Numerical investigation of rotating stall in centrifugal compressor with vaned and vaneless diffuser (original) (raw)
Related papers
Generation Mechanism of Diffuser Stall in a Centrifugal Compressor with Vaneless Diffuser
2020
The generation mechanism of a diffuser stall in a centrifugal compressor with a vaneless diffuser was experimentally and computationally investigated, and the main findings are presented in this paper. Diffuser stall is generated when the mass flow through the compressor decreases and approaches the compressor surge limit. It was observed that diffuser stall cells rotate at 25-30% of the impeller rotational speed, with diffuser stall fluctuations observed at 180°from the cutoff (volute tongue). It was found that the magnitude of the diffuser stall fluctuation gradually increased near 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. The diffuser stall fluctuations began at 180°f rom 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 the hub side boundary layer separation. Then, the diffuser stall cell further developed by the boundary layer separation accumulation and the induced low velocity area. The low velocity region formed a blockage across the diffuser passage span. The diffuser stall cell expanded due to the boundary layer separations occurring on the shroud and hub wall by turns. Finally, the diffuser stall cell vanished when it passed the cutoff, because the mass flow recovery occurred. The current paper presented the main results of this phenomena of great importance in centrifugal compressors.
An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers
Volume 7: Turbomachinery, Parts A, B, and C, 2011
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.
Journal of Mechanics, 2013
ABSTRACTThis paper is a numerical simulation that was made in the three-dimensional flow, carried out in a modified centrifugal compressor, having vaned diffuser stage, used as an auto-motive turbo charger. Moreover, the performance of the centrifugal compressor was dependent on the proper matching between compressor impeller and vaned diffuser, influencing significantly surge and the efficiency of centrifugal compressor stages. In addition, a modified compressor impeller, coupled with vane and vaneless diffuser, has been found to have similar internal flow patterns for both the vaneless and vaned diffuser design. The vaned diffuser effect has been paid particular attention in terms of better analysis where the diffuser was designed for high sub-sonic inlet conditions. Another aim of this research was to study and simulate the effect of vaned diffuser on the performance of a centrifugal compressor. The simulation was undertaken by using a commercial software, the so-called ANSYS CFX...
International Journal of Rotating Machinery, 2017
The transition process from a diffuser rotating stall to a stage stall in a centrifugal compressor with a vaned diffuser was investigated by experimental and numerical analyses. From the velocity measurements, it was found that the rotating stall existed on the shroud side of the diffuser passage in the off-design flow condition. The numerical results revealed the typical vortical structure of the diffuser stall. The diffuser stall cell was caused by the systematic vortical structure which consisted of the tornado-type vortex, the longitudinal vortex at the shroud/suction surface corner (i.e., leading edge vortex (LEV)), and the vortex in the throat area of the diffuser passages. Furthermore, the stage stall, which rotated within both the impeller and diffuser passages, occurred instead of the diffuser stall as the mass flow rate was decreased. According to the velocity measurements at the diffuser inlet, the diffuser stall which rotated on the shroud side was shifted to the hub sid...
Numerical Simulation of Stall Inception Mechanisms in a Centrifugal Compressor With Vaned Diffuser
Journal of Turbomachinery, 2016
The present paper numerically investigates the stall inception mechanisms in a centrifugal compressor stage composed of a splittered unshrouded impeller and a vaned diffuser. Unsteady numerical simulations have been conducted on a calculation domain comprising all the blade passages over 360 deg for the impeller and the diffuser. Three stable operating points are simulated along a speed line, and the full path to instability is investigated. The paper focusses first on the effects of the mass flow reduction on the flow topology at the inlet of both components. Then, a detailed analysis of stall inception mechanisms is proposed. It is shown that at the inlet of both components, the mass flow reduction induces boundary layer separation on the blade suction side, which results in a vortex tube having its upper end at the casing and its lower end at the blade wall. Some similarities with flows in axial compressor operating at stall condition are outlined. The stall inception process sta...
Journal of Mechanical Science and Technology, 2014
In compression systems, instability has long been an important issue. However, compared to axial machines, relatively little work has been done on the stability of centrifugal machines. Especially, many analytical models of stabilities have been developed to predict and control rotating stall, using compressor characteristic. However, stability models for centrifugal compressors are not scarce. Much research on compressor stability has focused on stalling flow coefficient and rotating stall phenomenon at the stalling flow coefficient. Given this situation, this paper presents a stability analysis of centrifugal compressors to predict rotating stall inception as well as the speed and number of cells. This analysis involves the use of compressor geometries, a steady compressor characteristic, and threedimensional flow analysis in the diffuser. The flow field perturbations at the axial inlet duct, impeller, and radial exit duct are determined via an eigenvalue analysis. The predictions are validated against experimental results from compressors with three different diffuser widths. The model accurately predicts the rotating stall inception flow coefficient. As the compressor characteristic becomes less steep with increasing diffuser width, the stalling flow coefficient increases. Also, experiment validates the model prediction that, depending on the dominant mode of flow perturbation, the number of rotating stall cells can be changed from three to two cells in the tested configurations. Furthermore, the cell speed increases as the flow coefficient decreases for a given number of stall cells. However, when the stall cell number is reduced, the cell speed decreases.
Analysis of Vaneless Diffuser Stall Instability in a Centrifugal Compressor
International Journal of Turbomachinery, Propulsion and Power
Numerical simulations based on the large eddy simulation approach were conducted with the aim to explore vaneless diffuser rotating stall instability in a centrifugal compressor. The effect of the impeller blade passage was included as an inlet boundary condition with sufficiently low flow angle relative to the tangent to provoke the instability and cause circulation in the diffuser core flow. Flow quantities, velocity and pressure, were extracted to accumulate statistics for calculating mean velocity and mean Reynolds stresses in the wall-to-wall direction. The paper focuses on the assessment of the complex response of the system to the velocity perturbations imposed, the resulting pressure gradient and flow curvature effects.
INFLUENCE OF VANELESS REGION DESIGN GEOMETRY ON THE PERFORMANCE OF CENTRIFUGAL COMPRESSOR
Influence of the vaneless region design geometry on the compressor performance and wide of stable operation have been experientially investigated. Two main geometries of the vaneless region design were investigated. The first design geometry is the clearance between the rotor exit and diffuser vanes inlet. This clearance was changed from minimum to maximum in five steps as C = 0.02, 0.1, 0.18, 0.26 and 0.34 relative to the distance between the rotor exit and the main diffuser vane inlet (original one). The second design geometry is considered by installing small vanes at the vaneless region between the rotor exit and the main diffuser vane inlet with different lengths and circumferential positions. The small vanes lengths were changed as, S = 0.9, 0.75, 0.5 and 0.25 relative to the clearance between the rotor exit and diffuser vane inlet. The circumferential positions of these small vanes were changed from the extension of the main diffuser vanes to 0.25 near pressure side of the main diffuser vanes to the middle or 0.5 of the diffuser channel and to 0.25 near suction side of the main diffuser vanes. The effects of all parameters on the limit of stable operation due to stall and surge at reduced flow conditions as well as on the compressor pressure coefficient were demonstrated. The pressure distributions at one of the diffuser passages at its inlet and exit were obtained. Simultaneously with the above conventional measurements, two pressure transducers with high sensitivity response were installed at the inlet and the exit of the diffuser to measure the time variation of static pressure. The data were processed using the Fast Fourier Transformation analysis (FFT) to estimate the Power Spectrum Density (PSD) for detecting the initiation of rotating stall and surge. The results show 13% improvements in stable flow range with clearance ratio of 0.18 and 5% in pressure coefficient and efficiency. Installing small vanes at 0.25 from the main diffuser vane pressure side gives about 36% improvement in stable flow range relative to the diffuser without small vanes. Comparisons between the experimental results and the available previous experimental and theoretical work show acceptable agreements.
Centrifugal Compressor Diffuser Rotating Stall: Vaned Vs. Vaneless
12th European Conference on Turbomachinery Fluid Dynamics and hermodynamics, 2017
Diffuser rotating stall (in both cases of a vaneless or a vaned configuration) is still one of the open questions which has never been fully understood because of the complexity of the phenomenon and the experimental difficulties to get reliable measurements in such a complex environment. Under this perspective, Computational Fluid Dynamics (CFD) is an interesting tool to "see" the flow and to provide a basic understanding of the associated physics. Several published work have shown that a simplified model of the diffuser without the upstream impeller and the downstream return channel, with some realistic boundary conditions entering the diffuser, can provide a qualitative analysis of the stall onset. Thanks to the simplified model it has been possible to change progressively the boundary conditions in such a way that the continuous reduction of flow in time, as it is the experimental procedure, has been fully simulated. Vaneless and vaned options have been compared.
Numerical Simulation of Flow inside a Vaned Diffuser Of a Modified Centrifugal Compressor
2012
In this paper, the flow inside a small modified centrifugal compressor with a vaned diffuser used in an automotive turbocharger application was simulated. It has been known that the performance of the centrifugal compressor depends on the interaction between the compressor impeller and the vaned diffuser. The modified compressor comprises two splitters, which are not extended to the leading edge. If the splitters extend to the leading edge, it would produce a sufficient blockage to cause choking at high speed. This research aimed to study and simulate the effect of a vaned diffuser on the performance of a modified turbocharger compressor. The simulation was undertaken using CFD analysis to predict the aerodynamic flow field and characteristics. The mesh generator of a CFD code was used to generate a polyhedral mesh. Steady state analysis was carried out for the stage with the mixing plane approach. According to the results of the simulation, the vaned diffuser flow is characterized by a subsonic flow and there is no choking in between impellers exit and vane inlet. It was also observed that the outlet diffuser velocity is lower than outlet impeller velocity. At the outlet of the vaned diffuser, the total pressure was found to decrease, and the static pressure increase.