Impeller Exit Width Research Articles

Influences of the Flow Cut and Axial Lift of the Impeller on the Aerodynamic Performance of a Transonic Centrifugal Compressor

In this study, the influences of the flow cut and axial lift of the impeller on the aerodynamic performance of a transonic centrifugal compressor were analyzed. The flow cut is a method to reduce the flow rate by decreasing the impeller passage height. The axial lift is a method of increasing the impeller passage height in the axial direction, which increases the impeller exit width (B2) and increases the total pressure. A NASA CC3 transonic centrifugal compressor with a backswept angle was used as a base compressor. After applying the flow cut, the total pressure at the target flow rate was lower than the total pressure at the design point due to the increase in the relative velocity at the impeller exit. After applying the axial lift, the total pressure at the design flow rate was increased, which was caused by the reduction in the relative velocity as the passage area at the impeller exit was increased. By applying the flow cut and axial lift methods, it was shown that the variation in relative velocity at the impeller exit has a significant effect on the variation in total pressure. In addition, it was found that the relative velocity at the impeller exit of the target flow rate is maintained similar to the base impeller when the flow cut and the axial lift are combined. Therefore, by combining the flow cut and the axial lift, three transonic centrifugal impellers with flow fractions of 0.7, 0.8, and 0.9 compared to the design flow rate were newly designed.

Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO2

This paper presents a design strategy for very low flow coefficient multistage compressors operating with supercritical CO2 for carbon capture and sequestration (CCS) and enhanced oil recovery (EOR). At flow coefficients less than 0.01, the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance. The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale reinjection compressor currently in service. Three-dimensional, steady, supercritical CO2 computational fluid dynamics (CFD) simulations of the full stage with leakage flows are carried out with the National Institute of Standards and Technology (NIST) real gas model. The design study suggests that a nondimensional impeller exit width parameter b2* = (b2/R)ϕ of six yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the nondimensional impeller exit width parameter b2* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO2, the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.

Experimental study of centrifugal compressor vaneless diffuser width

Seven different vaneless diffuser designs for a centrifugal compressor, varying only in diffuser width, were studied experimentally. The studied diffuser widths versus impeller exit width were 1.0, 0.903, 0.854, and 0.806. Three of the narrowed diffusers had the width reduced from the hub and shroud divided evenly, and the three others had the width reduced only from the shroud. The total and static pressures, the total temperature and the flow angles at the diffuser inlet and outlet were measured at the design rotational speed with three different mass flows. The impeller and diffuser performance was studied along with the axial distributions of flow angles and velocities in the diffuser. The results revealed that the pinch improved the compressor stage and impeller performance but deteriorated the diffuser performance. The pinch clearly decreased the secondary flow region present near the shroud. The pinch implemented in the shroud is more beneficial than pinch divided between the hub and the shroud. In order to obtain the beneficial effects of pinch, the pinch should be sufficient. However, excessive pinch deteriorates the compressor performance.

Performance Improvement of a Centrifugal Compressor Stage by Increasing Degree of Reaction and Optimizing Blade Loading of a 3D Impeller

Performance improvement of 3D impellers in a high specific speed range was investigated using computational fluid dynamics analyses and experimental tests. In order to reduce the loss production within the stator passages, the backsweep angle of the impellers was increased. At the same time, the inlet-to-exit relative velocity diffusion ratio was also increased by increasing the impeller exit width to prevent the reduction in the pressure ratio. Moreover, the blade loading distribution at the impeller shroud side was optimized to suppress the surge margin reduction caused by the increased relative velocity diffusion ratio. Five types of unshrouded impellers were designed, manufactured, and tested to evaluate the effects of blade loading, backsweep angle, and relative velocity diffusion ratio on the compressor performance. The design suction flow coefficient was 0.125 and the machine Mach number was 0.87. Test results showed that the compressor stage efficiency was increased by 5% compared with the base design without reducing the pressure coefficient and surge margin. It was concluded that an increased relative velocity diffusion ratio coupled with large backsweep angle was a very effective way to improve the compressor stage efficiency. An appropriate blade loading distribution was also important in order to achieve a wide operating range as well as high efficiency.

一枚羽根遠心羽根車の設計法に関する研究(流体工学,流体機械)

The sewage pump are demanded a high pump efficiency and a performance in passing foreign bodies. Therefore, the impeller used by these usages demands the large passing particle size (minimum particle size in the pump). However, because conventional design method of pump impeller calculates impeller exit width small, it is difficult to apply to single-blade centrifugal pump impeller which is used as a sewage pump. This paper proposes design method for single-blade centrifugal pump impeller. As a result, the head curve of the impeller by this design method satisfied a design point, and pump efficiency was over 62% more than conventional single-blade centrifugal pump impeller. Having compared design values and CFD analysis values, the suction velocity ratio of the design parameter accorded well, but the relative velocity ratio did not agree by the influence of the backflow of the impeller entrance.

Design Method for Single-Blade Centrifugal Pump Impeller

The sewage pumps are demanded a high pump efficiency and a performance in passing foreign bodies. Therefore, the impeller used by these usages requires the large passed particle size (minimum particle size in the pump). However, because conventional design method of pump impeller results in small impeller exit width, it is difficult to be applied to the design of single-blade centrifugal pump impeller which is used as a sewage pump. This paper proposes a design method for single-blade centrifugal pump impeller. As a result, the head curve of the impeller designed by the proposed design method satisfied design specifications, and pump efficiency was over 62% more than conventional single-blade centrifugal pump impeller. By comparing design values with CFD analysis values, the suction velocity ratio of the design parameter agreed well with each other, but the relative velocity ratio did not agree due to the influence of the backflow of the impeller entrance.

1729 一枚羽根遠心羽根車の設計法に関する研究(S19-6 流体機械に関連した流動現象とその解析・計測(6)流体機械の性能と内部流れ,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

It is important for the evaluation of the sewage pump to secure the passing diameter large compared to a pump diameter. But the impeller exit width is too small when we design by a conventional design method Therefore we cannot apply to non-clog type impellers for a sewage pump. A new design method for the centrifugal pump impeller theoretically was suggested This design method applied to a single blade centrifugal pump impeller. Pump performance, pressure distribution and behavior of radial thrust were measured about this impeller. Furthermore, we investigated the validity of the design by using CFD analysis.

Influence of the volute design parameters on the performance of a centrifugal compressor of an aircraft turbocharger

Theoretical and experimental studies on the effect of the volute design parameters on the centrifugal compressor range of stable operation and pressure rise coefficient were investigated. In the theoretical study, the effect of the symmetric and tangent volute area ratios (AR) on the diffuser pressure recovery factor and flow stability was investigated. In the experimental study, different configurations of a symmetrical and tangent (overhung) volutes with different ARs from 0.4 to 0.8 were investigated. Comparisons between the two different types of volute design parameters on compressor stable flow range and pressure rise coefficient were made. Effect of the gab between the diffuser vanes and the volute casing on the compressor stability was investigated. This gap was changed from 0 to 0.4 of the impeller exit width in five tests. The pressure fluctuation leading to stall was observed at the vaneless region. Stall and surge were detected by analysing both the fluctuations of pressure signals and the power spectrum density. The effect of different volute configurations and clearance on stall initiation, and hence compressor stable operation, were discussed. The theoretical results show that the increase in the symmetric volute exit radius increases the pressure recovery factor in the diffuser. Experimental data show that the increase in the AR of symmetric and tangent volutes increases the compressor range of stable operation as well as pressure rise coefficient.

Generation Mechanisms of Low-Frequency Centrifugal Fan Noise

Earlier investigations by Stahl & Argiiello (1986), who studied the sound field in a circular duct generated by a jet flow, indicated that flow separation occurring in a diffuser attached to the jet nozzle leads to increased sound levels in the low frequency regime. This observation formed the basis for the present study. Experimental investigations were carried out with five centrifugal fan impellers of different geometries. Cross correlation studies involving the unsteady wall pressures on various parts of the impeller and the fan casing lead to the following insights: The low frequency noise of centrifugal fans is generated by classical flow separation regions located on the shroud and the blade suction sides of the impeller. The design of the impeller shroud is of particular importance: A large radius of curvature in connection with a smaller impeller exit width improves the flow along the shroud. As a result, flow separation is reduced, the flow leaving the impeller is more uniform and less turbulent, and the low frequency noise is reduced. The flow in the fan casing also contributes to the low frequency noise: Here the flow along those parts of the volute is important which is close to the impeller periphery, specifically the cut-off area.

Prediction of Outlet Flow Characteristics of Centrifugal Impellers : 1st Report, Consideration of Velocity Distortion

The analytical method for predicting the flow characteristics at the outlet of centrifugal impellers is presented taking into account the influence of velocity distortion in both the peripheral and the hub-to-shroud directions and its effectiveness was confirmed with the flow measurements around pump impellers. The results show that the increase of wall friction due to a mixing of flow just after the impeller exit should be adequately considered for successful prediction, and the influence of velocity distortion in the hub-to-shroud direction should also be considered for the case of the parallel-walled diffuser width being larger than the impeller exit width, while it is negligible in the case of the diffuser width being equal to the latter. It is pointed out that the peripheral velocity component of the outlet flow used for the determination of slip factor and the theoretical head should be defined by the mass-averaged value of the section.