Diffuser Vanes Research Articles

A Three-Dimensional Design to Study the Shock Waves of Linear Cascade with Reduced Mass Flow Requirements

This paper presents the development of high-specific-speed mixed flow/centrifugal compressor vaned diffusers. Specifically, the design of a test rig that will make the visualization of shock waves on diffuser vanes manageable is addressed in the current study. In this particular case, linearization of an existing state-of-the-art compressor stage was used. For the computational modeling, a series of RANS analyses were conducted to examine the flow characteristics of the two cases explored: a complete transonic cascade and an idealized periodic passage. The distinct behavior exhibited by each vane passage within the entire cascade offers the opportunity to analyze the shockwave structures across a mass flow range of ±9% around the design point. Overall, the pressure coefficient distributions and flow field patterns appear to align with the single-passage conditions, although there are some minor lateral wall influences, particularly in the first passage close to the suction lateral wall. However, because of the nature of the flow, which is characterized by high velocity and density differences near the vanes, the equivalent mass flow per individual passage was difficult to estimate. This may also be attributed to the small endwall axial vortices; nonetheless, for the purposes of this paper, this was of little consequence.

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

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

Flow Measurements in Passages of Different Diffusers Of A Centrifugal Compressor at Design Condition

Flow field measurements in the passages of different types of diffusers of a centrifugal compressor are presented in this paper. The measurements are carried out using a precalibrated three hole pressure probe at the design condition. The measurements are carried out in the passages of the following diffusers: vaneless, vane, low solidity vane and partial vane diffusers. The results are presented as contours, axially averaged and mass averaged flow parameters. The partial vane diffuser shows slightly improved flow field.

Effect of Diffuser Vane Height and Position on Diffuser Wall Static Pressures in a Centrifugal Compressor

The present paper reports experimental investigations on the effect of diffuser vane height and its position on diffuser wall static pressures of a low speed centrifugal compressor. Apart from the standard types of diffusers, namely, vaneless, vane and low solidity vane diffusers, a partial vane diffuser, whose height is less than the diffuser width is also tested. The vane height of the partial vane diffuser is varied from 0.3, 0.4 and 0.5 times the diffuser width in vane and low solidity vane diffuser configurations. In addition, the effect of vane position is examined by fixing the partial vanes to the hub, shroud or hub and shroud. It is found that there is an optimum height for the diffuser vane height, which is found to be 0.3 times the diffuser width. The effect of position of the partial vanes on the hub or shroud on the diffuser wall static pressures is found to be negligible. However when partial vanes are fixed on the hub and shroud staggered at half spacing, the diffuser performance based on wall static pressures is improved substantially.

Flow Measurements in the Passages of Different Types of Diffusers of a Centrifugal Compressor at Off Design Conditions

Flow measurements in the passages of different types of diffusers of a centrifugal compressor at off design flow coefficients are presented and compared with those at design flow coefficient in this paper. The measurements are carried out using a precalibrated three hole pressure probe. The measurements are carried out in the passages of the following diffusers: vaneless, vane, low solidity vane and partial vane diffusers. The results are presented as contours, axially averaged and mass averaged flow parameters. The partial vane diffuser shows slightly improved flow field. At the above design flow coefficient, both vane and low solidity vane diffusers suffer large losses due to high incidences causing large drop in static pressure.

Laboratory-Scale Low-Speed Water Tunnel: Comparison of Experimental Flow Visualization and Computer-Aided Simulation

This study aimed at developing the laboratory-scale low-speed water tunnel design by first evaluating the effect of varying the number of wide-angle diffuser vanes and then confirming flow visualization against object geometries tested by experiments and computer-aided simulation. The low-speed water tunnel design in this study consists of a wide-angle diffuser, a honeycomb, a convergent section, and a working section. To ascertain the laminar flow of water would act in the experiment, a prior simulation using 3, 6, and 12 vanes and without a vane was conducted. Some geometries, including tube, triangle, aerofoil, and car models have been tested and then compared to the experimental results. The results indicate that the twelve vanes diffuser is more effective in minimizing turbulent flow and producing a streamlined pattern in the water tunnel. Laminar flow with a Reynolds number of 2238 and flow velocity of 0.008 m/s was obtained in the working section. The flow visualization by experiment and computational fluid dynamics simulation was almost similar.

Effect of Inlet Vane Angle of a Partial Vane Diffuser on the Performance and Flow Filed of a Centrifugal Compressor

The paper presents experimental results on the effect of inlet vane angle on the performance and flow field of Partial Vane Diffuser (PVD) of a low speed centrifugal compressor. Performance tests are conducted for eight configurations of PVDs. From these tests, an optimum configuration with 5° increase in inlet angle of partial diffuser vanes on shroud (PVDHS5) is selected for further measurements i.e. static pressure on the diffuser hub and shroud and diffuser passage flow field at four flow coefficients. From passage flow measurements flow parameters are obtained within the diffuser flow passage. Flow parameters are axially averaged and their contours are presented. Radial variation of these flow parameters is also presented. PVDHS5 gives 2.3% increase in peak energy coefficient and 4.9% increase in efficiency compared to that of vane less diffuser. The flow at the exit of PVDHS5 is more uniform.

P18: Improving Hydraulic Performance of the Left Atrial Assist Device Using Computational Fluid Dynamics

Purpose of Study: The purpose of this study was to refine the initial design of a new left atrial assist device (LAAD) using the results from computational fluid dynamics analyses (CFD). The CFD results would help guide design changes that could improve the hydraulic performance and flow patterns within the LAAD. The LAAD is a novel continuous-flow pump, implanted in the mitral plane, designed to treat patients with heart failure with preserved ejection fraction. Methods: Four different LAAD designs were evaluated. The key differences among the designs included changes to the shape and dimensions of the primary impeller blades, and the number, size, and curvature of the diffuser vanes. Steady state, frozen-rotor CFD simulations were run at flow rates and rotor speeds spanning the LAAD’s intended range of use. Of the four designs studied, the first and final designs were prototyped and underwent in vitro hydraulic performance testing. The in vitro studies were conducted by connecting a prototype LAAD to a mock circulatory loop system designed to simulate various levels of diastolic heart failure. Static pressure measurements, taken upstream, downstream, and at two other locations within the pump, were compared with CFD-predicted pressures to validate the CFD modeling approach and results. Results: The CFD-predicted static pressures agreed within 12% for design #1 and 10% for design #4 over a wide range of flow rate/rotor speed conditions providing confidence in the CFD modeling results. Regions of flow separation downstream of the impeller blade tips and the diffuser vanes were reduced for each design iteration. Increasing the impeller blade diameter, provided increased pump pressure rise at the expense of significantly higher torque and power requirements. A comparison of the hydraulic performance for the four designs, as shown in Figure 1, reveals that design #4 was able to provide the same pump pressure rise as design #1 at 3,800 vs. 4,400 rpm. Conclusions: Guided by the insight gained from each design iteration, the fourth design incorporated impeller blades with an improved alignment with the incoming flow and wider, more curved diffuser vanes that aligned with the approaching flow from the volute. These changes reduced flow separation within the impeller and diffuser regions which significantly improved the LAAD’s hydraulic performance and internal flow patterns. In vitro testing of this fourth generation LAAD design confirmed the predicted improvement in hydraulic performance.Figure 1. Comparison of the hydraulic performance for the four LAAD designs at a flow rate of 4.5 L/min and varying rotor speeds.

Experimental Investigation on Velocity Fluctuation in a Vaned Diffuser Centrifugal Pump Measured by Laser Doppler Anemometry

Turbulent flow, mainly originating from the rotor-stator interaction (RSI), is closely associated with the normal and safe operation of the centrifugal pump. In the current research, to clarify turbulent flow in the centrifugal pump with a vaned diffuser, the non-intrusive LDA (Laser Doppler Anemometry) system is applied to measure velocity pulsation signals at different regions when the pump operates at various flow rates. Time and frequency domain analysis methods are combined to investigate the velocity signals, and the velocity distribution around the volute tongue region is reconstructed from twenty measuring points. Results show that the velocity spectrum is characterized by the discrete components at the blade passing frequency and its higher harmonics, and it is caused by the RSI between the impeller and the diffuser. For the points in the volute spiral and diffusion sections, due to the significantly reduced RSI effect, the velocity spectrum shows an evident difference from comparison with the points between the impeller and diffuser, and the blade passing frequency is not always the dominant frequency. The comparison of velocity amplitudes and RMS* (root mean square of velocity) values at different points proves that the measuring position and flow rate affect velocity pulsations. As observed from velocity distribution reconstructed by LDA signals, high velocity regions are developed downstream of the diffuser channel for all the measured flow rates.

Hydrodynamic optimization of the impeller and diffuser vane of an axial-flow pump

Hydrodynamic optimization of the impeller and diffuser vane of an axial-flow pump

Influence of Diffuser Vane Thickness on Centrifugal Compressor Performance

The structure of diffuser vanes has a significant influence on the performance of centrifugal compressors. To research the influence of diffuser vane thickness on the overall performance and internal flow condition of the centrifugal compressor. This paper takes a centrifugal compressor as the research object. By changing the diffuser vane thickness, and using three-dimensional fluid simulation technology to carry out simulation research. The results show that the thinner diffuser vanes can greatly improve the efficiency, flow rate and outlet temperature of the centrifugal compressor. For internal local fluid characteristics, thin diffuser blades have lower internal heat load. However, the local flow of fluid is not consistent with the change of vane thickness.

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

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

Investigation of Surge in a Transonic Centrifugal Compressor With Vaned Diffuser—Part I: Surge Signature

Abstract The identification of stall inception mechanisms and stability-limiting components in a centrifugal compressor is required for the development of effective surge suppression approaches. Part I of this two-part paper investigates the surge signature of a centrifugal compressor at subsonic, transonic, and supersonic inlet tip conditions, and Part II considers the relationship of the surge signature with the compressor static pressure characteristics, as well as the impeller leading edge relative tip Mach number. Experiments were performed in the single stage centrifugal compressor (SSCC) facility at Purdue University with both steady performance and dynamic pressure data recorded. Results show the presence of both mild and deep surges on the compressor map. Deep surge occurs at subsonic and supersonic impeller tip relative Mach numbers while the mild surge is observed as the tip relative Mach number nears unity. Long length-scale modal oscillations can be detected only at the transonic operating condition while spike disturbances are identified as the precursor to instability at subsonic and supersonic tips relative Mach numbers. Finally, the impeller is determined to be the origin of instability throughout most of the compressor map. The observation of impeller-induced instability refreshes the conventional understanding that the diffuser is typically the stability-limiting component for centrifugal compressors with vaned diffusers and suggests various surge suppression approaches may be necessary to achieve range extension throughout a compressor map.

Cavitation Suppression and Design Optimization in a Cryogenic Liquid Turbine Expander Based on Thermodynamic Cavitation and Entropy Production Analysis

Abstract Cryogenic liquid turbine expanders have emerged quite recently as a replacement of J–T valve for enhancing energy efficiency of industrial systems, such as air separation units (ASUs) and Supercritical Compressed Air Energy Storage systems. In the liquid turbine expander, the rotating impeller-induced swirling flow and cavitation are essentially significant and intensive, which requests some in-depth work toward a thorough understanding flow physics and then effective attenuation. This study aims at effectively mitigating the swirling flow and cavitation. The entropy production analysis method (EPAM) is proposed to characterize the swirling flow and cavitation. It is then incorporated with the improved cavitation and turbulence models and validated through the simulation of the Hord's liquid nitrogen hydrofoil. To mitigate the swirling flow and subsequent cavitation, the design optimization method is developed, in which a novel optimization objective function is constituted by incorporating the local entropy production rate and vapor volume fraction to capture the mechanical energy dissipation and cryogenic cavitating flow physics; the non-uniform relational B-Splines and free form deformation (NURBS–FFD) parametric method is used to facilitate a flexible variation in impeller blade and diffuser vane geometries. It is solved within cfx frame by means of the particle swarm optimization (PSO) algorithm coupling the Kriging-based adaptive surrogate model. With the design optimization, the impeller and vaned diffuser tube geometries are collaboratively fine-tuned, and the mechanical energy dissipation and cavitating flow across both the impeller and vaned diffuser tube is effectively mitigated.

Numerical modeling of the dynamic unsteadiness with global mode decomposition approach in a high-speed compressor with abnormal diffuser vane

Adjustable vane diffuser is addressed as one of the main techniques in centrifugal compressor in seeking of wider stability margin. Flow instabilities, however, may be driven by the three-dimensional flow separation induced by the abnormal vane in the diffuser. In the present work, a numerical investigation of the radial vaned diffuser with abnormal vane was applied to identify the correlation between stall inception and asymmetric flow distribution. Unsteady numerical simulations of the centrifugal compressor with vaned diffuser was carried out and validated against experimental data. Then, the aerodynamic characteristics and evolution process of the flow instabilities in vaned diffuser was analyzed with the compressed dynamic mode decomposition in term of perturbation frequency and dominating flow mode. Finally, concerning the relative position of the volute tongue and abnormal vane, the influence of the abnormal vane on the matching between the diffuser and volute regarding the stall mechanism was summarized. The results indicate that the abnormal vane needs to be altered large enough, which is above 10°, to trigger the flow instability of vaned diffuser before the spike stall in impeller. With the increase of stagger angle alternation, the stall mechanism turns from a global flow separation in multiple passages into a regional and abrupt phenomenon in limited passages. With the mode composition approach, the flow separation near diffuser outlet and the tornado-type vortices at the leading edge of the abnormal vane are responsible for the flow instability of the case with 15° vane alternation. When combined with the volute, the case with abnormal vane close to the volute tongue tends to be more unstable than that 180° away from volute tongue. The tornado-type vortices are also moved to the trailing edge rather than the leading edge under the influence of the asymmetric volute.

Performance Enhancement of a Centrifugal Compressor by Designing a Tandem Cascade Diffuser

The performance of a vane diffuser plays an important role in transforming the high kinetic energy into pressure energy at the impeller outlet of a centrifugal compressor. In this study, the performance enhancement of a centrifugal compressor is achieved by designing a tandem cascade diffuser. An optimum value of the total bending angle of a tandem cascade diffuser is obtained through numerical simulation. The total bending angle is defined as the angle between the tangent line of the front blade arc line at the leading edge point and the tangent line of the rear blade arc line at the trailing edge point. The range of the total bending angle increases from zero to 21°. The simulation results show that the variation of the total bending angle has a great impact on the performances of the compressor stage. The best performance is achieved by the 10° model, by which the minimum total pressure loss coefficient and the maximum static pressure recovery coefficient are obtained. The mechanism of performance improvement by choosing a reasonable total bending angle is that the separated flow zone in the diffuser is constrained and the distribution of radial velocity at the inlet and outlet of the diffuser is more uniformed. With the 10° model, the separated zone almost completely disappears in the diffuser and a distribution of more uniform radial velocity along the meridional section is obtained. Compared with the stage of the prototype with a single row of vane diffuser, the stage with the newly designed tandem vane diffuser (10° model) achieved performance improvement, the efficiency increased about 6%, and the total pressure ratio increased about 3.5% at the flow coefficient of 0.15.

Stall Behavior in an Ultrahigh-Pressure-Ratio Centrifugal Compressor: Backward-Traveling Rotating Stall

AbstractThis paper describes the stall mechanism in an ultrahigh-pressure-ratio centrifugal compressor, composed of a double-splitter impeller, radial diffuser, and axial diffuser. A model comprising all impeller and diffuser blade passages is used to conduct unsteady simulations that trace the onset of instability in the compressor. Backward-traveling rotating stall waves appear at the inlet of the radial diffuser when the compressor is throttled. Six stall cells propagate circumferentially at approximately 0.7% of the impeller rotation speed. The detached shock of the radial diffuser leading edge and the number of stall cells determine the direction of stall propagation, which is opposite to the impeller rotation direction. Dynamic mode decomposition is applied to instantaneous flow fields to extract the flow structure related to the stall mode. This shows that intensive pressure fluctuations concentrate in the diffuser throat as a result of changes in the detached shock intensity. The diffuser passage stall and stall recovery are accompanied by changes in incidence angle and shock wave intensity. When the diffuser passage stalls, the shock-induced boundary–layer separation region near the diffuser vane suction surface gradually expands, increasing the incidence angle and decreasing the shock intensity. The shock is pushed from the diffuser's throat toward the diffuser leading edge. When the diffuser passage recovers from the stall, the shock wave gradually returns to the diffuser throat, with the incidence angle decreasing and the shock intensity increasing. Once the shock intensity reaches its maximum, the diffuser passage suffers severe shock-induced boundary–layer separation and stalls again.

Effect of diffuser vane slit on rotating stall behavior and pump performance in a centrifugal pump

Abstract In the previous study of a centrifugal pump with a vaned diffuser, it was confirmed that the flowrate at the onset of rotating stall was shifted to the lower flowrate by slitting the diffuser vanes. As the CFD result, it was found that the jet flow from pressure-side to suction-side impinges backflow near the suction surface and it suppresses to expand toward the diffuser inlet. However, it is difficult to say that the optimized shape and slitting position have been sufficiently studied. Therefore, in this study, we investigated the effect of slitting on the pump performance and internal flow by adding another slitting to the diffuser vane with slit. In order to clarify the suppression effect of the jet flow in detail, the effect of adding slits at two locations was verified. The pump performance was compared for these diffusers. The effect of slitting on the diffuser vanes on the occurrence of rotating stall was clarified by the experimental and numerical study.

Математическая модель метода универсального моделирования девятой версии для расчета и проектирования центробежных компрессоров: идентификация и верификация по экспериментальным данным

Identification and verification of a new version of mathematical model for universal modeling method for calculation of centrifugal compressors gas dynamic characteristics are carried out. To identify the loss model, the results of 70 tests of 20SE family model stages at LPI-SPbPU Compressor Engineering Problem Laboratory were used. A total of 420 efficiency values were compared. At the first stage, identification was carried out only for stages without a vane diffuser; the average error of efficiency calculation was 0.513 %, and, on the whole, for the characteristic is 1 %. When identifying vane diffuser stages, the search objects were coefficients influencing losses in the vane diffuser only, the average error of efficiency calculation was 0.525 %, for the whole characteristic is 1.12 %. To verify the empirical coefficients affecting losses in the axial separation impeller, test results of TKR-175E and TKR-140E compressors for turbocharger units were used. In the IDENT database we gave characteristics of TKR-140E compressor at conditional Mach numbers 0.437, 0.5836, 0.728, 0.8739; average error of efficiency calculation was 0.889 %, for the whole characteristic is 1.55 %. To verify the mathematical model, the results of the tests of Clark (USA) low-flow model stages were used. Ten tests of low-flow stages were considered; the average error of efficiency calculation was 1.08 %, for the whole characteristic is 1.31 %

Investigation of New Design Principles for the Centrifugal Compressor Vaned Diffusers

Diffuser’s aerodynamic performance is crucial for the centrifugal compressors, while at present the universal principles for the optimization design of the vaned diffusers are still not available. In this paper, three vaned diffusers with different inlet Mach numbers were numerically studied in order to explore new design principles for the centrifugal compressor vaned diffusers. It proved that there are practical and effective design principles for the vaned diffuser optimizations, the performance of the vaned diffuser can be improved by carefully control of two aerodynamic parameter distributions: Tangential velocity (Vt) and Meridional velocity (Vm). The vaned diffusers with subsonic, transonic and supersonic inlet conditions were optimized with the new design principles, and the peak efficiencies were increased by 4.23%, 2.15% and 2.59%, respectively. The stage pressure ratios were increased by 3.36%, 1.39% and 6.49%, respectively, and their surge margins were also improved substantially. Finally, since the Vt and Vm could affect each other during the optimization process, an interactive optimization design procedure was also presented in this paper in order to accelerate the optimization process.