
Flow Measurements ◽  
Flow Angle ◽  
Isentropic Efficiency

The performance and detailed flow characteristics of a high pressure ratio mixed flow turbine has been investigated under steady and pulsating flow conditions. The rotor has been designed to have a nominal constant incidence (based on free vortex flow in the volute) and it is for use in an automotive high speed diesel turbocharger. The results indicated a departure from the quasi-steady analysis commonly used in turbocharger turbine design. The pulsations from the engine have been followed through the inlet pipe and around the volute; the pulse has been shown to propagate close to the speed of sound and not according to the bulk flow velocity as stated by some researchers. The flow entering and exiting the blades has been quantified by a laser Doppler velocimetry system. The measurements were performed at a plane 3.0 mm ahead of the rotor leading edge and 9.5 mm behind the rotor trailing edge. The turbine test conditions corresponded to the peak efficiency point at 29,400 and 41,300 rpm. The results were resolved in a blade-to-blade sense to examine in greater detail the nature of the flow at turbocharger representative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been shown. The unsteady flow characteristics have been investigated at two flow pulse frequencies, corresponding to internal combustion engine speeds of 1600 and 2400 rpm. Four measurement planes have been investigated: one in the pipe feeding the volute, two in the volute (40° and 130° downstream of the tongue) and one at the exit of the turbine. The pulse propagation at these planes has been investigated; the effect of the different planes on the evaluation of the unsteady isentropic efficiency is shown to be significant. Overall, the unsteady performance efficiency results indicated a significant departure from the corresponding steady performance, in accordance with the inlet and exit flow measurements.

Internal Aerodynamic Performance Evaluation of Double Entrance S-Duct Intake at Moderately High Subsonic Mach Number

2021 ◽  
Author(s):  
Satpreet Sidhu ◽  
Asad Asghar ◽  
William D. E. Allan ◽  
R. A. Stowe ◽  
R. Pimentel
Keyword(s):  
Mach Number ◽  
Total Pressure ◽  
Swirling Flow ◽  
Essential Element ◽  
Aerodynamic Performance ◽  
Geometric Constraints ◽  
Pressure Recovery ◽  
Data Set ◽  
Similar Work ◽  
And Performance

Abstract Inlets are an essential element of aircraft propulsion systems. Aircraft with fuselage-embedded engines require intake ducts with bends to direct oncoming air into the engine. Consequently they often experience flow separation, losses, total pressure distortion, and swirling flow near the engine faces, all of which are detrimental to engine stability and performance. In some aircraft, double-entrance ducts are used to meet geometric constraints and maintain the required airflow. The present paper investigated aerodynamic performance of a bifurcated Y-duct with S-bends in both horizontal and vertical planes. Intake performance was evaluated at inlet Ma = 0.63 by measuring the surface static pressure along the four stream-wise rows of pressure taps and total pressure and 3D velocities using 5-hole probe across the exit plane of the intake duct. The data were used to determine the static and total pressure recovery, together with associated radial and circumferential distortion coefficients and swirl intensity. This work provides a rare experimental data-set for a twin-entrance, moderately high-subsonic, double S-duct intake. It compared reasonably with the most similar work published, that of single-entrance ducts at higher Mach number. Pressure recovery was on par while swirl was noted to be reduced when compared with those geometries. Complementary computational fluid dynamics was useful in the qualitative comparisons as well.

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