Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

2016 ◽  
Vol 138 (10) ◽  
Author(s):  
Xinqian Zheng ◽  
Qiangqiang Huang ◽  
Anxiong Liu
Keyword(s):  
Flow Control ◽  
Mass Flow ◽  
Centrifugal Compressor ◽  
Flow Separation ◽  
Flow Instability ◽  
Reverse Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Impeller Blade ◽  
Loss Mechanisms

Variable inlet prewhirl is an effective way to suppress compressor flow instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirls, discusses the mechanisms which are responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier–Stokes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was imposed by modifying the velocity direction of inlet boundary condition. Simulation results show that the peak efficiency at high inlet prewhirl is reduced by over 7.6% points compared with that at zero prewhirl. The extra loss occurs upstream and downstream of the impeller. Severe flow separation, which reduces efficiency by 2.3% points, was found near the inlet hub. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, which induces the separation. A dimensionless parameter C was defined to measure the centrifugal trend of gas and indicate the flow separation near the inlet hub. As for the extra loss which is produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes' leading edges. An analytical model was built to predict diffuser choking mass flow. It proves that the diffuser has already operated unstably at high prewhirl. Based on these two loss mechanisms, the hub curve and the diffuser stager angle were modified and adjusted, respectively, for higher efficiency at high prewhirl. The efficiency improvement benefited from the modification of the hub is 1.1% points, and that benefited from the combined optimization is 2.4% points. During optimizing, constant distribution of inlet prewhirl was found to be another factor for inducing reverse flow at the leading edge of the impeller blade root, which turned out being blamed on the misalignment of the swirl angle and the blade angle.

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

2015 ◽  
Author(s):  
Zheng Xinqian ◽  
Huang Qiangqiang ◽  
Liu Anxiong
Keyword(s):  
Flow Control ◽  
Mass Flow ◽  
Centrifugal Compressor ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Leading Edge ◽  
Flow Region ◽  
Percentage Points ◽  
Stable Conditions ◽  
Loss Mechanisms

Variable inlet prewhirl is an effective way to suppress compressor instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirl, discusses the mechanisms thought to be responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier-Stockes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was modeled by modifying the velocity condition at the inlet boundary. Simulation results show that the peak efficiency at high inlet prewhirl is reduced compared to that at zero prewhirl by over 7.6 percentage points. The extra loss is produced upstream and downstream of the impeller. Severe flow separation was found near the inlet hub which reduces efficiency by 2.3 percentage points. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, inducing the separation. A dimensionless parameter C is defined to measure the centrifugal component of flow. As for the extra loss produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes’ leading edges. An analytical model is built to predict diffuser choking mass flow which proves that the diffuser flow operates outside of stable conditions. Based on the two loss mechanisms, hub curve and diffuser stager angle were modified and adjusted for seeking higher efficiency at high prewhirl. The efficiency improvement of a modification of the hub is 1.1 percentage points and that of the combined optimization is 2.4 percentage points. During optimizing, constant distribution of inlet prewhirl was found to induce reverse flow at the leading edge of the blade root, which turned out being uncorrelated with blade angle. By revealing loss mechanisms and proposing flow control ideas, this paper lays a theoretical basis for overcoming the efficiency drop induced by high inlet prewhirl and for developing compressors with high inlet prewhirl.

scholarly journals Control of flow separation over a circular cylinder using synthetic jet

2021 ◽  
Vol 2119 (1) ◽  
pp. 012025
Author(s):  
A. S. Lebedev ◽  
M. I. Sorokin ◽  
D. M. Markovich
Keyword(s):  
Circular Cylinder ◽  
Flow Control ◽  
Flow Separation ◽  
Gas Turbines ◽  
Acoustic Noise ◽  
Reverse Flow ◽  
Flow Region ◽  
Synthetic Jet ◽  
Separation Flow ◽  
Longitudinal Length

Abstract The development of methods of active separation flow control is of great applied importance for many technical and engineering applications. Understanding the conditions for the flow separation from the surface of a bluff body is essential for the design of aircrafts, cars, hydro and gas turbines, bridges and buildings. Drag, acoustic noise, vibrations and active flow mixing depend drastically on the parameters of the vortex separation process. We investigated the possibility of reducing the longitudinal length of a reverse-flow region using the method of «synthetic jet» active separation flow control. The experiment was carried out on a compact straight-through wind channel with a 1-m long test section of a cross-section of 125x125 mm. The jet was placed at the rear stagnation point of a circular cylinder. The Reynolds number, based on the cylinder diameter and the free-stream velocity, was 5000 and the von Kármán street shedding frequency without the synthetic jet was equal to 64.8 Hz. For the first time, for such a set of parameters, we applied high speed PIV to demonstrate that the injection of the synthetic jet into the cylinder wake region leads to a significant reduction in the longitudinal length of the reverse-flow region.

Numerical Investigation of Diffuser Rotating Stall and System Instability in a Centrifugal Compressor With Vaned Diffuser

2018 ◽  
Author(s):  
Yang Zhao ◽  
Guang Xi ◽  
Jiayi Zhao
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Centrifugal Compressor ◽  
Flow Instability ◽  
Leading Edge ◽  
Rotating Stall ◽  
Vaned Diffuser ◽  
Corner Separation ◽  
Lumped Parameter

The operating range of a centrifugal compressor is often limited by the occurrence of the flow instability, such as diffuser rotating stall or system surge. In the paper, the unsteady numerical simulations are performed on a low-speed centrifugal compressor to investigate the characteristic of the rotating stall in the vaned diffuser. And also, the developed model of lumped parameter is used to predict the system instability. The flow field in the diffuser is firstly investigated at near stall condition. It is found that the leading-edge vortex and the secondary flow induce the hub-corner separation at the suction side of the diffuser blade. When the mass flow rate is reduced gradually, the fore part of the volute turns to act as a diffuser from a nozzle. Under the influence of the asymmetry induced by the volute, the hub-corner separation firstly develops into rotating stall in the passage with the lowest mass flow rate when at critical stall point. And then the diffuser rotating stall propagates along the circumferential direction at about 7% of the impeller speed. And also, the model of lumped parameter considering the effect of rotating stall is developed to analyze the system instability of mild surge. The predicted vibration frequency is within 5.8% of the measurement and the predicted transient process in mild surge matches well with the measurement. With different volume of the compressed air, the transient compressor characteristic tends to be stabilized or oscillates in a cycle along the counter-clockwise with different magnitude.

Numerical studies of active flow control on wing tip extension

2019 ◽  
Vol 91 (2) ◽  
pp. 346-352
Author(s):  
Petr Vrchota ◽  
Ales Prachar ◽  
Shia-Hui Peng ◽  
Magnus Tormalm ◽  
Peter Eliasson
Keyword(s):  
Flow Control ◽  
Mass Flow ◽  
Flow Separation ◽  
Physical Modeling ◽  
Active Flow Control ◽  
Leading Edge ◽  
Content Type ◽  
Tip Extension ◽  
Active Flow ◽  
Wing Tip

Purpose In the European project AFLoNext, active flow control (AFC) measures were adopted in the wing tip extension leading edge to suppress flow separation. It is expected that the designed wing tip extension may improve aerodynamic efficiency by about 2 per cent in terms of fuel consumption and emissions. As the leading edge of the wing tip is not protected with high-lift device, flow separation occurs earlier than over the inboard wing in the take-off/landing configuration. The aim of this study is the adoption of AFC to delay wing tip stall and to improve lift-to-drag ratio. Design/methodology/approach Several actuator locations and AFC strategies were tested with computational fluid dynamics. The first approach was “standard” one with physical modeling of the actuators, and the second one was focused on the volume forcing method. The actuators location and the forcing plane close to separation line of the reference configuration were chose to enhance the flow with steady and pulsed jet blowing. Dependence of the lift-to-drag benefit with respect to injected mass flow is investigated. Findings The mechanism of flow separation onset is identified as the interaction of slat-end and wing tip vortices. These vortices moving toward each other with increasing angle of attack (AoA) interact and cause the flow separation. AFC is applied to control the slat-end vortex and the inboard movement of the wing tip vortex to suppress their interaction. The separation onset has been postponed by about 2° of AoA; the value of ift-to-drag (L/D) was improved up to 22 per cent for the most beneficial cases. Practical implications The AFC using the steady or pulsed blowing (PB) was proved to be an effective tool for delaying the flow separation. Although better values of L/D have been reached using steady blowing, it is also shown that PB case with a duty cycle of 0.5 needs only one half of the mass flow. Originality/value Two approaches of different levels of complexity are studied and compared. The first is based on physical modeling of actuator cavities, while the second relies on volume forcing method which does not require detailed actuator modeling. Both approaches give consistent results.

scholarly journals Centrifugal Compressor Stall Control by the Application of Engineered Surface Roughness on Diffuser Shroud Using Numerical Simulations

Materials ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2033
Author(s):  
Amjid Khan ◽  
Muhammad Irfan ◽  
Usama Muhammad Niazi ◽  
Imran Shah ◽  
Stanislaw Legutko ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Centrifugal Compressor ◽  
Control Method ◽  
Flow Instability ◽  
Passive Flow Control ◽  
Passive Flow ◽  
Low Mass

Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective quantitative results in controlling stall phenomena, which results in increases of operating range from 16% to 18% and stall margin from 5.62% to 7.98%. Surface roughness as a passive flow control method to reduce flow instability in the diffuser section is the novelty of this research. Keeping in view the effects of surface roughness, it will help the turbocharger manufacturers to reduce the flow instabilities in the compressor with ease and improve the overall performance.

scholarly journals Model-Based Analysis of Flow Separation Control in a Curved Diffuser by a Vibration Wall

Energies ◽  
2021 ◽  
Vol 14 (6) ◽  
pp. 1781
Author(s):  
Weiyu Lu ◽  
Xin Fu ◽  
Jinchun Wang ◽  
Yuanchi Zou
Keyword(s):  
Flow Field ◽  
Flow Control ◽  
Flow Separation ◽  
Vibration Frequency ◽  
Flow Instability ◽  
Control Technique ◽  
Simplified Model ◽  
Main Stream ◽  
Control Performance ◽  
Flow Separation Control

Vibration wall control is an important active flow control technique studied by many researchers. Although current researches have shown that the control performance is greatly affected by the frequency and amplitude of the vibration wall, the mechanism hiding behind the phenomena is still not clear, due to the complex interaction between the vibration wall and flow separation. To reveal the control mechanism of vibration walls, we propose a simplified model to help us understand the interaction between the forced excitation (from the vibration wall) and self-excitation (from flow instability). The simplified model can explain vibration wall flow control behaviors obtained by numerical simulation, which show that the control performance will be optimized at a certain reduced vibration frequency or amplitude. Also, it is shown by the analysis of maximal Lyapunov exponents that the vibration wall is able to change the flow field from a disordered one into an ordered one. Consistent with these phenomena and bringing more physical insight, the simplified model implies that the tuned vibration frequency and amplitude will lock in the unsteady flow separation, promote momentum transfer from the main stream to the separation zone, and make the flow field more orderly and less chaotic, resulting in a reduction of flow loss.

Tip Leakage Flow Instability in a Centrifugal Compressor

2021 ◽  
Vol 143 (4) ◽  
Author(s):  
Teng Cao ◽  
Tadashi Kanzaka ◽  
Liping Xu ◽  
Tobias Brandvik
Keyword(s):  
Shear Layer ◽  
Centrifugal Compressor ◽  
Large Scale ◽  
Flow Instability ◽  
Leading Edge ◽  
Main Stream ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Unstable Flow ◽  
Tip Leakage

Abstract In this paper, an unsteady tip leakage flow phenomenon is identified and investigated in a centrifugal compressor with a vaneless diffuser at near-stall conditions. This phenomenon is associated with the inception of a rotating instability in the compressor. The study is based on numerical simulations that are supported by experimental measurements. The study confirms that the unstable flow is governed by a Kelvin–Helmholtz type instability of the shear layer formed between the main-stream flow and the tip leakage flow. The shear layer instability induces large-scale vortex roll-up and forms vortex tubes, which propagate circumferentially, resulting in measured pressure fluctuations with short wavelength and high amplitude which rotate at about half of the blade speed. The 3D vortex tube is also found to interact with the main blade leading edge, causing the reduction of the blade loading identified in the experiment. The paper also reveals that the downstream volute imposes a once-per-rev circumferential nonuniform back pressure at the impeller exit, inducing circumferential loading variation at the impeller inducer, and causing circumferential variation in the unsteady tip leakage flow.

Stall Inception in a High Speed Centrifugal Compressor During Speed Transients

2017 ◽  
Author(s):  
Fangyuan Lou ◽  
John C. Fabian ◽  
Nicole L. Key
Keyword(s):  
Heat Transfer ◽  
Centrifugal Compressor ◽  
High Speed ◽  
Flow Instability ◽  
Leading Edge ◽  
Fast Response ◽  
Rotating Stall ◽  
Transient Performance ◽  
Stall Inception ◽  
Pressure Transducers

The inception and evolution of rotating stall in a high-speed centrifugal compressor are characterized during speed transients. Experiments were performed in the Single Stage Centrifugal Compressor (SSCC) facility at Purdue University and include speed transients from sub-idle to full speed at different throttle settings while collecting transient performance data. Results show a substantial difference in the compressor transient performance for accelerations versus decelerations. This difference is associated with the heat transfer between the flow and the hardware. The heat transfer from the hardware to the flow during the decelerations locates the compressor operating condition closer to the surge line and results in a significant reduction in surge margin during decelerations. Additionally, data were acquired from fast-response pressure transducers along the impeller shroud, in the vaneless space, and along the diffuser passages. Two different patterns of flow instabilities, including mild surge and short-length-scale rotating stall, are observed during the decelerations. The instability starts with a small pressure perturbation at the impeller leading edge and quickly develops into a single-lobe rotating stall burst. The stall cell propagates in the direction opposite of impeller rotation at approximately one third of the rotor speed. The rotating stall bursts are observed in both the impeller and diffuser, with the largest magnitudes near the diffuser throat. Furthermore, the flow instability develops into a continuous high frequency stall and remains in the fully developed stall condition.

The Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaneless Diffuser

2020 ◽  
Author(s):  
Hiroshi Miida ◽  
Kenta Tajima ◽  
Nobumichi Fujisawa ◽  
Yutaka Ohta
Keyword(s):  
Mass Flow ◽  
Centrifugal Compressor ◽  
Rotational Speed ◽  
Flow Region ◽  
Cfd Analysis ◽  
Vaneless Diffuser ◽  
Low Velocity ◽  
Flow Angle ◽  
Boundary Separation ◽  
Low Mass

Abstract The unsteady diffuser stall behavior in a centrifugal compressor with a vaneless diffuser was investigated by experimental and computational analyses. The diffuser stall generated as the mass flow rate decreased. The diffuser stall cell rotated at 25–30% of the impeller rotational speed, with diffuser stall fluctuations observed at 180° from the cutoff. The diffuser stall fluctuation magnitude gradually increased near the cutoff. Based on diffuser inlet velocity measurements, the diffuser stall fluctuations generated near both the shroud and hub sides, and the diffuser stall appeared at 180° and 240° from the cutoff. According to the CFD analysis, the mass flow fluctuations at the diffuser exit showed a low mass flow region, rotating at approximately 25% of the impeller rotational speed. They began at 180° from the cutoff and developed as this region approached the cutoff. Therefore, the diffuser stall could be simulated by CFD analysis. First, the diffuser stall cell originated at 180° from the cutoff by interaction with boundary separation and impeller discharge vortex. Then, the diffuser stall cell further developed by boundary separation accumulation and the induced low velocity area, located at the stall cell center. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded in the impeller rotational direction due to boundary separation caused by a positive flow angle. Finally, the diffuser stall cell vanished when it passed the cutoff, because mass flow recovery occurred.

scholarly journals Interpretation of Four Unique Phenomena and the Mechanism in Unsteady Flow Separation Controls

Energies ◽  
2019 ◽  
Vol 12 (4) ◽  
pp. 587 ◽  
Author(s):  
Weiyu Lu ◽  
Guoping Huang ◽  
Jinchun Wang ◽  
Yuxuan Yang
Keyword(s):  
Numerical Simulation ◽  
Unsteady Flow ◽  
Flow Control ◽  
Flow Separation ◽  
Flow Instability ◽  
Control Flow ◽  
Separation Control ◽  
Pulsed Jet ◽  
Unsteady Separation ◽  
Flow Theories

Unsteady flow separation controls are effective in suppressing flow separations. However, the unique phenomena in unsteady separation control, including frequency-dependent, threshold, location-dependent, and lock-on effects, are not fully understood. Furthermore, the mechanism of the effectiveness that lies in unsteady flow controls remains unclear. Thus, this study aims to interpret further the unique phenomena and mechanism in unsteady flow separation controls. First, numerical simulation and some experimental results of a separated curved diffuser using pulsed jet flow control are discussed to show the four unique phenomena. Second, the bases of unsteady flow control, flow instability, and free shear flow theories are introduced to elucidate the unique phenomena and mechanism in unsteady flow separation controls. Subsequently, with the support of these theories, the unique phenomena of unsteady flow control are interpreted, and the mechanisms hidden in the phenomena are revealed.

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