Numerical Study of the Improvement in Stability and Performance by Use of a Partial Vaned Diffuser for a Centrifugal Compressor Stage (original) (raw)
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Numerical Studies of Twisted Vaned Diffuser on the Performance of a Centrifugal Compressor Stage
This paper examines the effect of twist in diffuser vane from hub to shroud on the performance of an industrial centrifugal compressor stage. The chosen diffuser has an aerofoil section with varying blade chord from hub to shroud due to blade twist or in other words, the solidity of the diffuser blade is varied from hub to shroud. The twisting is given to the diffuser blade by rotating the diffuser blade opposite to the direction of rotation of the impeller keeping its leading edge as origin resulting in different stagger angles from hub to shroud. The analysis was conducted at an impeller tip Mach number of 0.35. The overall stage performance is evaluated in terms of head coefficient, stage efficiency and power coefficient of the stage and static pressure recovery coefficient of diffuser vanes for different diffuser vane twist angles with varying flow coefficients. The observed optimum twist for the best performance is 9 o for the chosen impeller diffuser configuration.
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 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.
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.
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...
Numerical investigation of rotating stall in centrifugal compressor with vaned and vaneless diffuser
Journal of Thermal Science, 2015
This study presents a numerical simulation of the stall and surge in a centrifugal compressor and presents a description of the stall development in two different cases. The first case is for a compressor with vaneless diffuser and the second is for a compressor with vaned diffuser of the vane island shape. The main aim of this study is to compare the flow characteristics and behavior for the two compressors near the surge operating condition and provide further understanding of the diffuser role when back flow occurs at surge. Results showed that for a location near the diffuser entrance, the amplitude of the static pressure fluctuations for the vaneless diffuser case is higher than that for the vaned diffuser case near surge condition. These pressure fluctuations in the case of the vaneless diffuser appear with a gradual decrease of the mean pressure value as a part of the surge cycle. While for the case of the vaned diffuser, the pressure drop during surge occurs faster than the case of the vaneless diffuser. Also, results indicated that during surge in the case of vaneless diffuser, there is a region with low velocity and back flow that appears as a layer connecting all impeller passages near shroud surface and this layer develops in size with time. On the other hand, for the case of vaned diffuser during surge, the low velocity regions appear in random locations in some passages and these regions expand with time towards the shroud surface. Results showed that during stall, the impeller passages are exposed to identical impact from stall cells in the case of vaneless diffuser while the stall effect varies from passage to another in the case of the vaned diffuser.
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...
Energies, 2021
This study numerically investigates the beneficial effects of positive pre-swirl on the aerodynamic performance and internal flow field in a centrifugal compressor stage with variable inlet guide vanes (VIGVs) at low mass flow rates. Four positions of VIGV are considered, including 0◦, 30◦, 45◦, and 60◦ angles. The latter three positions of VIGV induce positive pre-swirl. Numerical results show that as positive pre-swirl increases, the aerodynamic performance curve of the stage moves in the low mass flow rate direction. In the three cases of positive pre-swirl, there was an improvement of approximately 9.95% of stall/surge margin greater than in conditions with no pre-swirl. The regulation of IGV can effectively improve the unstable flow of the compressor stage at low mass flow rates. A low frequency that has a great influence on the internal flow of the compressor stage is found, and the unstable flow caused by low frequency is analyzed by the combination of streamline distribution, spectrum analysis, vector, entropy increase, and modal decomposition method. Meanwhile, the modal decomposition method and flow field reconstruction techniques are used to investigate the coherent flow structures caused by low frequency under different guide vane openings.
CFD investigation of flow through a centrifugal compressor diffuser with splitter blades
Journal of Engineering and Applied Science, 2021
The aerodynamic losses in centrifugal compressors are mainly associated with the separated flow on the suction sides of impeller and diffuser vanes. The overall performance of such compressors can be improved by adding splitter vanes. The present work examines the effect of varying the geometrical location of the splitter vanes in the diffuser on the overall performance of a high-speed centrifugal compressor stage of a small gas turbine. To increase the pressure recovery through the diffuser, two radial sets of vanes are used. The first set of vanes (diffuser-1) is equipped with splitter vanes, placed mid-distance between the main vanes, while the vanes of the second set (diffuser-2) are conventional vanes. Flow through the compressor was simulated using the ANSYS 19 workbench program. Flow characteristics and compressor performance were obtained and analyzed for different circumferential positions of the splitting vanes relative to the main vanes of diffuser-1. The study covered se...
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.